Merits of a two stroke engine over four stroke engine

i) A two stroke engine has twice the number of power stroke than a four stroke engine at the same speed. Hence theoretically a two stroke engine develops double the power per cubic meter of the swept volume than the four stroke engine.

ii) The weight of two stroke engine is less than four stroke engine because of lighter flywheel due to more uniform torque on crankshaft.

iii) The scavenging is more complete in low -speed two stroke engines, since exhaust gases are not left in the clearance volume as in the four stroke engine.

iv) Since there are only two stroke in the cycle, the work required to overcome the friction and the exhaust strokes is saved.

v) Since there are no mechanical valves and valve gears, construction of two strokes engine is simple which reduces its initial costs.

vi) A two stroke engine can be easily reversed by a simple reversing gear mechanism.

vii) A two stroke engine can be easily started than a four stroke engine.

viii) A two stroke engine occupies less space.

ix) A lighter foundation will be sufficient for two stroke engine.

x) A two stroke engine has less maintenance cost since it requires only few parts.

Demerits of two stroke engine compared with four stroke engine

i) Since the firing takes place in every revolution, the time available for cooling will be less than in a four stroke engine.

ii) Incomplete scavenging results in mixing of exhaust gases with the fresh charge which will dilute it, hence lesser power output.

iii) Since the transfer port is kept open only during a short period, less quantity of the charge will be admitted into the cylinder which will reduce the power output.

iv) Since both the exhaust and the transfer port are kept open during the same period, there is possibility of escaping of the fresh charge through exhaust port which will reduce thermal efficiency.

v) For a given stroke and clearance volume, the effective compression stroke is less in a two stroke engine than in a four stroke engine.

vi) In a crankcase compressed type of two stroke engine, the volume of charge down in the crankcase is less due to the reduction in the crankcase volume because of rotating parts.

vii) A fan scavenged two stroke engine has less mechanical efficiency since some power is required to run the scavenged fan.

viii) A two stroke engine needs better cooling arrangements because of high operating temperature.

ix) A two stroke engine consumes more lubricating oil.

x) The exhaust in a two stroke engine is noisy due to sudden release of the burnt gases.