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Old 01-18-2013, 11:26 PM   #24 (permalink)
serialk11r
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Quote:
Originally Posted by beatr911 View Post
So to bringing this back from the dead, been doing more perusing.

Seems the modern "Atkinson" engines all use variable valve timing and even the "Mild Atkinson" engines too.

So, with the Prius retarding only the intake 33* during atkinson mode, I should be able to achieve a mild effect by retarding my cam one tooth which is 14.4*. If there is no piston contact, try for 28.8* and see what happens.

Also I found out that internal egr can occur if the exhaust is retarded as well, pulling exhaust back in during the intake stroke. Retarding up to 60* may be possible, however in a fixed cam engine idle egr can become excessive rather easily making idle quality really horrible. Hey, maybe try retarding both cams as as part of a second experiment.
I wouldn't touch the exhaust cam. The last thing a motorbike engine with its massive exhaust cams needs is more internal EGR. When you have a giant intake cam to work with and a good high compression ratio, you should try to maximize the "Atkinson cycle effect" (reduced VE via late intake valve closure) before resorting to methods that compromise combustion stability in an engine that already has problems with combustion stability.

Internal EGR is the least desirable method of pumping loss reduction because it raises the temperature of the charge significantly (knock, reduced heat capacity ratio).

Remember that all car engines these days have variable valve timing. What makes the "Atkinson cycle" engines different is the large intake cam, which you already have on your bike (in fact, your bike's intake cam probably has quite a bit more duration than a Prius cam).

Valves hitting the piston shouldn't be a problem, as retarding the intake cam makes them open later, as the piston is already moving away from the valves Retarding the exhaust cam could be a problem there though.
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