ABSTRACT
Petrol has an approximate composition of 15% hydrogen and 85% carbon. The oxygen for combustion is contained in the air supply and approximately 15 kg of air contains the amount of oxygen that will ensure complete combustion of 1 kg of petrol. This means that the airefuel ratio for complete combustion of petrol is approximately 15:1; a more precise figure is 14.8:1.
Combustion in spark ignition engines such as the petrol engine is initiated by the spark at the sparking plug and the burning process is aided by factors such as combustion chamber design, temperature in the cylinder, mixture strength, etc. Because petrol is volatile, each element of the fuel is
readily supplied with sufficient oxygen from the induced air to ensure complete combustion when the spark occurs. Petrol engine combustion chambers are designed so that the combustion that is initiated by the spark at the sparking plug is able to spread uniformly throughout the combustion chamber. For normal operation of a petrol engine, a range of
mixture strengths (airefuel ratios) are required from slightly weak mixtures e say, 20 parts of air to 1 part of petrol for economy cruising, to 10 parts of air to 1 part of petrol for cold starting. During normal motoring, a variety of mixture strengths within this range will occur e for example, acceleration requires approximately 12 parts of air to 1 part of petrol. These varying conditions plus other factors such as atmospheric conditions that affect engine performance lead to variations in combustion efficiency and undesirable combustion products known as exhaust emissions are produced. Exhaust emissions and engine performance are affected by conditions in the combustion chamber. Two effects that are associated with combustion in petrol engines are (1) detonation and (2) pre-ignition.
