Power Plant Engineering

8.3.2 DIESEL ENGINES/HEAVY OIL ENGINES
In contrast to the engines in which the fuel and air mixes before compression, in diesel engines:
air is compressed as the compression stroke begins and the fuel enters the cylinder at the end of compres-
sion stroke. Heat of compression is used for ignition of fuel.
In a typical diesel engine, air is compressed to about 30 bars, which increases the temperature
when finely atomised diesel fuel oil is sprayed into the heated air, it ignites and burns. High compression
ratio is therefore essential for reliable combustion and high efficiency. Compression ratios above those
needed to achieve ignition do not improve the efficiency.
The pressure ratio depends on engine speed, cylinder size and design factors. Typical compres-
sion pressures in diesel engines range from 30 bar to 42 bar. Small high-speed engines have higher com-
pression pressures.
8.3.3 DUEL FUEL ENGINES
In a duel fuel engine, a small quantity of pilot oil is injected near the end of the compression
stroke. It is ignited by the compression and the mixture burns like standard diesel fuel. The pilot oil
burning provides enough heat to the mixture of gas/air. Precise control of pilot oil injection and a sepa-
rate set of fuel pumps and nozzles are added. Means are provided to reduce air quantity at partial loads.

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POWER PLANT ENGINEERING
8.3.4 HIGH COMPRESSION GAS ENGINES
With operation solely on gas, without duel mixtures and pilot oil, the high Compression Gas
Engines of today use slightly richer mixtures of fuel and air, with lower compression ratios than duel
fuel engines. The compression ratios are higher than conventional gas engines and lower than duel fuel
engines. There is no need of pilot oil.
8.4 ADVANTAGE OF DIESEL POWER PLANT
The advantages of diesel power plants are listed below.
1. Very simple design also simple installation.
2. Limited cooling water requirement.
3. Standby losses are less as compared to other Power plants.
4. Low fuel cost.
5. Quickly started and put on load.
6. Smaller storage is needed for the fuel.
7. Layout of power plant is quite simple.
8. There is no problem of ash handling.
9. Less supervision required.
10. For small capacity, diesel power plant is more efficient as compared to steam power plant.
11. They can respond to varying loads without any difficulty.