Page 113 - 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 97

Figure 2.26 Precombustion chamber and swirl chamber (indirect injection)

effi ciency. During the combustion process, the burning gases are ejected from
this chamber with high pressure and energy. This ensures suffi cient turbulence in
the main combustion chamber for effi cient combustion.
The compression ratio of a direct injection engine is typically between 16:1 and
Key fact
21:1. This is suffi cient to raise the induced charge temperature for self-ignition
of the fuel under all engine operating conditions without creating excessive The compression ratio of a direct
injection engine is typically between
combustion noise (or diesel knock). Indirect injection engines have higher
16:1 and 21:1.
compression ratios of 22:1 to 25:1. This is necessary to generate the extra heat
energy required due to losses via the increased surface area of the cylinder head.
Diesel knock is less apparent in indirectly injected engines as the energy release
is more controlled and less spontaneous.
The diesel engine is designed to produce compression pressures that

generate suffi cient heat in the cylinder to ignite the fuel as it is injected into the
combustion chamber. This is known as compression ignition (CI). Petrol engines
are generally known as spark ignition (or SI) engines. During the combustion
stroke, the engine power output or work is generated, hence the name ‘power’
stroke in the four-stroke cycle of induction, compression, power and exhaust.
Engine combustion is a fundamental process in the operation of the engine. This
process must be effi ciently executed and controlled via the engine subsystems
(fuel, air, ignition, etc.) to ensure best effi ciency and performance, with minimum
harmful exhaust emissions.
Combustion in a diesel engine begins very rapidly as the fuel is being injected
into the combustion chamber and heated. This causes a rapid energy release
that generates the characteristic ‘diesel’ engine noise. For this reason, a simple
diesel engine is noisier than the equivalent petrol engine. The combustion
process is most rapid in a direct injection diesel engine ( Figs 2.27 and 2.28 ) and,
because of this, combustion losses are minimal and these are the most fuel-
effi cient type of internal combustion engine seen in road vehicles. Combustion is
not fully completed in the prechamber before the combustion gases are expelled
into the cylinder and continue the combustion process in the main combustion
chamber. The increased surface area means that more heat from combustion is
lost to the cylinder boundaries (walls, head, etc.) than in the direct injection type.
Indirect injection engines are however, quieter in operation because of the longer,
slower combustion process.