TABLE OF CONTENTS Liquid fuels such as petrol and diesel oil contain the energy that powers the engine. Hydrogen and carbon are the main constituents of these fuels and they need oxygen to make them burn and release the energy that they contain. The oxygen that is used for combustion comes from the atmosphere. Atmospheric air contains approximately 77% nitrogen and 23% oxygen by weight. For proper combustion, vehicle fuels require approximately 15 kg of air for every 1 kg of fuel. When the fuel and air are mixed and burned in the engine, chemical changes take place and a mixture of gases is produced. These gases are the exhaust. The principal exhaust gases are carbon dioxide, nitrogen and water (steam).
| Safety note | You should be aware that combustion is often not complete and carbon monoxide is produced in the exhaust. Carbon monoxide is deadly if inhaled. For this reason, engines should never be run in confined spaces. Workshops where engines are operated inside must be equipped with adequate exhaust extraction equipment. |
If all conditions were perfect the fuel and air would burn to produce carbon dioxide, super heated steam, and nitrogen (Fig. 4.1). Unfortunately these perfect conditions are rarely achieved and significant amounts of other harmful gases, namely carbon monoxide CO, hydrocarbons HC, oxides of nitrogen NOx and solids (particulates like soot and tiny metallic particles) are produced (Fig. 4.2).
