– Motorcycle News, Editorials, Product Reviews and Bike Reviews

Motorcycle News, Editorials, Product Reviews and Bike Reviews

The "Clean" Two-Stroke Mystery – Reader Responses

The mystery of the pyramids remains unsolved, but MD’s brilliant readers solved the mystery of missing two-stroke street bikes (at least arguably). These are just some of the responses (mostly unedited) MD received to its article on “clean” two-stroke technology dated April 27, 2001.

  • The interim technologies that are now marketed to clean up 2-strokes (FICHT,
    Orbital) are not clean enough to meet current or future street, or
    California’s off road CARB standards or expected future EPA off road
    standards. Even now, they are not very practical for engines that move
    rapidly up and down through the rpm range, like a typical bike engine.
    (Marine engines tend to stay at a fixed rpm for a relatively longer
    time, even in PWCs). The “driveability” (DR) just isn’t there for motorcycle applications. There are expensive R&D solutions to fix direct
    injection, but the costs are very high to move to production (relative to the
    number of bikes you could sell) and require extensive electronic management and
    catalysts. A few street riders might pay the high price, I doubt an
    off-road rider is likely to accept the fancy electronics and hardware.
    4-strokes are inherently cleaner, they have the potential to meet all
    future standards at a fraction of the cost of a “clean” 2-stroke while
    delivering high output, excellent DR and good fuel economy. There is extensive
    R&D going on by many OEMs which will keep prices down when products come to the market.
    So, while the mechanical simplicity of 2-strokes is attractive, the cost
    to clean them up is high and the results are not promising. Hope this

  • Honda is the real mystery, since they have raced an allegedly clean two
    stroke. Of course patents and almost pathological hatred of licensing
    from any non-Japanese firm (Suzuki seeming to be the only exception) are
    likely a bigger factor.

    Another thought is that emissions standards tend to be a moving target,
    and are set as much on what can be done as on what is needed. So two
    strokes are always at a disadvantage, and it wouldn’t surprise me to
    discover that the Big 4 are reluctant to tool up for volume production,
    only to have US and European standards revised below what they can

  • Dirck, I believe it was SportRider’s correspondent who went to HRC in
    Japan, and asked them this question about a year ago (I don’t happen to have that
    issue of SportRider in front of me). Honda said that they did in fact
    have ‘clean’ 2-stroke technology, but had concerns about putting a 2-stroke
    streetbike power-to-weight ratio in the hands of the general public.
    Basically, they could easily build a 300-350lb bike with say 130hp.
    Therefore they decided not to introduce the technology. But the HRC
    representative did not rule out using the technology in the future – which
    is probably more of a teaser than anything else.

    I don’t think that the reason Honda gave is entirely valid. Neither do I think
    that it is necessarily the entire reason. We may just have manufacturers (with
    the exception of Aprilia) who are fundamentally biased towards 4 stroke technology
    for reasons that we may never know.

  • I read with interest your article (I usually hit your site everyday)
    about the future of two strokes. I am a mechanic and mechanical
    engineer, and am currently studying for a doctorate at the Australian
    Defence Force Academy. Although my field of expertise is structural
    dynamics I have had considerable experience with engines from both a
    thermodynamic and practical point of view.

    I believe the answer as to why two strokes are not used more
    extensively comes down to efficiency. By efficiency I do not mean
    highway fuel consumption (although it’s related) or brake power per
    litre, but rather brake thermal efficiency. By definition brake
    thermal efficiency is brake power produced divided by fuel power
    consumed. In general two strokes have lower thermal efficiencies
    compared to four strokes for two main reasons: compression ratio and
    gas dynamics.

    Firstly, the thermal efficiency of either a two or four stroke engine
    is a function of compression ratio. The nature of this relationship
    is complex, but efficiency is higher for a higher compression ratio.
    Putting it simply, a higher compression ratio causes higher pressures
    and temperatures in the cylinder and allows more mechanical work to
    be extracted for the cycle. Two strokes tend to run lower
    compression ratios than four strokes (this is due to porting and
    scavanging requirements) and broadly speaking have reduced
    efficiencies due to this alone. The pressurising of the charge in a
    two stroke before it enters the combustion chamber raises the
    effective compression ratio (much like turbocharging), but this
    effect varies with engine speed and on the whole it’s worse off than
    a four stroke.

    The second reason two strokes have lower efficiency is due to gas
    dynamics. Gas flows are complex in a two stroke in comparison to a
    four stroke. Two strokes like to perform in certain rev ranges when
    scavanging is occuring in a favourable manner. This is usually
    refered to as being “on the pipe” or “in the powerband”. Here
    expansion waves are cleverly used to draw mixture through the
    crankcase and transfer ports and provide improved cylinder filling.
    Unfortunately, this only occurs in certain rev ranges depending on
    the pipe geometry and port timing and shape. Out of this favourable
    rev range the engine can suffer from inlet charge contamination by
    exhaust gases, or fresh charge being drawn unburned into the exhaust.
    The direct injection system will reduce or perhaps even eliminate the
    environmental side of the problem, but the gas flow problem will
    still exist. This will reduce combustion efficiency and hence brake
    thermal efficiency. The state of tune greatly affects this, but
    ultimately the control of gas flow will never be as good as a mildly
    tuned four stroke over the whole rev range.

    But why are they used in some applications? There are a few other
    considerations. The mechanical efficiency of a two stroke is higher
    than a four stroke since there is no valve gear etc. If the engine
    was designed to run at a set speed and throttle setting it could
    perhaps come close (maybe even better) the efficiency of a four
    stroke. This may be why they are used on outboards where revs and
    throttle are held constant for long periods of time. The other
    issues with outboards are weight, size and simplicity, areas where
    two strokes have a distinct advantage.

    In Canberra (here in Australia) a trial was run on government fleet
    vehicles using direct injection two strokes using Orbital’s
    technology. It was hoped that they would return improved fuel
    economy over a conventional engine. This was not the case – fuel
    economy was worse. To the defence of the two strokes it must be
    said that the cars may have been driven differently. Apparently the
    engines had more power than the originals and a power delivery that
    encouraged faster driving.

    If you are interested, most university level thermodynamics texts
    have more detail on the basic thormodynamics of engines. One book I
    thoroughly recommend for all rev heads is “Engines, an Introduction”
    by John L. Lumley from Cambridge University Press.

    The final point I will make is that I am not entirely familiar with
    the layout of a many of these modern direct injection designs. I
    might not be giving them due credit and they may very well have
    efficiency comparable to four strokes over the whole rev range.
    Orbital certainly seem to think they are the future.

  • The aprilia DiTech scooters will be in the US in a 2-3weeks. It uses
    technology developed by Orbital and Aprilia. It is a direct injection
    2-stroke engine. They are claiming awesome gas mileage(100+mpg) and low
    emissions (passes EURO2 standards now). DiTech will pave the way for the
    reintroduction of 2-stroke street bikes. Check them out at

  • Clean 2 strokes are still not as clean as good 4 strokes. Furthermore, it
    is hard to imagine a time when two strokes can overcome that deficit – each
    time they get better, the 4 strokes up the ante. Today’s ULEV (Ultra Low
    Emission Vehicle) standards would have been considered impossible ten years
    ago, yet many cars already meet it (many of these are Hondas, even the new
    MDX Ute makes it). ULEV requires infintessimal hydrocarbon, NOX and CO
    emissions (memory is telling me something on the order of 100 milligrams per
    mile). Even an incredible 2 stroke will have a hard time with that.

    Second, 2 strokes require mixing fuel, or at the least, buying special oil
    for the oil injection system. A pain in the butt (and expense which adds
    $0.50-1.00 per gallon to the cost of fuel) that most riders would rather do
    without. A 2 stroke that worked without mixed fuel would help quite a bit.

    I love the sound of 2 stroke dirt bikes and as an engineer, I love the
    simplicity. But my next dirt bike will be a thumper.

  • Over the past couple years, I have watched this trend and some of the
    technology available. There is a lot of discussion in the industry around
    2- vs. 4-stroke engines in the small/recreational vehicle markets. As
    4-strokes are approaching 2-stroke power density (when you include fuel and
    other system pieces to travel a certain distance), many manufactures are
    making guesses as to where the cost equation will fall out. Right now,
    2-stroke technology (specifically direct injection) is expensive, and for
    the most part needs to be licensed, at significant cost, from suppliers
    (Ficht and Orbital are the best known examples). In markets that are very
    cost/price sensitive, and embrace the low technology solutions they can work
    on, acceptance of these new technologies are relatively slow to come about.

    To move to fuel injection from carburetors is a huge change in the way a
    company must go about business. To make the jump to direct injection is an
    even greater change. For companies that have invested in the older
    technology solutions, there is a significant business risk in making the
    jumps. That is why you see new motorcycle companies coming out with fuel
    injection right away, while those who have carburetor solutions are slower
    to change. Big investments in both routes.

    Beyond the cost of entry, the capability in many of the current advanced
    systems are somewhat lacking in the rider/user drivability perspective.
    This was highlighted in the V-Due experiment, and reviews on many of the
    motorcycle systems (biggest example is the abruptness off idle, CV carbs
    offer some good benefits). Marine applications, where the throttle
    transients are minimal, application of this technology is reasonably
    straight forward and simple. Where there are large throttle transients,
    responding appropriately gets real difficult; both in the development of
    control strategies, and in calibrating them appropriately.

    As for Honda, I believe they are a 4-stroke company that did 2-strokes
    because they had to to survive. With the new engine technologies (including
    manufacturing and materials) to make the 4-strokes more viable, I believe
    Honda is moving to centralize their efforts on the one technology which
    should make them more able to survive as a company in the future. By
    focusing their efforts and resources, they will be able to afford to invest
    in engines (their primary purpose as a company) and rely less on
    high-technology controls to meet emissions regulations. This should provide
    more robust product for the various markets they compete in. Also Honda is
    a “green company”, and regardless of the technology employed, 2-strokes will
    be viewed by the uninformed masses as “dirty engines” for many years.

    In the end, it comes down to a fairly complicated cost equation. To get a
    certain level of performance, what does it cost to develop and support the 2
    engine technologies. With many of the racing endeavors changing to rules
    allowing 4-strokes to compete in the traditional 2-stroke arenas, the
    concept of needing to do 2-strokes to keep the corporate name visible is
    going away.

    Again, just some thoughts on the market/technology discussions from someone
    with a technical involvement. Not all of the information to make a rational
    decision on the future of 2-strokes is in yet, but it is coming fast.
    Trends point to 4-stroke due to cost (to make and to maintain), customer
    satisfaction (less noise, no mixing of gas, less smell), and the current
    lack of availability of a good 2-stroke technology alternative. In some
    applications, this is an easy change to 4-stroke. In others it is not, and
    the manufacturers are busily looking for a 2-stroke savior. Stay tuned.

  • FYI – Polaris has been working on FICHT for snowmobile application for
    several years (they have it working for PWC’s at present, but RPM is limited
    to 6000rpm – the “reliable” limit of current FICHT technology), the problem
    with the system is that it only works well when the injector cycles are
    lower than 7000rpm, far below what most applications require (IE. a high
    revving four cylinder.) FICHT is a great technology, the problem is the
    RPM… it will get worked out eventually. The beautiful thing with FICHT
    is the simplicity of the design when compared to Orbital. The Orbital
    system is FAR more complex making service and maintenance a bit of a pain
    (of course there is always the “more parts, more problems” theory about it

    I hope this helps you research about the great 2-stroke debate. FWIW –
    apparently auto manufacturers have shown some interest in this as well, the
    power/weight ratios of clean 2-strokes is awesome compared to four strokes
    and the simplicity of the overall engine design makes them attractive from a
    manufacturing point of view, as you may well know.

  • First, I’m an engineer but not a mechanical engineer so my opinions are no
    more valid than any other layman’s.

    Why are two strokes going away? I can tell you it is not because of
    licensing. Honda, and maybe others, have developed clean two strokes in
    house. Honda’s system is called Active Radical Combustion (ARC). ARC is
    simpler, lighter, cheaper and has fewer problems than Orbital’s system and
    was used both on race bikes (Paris-Dakar) and street bikes (a Japan only 250
    DP bike). I had the great pleasure of speaking to Kevin Cameron (along with
    Erv Kanamoto one of my life long heroes) about ARC. Kevin had been allowed
    to read the Honda research and testing documents and was very impressed.

    So that leaves the same question, why are they going away? I think there are
    two reasons. First, most people don’t like them. Yes, there are people who
    live and breath (cough, cough) two strokes but the large majority will take
    a four stroke if given a choice. The Japanese are not stupid, they learned in the seventies that, no matter
    how good your product, you can’t force people to buy what they don’t want.

    The second reason is that after you make the two stroke clean you still have
    a short lived (poor bottom end lubrication, pistons subject to almost twice
    the heat, pistons and rings have to live with cylinders with large holes in
    them), dirty (no mater how clean the combustion process you still have oil
    in the fuel and “symmetrical” exhaust timing), noisy (roller bearing bottom
    end, the nature of two stroke exhaust and intake noises) product that, with
    the very complex fuel injection required by most “clean burn” systems and
    the weight and bulk of the intake, cooling and exhausts required to make
    them street or even “green sticker” legal is nearly as expensive, bulky and
    heavy as a four stroke. Four strokes keep getting lighter and two strokes
    are being forced to get heavier and more complex.

  • I read your posting about two-strokes. When I visited Aprilia in 1999, I tried to get as much information about two-stroke sportbikes as possible. There were two major obstacles to a 500cc two-stroke sportbike coming out. First, they said they didn’t believe there was a strong enough market for it.

    “Aprilia, or any manufacturer, is always going to manufacture products that the market wants. The American market hasn’t been able to purchase a two-stroke bike for many years, so many people have forgotten the excitement that is part of two-stroke racing. On the other hand, Aprilia will definitely bring to market the products that the market wants. But the technology has to be there to be able to support bring those products to market. Right now, it’s definitely a serious subject in Aprilia whether to further devlop two-stroke technology that meets emissions and noise requirements but continues to have the high-performance that people want. Right now I can’t give you a clear answer, but it’s something that Aprilia is still very much interested in and evaluating. The sooner the U.S. subsidiary builds the inherent ability to understand clearly the U.S. market, the sooner they bring back to Italy the desire that the American market wants a two-stroke bike and the market is large enough to support then investment that would be needed to make that happen, the sooner Aprilia is going to make that product. ”

    As for the technology Aprilia uses, they use the DiTech system on their scooters, and Aprilia said that “it is not a performance-oriented product, at least not in the larger two-stroke engines.” It can’t be used in engines above 125cc. They added that they would have to use an Orbital-type direct-injection system for a 250cc bike, and presumably, for a 500cc bike.

    Aprilia does have a liscencing agreement with Orbital, and Aprilia said it is possible to develop a system for a 250cc bike. “But to industrialize that process would require a certain investment. >From what we see in the world motorcycle market today, the volumes of those two-stroke sales don’t justify the investment that’s needed to industrialize and develop that process. You won’t see in the very short term an Aprilia engine with Orbital technology until the U.S. market is able to bring back the market information that would be needed to justify the sales that are needed.”

    So what does all this mean? In my opinion, we’re not going to see a two-stroke sportbike from Aprilia, especially since dropping their 500cc V-Twin bike from the 500 Grand Prix class. And Honda, who is most able to actually produce an emissions-friendly two-stroker, has said it will no longer produce two-stroke bikes except for pure racing purposes.

    Incidentally, the Bimota Vdue, while an admirable concept, was a disaster for the little Italian company, and a large part of what brought them to bankruptcy. I rode the bike, and despite having an engineer present to continually modify the electronic injection, it ran terribly, and would be a safety hazard on the street.

    But riding the Vdue, with its light weight and capable chassis, did turn me on to the concept. And having a Yamaha RZ500 streetbike many years ago, I’d love to ride another one. For those who really want one, log on to and tell them. Only a strong market support will make this happen. Will we ever see a two-stroke sportbike again? I have my doubts. Top-level development of two-strokes is now over, thanks to the new rules in Grand Prix for 2002. C’est la vie.