[mamelukesabre]

Actually, just the opposite is the way it works. A highly efficient engine of any process tends to be very large for its power output.

The diesel engine outshines the gasoline (spark-ignited) engine every time in efficiency because of its higher compression ratio. However, it will weigh over twice as much as an equally powered SI engine.

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I'm not sure I totally buy your theory about higher compression ratios being the reason for the higher efficiency. I think it has more to do with the fact that diesels engines operate at higher temperatures and that there is a great deal of leftover heat "recycled" to vaporize each subsequent charge of fuel.

[John Jamieson]

Carnot cycle math from 100+ years ago shows direct relationship between compression ratio and eff. Also desiel fuel contains more BTUs.

[nightdriver]

"I'm not sure I totally buy your theory about higher compression ratios being the reason for the higher efficiency."

It works out that compression ratio is the main parameter that determines efficiency. There are gains that can be realised by making sure ALL the induced fuel gets burned (more turbulance, residence time, etc.) but this only goes so far. In order to increase the efficiency of any thermodynamic cycle, heat has to be added AT THE HIGHEST TEMPERATURE of the cycle.

Any hydrocarbon fuel has about 20,000btu/lb, whether it's natural gas or crude oil, so that's all we have to work with. With higher compression ratio, higher combustion temperatures are accomplished and benefit efficiency right up to the metallurgical and mechanical limit of the engine.

So a diesel engine with it's 21:1 ratio (it wont even run with less than about a 16:1 ratio) will always be more efficient.

[Doctor Stochastic]

In burning fuel-air mixtures, more energy is converted into mechanical motion at higher pressures. The rest of the energy is discharged into the manifold or goes out the exhaust. Burning is more complete at higher pressures, less carbon monoxide and unburned hydrocarbons.

Higher pressures also produce more nitric oxides and thus are not allowed leading to:
more unburned hydrocarbons and carbon monoxide in the exhaust leading to:
a post drive train burner (catalytic convertor) needing:
more expensive gasoline without lead, sulphur, phosphorous leading to:
more crude oil per gallon of gas.

The additional discharge of heat into the manifold leads to heavier radiators needing heavier engines. I'm guessing that the cost of complying with the nitrogen oxide regulations costs about 35% in efficiency, not to mention that photo-reactive partially burned hydrocarbons are still increased.