Page 45 - E BOOK ENGINE MECHANICAL M2

1. GENERAL

3. FEATURES OF DIESEL ENGINE
(1) Fuel Economy
This is most remarkable feature of a diesel engine which shows higher fuel economy than a gasoline
engine. The thermal efficiency of the diesel engine is about 30-40% while 22-28% of the gasoline en-
gine’s one, and the fuel economy is superior while driving at lower engine speed and lower engine
load. The following major differences between them are shown in the table below.
Item Diesel Engine Gasoline Engine
Compression ratio 15 to 23 7 to 10

Excess air ratio Larger Smaller
Pumping loss at partial load Smaller Larger

(2) Engine Performance
It is said in general that, a diesel engine output is lower than a gasoline engine due to the following
reasons.
1. A diesel engine has lower maximum engine speed.
The maximum engine RPM of the diesel engine is lower in comparison to the gasoline engine. The
combustion pressure generated during the combustion process in a diesel engine is higher than the
one generated in a gasoline engine. In order to withstand the high combustion pressure, the funda-
mental parts of the engine construction are heavier. Because reciprocating components, like crank-
shaft, connecting rods, and pistons, are heavier, the inertia in the engine operation becomes larger,
therefore the maximum speed of a diesel engine has to be suppressed.
2. A diesel engine has leaner air-fuel ratio in its operation.
The diesel engine requires a huge amount of intake air when running in high speed. Without using an
auxiliary device like a super-charger, it is hard to deliver enough air to the engine in order to cope
with the high speed running.

(3) Output Control Difference

<Gasoline Engine>
- The output of the gasoline engine is controlled by op-
erating the throttle valve to adjust the intake air mixture
amount -

A driver adjusts the throttle valve opening by depressing
the accelerator pedal to control the intake air amount
that is sucked by the cylinders. Fuel is injected and
mixed with the air in the intake manifold to make the air-
fuel mixture. The amount of the injected fuel amount is
calculated by the Engine-ECU depending on the engine
Fig. 1-2 operating condition in order to obtain optimum A/F ratio.
The engine output control is achieved by the combus-
tion power of the adjusted amount of the intake air-fuel
mixture into the cylinder in the combustion stage.

On MPI engines, through the crank angle sensor the
Engine-ECU identifies the engine speed, and through
the air flow sensor or the intake manifold pressure sen-
sor, the Engine-ECU identifies the intake air amount.
These have essential functions in the engine control.

1 - 2 Pub. No. PTAE1228