These engines can be distinguished based on their names alone as two-stroke engines complete the combustion cycle within two strokes of the piston, while four-stroke engine completes the combustion cycle within four strokes of the piston.
In these engines, a single power cycle is completed within the two strokes of the piston. As in two-stroke engines, the power stroke is produced during each crankshaft revolution, these engines have more uniform torque distribution than two-stroke engines. Also, these engines produce more power than four-stroke engines.
As shown in the below figure, the two-stroke engines have a simple construction.
In these engines, the crankcase is also involved in the engine cycle. During the upward stroke of the piston, the fresh charge enters into the crankcase through the intake port while during the downward stroke, it is transferred to the cylinder.
These internal combustion engines have ports (instead of valves) to exchange gases into the cylinder and crankcase. The opening of these ports is controlled by the movement of the piston.
In four-stroke engines, a single cycle is completed within the 4 strokes of the piston (i.e. suction, compression, power, and exhaust). It generates a single power stroke for every two revolutions of the crankshaft.
The above figure shows the basic construction of a four-stroke engine. As shown in the figure, the engine uses valves to control the flow of intake and exhaust gases in the cylinder.
These intake and exhaust valves are present at the surface of the cylinder head and actuated with the help of a valve actuating mechanism.
The working of these engines consists of the following 4 strokes:-
- Suction: In this stroke, the fresh charge from the intake manifold enters the cylinder.
- Compression: During this stroke, the charge trapped inside the cylinder is compressed with the help of the piston.
- Power: The compressed mixture undergoes combustion to raise the pressure inside the cylinder which helps to push the piston in a downward direction.
- Exhaust: The combustion products are expelled out through the exhaust port.
As each cycle in these engines consists of one power stroke and 3 idle strokes, these engines require the heavier flywheel to supply power to the idle strokes.
These engines have a valve actuating mechanism and dedicated lubricating system which makes them more complex than two-stroke engines.
Difference between 2-stroke and 4-stroke engine
|Sr. No.||Four-stroke engine||Four stroke engine|
|1||A single power cycle is completed within 2 strokes of the piston or one revolution of the crankshaft.||A single power cycle completes within 4 strokes of the piston or two revolutions of the crankshaft.|
|2||The power stroke is generated during every single revolution of the crankshaft.||The power stroke is generated during every 2 revolutions of the crankshaft.|
|3||Produces higher torque at lower RPM.||It produces less torque at higher RPM.|
|4||It has ports (Intake, exhaust, and transfer) for the exchange of gases in a cylinder.||This engine uses valves for the exchange of gases in a cylinder.|
|5||It has a simpler construction.||The valve timing mechanism controls the opening of valves.|
|6||The lubricant is directly added to the fuel.||The lubricant is not added to the fuel.|
|7||It has simpler construction.||Construction is more complex.|
|8||It requires more lubricant as it is burned or expelled with exhaust gases.||It requires less lubricant as it uses the same lubricant again.|
|9||The oil control rings are not necessary.||The piston requires an oil control ring.|
|10||Higher power-to-weight ratio.||Power to weight ratio is lower.|
|11||It generates higher emissions due to the incomplete combustion of fuel.||It has lower emissions.|
|12||Two-Stroke Engines are noisy.||It generates less noise.|
|13||Thermal Efficiency is low.||Thermal Efficiency is high.|
|14||It uses a lighter flywheel.||It requires a heavier flywheel.|
|15||Two-stroke engines are Cheaper.||Difficult to a cold start.|
|16||These are larger in size than two-stroke engines.||Four Stroke Engines are Costly.|
|17||These engines are more compact.||These are larger in size than two stroke engines.|
|18||Applications: Dirt bike, Chain saw, leaf blower, etc.||Applications: Cars, buses, trucks, etc.|
Which is a more efficient engine? two-stroke or four-stroke?
The four-stroke engines are more efficient than two-stroke engines.
Why do four-stroke engines last longer than two-stroke engines?
The two-stroke engines are subjected to more wear and tear due to the absence of a proper lubrication system. Thus two-stroke engines have a shorter life than four-stroke engines.
Does a two-stroke engine have a lower maintenance cost than a four-stroke engine?
The two-stroke engines have lower maintenance costs because of the simpler design and cheaper parts.