Spark-ignition engines can burn a variety of fuels including gasoline, propane and landfill gas. However the most common fuel for power generation applications is natural gas. Most are four-stroke engines and they are available in sizes up to around 6.5 MW.
Table 6.1 Piston engine speed as a function of size
Engine size (MW) Engine speed (rpm)
High speed 0.01-3.5
Medium speed 1.0-35.0 Slow speed 2.0-65.0
1000-3600 275-1000 58-275
Source: US Environmental Protection Agency.10
The spark-ignition engine uses a spark plug to ignite the fuel-air mixture which is admitted to each cylinder of the engine. In the simplest case this spark plug is located in the top of the cylinder and directly ignites the mixture within the cylinder. The fuel-air mixture within the cylinder will normally be close to the stoichiometric ratio required for complete combustion of the fuel although it may contain a slight excess of air (lean).
In larger, more sophisticated engines, the spark plug is contained within a pre-ignition chamber on top of the main cylinder. A fuel-rich mixture is ignited within the pre-ignition chamber and the flame shoots into the main chamber where it ignites the mixture there. The advantage of this system is that it allows the main mixture to contain a much larger excess of air over fuel. This results in a lower combustion temperature and this in turn reduces the quantity of nitrogen oxides produced.
The compression ratio of a spark-ignition engine (the amount by which the air-fuel mixture is compressed within the cylinder) must be limited to between 9: 1 and 12:1 to prevent the mixture becoming too hot and spontaneously igniting, a process known as knocking. With natural gas, the engine efficiency varies between 28% (lower heating value, LHV)* for smaller engines and 42% (LHV) for larger engines. An engine tuned for maximum efficiency will produce roughly twice as much nitrogen oxides as an engine tuned for low emissions.
Many natural gas engines are derived from diesel engines. However because they must operate a modest compression ratios, they will only produce 60-80% of the output of the original diesel. This tends to make them more expensive than diesel engines. In practice this loss may be offset by longer life and lower-maintenance costs as a result of the derating of the engine and the cleaner fuel. Higher power can be achieved with a dual-fuel engine (see p.80).
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