Still, it is a fact of life. All things being equal, your statement is certainly true. The problem is that there are a lot of variables - compression ratio, gearing, throttle losses, etc. If every chassis were tried with every available engine the most efficient would probably be one of the smallest, but we would find a lot of bumps in the graph of economy as a function of power or displacement. For example, the EPA MPG ratings
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for the BMW Z4 roadster 6 speed 3.0L is better than for the 5 speed 2.5L. Probably the improved gearing doing that, but there we are. In theory, gasoline engines (otto cycle) have a thermodynamic limit of efficiency around 65% while diesel engines (diesel cycle) have a limit of efficiency around 50%. But since those limits are approached as the compression ratio approaches infinity, the diesel wins almost every time.
----- Original Message ----- From: "Rob" Newsgroups: alt.autos.honda Sent: Friday, April 15, 2005 8:23 PM Subject: Re: Honda Civic SI Concept Information
Detonation (self-ignition) is the problem there. It isn't an issue in the diesel cycle because the fuel isn't introduced until the compression has heated the air charge to the ignition point.
That difference also accounts for the difference in theoretical limit of efficiency. The otto cycle adds heat at constant volume (less true at higher rpms than lower rpms, naturally), while the diesel cycle is deemed to add heat at constant pressure.
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