The energy content of petroleum fuels is roughly the same for a given weight regardless of the product. Diesel fuel is slightly heavier than gasoline, so there is a little bit more energy in a gallon of diesel.
The principle powere and efficiency advantages of the diesel engine are the lack of a throttle and the compression ratio. Gasoline engines must operate at a more or less constant fuel/air ratio at any power level. In order to control power output, the throttle varies the density of the air entering the cylinder by reducing the manifold pressure. This difference between the intake pressure and the exhaust pressure results in a braking effect on the engine, causing a loss of power and efficiency at any setting short of full power. The diesel engine controls power by altering just the fuel flow to the engine. The airflow remains constant for a given rpm, regardless of power output, so there is no need for a throttle body and therefore no throttle induced braking effect.
Increasing the compression ratio also increases the power and efficiency. There are two principle reasons for this. First, a higher compression ratio results in less exhaust gas in the cylinder. Any gasses in the cylinder that do not react must be heated and compressed by the fuel, absorbing energy that could be used to drive the piston. Second, the smaller combustion chamber of the high compression engine results in the combustion process being completed in a shorter period of time. (Even though the flame speed is reduced as pressure increases) Ideally, we would like to see the entire combustion process take place with the cylinder at top dead center. Any combustion taking place before or after TDC is wasting energy.
Rob
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