Quote:
Originally Posted by beatr911
I guess I'm still not getting it. It seems to me the restricted intake is the same as the throttle not being opened much relative to load. If I remember right, for the same engine output, the Atkinson cam timing has fewer losses than a more closed throttle.
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Let's put 2 examples of that head to head.
One is an ordinary Otto engine on half throttle, meaning the intake pressure is half the barometric pressure
The other an Atkinsoned Otto engine where the inlet valves close halfway the compression stroke, on full barometric intake pressure.
Both engines draw the same amount of air.
The Otto cycle engine draws air at half the atmospheric pressure, but regains a bit (about 25%?) of the lost energy during the first half of the compression stroke.
The Atkinson style engine draws air at atmospheric pressure, so no loss there and no gain through the first half of the compression stroke. After that the rest of the cycle is the same.
Both engines take the same amount of air but the Otto engine has to pull the intake air from a vacuum, while the Atkinson engine does not.
The Atkinson engine is therefore more efficient than a same sized normal engine on partial load. But where the Atkinson engine is at its peak, the Otto engine can produce more power. If you don't need that you could also use a smaller normal engine with the same peak power as the Atkinson engine, at roughly the same efficiency.
Atkinson engines however profit from the fact that at higher revs the intake air's momentum (flowing in) prevents it from flowing out again before the valve shuts; the higher the revs the less chance it will have to flow back. So the actual air intake volume on higher revs may be bigger than you'd expect.
As air intake momentum plays a role the air needs to have as much speed as possible, meaning narrow intake channels ending at single valves. And those reduce top revs.
(This all I searched and deducted after wondering why my Insight has only 8 valves while my '85 Civic with practically the same engine size had 12)
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