Which one of the following is a disadvantage of a four stroke engine?

Question

Question

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JKSSB JE Mechanical 02 Nov 2021 Shift 1 Official Paper

Answer 1

Concept:

Two-stroke engines: In this engine, one power stroke is obtained in each revolution of the crankshaft

Four-stroke engines: In this type of engine, one power stroke is obtained in two revolutions of the crankshaft

Advantages of the two-stroke engine over four-stroke engine are

Low cost
More uniform torque and hence lighter flywheel is needed
Because of one power stroke for one revolution, power produced for same size of engine is more
Lightweight and simplicity due to the absence of the valve mechanism.

Important Points

Four Stroke Engine	Two Stroke Engine
Four operations (suction, compression, power, and exhaust) take place in the four strokes of the piston	The four operations take place in two strokes of the piston
It gives one power stroke in the four strokes, i.e. in two revolutions of the crankshaft. As such three strokes are idle strokes	The power stroke takes place in every two strokes i.e. one power stroke for one revolution of the crankshaft
Due to more idle strokes and non-uniform load on the crankshaft, a heavier flywheel is required	The engine has a more uniform load as every time the piston comes down it is the power stroke. As such a lighter flywheel is used
The engine has more parts such as valves and its operating mechanism. Therefore, the engine is heavier	The engine has no valves and valve operating mechanism; Therefore, it is lighter in weight.
The engine is costlier as it has more parts	The engine is less expensive as it has a lesser number of parts
The engine efficiency is more as the charge gets completely burnt out. Consequently, fuel efficiency is more	The efficiency is less. A portion of the charge escapes through the exhaust port, and because of this, the fuel efficiency is less.