Page 44 - E BOOK ENGINE MECHANICAL M2

1. GENERAL

1. MAJOR OPERATIONAL CHARACTERISTICS
A diesel engine is essentially the same reciprocating engine as a gasoline engine in which the com-
bustion pressure causes the pistons to make reciprocating motion. The followings are major opera-
tional characteristics of the diesel engine.

◇ The air only is compressed to a high temperature (300 to 500 ) and a high pressure
(Approx.3MPa), and the fuel injected subsequently there for spontaneous ignition and combus-
tion.
◇ The engine output control is done by adjusting the fuel injection amount into the cylinders
through the injectors, and it is not like gasoline engine so that the engine output is controlled by
the intake air amount with the throttle valve.

2. OPERATING PROCESS
(1) Intake Stroke
On the intake stroke the inlet valve is opened. As the piston comes down from the top dead center,
the air only is drawn from the intake manifold through the inlet valve into the cylinder.

(2) Compression Stroke
When the piston begins to go up from the bottom dead center followed by the intake stroke and the
inlet valve closes, the air ceases to move in and out. The air in the cylinder is compressed as the pis-
ton moves up.

(3) Injection (Ignition) and Combustion Stroke
When the piston almost reaches at the top dead center in the compression stroke, the fuel is injected
into the compressed high temperature air and it starts to be atomized.
Then the engine operation proceeds to the combustion stroke as the particles of the injected fuel into
the combustion chamber change as describe below due to the high temperature/ pressurized air.

◇ The particles of fuel are heated and their surface temperature rise.
↓
◇ Fuel evaporation begins.
↓
◇ When they are mixed with the air and flammable mixture is made, the mixture is set on fire. (Fire
is not set to a specific point as in the gasoline engine.)
↓
◇ Ignition causes a sudden rise in temperature and pressure in the combustion chamber which ac-
celerates the evaporation and makes the remaining fuel particles into a flammable mixture, and
combustion instantly occurs.

(4) Exhaust Stroke
The combustion gas works to push the piston downward in the combustion stroke, and the burned
gas is expelled as the piston moves up through the exhaust valve to be opened in the exhaust stroke.
The diesel engine runs under the operation of above mentioned 4 strokes sequentially.

Fig. 1-1

Pub. No. PTAE1228
1 - 1