What is a Two-Stroke Engine and How Does It Work?

A 2-stroke engine is a type of internal combustion engine that completes a power cycle with two strokes of the piston during only one crankshaft revolution. This is in contrast to a 4-stroke engine that requires four strokes of the piston to complete a power cycle during two crankshaft revolutions.

A 2-stroke engine has the potential to produce more power, more torque, and more fuel efficiency than a 4-stroke engine of the same size. However, it also has some disadvantages, such as higher emissions, lower durability, and more noise.

History of 2-Stroke Engines

The first 2-stroke engine was developed by the German inventor Karl Benz in 1879. He patented his design in 1880 and used it to power his first automobile, the Benz Patent-Motorwagen. However, his engine had a separate charging cylinder that compressed the air-fuel mixture before sending it to the main cylinder. This made his engine bulky and complex.

The first practical 2-stroke engine that used the crankcase as a charging pump was invented by the English engineer Joseph Day in 1891. He simplified the design by eliminating the need for a separate charging cylinder and valve mechanism. He also introduced the concept of scavenging, which is the process of removing the exhaust gases from the cylinder and replacing them with fresh air-fuel mixture.

The first commercial 2-stroke engine that used cylinder compression was invented by the Scottish engineer Dugald Clerk in 1881. He improved the efficiency and performance of the engine by using a deflector piston that directed the incoming air-fuel mixture away from the exhaust port. He also used a spark plug to ignite the mixture instead of a hot tube.

The first water-cooled 2-stroke engine was invented by the Yorkshireman Alfred Angas Scott in 1908. He used a twin-cylinder design that increased the smoothness and balance of the engine. He also used a rotary valve to control the timing and duration of the intake and exhaust ports.

How Does a 2-Stroke Engine Work?

A 2-stroke engine works by performing two main functions in each piston stroke: compression and expansion. Compression is when the piston moves up and compresses the air-fuel mixture in the cylinder. Expansion is when the piston moves down and expands the burning gases, producing power. The intake and exhaust processes are performed simultaneously with these two functions, using ports in the cylinder wall that are opened and closed by the piston movement.

The basic working principle of a 2-stroke engine can be explained by following these steps:

1. Intake: As the piston moves down from its top position (top dead center or TDC), it creates a vacuum in the crankcase below it. This vacuum sucks in fresh air-fuel mixture from the carburetor through a reed valve into the crankcase. At the same time, the piston uncovers the intake port in the cylinder wall, allowing some of the air-fuel mixture to enter the cylinder from the crankcase.
2. Compression: As the piston moves up from its bottom position (bottom dead center or BDC), it compresses the air-fuel mixture in the cylinder. At the same time, it also compresses some of the air-fuel mixture in the crankcase below it. When the piston reaches its top position, it closes both the intake and exhaust ports in the cylinder wall.
3. Ignition: When the piston reaches its top position, a spark plug ignites the compressed air-fuel mixture in the cylinder, causing it to burn rapidly and expand. This expansion pushes the piston down with great force, producing power.
4. Exhaust: As the piston moves down from its top position, it uncovers the exhaust port in the cylinder wall, allowing some of the burnt gases to escape out of the cylinder into the exhaust system. At the same time, it also uncovers the transfer port in the cylinder wall, which connects the crankcase with the cylinder. The compressed air-fuel mixture in the crankcase then flows through the transfer port into the cylinder, pushing out most of the remaining exhaust gases and filling up the cylinder with fresh air-fuel mixture for the next cycle.

These four steps are repeated for every revolution of the crankshaft, resulting in one power stroke per revolution.

Advantages and Disadvantages of 2-Stroke Engines

A 2-stroke engine has some advantages and disadvantages compared to a 4-stroke engine. Some of them are:

Advantages:

• A 2-stroke engine has a simpler and lighter design, as it does not have valves, camshafts, or valve springs. This reduces the manufacturing cost and the friction losses in the engine.
• A 2-stroke engine has a higher power-to-weight ratio, as it produces one power stroke per revolution, while a 4-stroke engine produces one power stroke per two revolutions. This means that a 2-stroke engine can deliver more power with less weight and size than a 4-stroke engine of the same displacement.
• A 2-stroke engine can work in any orientation, as it does not rely on gravity to lubricate the moving parts. This makes it suitable for applications such as chainsaws, lawn mowers, and jet skis, where the engine may be tilted or inverted.

Disadvantages:

• A 2-stroke engine has higher emissions, as it burns oil along with the fuel and releases some of the unburned fuel and oil into the exhaust. This causes more pollution and reduces the fuel efficiency of the engine.
• A 2-stroke engine has lower durability, as it does not have a dedicated lubrication system and relies on the oil mixed with the fuel to lubricate the moving parts. This causes more wear and tear on the engine and reduces its lifespan.
• A 2-stroke engine has more noise, as it has more explosions per revolution and does not have a muffler to reduce the sound. This makes it louder and more annoying than a 4-stroke engine.

Types of 2-Stroke Engines

There are different types of 2-stroke engines based on their design, fuel type, and application. Some of them are:

1. Petrol 2-Stroke Engine: This is the most common type of 2-stroke engine that uses petrol (gasoline) as its fuel. It uses a carburetor to mix the air and fuel in a fixed ratio, usually around 32:1. It also uses a spark plug to ignite the mixture in the cylinder. It is mainly used in small and portable devices such as motorcycles, scooters, mopeds, chainsaws, lawn mowers, etc.
2. Diesel 2-Stroke Engine: This is a type of 2-stroke engine that uses diesel as its fuel. It uses a fuel injector to spray the fuel directly into the cylinder at high pressure. It also uses compression ignition to ignite the fuel-air mixture in the cylinder without a spark plug. It is mainly used in large and heavy-duty applications such as marine engines, locomotives, generators, etc.
3. Rotary Valve 2-Stroke Engine: This is a type of 2-stroke engine that uses a rotary valve to control the timing and duration of the intake and exhaust ports in the cylinder wall. The rotary valve is a disc that rotates along with the crankshaft and has holes that align with the ports at certain angles. This allows for better tuning and performance of the engine. It is mainly used in high-performance applications such as racing motorcycles, go-karts, etc.
4. Reed Valve 2-Stroke Engine: This is a type of 2-stroke engine that uses a reed valve to prevent the backflow of the air-fuel mixture from the crankcase into the carburetor during the compression stroke. The reed valve is a thin metal flap that opens when the pressure in the crankcase is lower than the pressure in the carburetor and closes when the pressure in the crankcase is higher than the pressure in the carburetor. This improves the efficiency and performance of the engine. It is mainly used in small and medium-sized applications such as motorcycles, scooters, mopeds, etc.

Conclusion

A 2-stroke engine is a type of internal combustion engine that completes a power cycle with two strokes of the piston during only one crankshaft revolution. It has some advantages and disadvantages compared to a 4-stroke engine. It also has different types based on its design, fuel type, and application. A 2-stroke engine is widely used in various devices and vehicles for its simplicity, lightness, and power.

Don’t forget to share it