Page 110 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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94 Automobile mechanical and electrical systems

Air 14.7 by mass (weight) Fuel 1 by mass (weight)

Chemically correct

Less than 1 gives a lean mixture

More than 1 gives a rich mixture

Figure 2.21 Mixture ratio

ratio, then the mixture strength is known as ‘rich’. Weak and rich mixtures are
Defi nition
less than optimum for the engine, although under certain conditions the mixture
Stoichiometric ratio strength is adjusted by the engine control system according to demand. For
For petrol, the correct ratio is example, for full power a slightly rich mixture is needed and this is provided when
approximately 14.7 air mass to 1 the engine is at full throttle. Extended running on rich or weak mixtures reduces
part fuel by mass. 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