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Combustion in CI Engine
Combustion in a CI engine is quite different
from that of an SI engine. While combustion in
an SI engine is essentially a flame front moving
through a homogeneous mixture, combustion
in a CI engine is an unsteady process
occurring simultaneously in many spots in a
very non-homogeneous mixture controlled by
fuel injection.
Air intake into the engine is unthrottled, with
engine torque and power output controlled
by the amount of fuel injected per cycle.
Only air is contained in the cylinder during
compression stroke, and a much higher
compression ratios (12 to 24) are used in CI
engines.
In addition to swirl and turbulence of the
air, a high injection velocity is needed to
spread the fuel throughout the cylinder and
cause it to mix with the air.
Fuel is injected into the cylinders late in
the compression stroke by one or more
injectors located in each cylinders.
Injection time is usually about 200 of
crankshaft rotation (150 bTDC and 50 aTDC).