Page 235 - 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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Engine systems 219

Figure 2.285 Glow plugs. (Source: Bosch Media)

The most important phase of controlled combustion is when fuel is being injected
into a burning mixture. This must be at a rate that maintains an even combustion
pressure onto the piston throughout the critical crankshaft rotational angles. This
gives maximum torque and effi cient fuel usage, because temperatures remain
controlled and the heat lost to the exhaust is minimized. The low temperatures
also help to keep NO emissions to a minimum.
x
The speed of fl ame spread in a diesel engine is affected by the air charge
temperature and the atomization of the fuel. These characteristics are shared
with the delay period. A suffi ciently high air charge temperature, of at least
450°C, is a minimum requirement for optimum ignition and combustion.

The delay phase or ignition lag for diesel fuel combustion lasts a few Key fact
milliseconds. It occurs immediately on injection as the fuel is heated up to
The delay phase or ignition lag for
the self-ignition temperature. The length of the delay is dependent on the
diesel fuel combustion lasts a few
compressed air charge temperature and the grade of fuel. The air charge
milliseconds.
temperature is also affected by the intake air temperature and the engine
temperature.
A long delay period allows a high volume of fuel to be injected before ignition and
fl ame spread occurs. In this situation diesel knock is at its most severe. When a
diesel engine is cold, there may be insuffi cient heat in the air charge to bring the
fuel up to the self-ignition temperature. When ignition is slow, heavy knocking
occurs.
To aid starting and to reduce diesel knock, cold-start devices may be used
( Fig. 2.285 ). For indirect injection engines, starting at lower than normal operating
temperatures requires additional combustion chamber heating. For direct
injection engines, cold-start devices are only required in frosty weather.

An initial delay, known as injection lag, occurs in the high-pressure fuel lines of
Key fact
rotary pumped systems. This occurs between the start of the pressure rise and
An initial delay, known as injection
the point when pressure is suffi cient to overcome the compression spring force
lag, occurs in the high-pressure fuel
in the injectors. Diesel fuel pipes and injectors are shown in Fig. 2.286 .
lines of rotary pumped systems.
Ignition of the fuel occurs in the combustion chamber at the time of injection into
the heated air charge. The injection point and the ignition timing are therefore