Ah, yes, another chance to expound my rather warped views on the world of
internal compbustion. I am always willing to talk until my brain is empty
and you have just chosen the topic of which it is quite full.
The Japanese and other experimented with stratified charge engines in the
late seventies and early eighties. The idea was to run the engine as lean
as possible to reduce polutants. Unfortunately, it is almost impossible to
burn a truly lean mixture. MCA Jet, CVCC, etc were schemes designed to
introduce a charge where a rich mixture surrounded the spark plug to aid
ignition while a leaner mixture filled the rest of the chamber. This did
reduce hydrocarbons but, by theory, should have sent Oxide of Nitrorgen
(NOx) emissions through the roof unless they could keep the mixture cool
enough. With the advent of the three way catalyst and its preferance for a
well mixed, stoichiometric (perfect fuel-air ratio) mixture combined with
closed loop electronic control, these stratified charge gizmos were no
longer a good thing.
I did read the other day that Mitsubishi was developing a direct injection
gasoline engine that was cabable of running extremely lean and put out a
ton of power. I also heard that they weren't planning on selling it in the
US because of cost and our unwillingness to adapt new technology. Sounds
interesting, though.
When I was young my family did quite a business doing performance
modifications to snowmobile engines and even marketed their own piston
design (Powerdome) for a short time. I do have quite a bit of
familiararity with 2 stroke engines. The main shortcoming of two strokes
for automotive use is their emmissions output and efficiency- they waste a
ton of gas. By nature, they have a lot of overlap between intake and
exhaust cycles. Most also burn oil as a matter of design. They generate
gobs of power for engine size, displacement, and in particular weight.
They also can be supercharged effectively, I believe although they don't
like turbochargers. I have always been surprised that there aren't more
two stroke aircraft engines since they would probably make a good one, at
least for low altitude applications.
Compared to four stroke engines, there are a lot more really neat things
you can do to increase power on a two stroke to the point that they can be
just a complicated and heavy as a four stroke but they will still smoke
them in power per displacement.
Wankels are an great example of engineering compromise. They are great for
mechanical packageing- small and light. They have terrible combusion
chambers- long and narrow with lots of quench surfaces. They also a a
bugger to seal and have some hot spot area issues.
I belive that the typical Wankel uses a three lobe rotor that undergoes to
compression / expansion cycles per revolution. In the "cold" cycle, the
rotor passes first exhaust then intake ports to expel the old charge and
load up a new one. Wankels do not use poppet valves. The "hot" cycle is
where the combustion occurs. This way Wankels fire every revolution, much
like a two stroke (although they are actually four stroke engines, they
just go through all four cycles every revolution). This is why Wankels put
our so much power for their size. Like two strokes, Wankels suffer from
poor efficience and are dirty by nature. However, I believe that they can
run without much port overlap, unlike a two stroke.
Also like a two stroke, Wankels burn a little oil by design.
Wankels need two sparkplugs because of the length of their combustion
chamber and also possibly because the intake process does not promote
adequate fuel-air mixing.
Again, if I am wrong or if anyone else wants to add something, go for it.
Regards
Bill Eastman
61 MGA which also burns a little oil but not by design.
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