2-Stroke vs. 4-Stroke Engine: Key Differences Explained

The machine that converts heat produced by burning fuel into useful work is known as a heat engine. In simpler terms, a heat engine is equipment that generates thermal energy and transforms it into mechanical energy. Heat engines can be classified in various ways, and one such classification is based on the working cycle: 2-stroke and 4-stroke.

2-stroke and 4-stroke engines are two distinct types of internal combustion engines widely used in applications like automobiles, motorcycles, small engines (lawnmowers, chainsaws), and some marine and industrial equipment. They differ significantly in their operation, design, and cycle of operation.

Two-Stroke Engine

In a 2-stroke engine, the entire sequence of events—intake, compression, power, and exhaust—is completed in just two strokes of the piston, which equals one revolution of the crankshaft.

A 2-stroke engine completes its cycle in two piston strokes: one upward and one downward.

• Upward Stroke: The engine compresses a mixture of air and fuel in the cylinder. Near the top, a spark plug ignites the compressed mixture, creating an explosion that forces the piston down (the power stroke).
• Downward Stroke: As the piston moves down, it uncovers exhaust ports, allowing burnt gases to escape. Simultaneously, a fresh air/fuel mixture is drawn into the cylinder through intake ports.

This cycle then repeats. Each stroke is either a power stroke or an exhaust/intake stroke.

Four-Stroke Engine

In a 4-stroke engine, all four events—intake, compression, power, and exhaust—take place inside the engine cylinder. These four events require four strokes of the piston, which is equivalent to two revolutions of the crankshaft.

A 4-stroke engine completes its cycle in four piston strokes: intake, compression, power, and exhaust.

• Intake Stroke: The piston moves downward, and the intake valve opens, allowing air to be drawn into the cylinder.
• Compression Stroke: Both intake and exhaust valves close, and the piston moves upward, compressing the air.
• Power Stroke: At the top of the compression stroke, a spark plug ignites the compressed air/fuel mixture, causing an explosion that drives the piston down.
• 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 performing a specific function.

Figure 1: Two-stroke engine (petrol type) and four-stroke engine (petrol/diesel type).

In a 4-stroke engine, all four events (intake, compression, power, and exhaust) occur in four distinct piston strokes. In contrast, a 2-stroke engine combines intake and compression into the upward stroke of the piston, while power and exhaust occur during the downward stroke.

Difference Between 2-Stroke and 4-Stroke Engines

The following table compares these engine types based on various features:

Note: While this table provides a concise overview of the main differences, there may be variations and exceptions based on specific engine designs and applications.

Let’s summarize the key differences below:

• 2-stroke engines have a simpler design and produce power in every revolution of the crankshaft. This makes them more lightweight and gives them a higher power-to-weight ratio compared to 4-stroke engines.
• 4-stroke engines are generally more fuel-efficient and produce less pollution. They have a dedicated exhaust stroke and better control over the fuel mixture and combustion process.
• 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 such as power output, emissions, fuel efficiency, and application requirements.