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INTRODUCTION—OUTLINE OF SOME DESCRIPTIVE SYSTEMS 7

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the charge gets compressed when E.P. (exhaust port) and T.P. are covered by the piston ; thus the
cycle is repeated.

L

E.P. M

V

L

E.P. M T. P.

V

C.C.

L

E.P.

M
T. P.

V

C.C.

T. P.

C.C.

Spark
plug

()i ()ii ()iii
L = Cylinder ; E.P. = Exhaust port ; T.P. = Transfer port ; V = Valve ; C.C. = Crank chamber
(i)(ii)(iii)
Fig. 1.5. Two-stroke petrol engine.
The power obtained from a two-stroke cycle engine is theoretically twice the power obtain-
able from a four-stroke cycle engine.

1.3.5. Compression ignition (C.I.) engines

The operation of C.I. engines (or diesel engines) is practically the same as those of S.I.
engines. The cycle in both the types, consists of suction, compression, ignition, expansion and
exhaust. However, the combustion process in a C.I. engine is different from that of a S.I. engine as
given below :
In C.I. engine, only air is sucked during the stroke and the fuel is injected in the cylinder
near the end of the compression stroke. Since the compression ratio is very high (between 14 : 1 to
22 : 1), the temperature of the air after compression is quite high. So when fuel is injected in the
form of a spray at this stage, it ignites and burns almost as soon as it is introduced. The burnt
gases are expanded and exhausted in the same way as is done in a S.I. engine.

1.4. Gas Turbines


1.4.1. General aspects

Probably a wind-mill was the first turbine to produce useful work, wherein there is no
precompression and no combustion. The characteristic features of a gas turbine as we think of the
name today include a compression process and an heat addition (or combustion) process. The gas
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