What is Stroke
When piston moves from TDC to BDC, it is called as 1 stroke. Stroke means movement of something in a direction. Here we mean movement of piston. Before defining 1 stroke we should know about some basic terminology used in four stroke engine.
Main Components of Four Stroke Engine
- Inlet Manifold: It is a passage for the entry of the fuel inside the cylinder.
- Exhaust manifold: Passage for the escape of the burnt gases.
- Intake valve: It is a cam operated valve through which the fuel or air enter within the cylinder.
- Exhaust valve: It is also a cam operated valve through which the burnt gases escapes out.
- Spark plug: It is used to generate the spark for the burning of the fuel.
- Cylinder: It is a part of the engine where piston does reciprocating motion in order to produce all four strokes necessary for the working of an engine.
- Piston: It is a part of engine which does reciprocation motion within the cylinder. It performs suction, compression, and exhaust process during the working of the four stroke engine.
- Connecting rod: It connects the piston and crankshaft of the engine. It transfers the power generated by the engine from piston to the crankshaft. And the reciprocating motion of the piston changes to the circular motion.
- Crankshaft: It is used to convert the reciprocating motion of the piston into circular motion.
- Crank case: It holds the crankshaft assembly in it and protects them from damage.
Basic Terminology Used:
- Bore: The inner diameter of the cylinder is called bore. It is generally expressed or measured in millimeter (mm).
- Stroke: The distance travelled by the piston from one of its dead center positon to other dead centre position is called stroke. The distance between the two dead centre is called stroke length.
- TDC: The full form of TDC is Top Dead Centre. It is the maximum upper limit up to which the piston can move.
- BDC: The full form of the BDC is Bottom Dead Centre. It is the lower maximum limit up to which the piston can move.
- Combustion chamber: It is a chamber where the combustion of the fuel takes place
- Clearance volume: It is the Volume included between the Piston and the Cylinder Head when the Piston is at its Top Dead Center in Vertical Engines and inner Dead Center in Horizontal Engines. The Clearance Volume is generally expressed as percentages of Swept Volume.
- Swept volume: It is the volume through which the Piston sweeps for its one Stroke. It is equal to the Area of cross section of the Piston multiplied by its Stroke Length. It is also known as swept volume.
- Total volume: The summation of the clearance and swept volume is called total volume.
- Compression ratio: The ratio of the total volume to the clearance volume is called compression ratio. For Petrol Engines the value of Compression Ratio is varies from 5:1 to 9:1 and for Diesel Engines varies from 14:1 to 22:1
What is Four Stroke?
There are four stroke in a 4 stroke engine. It means piston moves 4 times to complete its one cycle of power stroke. One cycle of power stroke includes suction, compression, power and exhaust stroke.
1 stroke: suction stroke
In this piston movement is from TDC to BDC
2 stroke: Compression stroke
In this piston movement is from BDC to TDC.
3 stroke: Power stoke
In this the piston movement is from TDC to BDC.
4 stoke: Exhaust stroke
In this the piston moves from BDC to TDC.
What is 4 Stroke Engine?
Any mechanical device which is capable of converting chemical energy of the fuel into mechanical energy is called an engine.
Also in four stroke engine, the chemical energy is converted into mechanical energy in which the piston does four times movement to produce a power stroke ( 2 times from TDC to BDC and 2 times from BDC to TDC).
Types of Four Stroke Engine
The four stroke engine are of two types and these are
1. Petrol engine/ gasoline engine: when petrol is used as a fuel in four stroke engine then it is called as four stroke petrol engine. The construction of petrol engine is slightly different from the diesel engine. In petrol engine there is a spark plug for the combustion of the fuel. And air-fuel mixture is sucked in the cylinder.
2. Diesel engine: When the fuel used in the four stroke engine is diesel than it is called as diesel engine. In diesel engine there is fuel injector for the injection of the fuel within the cylinder. During suction, only air is sucked within the cylinder. Hot compressed Air is used for the burning of the fuel in this type of four stroke engine.
The various strokes in four stroke engine are
1. Suction stroke
2. Compression stroke
3. Power stroke
4. Exhaust stroke
Let’s understand what actually happens in these strokes one by one in detail.
1. Suction Stroke:
- Piston moves from TDC to BDC
- Opening of intake valve
- Suction of the air or air fuel-mixture ( air in diesel engine and air-fuel mixture in petrol engine)
In suction stroke what happens, first the piston moves from TDC to BDC. As the piston moves the inlet valve opens and the air fuel mixture in case of petrol engine and only air in diesel engine enters into the cylinder. The exhaust valve remains closed during this stroke.
2. Compression Stroke
- Piston moves from BDC to TDC
- Compression of air or air-fuel mixture (air in diesel engine and air-fuel mixture in petrol engine)
- Intake and Exhaust valve remains closed
In compression stroke, the piston moves from BDC to TDC. The inlet and exhaust valve remains closed during this stroke. As the piston moves upward( from BDC to TDC) the compression of air- fuel mixture in case of petrol engine and only air in case of diesel engine takes place. The compression processes completes when piston reaches to the TDC. The compression is done to increase the temperature of the air or air-fuel mixture. The temperature is increased so that it can easily catch fire during sparking in case of petrol engine and spraying of diesel in case of diesel engine.
3. Power/ Expansion/Working Stroke
- Sparking and burning of the air-fuel mixture in Petrol engine.
- Burning of diesel by the hot gases.
- Piston moves from TDC to BDC.
- Intake and Exhaust valve remains closed.
Petrol engine: The air-fuel mixture is ignited by the spark plug. Due to the ignition the burning process starts. The burning of the air-fuel mixture creates a very high pressure burnt gases. This high pressure burnt gases exerts a thrust on the top face of the piston and it starts to move downward from TDC to BDC. This is the power stroke of the engine. In this stroke we get power which is utilized to run the vehicle. The intake and exhaust valve remains closed during this stroke.
Diesel engine: As the Piston approaches TDC the injection of the diesel in the form of spray by fuel injector takes place. As the diesel sprayed by the fuel injector come in contact with the hot compressed gases it catches fire and burning processes starts. Due to burning high pressure hot burnt gases originates and it puts a very high thrust on the top face of the piston. Due the thrust impact on the piston it starts to move in downward direction i.e. form TDC to BDC.
4. Exhaust Stroke
- Piston moves from DBC to TDC.
- Opening of the Exhaust Valve.
- Escaping of the hot burnt gases through exhaust valve.
In this stroke the piston moves upward i.e. from TDC to BDC. As the piston moves upward the exhaust valve opens and all the burnt gases left after power stroke starts escaping out of the cylinder. The burnt gases escape out in the environment through exhaust Valve. When the piston reaches at TDC the exhaust process completes. And after this again all the four stroke repeat themselves.
Video of Working of Four Stroke Engine (petrol engine)
Video of working of four stroke Diesel Engine
The four stroke engines are generally in motorcycles, cars buses trucks etc.
Advantages of Four Stroke Engine Over Two Stroke Engine
- It has high fuel efficiency
- It runs quitter than 2 stroke engine.
- It remains for last longer than 2 stroke engine and does not cease as quickly.
- We don’t have to mix oil in the fuel as done in the 2 stroke engine.