2 stroke vs 4 stroke engine

The machine which converts heat produced by burning of fuel into useful work is known as heat engine. In other words, heat engine is basically a equipment which generates thermal energy and transforms the same into mechanical energy form.

There are different types of classification of heat engines based on various criterias. There are two types of classification based on working cycle viz. 2 stroke and 4 stroke.

2-stroke and 4-stroke engines are two different types of internal combustion engines commonly used in various applications, including automobiles, motorcycles, small engines (such as lawnmowers and chainsaws), and some marine and industrial equipment. They differ in their operation, design, and cycle of operation.

Two stroke engine

In 2-stroke engine, the whole sequence of events i.e. suction, compression, power and exhaust are completed in two strokes of piston i.e. one revolution of crankshaft. A 2-stroke engine completes its cycle of operation in two strokes of the piston: one upward stroke and one downward stroke.

During the upward stroke, the engine compresses a mixture of air and fuel in the cylinder. Near the top of the stroke, a spark plug ignites the compressed mixture, causing an explosion that drives the piston down (the power stroke). As the piston reaches the bottom of the stroke, it uncovers exhaust ports, allowing burnt gases to be expelled from the cylinder. Simultaneously, fresh air/fuel mixture is drawn into the cylinder through intake ports. The cycle repeats, with each stroke being a power stroke or an exhaust/intake stroke.

Four stroke engine

In 4-stroke engine, all the four events (suction, compression, power and exhaust) take place inside the engine cylinder. The 4 events are completed in 4 strokes of the piston i.e. two revolutions of the crankshaft.

A 4-stroke engine completes its cycle of operation in four strokes of the piston: intake, compression, power, and exhaust. During the intake stroke, the piston moves downward, and the intake valve opens, allowing air to be drawn into the cylinder. In the compression stroke, both intake and exhaust valves are closed, and the piston moves upward, compressing the air. At the top of the compression stroke, a spark plug ignites the compressed air/fuel mixture, causing an explosion that drives the piston down (the power stroke). During the exhaust stroke, the exhaust valve opens, and the piston moves upward again, expelling the burnt gases from the cylinder. The cycle then repeats, with each stroke having a specific function.

2 stroke engine vs 4 stroke engine

The figure-1 depicts two stroke engine (petrol type) and four stroke engine (petrol/diesel type). In 4 stroke engine all the four events viz. suction, compression, power and exhaust occur in four different strokes of piston. In 2 stroke engine, suction and compression occur in upward stroke of the piston while power and exhaust occur in downward stroke of the piston.

Difference between 2 stroke and 4 stroke

Following table mentions comparison between these engine types with respect to various features. The table provides a concise overview of the main differences, there may be variations and exceptions based on specific engine designs and applications.

Parameters 2 stroke engine 4 stroke engine
Number of strokes When the cycle is completed in one revolution of crankshaft , it is called as 2-stroke cycle engine. When the cycle is completed in two revolutions of crankshaft, it is called as 4-stroke cycle engine.
Power stroke frequency Every revolution Every two revolutions
Fuel/Oil mixing Typically required Separate fuel and Oil
Lubrication system Mixed with fuel/oil Separate oil sump
Emissions Typically higher Typically lower
Noise Often noisier Quieter
Weight Lighter, Weight of engine per hp is comparatively low. Heavier, Weight of engine per hp is high.
Thermal efficiency comparatively low high
Removal of exhaust gases comparatively difficult easy
Power output For given weight, 2-stroke engine gives twice power compare to 4-stroke engine. For same weight, 4-stroke engine would give only half power of 2-stroke engine counterpart.
Engine speed Mostly high speed engines are in this type. All speed types are possible.
Rotating direction This engine can be operated in both the directions i.e. clockwise and anti-clockwise. This engine can be operated in one direction only.

Let us summarize key differences below.
➨2 stroke engines have a simpler design and produce power in every revolution of the crankshaft, making them more lightweight and providing a higher power-to-weight ratio compared to 4 stroke engines.
➨4 stroke engines are generally more fuel-efficient and produce less pollution because they have a dedicated exhaust stroke and better control over fuel mixture and combustion.
➨2 stroke engines tend to be noisier and produce more smoke due to their less efficient combustion process.
➨4 stroke engines are commonly used in most automobiles and larger machines, while 2-stroke engines are often found in smaller applications and some specialty vehicles (e.g., dirt bikes).

Conclusion : Both types of engines have their advantages and disadvantages, making them suitable for different purposes based on factors like power output, emissions, fuel efficiency, and application requirements.


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