Quote:
Originally Posted by Christ
The idea of the "Atkinson cam" is bunk, essentially. It may increase FE, but it sacrifices power to do it, which is exactly the opposite of the true Atkinson design.
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That's one way to look at it, but I think there's another perspective that would question whether it is indeed opposite of a "true" Atkinson.
The Atkinson-style cam is intended to simulate the operating parameters of the Atkinson cycle. The Atkinson cycle is fundamentally a "normal" compression ratio with an "enhanced" expansion ratio built into an otherwise Otto cycle engine.
In the "true Atkinson" engine this is done with a really trick crankshaft and/or connecting rod journals that lets the engine have a mechanical compression ratio of say 10:1 and a mechanical expansion ratio (i.e. "power stroke") of say 13:1. Since the swept volume will be less on the 10:1 intake stroke you have a smaller displacement engine than if you were to look at the swept volume of the power stroke. Personally I would rate the engine as the displacement indicated by intake stroke swept volume. In the "Atkinson-cam" engine of the same mechanical size with a 13:1 compression/expansion ratio the swept volume is larger than of the "true Atkinson" engine. Since intake charge is bled off during compression in the "cammer" then its specific power output is less than a "true Atkinson" but it is very likely to produce equal power to the "smaller true Atkinson" that has a lower volume.
So one may say that the "true Atkinson" doesn't sacrifice power the way the "atkinson cam" does, but the end result is essentially the same because the "true" engine is a smaller displacement not capable of producing as much power while the "cam" engine produces the same power because it's slightly less displacement-efficient. All in all, the two lumps o' iron (or Aluminum) are the same package size and weight, produce essentially the same usable power, and the cam engine is easier to build and control.
What's bunk about that?