First of all we need to clarify what we mean by a ‘gas engine’.
In this section we are talking about gaseous fuels such as propane, butane, methane, or a mixture of gases such as LPG, CNG or biogas.
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.
Most gas engines rely on spark-ignition, but some use diesel fuel to facilitate compression ignition, and these are known as ‘dual-fuel’ engines.
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.
- 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 large gas engines are set up to run lean in the interests of fuel economy.
However, on small and medium sized engines 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. Where NOx is a concern on large gas engines, it is normal practice to run them lean and use SCR technology to reduce the NOx. To find out more about SCR technology please follow this link.
The following graph illustrates the conversion efficiency against temperature of a typical three-way catalytic converter when fitted to a stoichiometric gas engine running on LPG