Compression engines

Compression-ignition engines (diesel engines) use no spark plug. Instead they use a high-compression ratio to heat air within the cylinder to such a temperature that when fuel is finally admitted towards the end of the compression stroke, it ignites spontaneously. The compression ratio is normally in the range 12: 1-17: 1.

* The energy content of a fuel may be expressed as either the higher heating value (HHV) or the lower heating value (LHV). The higher heating value represents the energy released when the fuel is burned and all the products of the combustion process are then cooled to 25°C. This energy then includes the latent heat of vapourisation released when any water produced by combustion of, for example, natural gas, is condensed to room temperature. The lower heating value does not include this latent heat and is hence around 10% lower than the higher heating value in the case of natural gas.

The efficiency of the diesel engine ranges from 30% (higher heating value, HHV) for small engines to 48% (HHV) for the largest engines. Research should push this to 52% (HHV)4 within the next few years. Diesel engines can be built to larger sizes than spark-ignition engines, with high-speed diesels available in sizes up to 4 MW and slow-speed diesels up to 65 MW. Large slow-speed engines can have enormous cylinders. For example, a nine-cylinder, 24 MW engine used in a power station in Macau has cylinders with a diameter of 800 mm.

The combustion temperature inside a compression-ignition engine cylinder is much higher than within a spark-ignition engine cylinder. As a consequence, nitrogen oxide emissions can be 5-20 times greater than from an engine burning natural gas. This can prove a problem and emission reduction measures may be required to comply with atmospheric emission regulations.

Diesel engines can burn a range of diesel fuels including both oil-derived fuels and biofuels. Smaller, high-speed engines normally use high-quality distillate but the large slow-speed engines can burn very low-quality heavy fuel oils which require a much longer combustion time to burn completely. These fuels tend to be dirty and plants burning them usually require additional emission mitigation measures.

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