What is 2-Stroke Engine? | How does a Two- Stroke Engine work? (2024)

Engines have become a vital part of all industries, vehicles, and homes. Engines have many types, and a 2-stroke engine is one of them. The two-stroke engine completes a power cycle in just two piston strokes per crankshaft revolution. However, a 4-stroke engine completes a power cycle in 4 strokes of the piston. This article explains the 2-stroke engine working, types, applications, and parts.

What is a 2-stroke Engine?

A 2-stroke engine is a type of reciprocating engine that uses only two strokes of the piston to complete a power cycle. In the two-stroke engine, one revolution of the crankshaft is completed after the completion of two piston strokes.

In this type of internal combustion engine, the piston reciprocates up and down in the cylinder twice for each combustion cycle. The intake and compression processes are completed in the first stroke of the piston. However, the power and exhaust processes are complete in the second stroke of the piston.

The thermal efficiency of the two-stroke engine is dependent on the design and model of the vehicle. A 2-stroke petrol engine usually converts only 20% of the fuel’s chemical power into mechanical power. Only 15% of this energy is used to drive the vehicle wheels, while the remaining 5% of energy loses to resist friction and others.

Two-stroke engines have a simpler design and fewer moving parts than four-stroke engines, which makes them lighter, more compact, and less expensive to manufacture.

It has more output power than the 4-stroke engine which completes a power cycle after the completion of four strokes of the piston or two revolutions of the crankshaft.

Due to these reasons, these engines have a higher power-to-weight ratio compared to 4-stroke engines. But they are not much flexible as the four-stroke engine, and also, they need high lubrication.

2-stroke engines are most commonly used in lightweight, small applications such as outboard motors, chainsaws, scooters, and motorcycles.

Types of 2-Stroke Engines

The two-stroke engines have the following types:

1. 2-stroke gasoline/petrol engine
2. 2-stroke Diesel engine

1) Two-Stroke Diesel Engine

A 2-stroke diesel engine works on the base of the diesel cycle. It completes a power cycle in two strokes of the piston. Hugo Guldner invented the 2-stroke diesel engine in 1899.

These types of 2-stroke engines are less famous compared to four-stroke diesel engines, but they are still employed in some specific applications where their high power-to-weight ratio is desirable, such as in stationary power plants, locomotives, and large marine vessels.

The two-stroke diesel engine is purged with fresh air rather than an air-fuel mixture. Fuel is only injected after all ports have been closed to prevent leaks.

These engines also have fewer moving parts and a simpler design than 4-stroke diesel engines. They are easy to maintain.

2) Two–Stroke Gasoline Engine

The 2-stroke petrol engine works on the otto cycle. In this engine, the intake, compression, power, and exhaust processes all occur within two strokes of the piston.

Two-stroke gasoline engines are usually used in lightweight, small applications, including small boats, scooters, mopeds, and chainsaws. They have a simpler design and lightweight than 4-stroke petrol engines.

These engines have higher emission rates than the 4-stroke petrol engine. They also need a proper supply of engine oil to the fuel mixture for lubricating the engine’s moving parts, such as the piston, camshaft, and crankshaft.

Read More: Different types of Engines

Two-Stroke Engine Working Principle

A two-stroke engine completes a power cycle in just two strokes of the piston. A 2-stroke engine works in the following way:

• 1st stroke (the Suction and the Compression strokes)
• 2^nd stroke (the Power and the Exhaust strokes)

1) Suction and Compression Stroke

• In a 2-stroke engine, the suction and compression strokes take place simultaneously.
• During this stroke, the piston moves upward from the bottom dead center (BDC) to the top dead center (TDC).
• During this upward movement of the piston, a vacuum starts producing inside the compression cylinder (combustion chamber) of the engine. Due to the creation of this vacuum, the air-fuel mixture enters the cylinder via an inlet port.
• After the suction process, the piston continues its upward movement and compresses the air-fuel mixture.
• At the end of the compression stroke, the compressed mixture is ignited due to the spark provided by a spark plug. As the mixture ignites, the power stroke piston starts.

2) Power and Exhaust Stroke

• Like suction and compression strokes, the power and exhaust process also complete in just one stroke of the piston.
• Due to the combustion process, the temperature, internal heat, and pressure of the air-fuel mixture become very high. The high-pressure gases produced during the combustion process exert a very high force on the piston and force it to move downward (from TDC to BCD).
• The downward motion of the piston rotates the crankshaft, which further rotates the flywheel of the vehicle.
• As the power is completed, the piston further moves downward and opens the exhaust valve.
• As the exhaust valve opens, the piston pushes the exhaust gases out of the combustion chamber.
• As the piston reaches at BDC, the piston completely expels the exhaust gases and fills the combustion chamber with the fresh air-fuel mixture, and the whole working cycle repeats. At BDC, one power stroke of the engine is completed, and now the piston is ready for the next power cycle.

Read More: Working of Four-stroke Engine

PV Diagram of Two-stroke Engine

The PV diagram of a 2-stroke engine cycle is given below. In a two-stroke engine, a working cycle completes in just two strokes of the piston.

In this cycle, the intake and compression processes occur simultaneously in the 1^st piston stroke, while the power and exhaust processes occur simultaneously in the second stroke of the piston. An explanation for the stages of the 2-stroke cycle is given below:

1. Ideal cycle (green line): The green line in the above-given diagram represents the suction stroke, while a 2-stroke engine doesn’t have this stroke. This is because when the 4-stroke engine is started, the piston is pulled upward, and the piston needs to be pulled downward to suck the air-fuel mixture. However, as shown (line1 to 2), the two-stroke engine can continue to suck the air-fuel mixture immediately.
2. Adiabatic Compression (1 to 2): At this stage, the intake port opens, and the piston moves downward. During the piston’s downward movement, it compresses the air-fuel mixture. Due to this compression process, the temperature and pressure of the mixture increase, but there will be no change in heat. Therefore, this process is known as an adiabatic process (no heat change). As the compression cycle is near its end (at point 2), a spark plug provides a spark to the compressed mixture and ignites it.
3. Isochoric Process (2 to 3): In this phase, as the compressed mixture reaches at point 2 (as shown in the above diagram), the compressed air-fuel mixture ignites due to that its temperature, thermal energy, and pressure become very high. But during this process, the volume of the mixture remains constant. Therefore, this process is known as the isochoric (same volume) process. The combustion process completes at point 3 where a high amount of thermal energy and pressure are stored in the air-fuel mixture, that is utilized in the power stroke.
4. Power Stroke (line 3 to 4): In this process, the stored heat and pressure (due to combustion) in the air-fuel mixture use to push the piston down, which further moves the crankshaft. This crankshaft further moves the flywheel and moves the vehicle. Therefore, this stroke is known as a power stroke. During this process, the volume of the compression cylinder increases.
5. Exhaust Stroke (line 4 to 1): During this process, the downward movement of the piston uses to remove the useless heat from the cylinder. When the useless heat leaves the cylinder, the kinetic energy of the molecules loses due to that pressure decrease. But in a two-stroke engine, there is no exhaust phase, and the process starts again, and a new air-fuel mixture is entered, and the whole process repeats.

Read Also: Working of Otto Cycle

Parts of the 2-Stroke Engine

A two-stroke engine has the following major components:

1. Cylinder
2. Cylinder head
3. Piston
4. Piston rings
5. Connecting rod
6. Crankcase
7. Crankshaft
8. Valves
9. Spark plug
10. Fuel injector
11. Fuel pump
12. EGR system

1) Cylinder

The engine cylinder is one of the most vital parts of the two-stroke engine. It is constructed of cast iron. This cylinder is also known as a combustion chamber. The piston moves upward and downward inside the cylinder.

The air-fuel mixture is compressed in the cylinder due to the upward and downward motion of the piston. The inlet valve is connected to one side of the cylinder while the outlet valve is connected to the other side.

Read More: Combustion Chamber Types and Working

2) Cylinder Head

This component of the engine is located at the upper side of the cylinder. In the case of the gasoline engine, the spark plug is located at the cylinder head. It has a fuel injector in the case of a two-stroke diesel engine.

Read More: Cylinder Head Function

3) Piston

It is one of the main components of two-stroke engines. It has a reciprocating motion inside the combustion chamber. It is constructed of an Aluminum alloy. It plays a big role to generate power according to the requirements.

Read More: Working and Types of Piston

4) Piston Rings

Each piston of the engine usually has 2 or 3 rings. The piston ring prevents the leakage from engine cylinder. It stops the highly pressured gases from leaking from the cylinder. It also assists to keep clean the cylinder walls.

5) Connecting Rod

It is made of aluminum or steel alloy. The connecting rod absorbs light and strong forces. This part of the engine links the crankshaft and the piston.

One end of the connecting rod is attached to the piston while the other end is attached to the crankshaft. It delivers the reciprocating motion of the piston to the crankshaft.

Read More: Connecting Rod Working and Applications

6) Crankcase

The crankcase is a major part of the two-stroke engine. It acts as a housing that surrounds the pistons, crankshafts, connecting rods, pistons, and other important parts of your engine.

It also works as a container for the engine oil, which is used to lubricate the rotating parts of your engine to stop wear and tear and minimize friction.

The crankcase also stops dust and other contaminants from going into the engine and causing damage.

Read More: Working of Crankcase

7) Crankshaft

Cast iron or forged steel is used to construct the crankshaft of the 2-stroke engine. It is connected to the piston through a connecting rod. It gets power from the engine piston, transfers this power to the vehicle wheels, and turns them.

Read More: Crankshaft Working and Applications

8) Ports

The engine uses ports for fuel suction and exhaust purposes. These ports are known as inlet and exit ports.

The two-stroke engine has the following three ports:

• Exhaust port
• Transfer port
• Intake port

9) Spark Plug

Spark plugs are only used on gasoline engines. It is installed on the cylinder head. It gets power from the vehicle battery. When the air-fuel mixture inside the engine cylinder is compressed according to the requirements, the spark plug produces a spark to ignite the mixture and initiates the combustion process.

Diesel engines don’t need a spark plug for the combustion process.

Read More: Bad Spark Plug Symptoms and Causes

10) Fuel Injector

The fuel injector is also installed on the engine cylinder head. It is used to spray the fuel in the engine cylinder.

11) Fuel Pump

The main purpose of the fuel pump is to take fuel from the fuel tank and pump it to the fuel lines. It ensures the proper supply of fuel to the fuel injector and fuel lines.

Read More: Bad Fuel Pump Symptoms and Causes

12) EGR System

The latest vehicle engines contain an EGR system. This system helps to control the emissions of exhaust gases.

Read More: Bad EGR Sensor Signs and Causes

Applications of Two-Stroke Engines

• Two-stroke engines have a compact design and lightweight. Therefore, they are most commonly used in scooters and small motorcycles. These engines also have relatively low manufacturing costs.
• They are used in snowmobiles because of their capability to work in cold weather conditions.
• Two-stroke engines are commonly used in chainsaws due to their ability to operate in any position and high power-to-weight ratio.
• These types of IC engines have a compact size and the capability to generate a lot of power. Therefore, they are usually employed in outboard motors for boats. These engines have a low manufacturing cost, which makes them an ideal choice for recreational boaters.
• They are employed in lawnmowers because of their relatively low cost and skill to deliver the desired power to cut grass efficiently.
• The 2-stroke engine is used in jet skis because of its capacity to generate extreme power in a small and lightweight package.

Advantages of 2-Stroke Engines

• It has low cost than four-stroke engines.
• A two-stroke engine has high power as compared to a 4-stroke engine.
• They are easy to maintain.
• A 2-stroke engine produces constant torque at the engine crankshaft.
• It has a simple design than a 4-stroke engine.
• These internal combustion engines are easy to start.
• 2-stroke engines have a compact size.
• They have the capacity to generate extreme power in a small and lightweight package.

Disadvantages of 2-Stroke Engine

• It requires high cooling than the four-stroke engine.
• It has low efficiency than the 4- stroke engine.
• These engines have low volumetric efficiency.
• These engines produce more pollution than 4-stroke engine engines.
• They are noisier than the 4-stroke engines.
• They produce more emissions than the 4-stroke engines.

FAQ Section

Read More

1. Different types of Engines
2. Working of 4-stroke Engine
3. Difference between 2-stroke and 4-stroke engines
4. Types of Reciprocating Engines