Page 46 - E BOOK ENGINE MECHANICAL M2

1. GENERAL

<Diesel Engine>
- The output of the diesel engine is controlled by adjust-
ing the injected fuel amount in the cylinder -
On the diesel engine the amount of air to be sucked into
the cylinders is almost constant regardless the engine
speed. The output control is achieved by adjusting the
amount of the fuel injected into the cylinders so that the
produced combustion power is changed.

On the diesel engine equipped with the mechanical type
injection pump, the governor that precisely controls the
fuel injection amount is normally mounted beside the
Fig. 1-3 injection pump. A governor is required because the driv-
er is not able to control the fuel injection amount correct-
ly by operating the accelerator pedal alone in order to
stable the engine speed or the output due to variation of
the engine load.
The governor is roughly comprised of a flyweight to re-
spond to the engine speed and a lever mechanism to be
connected to the accelerator lever. The fuel injection
quantity adjustment is carried out by the force balance
between the above parts in the governor.

On the diesel engine equipped with the common rail fuel injection system, the engine speed feed-
back control is able to carry out from the crank angle sensor output signal. The accelerator position
sensor output voltage relates to the accelerator pedal depression by the driver. These sensors have
important roles in the common rail fuel injection system.

(4) Engine Vibration and Noise
It is commonly understood that a diesel engine vibrates more and makes more noise than a gasoline
engine. The vibration source may be attributed to:
◇ Unbalance inertia resulting from reciprocating motion of piston
◇ Torque variations caused by cylinder pressure variation

(5) Exhaust Emission
A diesel engine is always operated at a leaner air/fuel ratio than stoichiometric to prevent smoke
emissions, so the exhaust emission contains excess oxygen. Therefore the diesel engine discharges
less CO and HC in comparison to the gasoline engine. On gasoline engines the fuel is injected into
the intake manifold. Some amount of the injected fuel may adhere to the intake manifold or the in-
take port, causing it to emit a higher HC emission amount than the diesel engine. However, the emis-
sion amount of NOx from a diesel engine is higher in comparison to the gasoline engine.

(6) Engine Start-ability
The combustion of a diesel engine mostly relies on spontaneous ignition of injected fuel with the
heated compressed air. That is why the engine is hard to start when it is cold even if the engine can
start easily by cranking it when it is hot. In order to enhance the engine start-ability during cold condi-
tion, the glow plug system is provided to the combustion chamber.

Pub. No. PTAE1228
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