Engine Efficiency - Compression Ratio

Compression Ratio

The efficiency of internal combustion engines depends on several factors, one of which is the compression ratio. Most gasoline engines have a geometric compression ratio (the compression ratio calculated purely from the geometry of the mechanical parts) of 10:1 (premium fuel) or 9:1 (regular fuel), with some high-performance engines reaching a ratio of 12:1 with special fuels. The greater the compression ratio the more efficient is the engine. Higher compression-ratio conventional engines need gasoline with higher octane value, which inhibits the fuel's tendency to burn nearly instantaneously (known as detonation or knock) at high compression/high heat conditions. However, in engines that utilize ignition by means of very high compression ratios (14-25:1), such as the diesel engine or Bourke engine, this is not necessary. In fact, lower-octane fuels are preferable in these applications because they are more easily ignited under compression.

Under part throttle conditions (i.e. when the throttle is less than fully open), the effective compression ratio is less than when the engine is operating at full throttle, due to the simple fact that the incoming fuel-air mixture is being restricted and cannot fill the chamber to full atmospheric pressure. The engine efficiency is less than when the engine is operating at full throttle. One solution to this fact is to shift the load in a multi-cylinder engine from some of the cylinders (by deactivating them) to the remaining cylinders so that they may operate under higher individual loads and with correspondingly higher effective compression ratios. This technique is known as variable displacement.

Diesel engines have a compression ratio between 14:1 to 25:1. In this case the general rule does not apply because Diesels with compression ratios over 20:1 are indirect injection diesels. These use a prechamber to make possible high RPM operation as is required in automobiles and light trucks. The thermal and gas dynamic losses from the prechamber result in direct injection Diesels (despite their lower compression ratio) being more efficient.

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