Alternative Fuels
Diesel Engines - More energy efficient
The diesel engine has been adapted for almost every use to which other kinds of engine have been put. A noteworthy characteristic of the diesel engine is its ability to produce consistently high torque at low crankshaft speeds. This ability enables it to "hang on" better than a petrol engine can. The most important characteristic of the high-speed diesel engine is its ability to burn a safe and cheap fuel and extract more useful energy for a given mass of fuel than a petrol engine. Moreover, the exhaust gases from a diesel engine are almost completely free of carbon monoxide whereas a certain amount of this toxic gas is invariably present in the exhaust of a petrol engine.Efficient use of fuel is made possible by the high internal pressures developed in the cylinder of the diesel engine. The thermal efficiency of petrol engines seldom exceeds 25 percent, but diesel engines under ordinary conditions have efficiencies as high as 40 percent.
In the petrol engine, combustion spreads through the entire airfuel mixture at a rate determined by the speed at which the charge can be brought to ignition temperature. As a result, burning of the fuel charge is completed early in the power stroke and, because the piston is still very near t.d.c., the gas pressure rises sharply to a value of approximately four times that of the compression. The heated gases expand during nearly all of the power stroke. In the diesel engine, fuel is injected into the cylinder over a definite period of time. Although the combustion pressure rises to a value of only about twice that of the compression, this pressure is maintained over a greater portion of the stroke. Hence the diesel engine can develop high torque at low speeds-a desirable feature in engines for heavy transport vehicles.
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