Page 234 - 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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218 Automobile mechanical and electrical systems

Figure 2.283 Types of diesel injection: direct using a rotary pump, common rail, unit injection
and pumped unit injection. (Source: Bosch Media)

TDC

80

a b c
60
Bar
40

20

BTDC 45 15 15 45 ATDC
Crank shaft angle

Figure 2.284 Combustion phases: a, delay; b, fl ame spread (rapid pressure rise); c, controlled
combustion (afterburn); BTDC, before top dead centre; ATDC, after top dead centre

Small high-speed diesel engine compression ratios are about 20:1 for direct
injection systems. This compression ratio is capable of raising the air charge to
temperatures of between 500°C and 800°C. Very rapid combustion of the fuel
therefore occurs when it is injected into the hot air charge.

The combustion process in a diesel engine follows three phases: ignition delay,
fl ame spread and controlled combustion ( Fig. 2.284 ). In addition, an injection lag
occurs in the high-pressure pipes of earlier systems, as the pressure builds up
just before injection.