Working of Two Stroke & Four Stroke Petrol & Diesel Engines

Definition

A two-stroke engine is an internal combustion engine in which one complete cycle of suction, compression, power, and exhaust is completed in two strokes of the piston and one crankshaft revolution.

A four-stroke engine is an internal combustion engine in which one complete cycle is completed in four strokes of the piston and two crankshaft revolutions.

A petrol engine is an engine that works on spark ignition and uses petrol as fuel, where the air-fuel mixture is ignited by a spark plug.

A diesel engine is an engine that works on compression ignition and uses diesel as fuel, where only air is compressed to a high temperature and fuel is injected into the hot air for self-ignition.

Main Content

1. Two-Stroke Engine Cycle

In a two-stroke engine, the entire working cycle is completed in only two strokes of the piston: one upward stroke and one downward stroke.
Unlike a four-stroke engine, separate intake, compression, power, and exhaust strokes are not fully distinct; instead, some of these processes overlap, which makes the engine simpler and capable of producing a power stroke in every crankshaft revolution.

A two-stroke petrol engine generally uses a crankcase for pre-compression of the air-fuel mixture and port openings for intake and exhaust. As the piston moves upward, it compresses the charge in the cylinder and simultaneously creates suction in the crankcase, drawing in a fresh air-fuel mixture. When the piston reaches near the top dead center, the spark plug ignites the compressed mixture. The expanding gases push the piston downward, producing power. Near the bottom dead center, the exhaust port and transfer port open, allowing burnt gases to escape and fresh charge to enter the cylinder.

A two-stroke diesel engine also completes the cycle in two strokes, but instead of a spark plug it uses fuel injection into compressed hot air. In many two-stroke diesel designs, a blower or supercharger is used to force fresh air into the cylinder because scavenging and charging cannot depend only on the piston movement. This improves the removal of exhaust gases and the supply of fresh air.

2. Four-Stroke Engine Cycle

In a four-stroke engine, one complete cycle is divided into suction, compression, power, and exhaust strokes.
The engine requires two crankshaft revolutions to complete one thermodynamic cycle, which makes the power strokes less frequent than in a two-stroke engine but generally improves efficiency and combustion control.

In a four-stroke petrol engine, the intake valve opens during the suction stroke and the air-fuel mixture enters the cylinder. During compression, both valves remain closed and the mixture is compressed. Near the end of compression, the spark plug ignites the mixture, producing a power stroke. Finally, during the exhaust stroke, the exhaust valve opens and burnt gases are expelled. This sequence allows precise valve timing and better control over mixture formation and exhaust removal.

In a four-stroke diesel engine, only air is admitted during the suction stroke. The air is compressed to a very high pressure and temperature during the compression stroke. At the end of compression, diesel fuel is injected into the hot air, auto-ignition occurs, and the resulting expansion produces the power stroke. During the exhaust stroke, the exhaust gases are removed through the exhaust valve. Because diesel engines operate at higher compression ratios, they generally achieve better thermal efficiency than petrol engines.

3. Petrol and Diesel Engine Differences

Petrol engines use spark ignition, while diesel engines use compression ignition.
Petrol engines usually operate with a premixed air-fuel charge, whereas diesel engines admit only air and inject fuel later into the cylinder.

Petrol engines are generally lighter, quieter, and capable of higher speeds. They are commonly used in motorcycles, small cars, and light vehicles. Diesel engines are stronger, more fuel-efficient, and suitable for heavy-duty applications such as buses, trucks, tractors, generators, ships, and industrial machinery. Petrol engines usually have lower compression ratios, typically around 8:1 to 12:1, whereas diesel engines have higher compression ratios, often ranging from 14:1 to 22:1. This high compression in diesel engines raises air temperature sufficiently for ignition of fuel without a spark plug.

In two-stroke petrol engines, simplicity and high power-to-weight ratio are major advantages, but fuel efficiency and emissions are poorer. In four-stroke petrol engines, the separate strokes allow better control, smoother operation, and improved mileage. In two-stroke diesel engines, power output is high and the design is suitable for large engines, but construction is more complex due to scavenging arrangements. In four-stroke diesel engines, efficiency, durability, and reliability are excellent, making them the preferred choice for commercial and industrial use.

Working / Process

1. Suction or Charging of Air-Fuel / Air

In a two-stroke petrol engine, a fresh air-fuel mixture enters the crankcase as the piston moves upward, while in a two-stroke diesel engine fresh air is supplied by a blower or scavenging system.
In a four-stroke petrol engine, the intake valve opens and the air-fuel mixture is drawn into the cylinder during the suction stroke.
In a four-stroke diesel engine, only air enters the cylinder during the suction stroke.

2. Compression and Ignition

In both two-stroke and four-stroke petrol engines, the compressed air-fuel mixture is ignited by a spark plug near the end of compression.
In diesel engines, the air is compressed to a high temperature and then fuel is injected at high pressure so that it ignites automatically.
Compression is essential because it increases temperature and pressure, improving combustion quality and engine output.

3. Power, Exhaust, and Scavenging

After ignition, burning gases expand and force the piston downward, producing mechanical work through the connecting rod and crankshaft.
In four-stroke engines, exhaust gases are removed during a separate exhaust stroke, making the process cleaner and more controlled.
In two-stroke engines, exhaust and fresh charge exchange occur almost simultaneously through ports, and this process is called scavenging. Proper scavenging is necessary to remove burnt gases effectively and avoid loss of fresh charge.

Advantages / Applications

Two-stroke engines are simple in construction, lighter in weight, and provide one power stroke for every crankshaft revolution, giving a high power-to-weight ratio.
Four-stroke engines offer better fuel economy, lower pollution, smoother operation, and longer service life due to better lubrication and separate strokes.
Petrol engines are widely used in motorcycles, scooters, small cars, lawn mowers, and portable machines, while diesel engines are used in trucks, buses, trains, tractors, ships, generators, pumps, and heavy machinery.

Summary

Two-stroke engines complete the cycle in two piston strokes and are compact, powerful, and simple.
Four-stroke engines complete the cycle in four piston strokes and are more efficient, durable, and cleaner in operation.
Petrol engines use spark ignition, while diesel engines use compression ignition, and this difference governs their working, construction, and applications.
Understanding the working of two-stroke and four-stroke petrol and diesel engines is essential for comparing performance, efficiency, and practical uses in engineering.