Page 109 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

2
Engine systems 93

Ignition point Maximum power
and torque

Figure 2.20 Ignition timing for best torque

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 ) are produced. These are harmful pollutants
x
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
Defi nition
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; MBT
therefore, the spark must be adjusted to occur at the correct time, according to Minimum spark advance for best
these conditions, to obtain the optimum torque from the engine. The optimum torque.
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