Emissions

Combustion processes produce products of combustion, which are discharged by the exhaust system.

Air comprises oxygen and nitrogen with very small amount of water vapour, carbon dioxide, argon and traces of many gases. The oxygen is required for the purpose of oxidising the hydrocarbons, comprising the fuel (with sulphur in diesel fuels), in the combustion chamber and the nitrogen is carried into and out of, the chamber without contributing to the task of converting the fuel’s chemical energy to heat energy.

Complete combustion should produce an exhaust gas mixture of steam (H20), carbon dioxide (CO2), surplur (S), oxygen (O2) and nitrogen (N2). This is indeed the case with correctly controlled gas turneries.

Incorrect combustion fails to complete the process, due to insufficient time, too low and to fuel ration, cold-running conditions and inadequate swirl of constituents in the combustion chamber.

This results in hydrocarbons and carbon monoxide being discharged with the above mentioned mixture. With very high combustion temperatures – normal in spark-ignition engines – a fusion of mixture assessment of nitrogen with mixture amount of oxygen takes place. This is not combustion since nitrogen is an inert gas and hence can not release chemical energy as heat. Such a fusion, in different clusters and sizes, causes minute amounts, called nitrous oxides (NO2), to be discharged with the exhaust gases.

The unburned hydrocarbons, carbon monoxide and nitrous oxides are called the EXHAUST EMISSIONS. Much attention has been drawn in recent years, to the destructive effects, on animals (particularly humans) and the environment by these emissions. With all three exhaust emissions, the air to fuel ratio is found to be a dominant influence. Carbon monoxide can be eliminated by lean mixtures and/or the use of a catalyst. Nitrous Oxides are drastically increased with increased combustion temperatures to a maximum at the lean mixtures which also cause the highest engine thermal efficiency. Hydrocarbons decrease to a minimum at this same lean mixture, after which they increase due to the very lean mixtures preventing high temperature with the excess air absorbing the heat energy.

The graphs show the dilemma of combustion engineering for spark-ignition engines.