the real reason that Diesels get better fuel milage


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Posted by George in MI on Sunday, April 09, 2006 at 1:12PM :

In Reply to: 454's not known for mpg posted by Chriscase on Sunday, April 09, 2006 at 12:12PM :

Diesels get better milage because the compression ratio of the engine is always high...maybe 18:1 They get the full charge of air all the time with the fuel injected independently.

Gas engines are throttled. Only when they run WOT do they achieve their maximum CR which may be only 8-10, ie never as high as the Diesels. When running at less that WOT, the cylinders are not fully filled so the effective CR is much less.

Additionally, it helps that Diesels tend to run at a lower rpm range which cuts friction. Their size and weight don't help fuel milage however. There are other minor considerations also like the energy content of the different fuels used.

At WOT, gas engines and Diesels engines are very close in terms of energy per unit of fuel.



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