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Old 01-28-2009, 12:59 AM   #32 (permalink)
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Quote:
Originally Posted by wwest40 View Post
"Atkinson cam is bunk.."

Well, since the actual Atkinson cycle engine does not make full use of the cylinder's intake "charge" capability it also sacrifices power in favor of efficiency. And remember that during the period the intake valve is left open is exactly in time with the least force capability (just leaving BDC) of the crankshaft acting to force the piston upward.
No, it doesn't. This is the part that makes most people walk away from any discussion about Atkinson engines. It has the same VE during intake stroke as any Otto engine with the same attributes. Nothing has changed about the intake cycle of the Atkinson ENGINE. It fills the same amount of space with the same amount of air. The part where Atkinson is supposedly better is the second "crank". It effectively makes for a longer power-stroke. That's all it does. Each piston is making power for more time. Each rod has more leverage on the crank, but only when the rods are actually pushing on the crank. When the crank is pushing the rod back up, or pulling it back down, there is less leverage, so the crank isn't working as much, and less work is wasted doing the intake/compression strokes. You can't logically compare this to an Otto engine with an Atkinson cam.
Quote:
Originally Posted by wwest40 View Post
Also take note that the Atkinson cycle doesn't come up "on the step" except at something like 70% or more cylinder charge. An elongated power stroke does no good unless there is enough charge to be still producing power at or near the bottom of the power stroke.
Source? I believe you're still mixing terms here. The Atkinson cycle engine is mechanically driven so that each power (combustion) stroke takes place on the longer stroke on the other side of an ellipsus on the crank shaft. Therefore, there is never a time when it's not in "Atkinson mode".
Quote:
Originally Posted by wwest40 View Post

An ideal system would leave the compression ratio at 13:1 for light cylinder charging, partial throttle, and only go into Atkinson cycle mode at 70% or more of charge.
See above.
Quote:
Originally Posted by wwest40 View Post
And by the by, the Atkinson cycle is at its MOST efficient at or near WOT when there is little of NO vacuum in the intake manifold.
Moot. Any engine is most efficient at lowest vacuum. It's called Volumetric Efficiency. This is the point of boost, initially. It increased the VE of airplane engines, so they could run better in higher atmosphere. It allowed thinner air to be compressed so that it would resemble air at or closer to sea level, and didnt' involve parasitic drag on the engine.

Using an Atkinson-style cam is no different. The point of what I said was simply to give light to the fact that the limited reversion from the cylinder isn't enough to defeat the inherent vacuum in the manifold. The fact is, even if there is LITTLE vacuum in the manifold, there is still SOME vacuum in the manifold. Since there is SOME vacuum in the manifold, there is still a high pressure spot outside the TB that keeps positive flow. This means that the atkinson cycle can work. The limit to this is when cylinder reversion causes an influx of pressure into the manifold. This prevents the pressure differential on either side of the TB plate, and allows airflow reversion from the TB. Boost solves this, hence the Miller Cycle.[/QUOTE]
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