During compression of the fuel/air mixture in a petrol engine, heat energy and kinetic energy (due to gas movement) are imparted into the mixture owing to the reducing volume and rising pressure. This creates a signifi cant temperature increase and the magnitude of this increase depends upon the speed of the compression process and the amount of heat rejected to the surroundings (via the cylinder combustion space, walls, head, etc.). The temperature rise elevates to a point just below the self-ignition temperature of the fuel/air charge, which will combust at or above the flashpoint when ignited via an external source (i.e. the spark plug). Note that if the temperature of the mixture was too high, spontaneous self-ignition could occur and this would be a limiting factor for the maximum compression ratio in a petrol engine
After compression of the inlet charge, combustion of the fuel creates heat and pressure energy, which is imparted on the piston to generate mechanical work. In a petrol engine, this process is initiated by the
high-voltage arc at the spark plug electrodes in the cylinder.
Combustion in the cylinder of an engine is a chemical reaction process between carbon and hydrogen in the fuel and oxygen present in the induced air. The carbon and oxygen combine to form carbon dioxide (CO2), and the hydrogen combines with oxygen to form water (H2O). Nitrogen passes through the engine as long as the combustion chamber temperatures remain below critical limits.
If the combustion process is not efficient, incomplete combustion will result and this produces carbon monoxide (CO). If combustion chamber temperatures are high, oxides of nitrogen (NOx) are produced. These are harmful pollutants and their emissions from motor vehicles are closely regulated and controlled by environmental protection agencies and bodies around the world.
The combustion process should occur in a rapid but controlled manner. The fl ame propagation and energy release in the cylinder should have a predictable, stable behaviour depending on the engine operating conditions. The timing of the spark ignition is critical to achieve appropriate energy release for maximum effi ciency in the energy conversion process that takes place in the combustion chamber. The burn duration of the fuel varies according to engine conditions; therefore, the spark must be adjusted to occur at the correct time, according to these conditions, to obtain the optimum torque from the engine. The optimum spark advance for a given engine condition is known as minimum spark advance for best torque (MBT)
The quality of petrol (gasoline) is measured by a parameter called the octane rating, which gives an indication of the fuel’s resistance to engine ‘knock’ or uncontrolled, spontaneous combustion, which causes engine damage. Fuels with a higher octane rating burn more slowly and in a more controlled manner, and hence have a greater resistance to knock. The octane rating of the fuel determines the limit of ignition advance for a given engine speed and load condition. Therefore, it is particularly important to operate the engine on the correct fuel, to prevent damage to the engine due to knocking.