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Old 01-17-2014, 04:10 PM   #182 (permalink)
RustyLugNut
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Very good points.

Quote:
Originally Posted by P-hack View Post
any pressure build up (aside from normal compression) that occurs prior to tdc is counteractive, so, under certain conditions, you can get some improvement by increasing the rate of combustion. Also it is generally accepted that a pressure peak of about 10 degrees ATDC is most efficient.

Since lean burn is "slower", as is lower pressure, the pressure curve is wider (or as the curve is wider in relation to higher rpms), adding n02 to enhance combustion makes sense so that you can narrow the pressure curve and put more on the expansion side of tdc. Again, the effect depends on conditions.

As well as "leaning out" an engine is somewhat counter productive, the engine is oversized and moving more air than is needed for combustion. Lean AND high rpm is a bad combination.
I would like to add that lean-burn engines can operate with little to no throttling resulting in much reduced pumping losses at a given power output. That is the whole idea behind pulse and glide driving - to use the engine in as open a throttle setting as possible.

Lean-burn is akin to cylinder deactivation in that a "large" engine can act like a much "smaller" engine by virtue of the greatly reduced fuel use at cruise settings without the complexity of cylinder deactivation.

And, yes, lower engine speeds (rpm) more easily allows the use of lean-burn.
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