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If the combustion process is not effi cient, incomplete combustion will result and this produces carbon monoxide (CO). If combustion chamber temperatures are high, oxides of nitrogen (NO x ) 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) ( Fig. 2.20 ).
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.
A chemically correct air and fuel ratio mixture must exist to ensure that suffi cient oxygen is present to completely combust all of the fuel. This is known as mixture strength and is the ratio of air mass to fuel mass ( Fig. 2.21 ). For petrol, the correct ratio is approximately 14.7 air mass to 1 part fuel mass. If more air is present then the mixture strength is known as ‘weak’. If less than a 14.7 air/fuel
Ignition point Maximum power
and torque
Figure 2.20 Ignition timing for best torque
Defi nition
MBT
Minimum spark advance for best torque.
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Automobile mechanical and electrical systemsratio, then the mixture strength is known as ‘rich’. Weak and rich mixtures are less than optimum for the engine, although under certain conditions the mixture strength is adjusted by the engine control system according to demand. For example, for full power a slightly rich mixture is needed and this is provided when the engine is at full throttle. Extended running on rich or weak mixtures reduces engine effi ciency and can cause damage to the engine and its subsystems. The combustion process creates energy within the cylinder in the form of heat from the burning fuel/air mixture. Owing to the enclosed nature of the cylinder, this heat energy creates a pressure rise in the cylinder above the piston. This pressure, applied over the piston area, in turn, creates a force pushing down on the piston and turning the crankshaft via the connecting rod, thus producing torque at the crankshaft. The pressure in the cylinder is shown plotted against cylinder volume in Fig. 2.22 . This is known as an indicator diagram.
The torque at the crankshaft is a function of the cylinder pressure and crankshaft angle; the maximum torque is produced when the connecting rod and crankshaft main/big-end bearings are at right angles (i.e. 90° crank rotation from TDC position). Note that at TDC, any pressure on the piston produces no work as there is no turning moment (torque), just a force pushing down on the bearings. The ignition and fuel settings of an engine are set by the manufacturer at the optimum position to achieve the best compromise of performance, economy and minimal exhaust emissions. With respect to combustion, it is important that the maximum cylinder pressure and energy release occur at the correct angle. Damage to the engine can occur if this happens too early or late in the engine cycle ( Fig. 2.23 ). An example is early or advanced ignition, which causes engine knock and damages the piston if allowed to occur for any signifi cant period. This is a characteristic noise caused by preignition or early ignition of the fuel/air mixture. Advanced or early ignition causes an early pressure rise that is applied to the piston at TDC. At this crank angle, no engine torque can be produced and this means that all the combustion energy is applied directly to the engine mechanical components (piston crown, bearings, etc.), causing them to generate
Fuel 1 by mass (weight) Air 14.7 by mass (weight)
Chemically correct
Less than 1 gives a lean mixture More than 1 gives a rich mixture
Figure 2.21 Mixture ratio
Defi nition
Stoichiometric ratio For petrol, the correct ratio is approximately 14.7 air mass to 1 part fuel by mass.
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the ‘pinking’ noise. Although the noise is quite subtle, the forces are massive and cause considerable damage to the engine.
When pinking occurs, the combustion energy precipitates through the engine components, causing damage. In addition, heat is generated that is not dissipated normally and this causes excessive temperature of engine components (e.g. pistons, valves and valve seats) and consequent heat-related damage. Overretarded ignition causes incorrect timing of the energy release from the fuel that, in turn, means less energy to do work and therefore more energy to dissipate via the cylinder boundaries. This causes an increase in engine temperatures, damages components and reduces overall engine effi ciency. This excess energy also has to be rejected via the exhaust and this causes increased exhaust gas temperatures that can damage exhaust valves and seats, as well as exhaust gas components (catalytic converter).