Quote:
Originally Posted by AndrewJ
So this one hit me as I was looking around wikipedia last night. I seem to recall reading somewhere a long time ago that the CRX HF engine ran on a different "cycle" than a normal engine, partially accounting for it's increased fuel efficiency.
A few minutes of searching turned up the Otto cycle (the "normal" cycle) and the Atkinson cycle (the high FE cycle).
It looks to me as if the only appreciable difference is in the lift duration of the intake cam. If it was possible to have a new cam ground to provide this increased intake duration, you could effectively convert your engine to the Atkinson cycle, thereby raising your FE.
So the question I'm posing for all of you engine gurus out there is this:
Would this be a feasible mod for all us folks out here with Otto cycle engines? |
First, what is currently being recognized as an Atkinson Cycle engine is not. Only a close approximation.
The current "Atkinson" implementation, delaying the intake valve closing so that a portion of the cylinder A/F mixture, CHARGE, can be pushed back into the intake manifold, only works satisfactorily with 4 cylinder engines or multiples thereof wherein an "opposite" cylinder is in an "intake" cycle and will threfore "absorb" the A/F charge being "pushed" back into the intake manifold.
Another approach might be to use a "reed" valve, one-way shuttle valve, like that used in 2-cycle engines, to prevent reverse flow out of the intake system.
A more common approach is that developed in teh late forties, called the Miller Cycle, using a positive displacement supercharger to prevent the reverse flow and at the same time provide "make-up" for the lost efficiency HP/torque wise of the Atkinson cycle engine.
And just changing the cam profile isn't enough. The modern implementation of the Atkinson cycle typically uses a "native" cylinder compression ratio of about 13:1. So once you "push" 20-30% of the mixture back out of the cylinder you end up with a "net", effective, compression ratio of 10:1, pretty standard.