First of all we need to clarify what we mean by a ‘gas engine’. On this page we are talking about an engine which has spark plugs and runs on a gaseous fuel such as propane, butane, methane, or a mixture of gases such as biogas or landfill gas. To avoid confusion we are not including engines running on petrol (gasoline) or ethanol because these fuels are rarely used in the market sectors in which we are active.
One of the fundamental distinctions between a gas engine and a diesel engine is the air/fuel ratio. Whereas diesel engines always run with a lean mixture, gas engines may run lean, rich or stoichiometric (neither lean nor rich). The air/fuel ratio has a major influence over both the quantity and type of harmful emissions, and determines the ability of a catalytic converter to clean them up. Catalytic converters on engines running with a lean mixture are only really effective against carbon monoxide and hydrocarbons and are therefore described as ‘two-way’. If the engine runs rich, the catalytic converter will be good at reducing oxides of nitrogen but will show little effect against carbon monoxide or hydrocarbons. If the engine runs with a stoichiometric mixture, the catalytic converter will exhibit high conversion efficiency over all three major pollutant groups, and will therefore be known as ‘three-way’. The following graph illustrates this
In practice, most gas engines are set up to run lean in the interests of fuel economy. However, because it is difficult to reduce Nox emissions from a lean engine, if Nox is a concern the engine will usually be set to run with a stoichiometric mixture so that it can be fitted with a three-way catalytic converter.
The following graph illustrates the conversion efficiency against temperature of a typical three-way catalytic converter when fitted to a stoichiometric gas engine
Since the automotive sector is enormous, most catalytic converter technology was developed for automobile engines running on petrol. This technology can be (and frequently is) used for engines running on gaseous fuels, but it cannot match the performance of technology which was specifically designed for this purpose. The main differences (from an emissions perspective) between automotive engines running on petrol and non-automotive engines running on gas are as follows:
- Gas engines tend to run at higher temperatures, which leads to higher Nox emissions
- Although both petrol and gas engines can be set to run with a stoichiometric mixture, in practice a gas engine used on an application such as a forklift truck will deviate further from the ideal mixture, partly because of lack of sophistication in the fuel control and partly because of the inherent variability in the gas itself.
For these reasons, if very high reductions in exhaust emissions are required from a gas engine it is important that the coating of the catalytic converter was designed for that purpose.