What is 2-Stroke Engine? | How does a 2 Stroke Engine work?

 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. A two-stroke engine completes one revolution of the crankshaft after the completion of two piston strokes. The two-stroke engine works in such a way that:

1. The intake and compression processes are completed on the first stroke of the piston.

1. The power and exhaust processes are completed on the second stroke of the piston.

 The thermal efficiency of a two-stroke engine is dependent on the design and model of the vehicle. But generally, a 2-stroke petrol engine only converts 20% of the chemical (fuel) power into mechanical power. Only 15% of this energy is using to drive the vehicle wheels, while the remaining 5% of energy is lost to friction and other losses.

 A diagram of a typical 2-stroke engine is show above. Instead of suction and exhaust valves, a two-stroke engine uses back pressure ports. 

 Two-stroke engines have low costs, are highly efficient, and have lower 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 larger size and a higher weight than two-stroke engines because four-stroke engines have more parts than two-stroke engines.

 Background of the Two-Stroke Engine

1. A Scottish engineer, Dual Clerking, invented the first 2-stroke engine, which had compression chambers. In 1881, he patented his model.

1. The crankcase was sweeping the engine by using the area under the piston as a booster pump. This is usually due to St. Joseph’s Day in the UK.

1. A German discoverer, Carl Benz, manufactured a two-stroke gas engine on December 31, 1879, which was patented by Germany in 1880.

1. The first practical two-stroke engine was designed by YorkshiremanAlfred Angus Scott. In 1908, he started to produce water-cooled two-cylinder bikes.

 In these engines, the exhaust gases send less heat to the cooling system compared to the 4-stroke engines. That means more energy is available to run the piston and, if possible, the turbocharger.

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

 When the engine is working, it generates extreme heat. Therefore, a radiator fan is used to cool the engine.

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Types of 2-stroke engines

 The following are two main types of 2-stroke engines:

1. 2-stroke gasoline/petrol engine

1. 2-stroke diesel engine

1) Diesel engine with two strokes,

 A 2-stroke diesel engine works on the basis of the diesel cycle. Hugo Glider invented the 2-stroke diesel engine in 1899. The two-stroke petrol engine is simpler in design than a two-stroke diesel engine, but more complex in an aerodynamic and thermodynamic process.

This type of internal combustion engine is used as the main source of power not only in automobiles but also in off-road engine applications. These engines are most commonly used in high-performance applications like hydroelectric power plants and ships.

 A two-stroke diesel engine is purge with fresh air rather than an air-fuel mixture. To prevent leaks, fuel is only inject after all ports have been close. These are more efficient and have less exhaust gas emissions as compared to a 4-stroke diesel engine.

2) Gasoline or Petrol Two-Stroke Engine

When high lightweight and simplicity are required, the 2-stroke petrol engine takes precedence. Also used where a high power-to-weight ratio is required.

 Commercially available 2-stroke diesel devices/engines are not available because they do not provide the required compression ratio.

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Working Principle of a Two-Stroke Engine

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) Stroke of Suction and Compression

• 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 to form 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 by the spark provided by a spark plug. As the mixture ignites, the power stroke piston starts.

2) Stroke of Power and Exhaustion

• Like suction and compression strokes, the power and exhaust strokes also take place at the same time.

• 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 due to the fact that the piston moves 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 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 two-stroke engine is completed, and now the piston is ready for the next power cycle. 

   PV diagram of a 2-stroke Cycle

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

  In this cycle, the intake and compression processes occur simultaneously in the 1^st piston stroke, while power and exhaust processes occur simultaneously in the second stroke of the piston. An explanation of the stages of the 2-stroke cycle is give 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 starting, the piston is pulling upward, and the piston needs to be pulled downward to suck the air-fuel mixture. However, as shown (lines 1 to 2), the two-stroke engine can continue to suck the air-fuel mixture immediately.
  1. 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 will 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 nears completion (at point 2), a spark plug ignites the compressed mixture by providing a spark to it.
  1. Isotropic Process (2 to 3): In this phase, as the compressed mixture reaches point 2 (as shown in the above diagram), the compressed air-fuel mixture ignites due to its temperature, thermal energy, and pressure becoming very high. But during this process, the volume of the mixture remains constant. Therefore, this process is known as the isotropic (same volume) process. The combustion process completes at point 3, and at this point, a high amount of thermal energy and pressure are storing in the mixture, which is used in the power stroke.
  1. In this process, the stored heat and pressure (due to combustion) in the air-fuel mixture are using to push the piston down, which further moves the crankshaft. This crankshaft further moves the flywheel and moves the vehicle. Therefore, this stroke is a power stroke. During this process, the volume of the compression cylinder increases.
  2. Exhaust stroke (line 4 to 1): During this process, the downward movement of the piston is used to remove the useless heat from the cylinder. When the useless heat leaves the cylinder, the kinetic energy of the molecules is lost due to the 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 entering, and the whole process repeats.

      Parts of the 2-stroke engine

      A two-stroke engine has the following major components: 

     1. Cylinder
     1. Cylinder head
     1. Piston
     1. Piston rings
     1. Connecting rod
     1. Crankcase
     1. Crankshaft
     1. Valves
     1. Spark plug (for petrol engines)
     • Fuel Injector (for diesel engines)

     1) Cylinder

      The cylinder of a 2-stroke engine is constructed of cast iron. This cylinder is a "combustion chamber." This is the place where the piston reciprocates. Due to piston reciprocation, the air-fuel mixture compresses in the cylinder. It has inlet and outlet ports.

     2) Head of a Cylinder

     Main Article: Cylinder Head

      This component of the engine is locating on the upper side of the engine’s cylinder. In the case of the gasoline engine, the cylinder head contains a spark plug. It has a fuel injector in the case of a two-stroke diesel engine.

     3) Piston

      It is the main part of the cylinder that has reciprocating motion inside the combustion chamber. It is constructed of an aluminum alloy.

     Piston Rings 4)

      Each piston of the engine usually has 2 or 3 rings. This piston ring stops the highly pressurized gas from escaping from the cylinder. This is also helpful to keep the cylinder walls clean.

     5) Joining Rod

     Main Article: Connecting Rod,

      It is made of aluminum or steel alloy and absorbs light and strong forces. This part of the engine links the crankshaft and the piston. One end of a crankshaft is attached to the piston and the other end to the crankshaft. It sends the motion of the piston toward the crankshaft.

     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 make of cast iron or cast aluminum using sand casting.

     More on Crankcase Operation

     7) Crankshaft

     Main Article: Crankshaft

      Cast iron or forged steel is used to construct the crankshaft of the 2-stroke engine. The diesel engines have steel crankshafts to relieve stress. This shaft changes the vertical movement of the reciprocating piston into a rotary motion.

     8) Ports

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

     The two-stroke engine has the following three ports:

     • Exhaust port
     • transfer port
     • Intake port

     9) The Spark Plug

      On gasoline engines, spark plugs are only using. The objective of this engine component inside the cylinder is to ignite the fuel-air mixture. It provides a spark to the compressed fuel. 

     10) Injector Fuel

      In a two-stroke or four-stroke diesel engine, fuel is spraying into the engine cylinder through an injector known as a fuel injector. It is installed on the cylinder head. 

      Applications of the Two-Stroke Engine

     • These types of IC engines are most commonly used for off-road motorcycles, racing, lawn mowers, bikes, and ships.
     • These engines are also used for outboard motors, weed eaters, chainsaws, and other applications.
     • Two-stroke petrol engines are using in small vehicle applications such as motorcycles, mopeds, and SUVs.
     • These are used for ship propulsion.
     • These engines are using for small outboard motorboats, pumping, and generator sets.
     • Two-stroke engines are not perfect for heavy-duty applications.

      Advantages and Disadvantages of 2-Stroke Engines

     The two-stroke engine has the following advantages and disadvantages:

      Advantages of a 2-stroke engine

     • It has a lower cost than four-stroke engines.
     • A two-stroke engine has high power as compared to a four-stroke engine.
     • They are easy to maintain.
     • A 2-stroke engine produces constant torque at the engine crankshaft.
     • It has a simpler design than a 4-stroke engine.
     • These internal combustion engines are easy to start.

      Disadvantages of a 2-stroke engine

     • It requires more cooling than a 4-stroke engine.
     • It has lower efficiency than a 4-stroke engine.
     • These engines have low volumetric efficiency.
     • These engines produce more pollution than 4-stroke engines.
     • They have a noisier operation than a 4-stroke engine.

      FAQ Section

     Is there a 2-stroke engine?

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

     Does a 2-stroke engine need oil?

      A 2-stroke engine doesn’t require oil.

     Why are 2-stroke engines so fast?

      The two-stroke engine has a lower number of parts than a four-stroke engine. It completes a working cycle in just 2 piston strokes instead of 4 strokes. For this reason, it is so fast.

     How many strokes are in a two-stroke engine?

      The name of the two-stroke engine implies 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. The 2-stroke engines have a simple design and are fast in operation. Therefore, it is more powerful.

     What happens if you don't put oil in a 2-stroke?

      If your 2-stroke engine is running with too little oil, it may damage the equipment. The oil is very important for lubrication because it cools the cylinder and pistons by reducing their temperature. If your engine is not lubricated properly, the metals may melt the lubricant and damage each other, causing the metals to move against each other and permanently deform.

     Is a 2-stroke or 4-stroke faster?

      In the case of an engine, the piston movement is a "stroke." A two-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 a higher top speed and consistency, and the top speed will be higher.

     Conclusion

      Two-stroke engines have some advantages, primarily for trail riding, where their lightweight is a big plus. They have downsides compared to 4-stroke engines, like a big hit, a narrow power band, poor gas mileage, and the need for fairly frequent teardowns (which thankfully isn’t hard). The 2-stroke engine is using in chainsaws, lawn tools, remote control cars, dirt bikes, and boat motors, etc.