cam drive engine

[bernie007]

The geometry between a piston, piston rod and a crankshaft in an IC engine appear to leave a lot to be desired since the gas expansion only acts on the crankshaft in any usable sense for about 35 to 40 degrees of rotation. And never at a 90 degree angle. The side loads etc would have to be absorded by the casing meaning a more robust structure. It seems to me that a cam arrangement would be more advantageous since the piston thrust would act on the cam at 90 degrees for a much larger part of the crank's rotation. So the question is, are they are more effecient, and if so, why don't we see them in production?

 

[IceStationZebra]

Its tough to change a paradigm that has existed for so long.
- entrenched thinking
- millions of hours and dollars spent optimizing emissions
- established service industry
- established suppliers
- etc. etc.

You will not change the establishment unless you can prove substantial advantages.

ISZ

 

[MikeHalloran]

It's been done.

http://www.prototype-design.com/Camengin.htm

Mike Halloran
Pembroke Pines, FL, USA

 

[hydtools]

There is the Wankel rotary engine. But it has not displaced the reciprocating piston engine.

Ted

 

[Artisi]

Not quite what was raised - but close.

http://www.sa.hillman.org.au/TS3.htm

 

[GregLocock]

and if you can stand the hype

http://en.wikipedia.org/wiki/Revetec

I think fundamentally the issue is that you don't lose much by 'pushing' at the wrong time. In energy terms over a cycle it doesn't matter whether you push very hard against a short lever arm, or less hard against a longer lever arm.

Not that the nutters ever understand that.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[zekeman]

The actual work done during the combustion expansion cycle has very little to do with the perceived usable angle the crank makes and everything to do with the thermodynamic efficiency of the cycle. Looking at the ideal Otto cycle. the work done is a function of the compression ratio, and the heating value of the fuel. And the actual net work is this work less the losses in less than ideal combustion, fuel -air mixture , heat losses, not so perfect expansion of the piston and friction.
Ideal camming probably could help during the ignition stage, and perhaps during the expansion to to make it closer to isentropic than presently done, but I doubt that the designers have overlooked this.

 

[JStephen]

You could arrange the crankshaft differently or add extra linkages in if the standard arrangement was really that bad.

 

[bernie007]

Thanks for all your input guys. I'm not trying to change the world but was curious about the the efficency of a cam based engine. I've done some vector analysis of a power stroke in a regular engine vs a cam based one and noted a 38% increase in crank torque given all other factors remain the same. Whether or not that holds true in realty remains to be seen.

Thnx, Bernie