(this precedes the "salmonella" quote:)
>In the theoretical world, the exhaust stroke of a four-cycle engine
>occurs when the piston pushes a cylinder's worth of burned gas and
>air out the exhaust pipe. A piston engine being fairly discrete in
>its operations, the stream of gas isn't continuous but rather happens
>in slugs -- think of them as little sausages being shoved out of a
>meat grinder. (Hmmm, you're not a vegetarian, are you, Lydia? :-)
>
>The problem is that in most sporting engines, the exhaust valve and
>the intake valves are both open for a short period of time. When this
>happens, three pressure areas cause a pressure differential across two
>restrictions; the areas are respectively the intake manifold, the
>cylinder, and the exhaust manifold, and the restrictions are the
>intake and exhaust valve. If there's any sausage left in the exhaust
>manifold, it can sometimes get sucked back into the cylinder if the
>pressure of the incoming meat chunks is lower than the pressure of
>trying to shove the sausages down the chute. And just as leaving
>raw meat on your cutting board can give you salmonella if you don't
>clean it carefully, letting the exhaust charge get sucked back into
>the cylinder will reduce the amount of gas and air that can be drawn
>in from the intake manifold.
[...]
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