More on the Diesel Cycle and Thermal Efficiency

Thermal Efficiency is basically a performance measure of devices, such as diesel engines, that use thermal energy. It basically measures energy output over energy input. Because you can never get more energy than you put it, thermal efficiency will always be less than one.

Maximum diesel efficiency is dependent on the cut-off ratio and the compression ratio shown by the following equation:


where nth is the thermal efficiency, α (alpha) is the cut-off ratio (V3/V2), r is the compression ratio (V1/V2), and γ is the ratio of the specific heats (Cp/Cv). This equation is based off of this graph that represents the idealized diesel cycle:


Gasses act ideal at high temperatures and low pressures.

The cycle can be summarized as follows. Work is put in at point one. As this happens, volume decreases and pressure increases. Basically, the pressure compresses the gas. The heat is put in increasing the volume of the gas, expanding it, at constant pressure. From there the pressure decreases as the volume increases due to expansion of the gas, releasing work that powers the engine. Then, pressure goes down at constant volume as heat is given off to return to the beginning of the cycle.

Higher thermal efficiency is not only more cost effective than lower thermal efficiency, but it yields a longer engine life. This is why diesel engines are becoming more popular, they last longer, and fuel is cheaper. This makes them perfect for heavy hauling trucks and now some companies are even making cars that have them that everyday people can use.

Eddie McClain


One response to “More on the Diesel Cycle and Thermal Efficiency”

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