TABLE OF CONTENTS Petrol is a colourless liquid and is one of the fuels most commonly used in the motor vehicle engine. This is because it is a clean liquid, is easily stored and flows freely. It gives off an inflammable vapour even at very low temperatures and when burnt gives off a large amount of heat.
Before petrol can be burnt it must be atomized (that is it has to be broken down into very small droplets like a mist) so that it can be mixed with a suitable quantity of air. This is usually done by the carburettor. The combustion process (the burning of the air/fuel mixture) involves the chemical combination of a fuel with oxyge; during this process heat is given off. This heat given off by the complete combustion of a unit mass of a fuel is called the calorific value of a fuel, e.g. an average sample of petrol has a calorific value of 44 MJ/kg. Crude petroleum, from which petrol is refined, is a mixture of various compounds of hydrogen and carbon (called hydrocarbons).
The mass of air per kilogram of fuel in a mixture of air and fuel gives the air/fuel ratio. For complete combustion the chemically correct mixture is approximately 14.7 parts of air to 1 part of fuel ( 14.7:1). Ratios of less than about 8:1 (rich) or more than 22:1 (weak) cannot normally be ignited in petrol engine cylinders. The air/fuel ratio has a considerable...
