Practical solution to variable compression

[Matthew B. Watts]

I was thinking about this while designing a model of a 200cc
two stroke engine (single bore only) that uses opposing pistons...

What if one tied the cranks together with a gear drive mechanism
having a gear on each crank and a pair of idler gears coupled
together with a clutch that could only slip say, 15 degrees.  By
effectively putting the two cranks out of exact synchronization,
wouldn't you change the engine's compression?

Think about it a minute.  If at exact alignment, the two pistons
reach TDC at the same time, then you have MAX compression.
If the alignment is offset by allowing the clutch to slip just a little
and then lock back up say ten degrees off sync, then you would
never have the two pistons reaching TDC at the same time.  Thus,
your compression ratio would drop.


Now, my next question is...  Would you gain significant performance
by allowing high compression at low throttle settings and then use
a lower compression at WOT?

Having variable compression seems like a cool idea, and I
believe an engine and management system could be built to
do it.  Looking this whole thing over, I'm pretty sure that
MAX performance would still be limited the way any other
engine is, but couldn't the whole power curve starting from
way down low near idle be boosted considerably?

Since you don't have a complete fuel charge in the cylinder
at low throttle settings, wouldn't you want to increase the
compression ratio to compensate?

Does this idea warrant any merit?

Can anyone shoot BIG holes in it?

>The only real problem I can see is that one of the pistons would
>be potentially moving into the flame front, which could be a
>problem.  I think it would be more useful with a gas engine,
>where too much compression detonates things, and less useful in a
>diesel engine where this is mostly normal operation.

Pump gasoline with added lubricant is my choice of fuel.

>Still it may be a problem with the piston trying to compress an
>expanding gas, knowing what knock does to an engine
>
> Roger


My line of reasoning is that the expanding combustion would
take the path of least resistance which would be the piston
moving away up to the 10 degrees offset, at which point both
pistons start to move away.  Thinking about the geometry of
the crank, the piston moving away should be moving faster
than the one approaching TDC.

Another avenue is to keep the ignition synchronized to the
piston that is lagging behind so that spark never occurs until
at least one piston is moving away.  I would bet with the
right combination of ignition timing and compression adjustment,
engine knock could be eliminated completely.  There would
have to be another entry in the computer's table to coordinate
the extra factor for sure.

Might work, say what?

Matt
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