Two Stroke Cycle Engine 

Merits:   One   power   stroke   in   one revolution of the crankshaft 

Power  developed  for  the  same  engine speed  theoretically  twice  that  of  a  four stroke engine 

Simple design and lighter in construction for the same power 

Merits: Uniform torque is obtained. Hence a lighter fly wheel can be used 

Design  of  ports  is  simpler.  Hence  initial cost is less 

Mechanical efficiency is high. No moving parts like cam, follower, rocker arm valves etc., 

Merits: Starting is easy 

These engines are generally air cooled

Four Stroke Cycle Engine

 Demerits:  One  power  stroke  in  two revolutions of the crank shaft Power  developed  for  the  same  engine  speed  is  theoretically  half  that  of  two stroke engine.

 For the same power complicated design and heavier in construction.

Demerits:  Non  uniform  torque  on  the crankshaft.  Hence  a  heavier  flywheel  is required for balancing. Design  valve  mechanism  is  difficult.  Hence initial cost is more.Mechanical  efficiency  is  less.  Power  is lost  due  to  friction  caused  by  valve mechanism

Demerits: Starting is not so easy. These engines are generally water cooled.

 COMPARISON BETWEEN FOUR STROKE CYCLE AND TWO STROKE CYCLE ENGINE (MERITS AND DEMERITS):

Two Stroke Cycle Engine

 DeMerits: Consumption of lubricating oil is more, because less time is available to remove the heat More wear and tear of moving parts.

 Some of the fresh air fuel mixture may escape with exhaust gases. Hence fuel consumption is more

 DeMerits: Thermal efficiency is less. It produces more noise due to sudden release of exhaust gases Scavenging is poor, since exhaust port is open only for a short time

 Merits: Poor scavenging leads to mixing of fresh charge with exhaust gases. This results in poor performance, slow running  Used in light vehicles, like bikes, scooters, mopeds, etc.

Four Stroke Cycle Engine 

Merits: Consumption of lubricating oil is less, because more time is allowed for removing heat from the cylinder. Less wear and tear of parts is less

 Fuel cannot escape with exhaust gases. Hence fuel consumption is less.

 Merits: Thermal efficiency is more.

 Noise is less is less. Exhaust gases are released in separate stroke.

 Scavenging is better, since there is a separate exhaust stroke for the removal of exhaust gases

 Demerits: Better performance and efficiency is more

 Used in heavy vehicles, like buses, lorries, trucks etc.