Here's a description of new combustion engine improvement by Mazda, called SkyActive-G. They claim that in a "generic" engine...

when the exhaust manifold is short, the high pressure wave from the gas emerging immediately after cylinder No. 3’s exhaust valves open, for example, arrives at cylinder No.1 as it finishes its exhaust stroke and enters its intake stroke. As a result, exhaust gas which has just moved out of the cylinder is forced back inside the combustion chamber, increasing the amount of hot residual gas

I always though, that exhaust manifold and intake manifold are separated, so exhaust gases just can't possibly enter other cylinders combustion chambers as described in the above quoted text.

Can exhaust gases be diverted into other cylinders as claimed in that text?


The exhaust and the intake are separate. What they are claiming is that exhaust gases that have just left a piston, may be forced back into it, before the valve closes, due to a high pressure wave originated at a different piston's exhaust.

I am not sure how innovative that is though, since some 20 years ago they were already teaching Mechanical engineers like myself that an intelligently designed exhaust manifold can take advantage of reflections of compression and rarefaction waves to improve cylinder emptying.

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  • $\begingroup$ I agree with this. It would be foolish of Mazda to suggest that manufacturers don't already know about this behaviour. My advice, don't buy a Mazda. $\endgroup$ – MoonKnight Nov 15 '12 at 18:00

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