SPARK-IGNITION ENGINES
Spark-ignition engines are capable of burning a variety of fuels, including gasoline, propane, biogas, and landfill gas. In practice, however, many of these engines burn natural gas when used for power generation applications because of the lower emissions. These power-generating units are generally four-stroke engines and they are available in sizes from less than 1 kW up to around 6.5 MW (Table 5.2).
The spark-ignition engine uses a spark plug to ignite the fuel–air mixture that 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 composition of the fuel–air mixture in the cylinder may be close to the stoichiometric ratio required for complete combustion of the fuel, but more often it will contain a significant excess of air. More technically com- plex engines can use a preignition chamber in which a small amount of a fuel– air mixture rich in fuel is admitted and ignited. This preignition then spreads into the main cylinder where a fuel–air mixture containing a much greater proportion of air is ignited.
In common with all thermodynamic heat engines, the efficiency that a reciprocating engine can achieve increases with the temperature of the working fluid: air. For a spark-ignition engine the highest cylinder temperature is reached when the air-to-fuel ratio is around 16:1, the ratio at which a stoichiometric amount of oxygen is available to react with the fuel. An engine that operates with this air– fuel mixture is described as a rich-burn engine. A rich mixture leads to the highest temperature but it also leads to the greatest formation of nitrogen oxide, as well as significant amounts of carbon monoxide and unburned hydrocarbon particles as a result of incomplete combustion of some of the fuel. Under most circumstances, therefore, engines operating on a rich mixture will require emission control systems to limit the release of these potential pollutants.
If engine emissions are to be reduced during combustion, then the combustion temperature must be lowered and a greater amount of oxygen introduced to allow complete combustion of the fuel. Such engines are described as lean-burn engines and can operate with an air-to-fuel ratio between 20:1 and 50:1, significantly higher than in the rich-burn engine. The greater proportion of air lowers the overall combustion temperature (there will be less fuel entering the combustion chamber in the lean mixture), reducing the production of nitrogen oxide from nitrogen in the air, and provides the conditions for much more complete combustion of the fuel. This will reduce the amounts of carbon monoxide and unburned hydrocarbons in the exhaust gases. Against this, the lower temperature reduces overall efficiency. Lean-burn engines achieve a typical efficiency of only 28% (LHV),3 compared to up to 42% (LHV) for a rich-burn engine. An engine tuned for maximum efficiency will produce roughly twice as much nitro- gen oxide as one tuned for low emissions. Typical nitrogen oxide emission levels for spark-ignition engines are 45 ppmV to 150 ppmV.
The compression ratio of a spark-ignition engine (the amount by which the air–fuel mixture is compressed within the cylinder) is normally limited to a maximum between 9:1 and 12:1 to prevent the mixture from becoming too hot and spontaneously igniting, a process known as knocking. Lean natural gas–air mixtures have a much higher resistance to knocking than stoichiometic mixtures and can tolerate higher compression ratios than gasoline.