Page 114 - 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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98 Automobile mechanical and electrical systems

Figure 2.27 Indirect diesel injection

Figure 2.28 Swirl chamber

Diesel engines always operate with excess air (or weak mixture) and with high
exhaust gas temperatures. This oxygen combines with nitrogen to form the
pollutant NO (oxides of nitrogen). Exhaust gas recirculation (EGR) is often used
x
to reduce cylinder temperatures and to reduce the amount of oxygen in the
Key fact cylinder air charge, thus preventing the formation of NO ( Figs 2.29 and 2.30 ).
x
For diesel engines, there are three main phases in the combustion process
For diesel engines, there are three
main phases in the combustion ( Fig. 2.31 ). The fi rst is the delay phase as the fuel absorbs heat from the cylinder
process: air charge and vaporizes. The next phase occurs when the fuel has reached a
1 start of injection and delay suffi cient temperature to self-ignite. This causes combustion and the fl ame front
2 start of combustion and fl ame propagates rapidly out across the piston crown. This is where the rapid energy
spread release occurs and causes the characteristic diesel engine noise. Once initial
3 controlled combustion. burning of the fuel takes place, continued injection of fuel provides a controlled
burning and energy release to provide sustained pressure on the piston and
good torque generation at the crankshaft.

Effi cient and effective combustion promotes an engine with good power output
and with minimal harmful emissions. This can only be achieved when the engine
mechanical parts are in good condition and the engine control systems for fuel
delivery, ignition and emission control are correctly optimized and set.