Practical solution to variable compression

Raymond C Drouillard cosmic.ray at juno.com
Sat Mar 14 05:44:42 GMT 1998


The only problem I could see with the dual piston setup is... ummm...
where do you put the valves?

Ray Drouillard


On Fri, 13 Mar 1998 10:02:00 -0500 jb24 at chrysler.com writes:
>Variable compression will see you huge gains in part-throttle
>fuel-economy.  It's that thermodynamics thing that expresses work 
>being
>done is the area between max pressure and min pressure on a
>pressure/volume diagram.  If you up expansion ratio (by way of upping
>compression ratio) you get more work for a given amount of pressure.
>When my variable compression ratio two-stroke patent gets approved, I
>will share it with the list (and no, it is totally different than the
>below mechanism, you hope :).
>---------------------- Forwarded by John R Bucknell/JTE/Chrysler on
>03/13/98 09:50 AM ---------------------------
>
>        owner-diy_efi @ efi332.eng.ohio-state.edu
>        03/12/98 12:24 PM
>Please respond to diy_efi at efi332.eng.ohio-state.edu @ SMTP
>To: diy_efi @ efi332.eng.ohio-state.edu @ SMTP
>cc:
>Subject: Re: Practical solution to variable compression
>
>>
>> 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 alittle
>> 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?
>>
>> 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.   Still
>it may be a problem with the piston trying to compress an expanding
>gas, knowing what knock does to an engine
>
>   Roger
>> Matt

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