How does a Two-Stroke Engine work? | 2-Stroke Engine Function and Parts

The engines are most commonly used in vehicles. There are multiple types of engines, and a two stroke engine is one of them. A 2-stroke engine is a type of IC engine. It completes a power cycle with two strokes of the piston during only one crankshaft revolution. This article mainly explains the two-stroke engine working, types, and parts.

What is a Two-stroke Engine?

A two-stroke engine is a reciprocating engine that uses only two piston strokes to complete a power cycle. A 2-stroke engine completes one revolution of the crankshaft after the completion of two piston strokes. A two-stroke engine work in such a way that:

  1. The intake and compression processes are completed in the first stroke of the piston.
  2. The power and exhaust processes are complete in the second stroke of the piston.

The thermal efficiency of two-stroke engines depends on the vehicle’s design and model. But generally, a 2-stroke petrol engine only converts 20% of the chemical (fuel) 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.

These engines have low costs, are highly efficient, and are low in weight than four-stroke engines. This type of IC engine has more output power than a 4-stroke engine which completes a power cycle after the completion of four strokes of the piston or two revolutions of the crankshaft.

Four-stroke engines have a large size and high weight than two-stroke engines because four-stroke engines have more parts than two-stroke engines.

Due to these reasons, 2-stroke 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.

These types of IC engines are most commonly used for small applications such as lawnmowers, bikes, ships, and other small vehicles. However, these are not perfect for heavy-duty applications.

Working of 2-stroke Engine

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

  • Upward stroke (the Suction and the Compression strokes)
  • Downward stroke (the Power and the Exhaust strokes)

Working of two stroke engine

1) Upward 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) Downward Stroke

  • As soon as the combustion of the fresh charge takes place, a large amount of the hot gases is produced which exerts a very high-pressure force on the top of the piston. Due to this high-pressure force, the piston moves downward, rotates the crankshaft, and does useful work.
  • During this stroke, the piston covers the inlet port, and the new charge is compressed in the crankcase.
  • Further downward movement of the piston uncovers first the exhaust port and the transfer port, and the exhaust starts through the exhaust port.
  • As soon as the transfer port opens, the charge through it is forced into the cylinder.
  • The charge strikes the deflector on the piston crown, rises to the top of the cylinder and pushes out most of the exhaust gases.
  • The piston is now at the BDC position. The cylinder is completely filled with the fresh charge, but it is somewhat diluted with the exhaust gases.
  • Finally, the cycle event is then repeated. We get two strokes for the single revolution of the crankshaft.

PV Diagram of 2-stroke Cycle

PV diagram of a 2-stroke engine cycle is given below. A two-stroke engine completes a working cycle in just two strokes of the piston. In this cycle, the intake and compression processes occur simultaneously in the 1st piston stroke, while power and exhaust processes occur simultaneously in the second stroke of the piston.

PV diagram of 2-stroke cycle

An explanation for stages of the 2-stroke cycle is given below:

  1. Ideal cycle (green line): The green line in the above-given diagram represents 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): During this phase, the inlet port opens, and the piston will be drawn up, so it can compress the fuel-air mixture that entered the chamber. The compression causes the mixture to increase slightly in pressure and temperature—however, no heat is exchanged. In terms of thermodynamics, this is referred to as an adiabatic When the cycle reaches point 2, it is when the fuel is met by the spark plug to be ignited.
  3. Isochoric Process (2 to 3): This is where combustion occurs due to the ignition of fuel by the spark plug. The combustion of the gas is completed at point 3, resulting in a highly pressurized chamber with lots of heat (thermal energy). In terms of thermodynamics, this is referred to as an isochoric
  4. Power Stroke (line 3 to 4): In this process, the stored heat and pressure (due to combustion) in the air-fuel mixture 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):From process 4 to 1, all the waste heat is expelled from the engine chamber. As the heat leaves the gas, the molecules lose kinetic energy causing a decrease in pressure. However, there is no exhaust phase in a two-stroke engine, so the cycle begins (1 to 2) again by allowing a new mixture of fuel and air to be compressed.

Types of 2-stroke Engines

1) Piston-controlled inlet port

The piston port is the simplest of designs and the most common in small two-stroke engines. All functions are controlled solely by the piston covering and uncovering the ports as it moves up and down in the cylinder.

A maximum of 70% of bore width is possible in racing engines, where rings are changed every few races. Intake duration is between 120 and 160°. Transfer port time is set at a minimum of 26°.

The strong, low-pressure pulse of a racing two-stroke expansion chamber can drop the pressure to -7 psi when the piston is at the bottom dead center and the transfer ports nearly wide open.

One of the reasons for high fuel consumption in two-strokes is that some of the incoming pressurized fuel-air mixture is forced across the top of the piston, where it has a cooling action, and straight out the exhaust pipe. An expansion chamber with a strong reverse pulse stops this outgoing flow.

2) Rotary Inlet Valve

The intake pathway is opened and closed by a rotating member. A familiar type sometimes seen on small motorcycles is a slotted disk attached to the crankshaft, which covers and uncovers an opening at the end of the crankcase, allowing charge to enter during one portion of the cycle (called a disc valve).

Another form of rotary inlet valve used on two-stroke engines employs two cylindrical members with suitable cutouts arranged to rotate one within the other – the inlet pipe having passage to the crankcase only when the two cutouts coincide.

The advantage of a rotary valve is that it enables the two-stroke engine’s intake timing to be asymmetrical, which is not possible with piston-port type engines.

The piston-port type engine’s intake timing opens and closes before and after the top dead center at the same crank angle, making it symmetrical. In contrast, the rotary valve allows the opening to begin and close earlier.

Rotary valve engines can be tailored to deliver power over a wider speed range or higher power over a narrower speed range than either a piston-port or reed-valve engine. Where a portion of the rotary valve is a portion of the crankcase itself, of particular importance, no wear should be allowed to take place.

3) Reed Inlet Valve

The reed valve is a simple but highly effective form of check valve commonly fitted in the intake tract of the piston-controlled port. It allows the asymmetric intake of the fuel charge, improving power and economy while widening the power band. Such valves are widely used in motorcycles, ATVs, and marine outboard engines.

4) Stepped Piston Engine

The piston of this engine is “top-hat”-shaped; the upper section forms the regular cylinder, and the lower section performs a scavenging function. The units run in pairs, with the lower half of one piston charging an adjacent combustion chamber.

5) Loop Scavenging

This method of scavenging uses carefully shaped and positioned transfer ports to direct the flow of fresh mixture toward the combustion chamber as it enters the cylinder. The fuel-air mixture strikes the cylinder head, then follows the curvature of the combustion chamber, and then is deflected downward.

This prevents the fuel/air mixture from traveling directly out of the exhaust port and creates swirling turbulence that improves combustion efficiency, power, and economy. Usually, a piston deflector is not required, so this approach has a distinct advantage over the cross-flow scheme.

Loop scavenging is the most common type of fuel/air mixture transfer used on modern two-stroke engines.

Suzuki was one of the first manufacturers outside of Europe to adopt loop-scavenged, two-stroke engines. This operational feature was used in conjunction with the expansion chamber exhaust developed by the German motorcycle manufacturer MZ and Walter Kaaden.

6) Cross-Flow Scavenging

In a cross-flow engine, the transfer and exhaust ports are on opposite sides of the cylinder, and a deflector on the top of the piston directs the fresh intake charge into the upper part of the cylinder, pushing the residual exhaust gas down the other side of the deflector and out the exhaust port.

The deflector increases the piston’s weight and exposed surface area, making the piston cool and achieving an effective combustion chamber shape.

7) Uniflow Scavenging

In a uniflow engine, the mixture, or “charge air” in the case of a diesel, enters at one end of the cylinder, controlled by the piston, and the exhaust exits at the other end, controlled by an exhaust valve or piston. Therefore, the scavenging gas flow is in one direction only, hence the name uniflow.

Parts of the 2-stroke Engine

Following are the main parts of 2 stroke engine:

  1. Valves
  2. Cylinder
  3. Crankshaft
  4. Connecting rod
  5. Cylinder head
  6. Piston
  7. Crankcase
  8. Piston rings
  9. Fuel Injector (for diesel engine)
  10. Spark plug (for petrol engine)

parts of 2 stroke engine

1) Cylinder Head

It is switched at the upper side of an Engine’s cylinder, and sometimes it has a spark plug for gasoline engines and a fuel injector for a diesel engine.

2) Fuel Injector

For a two-stroke diesel engine, fuel is sprayed in the engine cylinder through an injector.

3) Spark Plug

Spark plugs are only used on gasoline engines. The spark plug inside the cylinder aims to ignite the fuel-air mixture.

4) Piston rings

Made of cast iron and the pistons have two or three piston rings. This piston ring prevents escaping the highly pressured gas from the cylinder to the valve body. And it is useful for cleaning the wall.

5) Connecting rod

It is made of aluminum or steel alloy, absorbs light, and has a strong impact. Also made of titanium but a little more. It links the crank and the piston. It transfers the motion of the crankshaft towards the piston.

6) Crankcase

It is the most significant part of the two-stroke engine. It Includes cranks, crankshafts, lubricants, and other powerful engine components. Engine crankcases are made of cast iron or cast aluminum using sand casting.

7) Crank

Made up of steel. It rotates inside the engine’s cylinder. One end of a crank is attached to the piston and the other end to the crankshaft.

8) Ports

Ports use in two-stroke gasoline engines. These ports are known as inlet and exit ports.

The two-stroke engine has three ports.

  • Exhaust port
  • Transfer port
  • Intake port
9) Crankshaft

Crankshafts are made of cast iron or forged steel, and these are commonly used of steel to relieve stress in diesel engines. This part of the engine changes the vertical movement of the reciprocating piston into a movement of horizontal.

10) Piston

It is the main part of the cylinder that moves reciprocating and is made of an Aluminum alloy.

11) Cylinder

These are made of cast iron. Because inside a cylinder, the pressures and the temperatures increase highly. This is a place where the piston reciprocates. It houses exhaust as well as the transfer port.

 Advantages of Two-stroke Engine

  • A two-stroke engine completes a working stroke for each revolution of the crankshaft. The four-stroke engine completes a working stroke for every two revolutions of the crankshaft.
  • There is no valve and valve mechanism in it. The ports can be easily designed, covered, and uncovered by the movement of the piston itself.
  • Power developed by the two-stroke engine is twice that developed by the four-stroke engine for the same engine speed and volume.
  • For the same power, a two-stroke engine is more compact and light and requires less space than a four-stroke engine; therefore, it is used in motorcycles and scooters.
  • A two-stroke engine requires a lighter flywheel because of the more turning moment on the crankshaft.
  • It is simpler in construction and mechanism.
  • It requires fewer spare parts due to its simple design.
  • It has high mechanical efficiency.

Disadvantages of Two-stroke Engine

  1. In a two-stroke cycle Otto engine, the fuel consumption is high because the fresh charge is likely to be wasted by escaping through the exhaust port.
  2. It gives greater noise.
  3. The actual compression starts when the ports are completely closed by the upward movement of the piston, after a few degree revolutions of the crankshafts. Thus, the actual compression ratio and hence the thermal efficiency of a two-stroke engine is less than that of the four-stroke for the same dimensions.
  4. The burnt gases dilute the charge due to incomplete scavenging.
  5. There are greater wear and tear of moving parts.
  6. It consumes more lubricating oil.

Applications of 2-stroke Engine

  1. Two-stroke engines are less expensive to build.
  2. It is used in lightweight vehicles like scooters, motorcycles, mopeds, etc., which use gasoline as fuel.
  3. They are used in chainsaws and weed-eaters.
  4. They are also used in off-road motorcycles and racing vehicles.

Two-Stroke Engine vs Four-Stroke Engine

Two-Stroke EngineFour-Stroke Engine
The two-stroke engine completes a power cycle after one revolution of the crankshaft.The four-stroke engine completes a power cycle after two revolutions of the crankshaft.  
The 2-stroke engine produces higher torque at a higher speed (rpm).The 4-stroke engine produces less torque at high speed (rpm).
It completes a working cycle in two strokes of the piston. It generates high torque.It completes a working cycle in four strokes of the piston.  of the
It has low thermal efficiency.It has high thermal efficiency.
When the oil ignites with the fuel, the engine needs more lubricating oil.This engine needs lower lubricating oil.
A 2-stroke engine has inlet and outlet ports for fuel suction and discharge.  A 4-stroke engine has an inlet valve and outlet valve for the fuel suction and discharge.  
It can’t produce higher torque at low speed (rpm).This engine produces higher torque at low speed (rpm).
This engine has a higher power-to-weight ratio.  This engine has a lower power-to-weight ratio.  
The 2-stroke engine produces more smoke.4-stroke engines produce less smoke.
These engines have a low cost. Engines are cheaper and are simple for manufacturing.These have a high cost due to extra rotating parts.  
It has high wear & tear issues because of poor lubrication.These engines have lower wear & tear issues.
These engines are not environmental-friendly.4-stroke engines are environmental-friendly.
They have a simple design.These have complex construction due to the additional parts.
They have high operating noise.They have quiet operation.
They have low weight and small size.They have high weight and large size.
The 2-stroke engine has less volumetric efficiency.The 4-stroke engine has higher volumetric efficiency.

FAQ Section

Is there a 2-stroke engine?

A 2-stroke engine is an IC engine that uses only two piston strokes for a working cycle. It is mainly widely used in small vehicles like motorcycles, mopeds, etc.

What is a 2-stroke engine?

A two-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 four-stroke engine that requires four strokes of the piston to complete a power cycle during two crankshaft revolutions.

Why is a 2-stroke engine so fast?

The two-stroke engine has a low number of parts than the four-stroke engine. It completes a working cycle in just two piston strokes instead of 4 strokes. Due to this reason, it is so fast.

What is a Two-stroke diesel engine?

A two-stroke diesel engine is an internal combustion engine that uses compression ignition with a two-stroke combustion cycle.

How many stokes are in a two-stroke engine?

The name of the two-stroke engine represents that it uses only two piston strokes to complete a power cycle.

Are 2-stroke engines more powerful?

A four-stroke engine generates the highest torque at high speed, while a two-stroke engine generates the highest torque at low speed. 2-stroke engines are simple in design and fast working. Therefore, it is more powerful.

Does a 2-stroke have valves?

Instead of intake and exhaust valves, the two-stroke engine uses pressure check valves and cylinder wall ports that are covered and uncovered by the movement of the piston. In the two-stroke engine, both the volume above the piston and the volume below it are used in gas transfer engine operation.

What Is 2 Stroke Fuel?

Two-stroke fuel is basically unleaded petrol mixed with 2-stroke oil. The oil-to-fuel mix ratio should be specified in your engine’s instruction manual. The oil in 2 stroke fuel is extremely important in lubricating your engine as two-stroke engines do not have an internal oil reservoir.

Is a 2-stroke or 4-stroke faster?

In the case of the engine, the piston movement is known as a stroke. That is, a 2-stroke off-road vehicle has two different piston movements, and a four-stroke vehicle has four-piston movements.

The 2-stroke is generally more unbalanced and accelerates faster, while the 4-stroke has higher top speed and consistency, and the top speed will be higher.

How many Pistons are in a 2-stroke engine?

As the name implies, the two-stroke engine only requires two piston movements (one cycle) to generate power.

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