DIESEL POWER PLANT 249
The blowers are usually driven from the engine cranks shaft by mean of Spur, helical or herring-
bone gears, silent chains or V-belts at a speed 2-3 times the engine speed. In the case of turbo-super-
charger, the super charger is coupled to a gas turbine in which the exhaust of the engine is expanded.
The positive displacement types are used for low and medium speed engines with speeds not
over 4000 rev/min. Positive displacement type used with many reciprocating engines in stationary plants,
vehicles and marine installations.
The piston cylinder type is used on large and slow speed stationary engines, but their use is
limited since they are bulkier, more expensive and less dependable than the rotary type blowers.
Centrifugal blowers are used both on low speed and high-speed engines. Centrifugal blowers
driven by exhaust gas turbine are small and light and are used for stationary, locomotive, and marine and
aircraft engine. The speed of the centrifugal type of blowers is high about 10000 to 15000 rev/min for
low speed engines and 15000 to 30000 rev/min for high-speed engines such as aircraft engines.
Centrifugal type widely used as the supercharger for reciprocating engines, as well as compres-
sor for gas turbines. It is almost exclusively used as the supercharger with reciprocating power plants for
aircraft because it is relatively light and compact, and produces continuous flow rather than pulsating
flow as in some positive displacement types.
8.12.2 ADVANTAGE OF SUPERCHARGING
Due to a number of advantages of supercharging the modern diesel engines used in diesel plants
are generally supercharged. The various advantages of supercharging are as follows:
- Power Increase. Mean effective pressure of the engine can be easily increased by 30 to 50%
by supercharging which will result in the increase the power output.
- Fuel Economy. Due to better combustion because of increased turbulence, better mixing of
the fuel and air, and of an increased mechanical efficiency, the specific fuel consumption in most cases,
though supercharging reduces not all.
- Mechanical Efficiency. The mechanical efficiency referred to maximum load is increased
since the increase of frictional losses with a supercharger driven directly from the engine is quite smaller
as compared to the power gained by supercharging.
- Fuel Knock. It is decreased due to increased compression pressure because increasing the
inlet pressure decreases the ignition lag and this reduces the rate of pressure rise in the cylinder resulting
in increasing smoothness of operation.
- Volumetric Efficiency. Volumetric efficiency is increased since the clearance gases are com-
pressed by the induced charge that is at a higher pressure than the exhaust pressure.
8.13 Exhaust System of Diesel Power Plant
The purpose of the exhaust system is to discharge the engine exhaust to the atmosphere outside
the main building.
For designing of exhaust system of a big power plant, following points should be taken into
consideration
