Genuine 2 stroke comeback - soon?

Nice to see the FIM still pushing. Better late than never

never gonna happen. 2 strokes are done

2 strokes are just beginning their 2nd round of kicking 4 stroke butt. Wait until the 2011 mx season and tell me the same thing. Go big or go home. ;-)

Serious question:
If you could have a bike with a little more power and the same power characteristics. Fewer moving parts, less reciprocating mass with better fuel economy and less maintenance than your four stroke would you want it?

My 250f is toast after one season. New crank, piston, valves, and baskets, I could rebuild a two stroke like 3 times. I hate my four stroke.

i love 2strokes! But here we go AGAIN!!!

If 2 strokes never existed until now and people felt like it was the "new thing" there would be nothing but smokers at the track.
Plain and simple, if the rules were equal there would be no thumpers.

"......Americans are a bovine, manipulable herd - putty in the hands of advertisers who can manufacture demand for whatever products manufacturers want to produce." ~ George F. Will

Yeah, that's pretty much it, Racer92. Man, you're quick with the quotes.

Perfect.

I am doing my part.......got rid of 2 four strokes in the last year, and picked up 3 2 strokes.
rmz450 replaced by rm250
crf50 replaced by rm65
and just got pw50 for the man child

still have one crf50 left......

And you aren't even into the expensive years yet.

I've been saying this allllll along. 2 strokes can, and always have been, easier to make a lot of power with a smaller engine. Top it off with superior reliability, low cost of maintenance, lower production costs and the most important issue (to the powers that be): they are inherently easier to make reasonably quiet. FI and clean burn technology takes care of the emissions deal so whats not to understand?

http://www.dirt-bike-tips-and-pics.com/consumer-blindness.html

Just sayin.

when I got into dirt bikes in the early 70's thats exactly what was going on.

The 2 has distinct advantages, though you refuse to admit it, and there is significant interest in bringing them back, especially outside the blind little microcosm(sp) you live in (American MX, and even within those ranks there is dissent). There are quite a few manufacturers investing research dollars into them (I guess they should be consulting you first), and those are just the ones that are being upfront about it.

You don't want to see them come back because you are in LOVE with the 4-stroke. I get it. No different than the guys who refuse to let go of the 2-stroke, because of a nostalgic love affair. Both are wrong reasons. How about a third group that can rationally see the inherent advantages both have. The advantages that the 4 has, will influence the goals in the development of the new 2's (torque, more forgiving powerband).

Tiki, you are the only one still beating that tired old drum (emissions). The current crop of 2's can meet any emission requirements that the 4's can meet. Give it up already. The EPA has no laws against 2-strokes, just emission standards to meet. You are the one with suspect intelligence, my friend.

Rant on Mr. Hammer. I'm sure you can tell me how it is.

Tiki - get a clue.

Maybe this will help -

From - http://www.dirt-bike-tips-and-pics.com/future-of-two-strokes.html



The Future Of Two Strokes And Your Dirt Bike

This is a unique page on my site. It relates to the future of two strokes in general, but ties in with how this will affect you and your choice of dirt bikes. The information here comes from Tim Hickox who will be putting up regular posts using his many years of experience with engine design.
So, if you are interested in what's happening out there in regards to engine design and dirt bikes... read on and be sure to check back as this page is updated!
A quick introduction with Tim...
"I can't put a lifetime into a bottle, but: I got my first motorcycle when I was 14. After owning a Greeves, CZ, OSSA, Honda, Suzuki, et al., I had a pretty good idea of what worked and what didn't. Engineering was my destiny. I was service manager for a motorcycle importer/manufacturer; I was Technical Editor of a motorcycle magazine and then Technical Analyst for the Motorcycle Industry Council. I worked for Honda on the early development and testing of their trials machines, and so on. I wanted to stay in the motorcycle industry, but it was going in the "wrong" direction for me (pushing everything up-market to maintain dollar-volume as unit-volume fell). So I went into aerospace. I worked on the Space Shuttle and on many earth-observation satellites. I am now writing a book (philosophy) and working on a unique vehicle (with a two-stroke engine!) that can get 300-mpg at 65-mph."
Tim Hickox
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3rd Nov 2008:
I understand the point on dirt-bike specifics, but if we really try to limit it so, there isn't much to talk about! This is the problem! What I really think is needed is to offer an alternative to the misinformation that has confused so many people.
There are many ways to build two-stroke engines. People must understand that what has been offered to motorcyclists in the past is not a 'two-stroke engine'. That is, the future of two-stroke dirt bikes, if there is going to be any future, need not be, in fact cannot be, more of the same. The guys seem to think that what was, is no longer acceptable, therefore we have to chuck the whole idea of 'two-stroke engines' and switch to four-stroke engines. You see, what we really want to talk about is... two-stroke engines of the future! But that future is not here, so how can we possibly put limits on what it is? The 2009 Skidoo 600 E-TEC is the best two-stroke going now that could represent the direction in which some future motorcycle engines may go.
But I must make this point: If the two-stroke engine does not return as a viable transportation option for two-wheelers in developing countries, I think it will die. I think this is really what the Japanese companies have been saying. If they can't build and sell two-stroke bikes to the masses, at the low-end of the spectrum, they aren't going to support the type anywhere. So... what may be needed now, to ensure the future of "two-stroke dirt bikes", is a low-tech (i.e. cheap), clean, economical two-stroke motorcycle. The 'death' of two-stroke dirt bikes has come after the loss of two-stroke motorcycles in general; and I believe that the future of two-stroke dirt bikes may have to come after a revival of the wider market.
What I want to get across to people now, which is about all there is to talk about, is the potential... what two-stroke engines can be - the future of two strokes. Whether one sticks such an engine in a dirt bike, a street bike, a lawnmower, or whatever, is an issue somewhat outside the realm that I will address. Within that realm, I must refer to a lot of things that are even outside the world of motorcycles, like the Skidoo. Because that is what we have or have had. Otherwise, I can only talk of theory, of thermodynamics, which most people won't understand, and which would leave me open to any and all contradicting theories.
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17th Nov 2008:
Let's get right to the question: What is wrong with the two-stroke engine? Nobody seems to dispute that the two-stroke is smaller, lighter, and cheaper to make than a four-stroke of equal performance. I think it has also been conclusively demonstrated that a high-performance four-stroke engine is more expensive to maintain. This is a very impressive list of advantages.
So what's wrong? Somebody is going to say that fuel consumption and exhaust emissions are too high. But wait! Is anybody screaming about the fuel consumption of Formula-One cars? Does anybody care about NOx emissions in NASCAR? Why is the racing two-stroke going away? A lot of people blame the government. But the 'EPA ban' on two-strokes is a myth. The EPA has said over and over that their regulations can be met by four-stroke and fuel-injected two-stroke engines.
The problem is: there are no fuel-injected two-stroke motorcycle engines in production. (I'm ignoring a few scooters.) So we have to look first at the fuel-injected two-stroke engines made for other applications, which could be adapted to motorcycles.
Let's skip over motocross and other closed-course racing that is run on private property - where (like NASCAR) anything goes. Enduro bikes may operate on public land, and the government does regulate what happens there. You should not be surprised to hear that a snowmobile operated on public land gets treated the same as a motorcycle. So, back in year-2000, the SAE (Society of Automotive Engineers) created the SAE Clean Snowmobile Challenge, a competition to stimulate the development of cleaner snowmobiles. (Too bad they didn't choose motorcycles!) What happened? I am going to tell you why - technologically - the future of the two-stroke engine looks extremely bright!
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1st Dec 2008:
As expected, the CSC was a four-stroke affair. Nobody thought of building a 'clean snowmobile' with a 'dirty' two-stroke engine. But then, somebody always has to show off. After winning the CSC in 2002 and 2003 with a BMW, fuel-injected, four-stroke engine, the University of Idaho decided to do the unthinkable. The fact remained, they said, that "...avid snowmobile riders still prefer the lighter and more powerful two-stroke engine." They pointed out that a 'simple' four-stroke snowmobile only made a little more than 50-hp. A high-tech, 750-cc with turbocharger and fuel injection made about 85-hp. The lighter Ski Doo, two-stroke, made 120-hp. So the UICSC team switched to a stinker. They won again in 2007.
What they did was adapt a direct-fuel-injection system (E-tec) from an Evinrude outboard motor. What you should appreciate is that everyone else in this competition was trying to get the best out of some four-stroke. And all the testing was done by engineers, based on EPA 2012 standards. No hype, no bias. The two-stroke won!
To put this into a dirt-bike perspective, a snowmobile is much heavier than a dirt bike - 500+ pounds. And the power transmission (a belt-track) is very inefficient (which is why they need so much power). So the snowmobile engine must work much harder, burn more fuel, and create higher exhaust emissions than a dirt bike, given the same use. If a two-stroke snowmobile is not a problem, a two-stroke dirt bike would be a party.
For 2009, there is the Ski Doo 600 E-tec (do a Google, and you will find pics and details). So let's imagine that we wanted to build a killer enduro bike. One half of that Ski Doo motor would give us a 300-cc, single-cylinder motor with 60-hp. It would have lower exhaust emissions and lower fuel consumption than any four-stroke of equal performance. And, of course, it would still be a simple, compact, light, easy-to-maintain two-stroke. All the real engineering has already been done - it works, the parts are in production.
If we went to Rotax, in Austria (Evinrude, Ski Doo, and Rotax are owned by Bombardier, in Canada), who builds the Ski Doo engine, and motorcycle engines, we could just tell them to take parts that they are already making and put them together into this killer enduro bike engine/transmission assembly. I'm sure they would be very happy to do this (for a nice chunk of money, of course). The UICSC team estimated that the cost of E-tec, for the twin-cylinder, would be about $300.
So what's wrong with the two-stroke? And where is the future of two strokes? Hold on, it gets better!
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15th Dec 2008:
To reiterate: Clean, efficient two-stroke engines (making 120-hp!) are in production and used in off-road vehicles on public lands subject to EPA regulations. Unfortunately, those vehicles are not motorcycles!
Let's look now at what Honda was doing just before they announced to the world, "No more two-strokes!" The story really begins a long time ago when people discovered that two-stroke engines would sometimes run without spark-ignition. That is, one could pull off the sparkplug lead, while the engine was running, and it would continue to run as if nothing had changed. People said, "It's dieseling." Actually, what was happening was very different from combustion in a diesel engine. The phenomenon was best explored by Shigeru Onishi in the 1970s. He called it "Active Thermo-Atmosphere Combustion (ATAC)." He said, "With ATAC the fuel consumption and exhaust emissions of two-stroke cycle, spark-ignition engines are remarkably improved, and noise and vibration are reduced." He eventually showed that a carbureted two-stroke engine could be more efficient (lower fuel consumption) than a diesel engine. But only within a narrow range of speed-load conditions.
In the 1990s, Honda R&D Chief Engineer, Yoichi Ishibashi, wanted to clean up two-stroke motorcycle engines. He called ATAC: "Activated Radical (AR) Combustion." Onishi's engines ran generators at nearly a constant speed. Ishibashi needed to greatly extend the range where AR combustion was stable. He found that throttling the exhaust was the secret. He developed a 400-cc single-cylinder engine. For real-world testing, several of these EXP-2 engines were put in endurance-racing chassis. They ran in the Granada-Dakar Rally and the Baja 1000. The results were good enough for Ishibashi to press on and develop the engine further.
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29th Dec 2008:
Two bikes were entered in the Dakar - both finished. As Ely Kumli reported in 1997: "The race results were very good even though the bike was not designed to win races, but to test new technology. When the dust settled, the EXP-2 had earned 5th overall and first in both the under-500cc and experimental classes..."
As it turned out, the second-string riders had been given the better equipment! "Compared to Honda's NXR780 four-stroke twin rally race bike," said Kumli, "the EXP-2 has very similar performance, with several advantages. While the single-cylinder EXP-2 produces 54hp to the big NXR's 71hp, they both make 58 lbs-ft of torque, but the EXP-2 is 118 pounds lighter giving it a slightly better power-to-weight ratio. What all this boils down to is that the EXP-2 has about the same real-world performance as the 780, but with substantially better fuel economy and lower emissions."
As good as that sounds, Ishibashi was just beginning. The exhaust valve prevented most of the fresh charge (fuel-air mixture) from getting lost out the exhaust port, but he needed to scavenge the cylinder with air only, and admit the fuel late in the cycle. He came up with a 'pneumatic injection system' that used a standard four-stroke-type fuel injector. Ishibashi summarized the results: "...PDI-AR Combustion drastically decreases HC emission close to the level of four-stroke, and CO and NOx level is 1/5 and 1/7 of four-stroke level respectively. Furthermore, fuel consumption is improved 15% compared with the four-stroke." His English isn't perfect.
I will add: A Honda four-stroke engine giving equal performance had carbon monoxide emissions 500% higher and oxides of nitrogen emissions 700% higher than his two-stroke. Without an oxidizing catalyst, the two-stroke hydrocarbon emissions were slightly more than the four-stroke's, but with a cat, the levels of HC were the same. The CO levels dropped, but the two-strokes advantage over the four-stroke was actually greater. (NOx is unaffected by an oxidizing catalyst).
Obviously, Honda had a good engineer doing good work. He had one more song to sing before the bean-counters dropped the axe on him...
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12th Jan 2009:
He did not begin with the long-stroke CR250, but with half of Honda's NSR500V. This was a case-reed road-racing engine with a square bore-stroke ratio - like the McGrath generation YZ250s. Honda said that it made "135-plus horsepower" at 10,500 rpm; about 68-hp for a 250.
If ten-five makes it sound like all the power was on the top end, one rider called it a "torque monster" and said the power was "similar to an open-class motocrosser". This engine was introduced at the first 500 GP, 1996. I have given these details because there has been, at least, the suggestion that the long-stroke 250 two-stroke had reached some sort of a technological limit (at less than 50-hp) and that the 450 four-stroke was some sort of a 'solution' to this 'problem'. In 2005, MotoVerde magazine (Spain) dyno tested the CRF450 and the RMZ450 and got 56-hp and 55.2-hp, respectively.
So this was the basis of Ishibashis new 'environmental conscious power unit'. But he really only used the parameters; he built a new design. He got AR Combustion to work from about 3000 rpm to peak power - meaning that the engine could operate normally within that range without an ignition system! At the very bottom, the sparkplug initiated combustion and his Pneumatic Direct Injection limited HC and CO emissions. The engine made 60-hp at 11,000 rpm.
Because the solenoid injector fed an anti-chamber - not the cylinder - its high-frequency limit did not restrict high-end power. In other words, Ishibashis approach did not require the invention of any new technology, only basic engineering. The technology required existed in the 1970s.
There was a rumor that the NSR500V might be turned into a killer street bike. Of course, that never happened. Instead, Ishibashi was sent off to design Civic door latches, or whatever...
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27th Jan 2009:
About now, someone should be asking: "If Honda knew how to make two-strokes that are cleaner and more economical than their four-strokes, that do not require any new technology, and that could win endurance races, why didn't they produce them?"
Back in 1984, Steve Anderson (then Technical Editor of Cycle World Magazine) went to Japan for a VIP tour of Honda's (then) new racing R&D facility. There, Takeo Fukui, Design Director, "...made it clear that Honda views itself as a four-stroke company that will not be satisfied until it wins all of its championships with four-strokes." Simply put, the four-stroke engine has been company policy; in fact, it has been more of a religion. Anderson saw, "...shelf after shelf of oval pistons..." and was told that engines were running "...with at least eight valves per cylinder..." and turbo charging. We haven't heard anything about oval pistons in a long time because, after spending umpteen millions of dollars, somebody saw that it was a really stupid idea. It was a way of getting around the racing rules. Four-strokes couldn't win within the rules, so Honda had to find some way to beat the two-strokes and/or the rules. The trouble was, no matter what the shape of the pistons, or how many valves they could stuff in, or how many rpm they could get out of them, the two-strokes kept getting faster and the four-strokes weren't even able to keep pace. In 1995, Mick Doohan was asked how much power his NSR500 two-stroke made: "I can't tell you that, but I can tell you that if Honda produced a one-liter motor it would make upwards of 400 horsepower."
It was about that time, it seems, when one of the four-stroke monks had an epiphany: "Instead of spending more money on exotic engines," he might have said, "which hasn't been getting us anywhere, why don't we just buy the AMA and FIM and make our own rules? For instance, we could require that all two-stroke riders wear their helmets backward - if they can't find the first turn, we have to win!" And that is - sort of - what happened.
Honda also likes to be seen as an 'engineering company'. I must point out that trying to force a particular technology (that is not working!) on a naive public by denying options, rewriting race rules, etc. is not any sort of 'engineering'. Jeremy McGrath defined the engineer's job: "I think the effort should be to build the best bike, period, no matter what it is." Thank you, Jeremy.
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10th Feb 2009:
So, what is the 'best bike'? What does that mean - and who decides? Honda said, "No more two-strokes!" All over the world, riders dumped their two-strokes and rushed to buy four-strokes.
Back in the 1950's, a few intellectuals, like J.K. Galbraith, issued a warning. As George F. Will put it: "...Americans are a bovine, manipulable herd - putty in the hands of advertisers who can manufacture demand for whatever products manufacturers want to produce." As L.J.K. Setright put it: "If a camel is a horse designed by a committee, then it is the job of the public-relations and advertising men to persuade the customers that the camel is really a horse after all, or that what they really wanted was a camel." Do you know what you want? For the manufacturers, the 'best bike' is the one that gets you to part with the most money. As E.F. Schumacher told us, "...in economics, the only measure of 'good' is profit. I tell people that in engineering, goodness is efficiency. The two are opposites: profit leads to more and more; efficiency to less and less."
For you and me, 'best bike' must mean something else. What we are trying to do here is to give riders more information so that you can decide what the best bike is for you. I don't want my fellow riders to get sucked-in the next time somebody says, "Da hump is a new feature, and next year we'll have a model with two humps!" If the manufacturers succeed by such deceptions, I too will end up riding a camel, because there won't be any more horses! I already know that the best bike for me is the one not being made.
What I will try to do in the next few segments is to look at this notion of the 'best bike' and point out how much the aim depends on you - the individual rider. I will also point to the dirt bikes available today and stress how limited our options are. Where I am going, in the long run, is in the direction of answering this question: If the two-stroke engine has a future in our field of interest, what sort of dirt bikes might we see and how might these new options bring you closer to your 'best bike'?
I will begin by recommending a particular model, available now. I will put this in the form of a two-stroke/four-stroke comparison. I think the contrast is surprising. This bike could be an arrow pointing to much better dirt bikes in the future - if riders can get past their herd instincts.
The future of two strokes continues...
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23rd Feb 2009:
The antagonist in my little play is Suzuki's RMZE 250. It says "Enduro Racing" on the swingarm. This is the motocrosser with minimum chassis changes to qualify as an enduro model. There is no electric starter or other non-essential to add weight. It weighs 229 lbs - less gas. It has five speeds and makes 33.4 hp @ 11.250 rpm. (All numbers in this comparison are from MotoVerde magazine tests, not maker's claims.)
The protagonist (drum roll, please) is the KTM 105 JMC. (I will explain the odd suffix.) This little 105-cc wonder rips 30.5 hp @ 12,000 rpm and has six speeds. But what makes it fly is that the all-up weight, less gas, is under 155 pounds! No, that is not a misprint; Suzy weighs 75 pounds more.
Do this: Go out to the garage and find something that weighs about 75 lbs - a washing machine maybe. Pick it up. Do you really want to take that riding with you?
Let me interrupt this for a brief trip through the Way-Back machine. In 1980, the AMA dropped the 'claiming rule' and the factories gave their top riders unlimited works bikes. Dave Hawkins wrote then: "In motocross racing, it's obvious that power is important. But how much is enough?" In the 125 class: "The problem was not to make more power (30 bhp seemed quite enough for a 175-pound bike)... [and] only the best can fully use these bikes." Today, 30 years later, we still can't go into a dealer and buy a 175-pound bike with 30 hp. Instead, we are expected to dance with Miss Piggy from Hamatsu. What we can do is buy a 105 SX and bring it up to that old works-bike standard. And unless you are a better rider than the likes of Jeff Ward, Mark Barnett, and Broc Glover, that should be "quite enough".
Let's get into the details. First, what is JMC? That is the establishment (in Spain) of Jesus (which in Spanish sounds like Hey-sus) Balaguer. I cannot avoid calling him one of the top two-stroke tuners in the world. He has 'the gift'. He tweaked more than a 50% increase in horsepower from KTM's standard 105 SX. But that statement will make you think that it must be all top-end and could only make noise below ten-grand. In fact, the JMC is stronger everywhere. The smallest percentage increase is at 6000 rpm, where it still makes two ponies more than stock. And the stock motor is in no way peaky; it actually makes less peak power than the 85 SX. The 105 is more of an enduro motor. The JMC pulled the MotoVerde dyno from 3000 rpm to 13,500 rpm. I cannot point to another engine that makes almost 300 hp/liter and has such a wide powerband. And while dynos can give one the wrong impression, the MotoVerde crew said that the track tests confirmed "without a doubt" what the dyno promised. The bike rockets out of turns like...a works bike!
* In reference to the KTM 105 JMC, the test was in MotoVerde number 360, July 2008. For full specs, see number 345, April 2007. The Suzuki RMZE 250 was in number 358, May 2008.
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10th March 2009:
It is time to say that I am not recommending the 105 JMC for motocross. I see it - with the right mods - as a super enduro bike, and that is why I set it against the RMZE. And so I must point out that when the going gets tough and one must slog through at 3-4000 rpm, the 250 has much more gusto. But wait! What should concern us is not torque at the crankshaft, but torque at the rear wheel. Remember, the KTM has a six-speed. It so happens that if both bikes are geared for the same top speed, low-gear on the RMZE is about equal to second on the KTM. Shift the 105 to low and the torque at the rear wheel jumpsup. The extra gear, and the lack of a washing machine, give the 105 the edge everywhere. And it is just those toughest sections where light weight and ease of handling will keep the two-stroke rider smiling and the four-stroke victim puffing. And should you put a wheel wrong and stall the engine, which would you rather kickstart?
Of course, the 105 SX needs some alterations; I'm suggesting a use for which it was not intended. Consider the JMC engine mods, a larger gas tank, an 18-inch rear wheel, and a steering damper, to start with. These are just the changes that are frequently made to other machines, and the aftermarket is there to serve us. And this brings us to the prices. I don't have U.S prices at hand (and the Feds have put sand in our tanks) so I will give the prices from the MotoVerde tests. It is only the difference that concerns us here. The RMZE was 7,699 (Euros) in 2008; the standard KTM 105 SX is 4,690 (Euros) for 2009. Obviously, at the price of the Suzuki, you can do quite a bit to customize the KTM.
I must say more about Balaguer's work on this little motor. The huge improvement from stock might suggest radical changes. In fact, he made only minor alterations - but the right ones! After testing several vendor pipes, he went with one made by DEP. He replaced the standard 28-mm carb (same as the 85 SX) with a 30-mm Mikuni. He cleaned up the ports and set the squish clearance. But 'the secret' was adopting a CRD programmable ignition module; then spending hours finding the right curve. Ignition timing has been largely ignored by tuners. These programmable units are a rather new option and the dyno time required is expensive because any curve only fits a specific engine configuration. Change the pipe (to one with a longer convergent-cone, for example) and the ignition curve has to be gone over. But what Balaguer's results show is that two-stroke engines are much more sensitive to ignition timing than we ever guessed in the old days - when we couldn't do anything about it anyway!
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23rd March 2009:
KTM's 105 has more potential than the Austrians seem to recognize. It is currently the only approach to works-bike performance. For certain persons, it is the way to go. Of course, someone will say, "But that KTM is too..." Indeed, no single model can be a universal dirt bike. As Grant Langston put it: "Let's say you have ten guys riding Kawis, then you might need three or four different frame choices to give each of those guys the best fitting bike."
We used to refer to the 'UJM', the Universal Japanese Motorcycle. Sadly, the Orient has given us the choice of red, green, blue, or yellow. Langston was deploring the one-size-fits-all formula. But it isn't just a matter of physical size. There was a time when a rider could have a Greeves, a Husky, a CZ, or a Bultaco (and the list goes on). Each bike was quite different from the next. The Europeans were not given to committee-think that transcended a particular brand. There was real competition between manufacturers to build a better motorcycle. Dirt bikes evolved rapidly in that period. In contrast, the last 25 years of Japanese domination has been an age of refinement, of developmental stagnation. It is too easy for younger riders to think that the off-road motorcycle has reached some state of perfection and there is nowhere to go from here. Perhaps this is why so many were keen to jump on four-strokes - conned into thinking that this was at last, the next leap forward. In fact, it was a giant leap backward.
When it first looked as if 250 four-strokes might dominate the '125 class', Mitch Payton (owner of Pro Circuit) said that, "...a 125 two-stroke is as fast as a 250 four-stroke around a track. The only thing holding the 125 back is the fact that four-strokes tend to get better starts." He then predicted that soon, "...the Yamaha 125 and 250 two-strokes will be very close to the AMA's minimum weight limit, which would make for an incredible two-stroke. And let's not forget how important weight is... that's why many riders still prefer two-strokes. They handle so much better on tight tracks..." Better yet, throw out those stupid old weight limits. As Colin Chapman (of Formula One car fame) used to say: To go faster, "add lightness!" Payton could not predict Yamaha's future, but he did see a potential future where weight - not horsepower - would win the day. Andif weight is the criterion, we cannot begin with a four-stroke engine. If you want to win a horse race...
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6th April 2009:
A very long time ago, racing classes were delimited according to engine displacement. These rules placed all the emphasis on specific-power (power in relation to a certain displacement). Riders were always crying for more power, so engine development has remained the highest priority. Hello! The horsepower race is over! A small, light two-stroke engine can make all the horsepower any rider can use. What we should learn from recent events is that this traditional relationship between displacement and power is no longer meaningful.
In the beginning, the whole point of class distinctions was not to create an environment in which one rider might out-power the next. Quite the contrary, the intention was to take machine differences out of the equation and find out which rider is better. It is still true that, as Rick Johnson said, "...the rider is 95% of the equation, and the bike is only 5%." But really, the bike and the rider must function together as one biomechanical unit.
Once upon a time, there were many classes. When racers made the rules, the aim was not to force people to buy a bike of a certain size. They tried to make it possible for everybody to race competitively, no matter what size bike anyone happened to own.
I do not expect the powers-that-be to drop this traditional, now obsolete, classification. Instead, I will ask you to try to step away from all the psychological baggage that race rules have forced us to carry. Let's consider, as McGrath suggested, what might be 'the best bike'. Let's play What If... .
The Open-class two-strokes disappeared when the 250s proved to be as fast, and the 500s only harder to ride. I suggest that it is time for a similar change. But I am not saying that 125-cc is the answer. Forget the 125 class. Forget the 250 class. Right now, I think that most riders would find that they perform best with something around 175-cc, when the weight is under 175 pounds. That engine might make as little (?!) as 30 horsepower for enduros, or as much as 45 for a pro MXer. I won't go into details, but you can imagine that this mythical 175 could be a sort of big brother to the KTM 105 JMC. It might be 20 pounds heavier. It would have much more torque at the bottom end. It would fit larger, heavier riders and it would be more stable through the fast sections. It would require no new technology. So why is nothing like this being built?
There was a time when a 650 Triumph was a 'good' dirt bike. Times change. The current four-strokes are also 'good' dirt bikes. And maybe it's time for another change.
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20th April 2009:
Perhaps the message has been received, but I want to make it explicit. The fact that four-strokes are winning races does not mean that four-stroke engines are 'better'. Rules - arbitrary and often pointless - not engineering, determine the end result.
I learned an important lesson from Gabriel Voisin (one of the 'Great Designers' of airplanes and automobiles). He said that only the engineer can be responsible for the machines that have changed the world. The scientist is not responsible, for his realm is epistemology; he is only trying to understand how the universe works. The customer is not culpable, for he hasn't a clue how to go from a clean sheet of paper to a practical, safe, efficient machine. The businessman must be concerned with profit. His interests are too one-sided and he too can claim ignorance. Only the engineer can understand (if he is worthy!) the complexities, priorities, and compromises that go into a sophisticated machine.
That said, engineers are not responsible! Samuel C. Florman, in The Existential Pleasures of Engineering, said that 'engineering' ended with the dropping of the first hydrogen bomb. In the days of Henry Ford and Alfred Scott, engineers were responsible. The Model T Ford was exactly what Henry wanted it to be. The man who bought a Scott motorcycle got a creation of Alfred Scott. With mass production came mass marketing. Many decisions shifted to Board Rooms (what is 'good' for business) and Marketing Departments (what is 'good' for the customer). The Engineering Department lost the right to make certain decisions, such as: What should we make? When the new decision makers made products that were poorly conceived, unreliable, even dangerous (does anybody remember Nader and the Corvair?), the government stepped in. The engineers set up shop in a corner of the basement. By the time all the directives, laws, regulations, standards, and restrictions hit the engineers, the problems have become something like: What color curtains do you hang in a submarine?
Do you get it? This question of whether a certain motorcycle should, or should not, have a two-stroke or a four-stroke engine, is not an 'engineering' problem.
The great danger that institutions portend in general (AMA, FIM, UAW), and governments in particular (EPA, CPSC, NHTSA, BLM), is that decisions are made, the world changes, and nobody is responsible.
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4th May 2009:
On to matters more practical, specific, technical. Let's consider the claim that four-strokes are easier to ride. Looking at the chassis, a heavier bike with a higher center-of-gravity will be more stable, less responsive. These two qualities are contradictory. The engineer must find a compromise. The best compromise will always depend on the rider. But look at it this way: If you want to make a two-stroke bike 'easier to ride', you can always bolt on a chunk of iron and increase the rake and trail (slow the steering). The four-stroke takes away the more-responsive option because you can't take the lead out!
As to the engine, there have been a lot of very mellow two-strokes. I have been arguing that engines like that have not been offered in a long time. If you find a motocross two-stroke too hard to handle, a four-stroke is not the answer. You need a two-stroke engine and motorcycle designed for riders like you. Right now, this is the option not offered!
KTM, on some models, has addressed the rideablity question two ways. They let you select and adjust the ignition advance curve and the exhaust (power) valve opening. For some riders, that's good enough.
What has been 'wrong' - for many riders - is insufficient flywheel-effect. Long ago I took two identical motocrossers and put a very heavy ignition flywheel on one of them. In drag races (on dirt, of course), whoever rode the modified bike won. But it depends on the ground. It's simple: If the rear wheel is spinning, more flywheel will make the bike faster and easier to ride.
So why is it that our bikes don't have enough flywheel? Firstly, if you want to keep the overall machine weight down, it is very tempting to take a big lump out of the motor. Secondly, a bike that is lacking flywheel effect, and spinning the wheel every time you crank the throttle, feels 'faster' to most riders. Thirdly, hanging on a heavier ignition flywheel is not the way to do it. With the drive on one end of the crank and a great mass on the other end, the crank-webs will shift on the crankpin. (I tried welding the pins, and the welds broke!) The additional mass needs to be on the drive-end of the crankshaft; but the conventional layout puts the clutch in the way.
Here is the solution for our bike-of-the-future. Take the clutch off the transmission mainshaft and put it on the crankshaft. The 250 CZ that Joel Robert used to win his first (of six) World Motocross Championships was made this way. The new BMW G 450 X (a four-stroke, of course) also has a crankshaft-mounted clutch. There is no new technology here. As a bonus, because the torque is not multiplied by the primary gears before the clutch, the clutch can be smaller and lighter, and the ignition flywheel can also be made lighter (in many cases). The whole bike gets faster, lighter, and easier to ride.
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23rd May 2009:
This is a good time to get into a subject that has been the source of much confusion: Lubrication. Many riders seem to think that this is 'the problem'. Oil must be mixed with the gas, it goes through combustion and out the exhaust, and this is 'the problem'. I will begin the refutation with a little story.
Not long ago, a friend of mine who is into airplanes as well as motocross, told me that Rotax two-stroke aircraft engines were seizing pistons. He explained that they would take off and climb to altitude no problem. At some point they would idle the engine and glide to a lower level. This might take two or three minutes. Then they would open the throttle, to increase speed or gain altitude, and that's when it would stick. He asked me, "Why?"
I took out a carburetor pilot jet. Most riders know that these control the amount of fuel the engine gets at idle and small throttle openings. Holding the jet at eye level I said, "Try to imagine that 1/50th of the liquid that passes through that tiny hole is oil. Then imagine that most of that oil, maybe 80%, goes through the engine and out the exhaust without ever coming in contact with any of the parts that desperately need lubrication." He looked at me, "No oil!"
Phil Irving, who designed both two and four-stroke engines, calculated that at 4000 rpm, with a fuel/oil ratio of 20:1, said it "...works out to... one drop in about twenty revs." He also said that, "...if the engine is driven downhill for some distance with the throttle shut, especially if a low gear is engaged to provide more engine braking, little or no fuel passes through therefore no lubricant will be going in and some component, usually the big-end (connecting-rod bearing) may suffer in consequence."
When Yamaha first started getting serious horsepower from their racing engines, piston seizures were too common. So they did some tests to find out how much oil was enough. They found out - no surprise - that more power requires more oil. And they found a point beyond which 20:1 was not enough oil; a point they had been exceeding. They concluded that mixing the oil with the gas was not good enough. They then developed a pump system that metered the oil better in relation to power. Later, they were having big-end failures and they switched to a system that delivered the oil directly to the bearings. Really, they copied the system that Suzuki was already using. But Yamaha only used this method on their factory bikes. For the racers that they sold, they told the riders to mix the oil with the gas at 10:1 for break-in and 16:1 for racing.


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1st June 2009:
I ran a 10:1 fuel/oil mix in a TZ250 Yamaha. It ran great. Of course, all the jets must be opened up. For a given jet size, more oil must mean less fuel; i.e., the air/fuel ratio goes lean. McCulloch Corporation - who made not only chainsaw and kart engines, but four and six-cylinder two-strokes for the military - also tested fuel/oil ratios. They found that the power output increased with the proportion of oil. It isn't just a matter of lubrication, reducing friction. Oil helps the rings to seal and provides a thermal bridge between parts that aids cooling.
Suzuki obviously understood all this. They used their 'posi-force' system on everything from a three-cylinder, 50-cc factory road racer to their production 400-cc motocrosser. Then they stopped. A kind of macho mentality took over the motocross world and oil-injection became effeminate. The oil pumps were replaced with a stencil on the gas tank telling riders to add oil to the gas at a ratio of 20:1. When I asked, I never found anyone who used that much oil. "Real men don't use oil!" Suzuki’s position seemed to be: "We can tell them the right thing to do, and if they don't do it, we can sell them a lot of spare parts." Actually, the manufacturers were deferring responsibility for the engines lubrication. "It's not our job!" So, 'the job' was given to people who didn't know what they were doing... the riders!
When just the right amount of oil - not too much or too little - is metered to all the parts that require lubrication, the amount of oil consumed is surprisingly small. The last Suzuki two-stroke road bike that I tested went 1700 miles on a quart to oil. That was a 550-cc triple, in late 1977. (1978 was the last model/year for Suzuki's big two-stroke road bikes.) Besides all the road miles, that bike survived a full dyno test and several all-out runs at the drag strip.
All this history brings us to the future. Any sort of fuel-injected two-stroke must use oil-injection. That will eliminate the smoky exhaust and the odor that makes following a two-stroke not-fun. But the picture gets brighter. You have probably discovered that gasoline is a good solvent, good for washing the oil off of parts. That's what it does in the crankcase of a two-stroke. Eliminate that gas, with fuel-injection, and less oil will do a better job of keeping everything lubed. But do not expect a return to 'Posi-force'. Those engine-driven pumps varied quite a lot in production. Metering tiny amounts of oil is not easy. The new BRP (Evinrude-Skidoo) pump is like one of their fuel-injectors, electric, and is controlled by the fuel-injection computer. All the pumps are checked in production and the inevitable differences are adjusted for by the computer. What's more, if the engine (or oil) is cold, or the engine overheats, the computer can increase the amount of oil. Besides cleaning the air and conserving natural resources, the system pays for itself by saving the owner a bunch of oil-money.
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15th June 2009:
I had something else in mind for this entry, however I've come across a very interesting article released by the CEO of KTM, Stephan Pierer, that will help point out what I'm trying to get across about the future of two strokes.
Stephan finished by saying this... "At KTM the two-stroke will continue to remain an integral part of the future model planning. We have already recognised new topics and initiated the development of even more powerful, quieter and above all less polluting two-stroke sports models within the framework of a specific project grouping."
To read the full article, www.ktm.com
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29th June 2009:
KTM's fuel-injected two-strokes are on the horizon. As we wait for them to release the juicy details, I want to keep the pot boiling, so I will make a statement intended to shock: The four-stroke engine - NOT the two-stroke - is now obsolete! And I don't mean just for dirt bikes!
Suppose that I approached some 'environmental types' and told them that I had developed a new engine. It has some advantages, but it will consume 30% more fuel. I would expect them to reject this without a blink. "It is unacceptable," they might say, "to use 30% more irreplaceable energy and dump 30% more CO2 into our already overtaxed atmosphere!" Hey, I agree!
So, if our values have been thus established, we can now switch to reality. Remember the University of Idaho? When they won the Clean Snowmobile Challenge their two-stroke got 30% better mileage (with better performance) than the four-strokes. And remember that more engineers were doing all they could to get the best out of the four-strokes. What was true for snowmobiles will be true, more or less, for cars, motorcycles, etc.; for it was the brake-thermal-efficiency of the engines, not the application, that determined the result.
Back in 1992, GM introduced the 'Ultralite'. Don Sherman reported that "the magic 100-mpg fuel-economy bogey is met at a steady speed of 50 mph." In the rear of the Ultralite was, "...according to Advanced Engineering Staff experts, the most efficient powerplant currently available at GM: the CDS three-cylinder two-stroke...". Since that engine was not in production, "available" meant: The best that GM knew how to make.
Neither the Ultralite nor their two-stroke ever saw production. As you read the latest news on the energy crisis and global warming, you might wonder why the roads are not filled with cars like that (with engines like that!), why GM is going out of business, and why the government has been trying to make dirt bikes into a 'problem'?



13th July 2009:
I have said that any modern (not traditional) two-stroke must have fuel-injection. I must now explain that 'fuel-injection' must have a different meaning for a two-stroke than for a four-stroke. Fuel-injection (FI) on a four-stroke is just an alternative to a carburetor. Both schemes meter a quantity of fuel in relation to some mass of air. Take a short ride on a four-stroke with a carb, then ride the same model with FI, and you might be hard pressed to tell the difference. Ride a two-stroke likewise, and you may not believe that the same basic engine is at work.
To differentiate two-stroke technology, I refer to 'DFI', meaning 'Direct Fuel-Injection'; that is, direct into the cylinder. My definition of DFI results from the need to classify all of the schemes that have proven effective - which covers a surprisingly wide range (including systems with carburetors!). Here it is: Any system that admits fuel independent of the scavenging air. To make that clear, the air that enters the crankcase and passes into the cylinder through the transfer ports 'scavenges' the cylinder of the exhaust gases left over from the previous cycle. In the traditional two-stroke, this air carries with it the fuel. This is 'the problem', with the traditional two-stroke. So (to oversimplify the solution), don't do that!
To understand why DFI is such a vast improvement over the traditional scheme, and why the development of the technology has kept a few engineers awake at night, we need to understand that three requirements must be met for an engine to operate efficiently. One: The fuel must be admitted to the cylinder in such a manner that none (or almost none) of the fuel escapes out the exhaust port before it closes. Two: The fuel must be mixed with the air - 'atomized' - so that all (or nearly all) of the fuel chemically reacts with the oxygen in the air during combustion. Three: A combustible mixture must be in the vicinity of the spark plug when it fires, or there will be a misfire.
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27th July 2009:
Three problems can be more than three times as tough as one problem. Solving the first problem often made the second problem worse. Solving the first two problems could leave the third problem unresolved. A lot of good ideas were not good enough.
The first really effective scheme came from the Orbital Engine group of Australia. Mercury Marine has been using the Orbital system for more than a decade with excellent results. It is certainly the most-proven solution out there. But the Orbital injector must pass fuel and air. Air cannot be forced the way fuel can (because it compresses). They then have been fighting the fourth problem: Their injector has had a rather low frequency-limit. It stops working at about 6000 revs. They may have found another way around this, but the only solution I know of - if the engine is to turn 10k - is to use two injectors per cylinder. This makes it rather more complicated and expensive than the E-TEC option. The Orbital system also requires a small air-pump, which E-TEC doesn't need. But I don't want to suggest that these are the only two options, they are just at the head of the pack right now.
I will go into the third problem mentioned in some detail, because it is the most difficult to understand. To reduce power, the throttle restricts the inlet to the crankcase. The amount of air delivered to the cylinder (through the transfer ports) drops in proportion. Remember that the cylinder is filled with exhaust gas from the previous cycle. The fresh air must displace that exhaust. As the throttle is closed, there comes a point where the fresh charge reaching the cylinder is insufficient, the cylinder retains a large percentage of exhaust gas, and the engine misfires (because exhaust gas won't burn).
One way to deal with this is to create a 'stratified charge'. A combustible fuel/air mixture is concentrated near the sparkplug, which ensures combustion. The remainder of the cylinder may have exhaust gas or fresh air (no fuel) or a combination of these. With this arrangement, the traditional bad habits of two-strokes are history. Gone is the irregular idle and the surging at light-loads. As a side benefit, it is almost impossible for the engine to not start on the first kick! The exception is that, in the off-road scheme, a capacitor replaces the usual battery. After a few days of not running, this capacitor will run down. The first kick may then only charge the capacitor, and the engine then starts on the second kick.
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10th August 2009:
There is another way to deal with over-throttling. Just before that point where misfiring would occur, the DFI computer switches to an alternate-firing (AF) mode. Suddenly, our two-stroke becomes a four-stroke! The power then drops about 40-percent. This feature opens the door to a new option: Traction Control!
We add simple speed sensors to both wheels (much like an electronic speedometer). The outputs from those sensors go to the computer which compares them. The result is a speed differential, which in this control system is called hysteresis.
It works very simply. If the speed of the rear wheel exceeds the speed of the front wheel by a certain amount (the hysteresis), the computer switches to AF mode. But understand that the engine turns much faster than the rear wheel, and the computer can switch modes, back and forth, very quickly, so we can maintain maximum power to the rear wheel with respect to traction.
Traction control was used in Formula One cars, then outlawed. The critics said that it made driving too easy. Some even said that if such technology continued, monkeys could drive the cars. (Given how much F1 drivers are paid...!) For those who say four-strokes are easier to ride, here is the ultimate solution. But more than that, the bike will accelerate faster. Of course, the front wheel can still lift, and this will probably keep out the monkeys.
Because wheelspin is sometimes desirable, we can give the rider options. A button on each handlebar (like a horn button) can vary the hysteresis. Each tap of the right button would give an incremental increase in hysteresis, i.e., more wheelspin. The left button does the opposite. The hysteresis could also vary depending on the gear selected.
I will end this segment with a few numbers from our friends at MotoVerde. Let's say, after a full season of racing, you want to renew the top-end - replace all the worn parts from the base-gasket up. The parts will cost: For a YZ250, 1172.16; for a YZF450, 3617.53; for a KTM 250 SX, 668.68; for a 450 SXF, 4253.14. All prices are in Euros, in Spain. Too bad those four-strokes don't last forever.
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28th August 2009:
Please don't ask me when you will be able to buy a bike with traction control. I can't predict the future. More than 30 years ago I wrote an article, "The Source of Power" (published, January 1978). I said then: "Carbureted two-stokes produce high levels of unburned hydrocarbons because of the fuel lost to the exhaust before combustion. Direct-cylinder fuel injection would solve this problem... (In) one test of this principle ... the injected two-stroke ... showed both a higher thermal efficiency and a lower specific fuel consumption than the four-stroke... Another experimental direct-fuel-injected two-stroke showed full-load specific fuel consumption figures rivaling those of many diesel engines." ... I'm still waiting for that future that I predicted then!
We started this with the intention of talking about two-stroke dirt bikes of the future. What I have been giving you is a lot of history. To clear the air, I had to bring you back to the 'future' of about 1995. Do you really want me to predict the future? How long do you intend to live?
With traction control, I did take you where no man has gone before. The technology involved is very simple, but it depends on DFI. Two-strokes with DFI have run an alternate-firing mode, so we know that this works very well. I'm not selling moonbeams. And the control scheme that I laid out is just a variation on the principles used in anti-lock braking systems. Old technology, new applications.
As uncertain as any future may be, I do want to tell you about some possibilities, like traction control, because I think the future of the two-stroke dirt bike could be very exciting. DFI is not the end of two-stroke development, but can be a new beginning. So let's go on.
If you look at the torque curve of any high-performance two-stroke, you will see a dip, or a valley. In the case of the KTM 105 JMC, this runs from 5500 to 7500 rpm. The engine actually makes less torque at 7500 than at 5500. Immediately after this depression, the torque curve rises very steeply. This is why these engines have that sudden rush, which some riders like, and other riders find difficult to control.
A long time ago, I went looking for a way to fill in this valley, to bring up the bottom end of the torque curve. This result would not only improve performance 'off the pipe', but it would make the bike easier to ride. Here then could be the next revolution.
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08th September 2009:
It will help us to consider the 'cause' of this low-torque depression. Some of you already know to blame the expansion chamber. The positive (high pressure) wave travels down the pipe (at the speed of sound) and returns to the port too early, at 'low' speeds - when the pipe is tuned for high speed. When the torque falls off, we say that the pressure-wave cycle in the expansion chamber is out of phase with the scavenge cycle.
I know I'm getting very technical, and that I may be pushing what most riders can comprehend, but there is no way to understand engines using kitchen terms. The principle I'm referring to is so crucial that I will explain it by way of a simple analogy:
Most everyone has pushed a child on a swing. We learn very quickly that if we push just after the highest point, the child will go higher and higher. But if we push before that high point, repeatedly, we can bring the swing to a stop. The outcome depends on the timing between the natural period of the swing and the pusher's effort. We call that the phase relationship. If we push after the top, we put energy into the (swing) system. If we push before the top, we take energy out of the system.
These pressure waves in the exhaust have (are) energy, so they can push (a positive wave) or pull (a negative wave). But whether this pushing and pulling helps us or hurts us depends on whether or not these events are in-phase or out-of-phase with the scavenging of the cylinder. Because the length of the exhaust system is fixed and because the waves travel at the speed of sound (consider it a constant), the systems will be in-phase at the (high) speed that the expansion chamber was designed for, and out-of-phase at low speeds, which is where that valley shows up on the dyno charts.
The 'elementary solution' to this problem would seem to be a variable expansion chamber. In most cases today, we have this. The exhaust (power) valve delays and modifies the waves. This works well, but it mitigates, it does not eliminate, the problem.
If you really want to understand two-stroke engines, think of 'it' as three 'pumps' connected in series - the crankcase (sometimes called the 'primary pump'), the 'working cylinder', and the expansion chamber. The trick then is to get all three of these pumps working together (in-phase) so that each helps (so to speak) and does not conflict with what the others are doing. And making that work over a 10,000-rpm range, really is a trick!
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21st September 2009:
To say that the expansion chamber is the 'cause' of this low-torque region gives the wrong idea. The two-stroke engine cannot be analyzed as 'parts' with cause-and-effect relations. The mathematician would say that there are too many nonlinearities. This is why the evolution of the two-stroke has been a process of trial and error.
If we look at what may happen with the piston at BDC (Bottom Dead Center), we see that the exhaust and transfer ports are open and, functionally, we cannot say where one 'pump' ends and another begins. (I told you to think of the two-stoke engine as three 'pumps'. Now, I'm saying that you can't think of it that way! The two-stroke is a paradox!) So, let's look at the whole process again and see if we missed something important.
As the piston descends, it compresses the air in the crankcase. The exhaust port opens and the cylinder pressure drops. The transfer ports open and air flows from the crankcase into the cylinder because of the pressure differential. But wait. There is something wrong here. Before the piston reaches BDC, the pressures reach equilibrium. Before the scavenge cycle is even half through, the crankcase stops pumping. What's worse, as the piston passes BDC, the volume of the crankcase increases and the volume of the cylinder decreases. Simply put, the working-cylinder tries to pump its contents back into the crankcase! What actually happens depends on a lot of variables, but I can now tell you in familiar terms that the phase relationship between the crankcase pump and the scavenge cycle is all wrong. If we say that the expansion chamber has a problem because it is sometimes out of phase, how are we to think about the crankcase pump which is always out of phase? "Well," a fellow engineer might say, "we don't think about it at all, because we can't change it." Well...what if we could?
An engineer, Manuel Sevilla, asked that question about 25 years ago. And picking up a scheme that DKW engineers had played with in the 1930s, he went to work. The beauty of Sevilla's approach was that he modified production engines. He was then able to demonstrate the difference before and after the phasing error was corrected. The difference is shocking! The torque characteristic (the shape of the curve) was transformed. Show an engineer the two torque curves and tell him that both engines have the same intake, porting, and exhaust systems. "That's impossible!", would be the natural response.
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5th October 2009:
Sevilla's experiment was a great proof-of-concept, but his engines were bigger, heavier, less efficient (they needed DFI) and more expensive - or would have been if they had gone into production. He showed us what we needed to do, but not how to do it.
So we come to what I call AST - Assisted Scavenging Technology. To our traditional two-stroke - with DFI - we add a plenum chamber. This is nothing more than an empty space in the crankcase/gear casing. The only requirements are that it be air-tight and that the volume be at least equal to the piston displacement. (There are also what we call 'packaging' considerations.) This chamber is connected to the crankcase through a reed valve. If the pressure is higher in the crankcase, air will flow into and charge the plenum. When the pressure in the crankcase falls, the higher pressure remains in the plenum. There are other changes required, but these are mostly modifications to existing parts (like cast-in ports that permit transportation of the air), and not additional parts. It sounds too simple, but let's run through it.
As the piston compresses the air in the crankcase, the plenum chamber is also charged. The exhaust port opens, then the transfer ports. The pressure quickly drops in the crankcase until it is equal to the pressure in the cylinder. So far, everything is normal, except that the crankcase pressure, when the ports open, is lower; this is because we have lowered the primary compression ratio through the additional volume of the plenum. This is not a problem. This lower pressure, at this point in the cycle, is consistent with Sevilla's engine. At low speeds - where we want to pick things up - lower pressure means lower velocities through the ports, less mixing of fresh charge with exhaust residuals, and less charge loss out the exhaust. In technical terms, the 'trapping efficiency' is improved. Of course, if we only lowered the primary compression ratio, we would see a power loss at high speeds.
Now, as the piston nears BDC, a valve opens and the compressed air in the plenum chamber discharges into the cylinder. The pressure at this time is actually higher than when the transfers first open, because the pressure in the cylinder is lower. The only 'pressure' that means anything is the differential.
A simple way to think of this is that we have added a fourth 'pump'. The traditional crankcase works through the transfer ports, very much like always. Call this 'phase one'. Then, near BDC, when that system has run out of steam, the air in the plenum is released and AST keeps air moving into the cylinder until the AST ports close. Call this, 'phase two'. From here, it gets interesting!
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20th October 2009:
Is anybody still with me? AST is very simple mechanically, but the thermodynamics are very sophisticated. In the traditional two-stroke, when the transfers open, there is a great inrush of air, then... casi nada. This pattern is dictated by the mechanical constraints of the simple engine. From the standpoint of thermodynamic efficiency, it would be much better if the pressure started low, when the ports open, and then the pressure increased until the ports closed. AST doesn't quite do that, but it is a step in the right direction, and it is simple. But what I have described is only the beginning. AST changes all the rules.
The AST ports are independent of the normal transfer ports (which work the same way as always). This means that phase-one can be optimized for scavenging - displacing the residual exhaust gas. Phase-two can be optimized so as to retain as much fresh air as possible in the cylinder - what we mean by trapping-efficiency. These are really two different problems, but until now we had to accept a single compromised solution.
A very neat trick is now possible. The AST ports can be as high as the exhaust port. Phase-one requires a 'blowdown' period, the time between the exhaust opening and the transfers opening. But the AST valve is closed until nearly BDC, so nothing can blow-back through those ports. Phase-one can then be about 60-degrees, and phase-two can be, maybe, 80-degrees. In effect: asymmetric timing!
I should make it clear that the AST valve is not something that can go BANG! We get the necessary timing function by modifying parts that we already (in any simple engine) must have. AST requires no new moving parts except the reed valve between the crankcase and plenum. And so the stage is set.
At the same time that I was working on AST, I was putting together a simple DFI system. But I was aiming at a different target from the computer nerds. I wanted engines for any application. It has been estimated that there are about 100-million small, two-stroke engines operating in Southeast Asia today. Most of those are in small motorcycles. People bought these things because they were cheap (but also because of performance, ease of repair, etc.). If you are trying to live on $1000 - $1500/year, cheap is good! These folks aren't going to buy any fuel-cells, or hybrids, or computers. Of course, Honda, et al., would like to sell them four-strokes, but they rejected that option last time - they bought two-strokes!
So we know what they don't want! What's left? How about more simple, cheap two-strokes with lower fuel consumption and emissions than the four-strokes? As it turned out, AST was the secret to making a really effective, carburetor-based DFI system.
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6th November 2009:
If AST is given, we get DFI very simply. The traditional carburetor, of course, is replaced with a throttle valve at the inlet to the crankcase. (Reed valves are retained.) A much smaller carburetor is then connected, through another reed valve, to the crankcase via a long manifold. This carburetor functions as always, but the (rich) air-fuel mixture never quite gets to the crankcase. On each inlet cycle, the manifold becomes charged with mixture, while the crankcase remains fuel-free.
Here is the trick: This 'manifold' is also an AST port. So when the AST valve opens, the pressure in the plenum chamber 'blows' the fuel mixture into the cylinder. (This clears the manifold/port for the next cycle.) Presto! 'Fuel injection'! But now it gets good. The AST fuel port directs the mixture to the sparkplug - it's a straight shot. At the same time, other AST ports open. These air-only ports create streams that converge just above the exhaust port. Some of the air will be lost out of the port, but it's only air. Mostly, this air prevents the fuel mixture from migrating toward the exhaust port. It also prevents the fuel (generally speaking) from contacting the cylinder wall, which adversely affects lubrication and emissions. Finally, it helps direct the mixture to the sparkplug and confine it to the combustion chamber.
Of course, all of the air delivered by AST is in addition to the air that passes through the four, main transfer ports; but the AST airflow is against the 'loop' pattern that those ports establish. The air

Is that officialy the longest post on Vital?

Two strokes are going to have do some serious catch-up. I hope they do- but they are five years behind in technology right now.

Currently I ride/race a 2004 RM250 and a 2008 RMZ450 (actually haven't raced the 450 yet). On the track, there is absolutely no comparison. The 450 I barely have to shift, it sticks like a dart in flat corners, the power delivery is 100% more predictable, and I can ride far longer before my forearms explode. Suspension is way better also.

I love my 250 but it feels antiquated at this point.

That is what is going to be tough to fix (on the two stroke) : the rideability

It would take just a very short time for the japanese to shoehorn their existing 250cc two strokes into a modern 250F chassis and be instantly current as far as that everything else goes, but we are all spoiled with massively wide powerbands , the four stroke ability to hook up and just generally haul with less wheelspin

There is no doubt that it takes more effort and skill to go fast on a two stroke. The question is, is anyone interested in putting forth that effort

hell, I am even considering getting a KTM because I am sick of kickstarting my bikes..lazy?? maybe.....but some of the best riding memories I have from recent history are on my 05 YZ250 on a tight sandy track...that is 2 stroke territory

Some people lust after that old two-stroke feel. And I agree there is nothing like the sound of a 250 wide open up a hill at Budds- but to me progress is progress. I am going to ride the bike I can go faster on with less fatigue. And this is after four seasons of racing my 250 against 450s.

I am looking forward to some roost payback too!

two strokes will never be dead as far as I'm concerned. Maybe for the professionals racing, but not for me. Keep dreaming, 4-thump lovers.

It's not the 2 stroke motor that is anticuated, but the 2 stroke motors that we have in mx bikes.

You should read Kinetic's post, before pointing a problem (rideability) that has already been solved.

Uhm yeah, I didn't read all that... BUT, I had a brand new YZ400 when it came out and rode thumpers ever since, except for a brief affair with a sexy YZ250...(bitch ruptured my spleen).

That said, 2010 I will be picking up my 300XC where I will be seeing TIKI at the worcs races..

Suspension is a no-brainer. New stuff simply bolts on. Differences in valving and spring rates are a non-issue, as they differ greatly rider to rider anyway.

Chassis? Yes, there will be some necessary chassis tuning to account for the differences that dropping the 2-stroke into a current chassis that is designed around the 4, will bring to light.

Traction, breadth of powerband. Ah yes, the big variable in the 2 vs. 4 debate. Like I said earlier, the advantage that the 4 has here will most certainly play a role in the development, but also, will remain as the big difference between them, and also those very characteristics become advantages for the 2-stroke under some conditions, soil types, etc.

It is clear that some riders don't mind making their left foot move a little more often, and don't consider that to be a drawback, nor does it effect their laptimes negatively in a majority of cases.

You're right. Big time catch-up. Most of it could be achieved in a chassis update, then it is still back to personal preferences. Some very talented riders still prefer the 2-stroke delivery, but the 4-stroke favors the less talented, hence it's success and favor.

The reason many riders switched, though, wasn't because of the easy-ride powerband, but rather the sheer advantage of displacement (= horsepower and torque). They should be compared cc to cc, but that is another argument all together.

After going back to a 2stroke last year, I hate riding 450's and makes me wonder what attracted people to them in the first place (other than the motor). I'm slower on the 450, although after a couple weeks on it I'm sure it would equal out. They also feel ridiculously heavy no matter what the marketing says. People that say a new 450 feels light haven't ridden a 2 stroke in a while. Every once in a while I'll hop on my brothers 450 just to double check, and I never enjoy it. It does make me realize that the power advantage is huge though. It just takes the fun out of riding for me. For slower riders, the 2 strokes can be a handful though and I can see why they pick the 4 strokes. To me it's just preference and I would like both models to stick around, but in the meantime I'll just enjoy being the odd man out.

TerryB, your avatar is the perfect representation of this topic.

I had to work the Long Beach motorcycle show a couple of weeks ago and stopped at the Yamaha display, while I was there I asked a couple of the Yamaha reps if this is going to be the last year for the 2-strokes and they said that last year was supposed to have been, but because sales have been strong on YZ250's and with the new amateur rules allowing YZ250's to compete with 250f's they will continue to make them as long as they sale them.

5 years behind, and then you compare '04 to '08, thats 4 years. If I took a 2010 ktm 250sx, compared it to a 2005 rmz, is that 4 years or 5? Then theirs the 2010 cr250, oh yeah, that one is just imagination. So it depends on the manufacturer on 'years behind'. Have you seen that ossa 280cc trials bike? It's a 2-stroke, the motor is about the size of a volleyball, go check it out.