An engine is a machine that converts one form of energy into mechanical energy. Heat engines particularly used in vehicles, use chemical fuel to produce mechanical energy. In heat engines, the fuel is burnt and the resultant steam provided, is used to cause a rotating motion. These heat engines are classified into
1. Internal combustion Engine
2. External Combustion Engine
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Credit (Internal Combustion Engine): By MichaelFrey - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=41646424 |
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External Combustion Engine |
An internal combustion engine burns the fuel within it and uses the developed steam to produce mechanical energy. Whereas in an external combustion engine, the fuel is burnt in a separate chamber and the generated steam is carried out through a pipe and then fed to the engine to produce mechanical energy. The internal combustion engine is also classified in many ways, namely,
1. Reciprocating Engines
2. Rotary Engines
3. Continuous Combustion Engines
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Credit (Rotary Engine): By MichaelFrey - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=41587908 |
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Credit (Continuous Combustion Engine): By Jeff Dahl, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=3235265 |
Among these, the reciprocating engine is a normal type of engine that is used in most land vehicles today. It functions using piston strokes. Here the stroke of the piston produces the rotational motion with respect to linear motion caused by the pressure of the blast created during the combustion of fuel. The rotary engine is similar to the reciprocating type engines, but instead of piston strokes, it performs rotating strokes. The Combustion Engines burn the fuel continuously and produce the rotating movement whereas in reciprocating and rotary engines, burning the fuel takes place only at specific intervals.
Reciprocating engines
Reciprocating type engines generally use petrol (gasoline) and diesel for their combustion. All type of reciprocating engines consists of a piston, which converts linear motion into rotating motion, a crankshaft, and finally inlet and outlet valves. The only main difference between the petrol and diesel engine is that the petrol engine has a spark plug, whereas a diesel engine has a fuel injector. The reason behind this is; that petrol can easily be evaporated and can be easily ignited by a little spark, but diesel does not evaporate as much as expected and cannot be ignited easily. So it must be sprayed into a high-temperature air or something, to start ignition. That is why to spray the diesel; a fuel injector is used in a diesel engine, while a spark plug is used in a petrol engine to produce a spark. But nowadays fuel injectors are also used in petrol engines by slight engine modification. Based on the usage, the reciprocating type engines are further classified as,
1. 2- Stroke engine
2. 4-Stroke engine
The 2-stroke and the 4-stroke engine is applicable for both petrol and diesel engines. Let’s look at the operation of petrol and diesel engines in both 2 strokes and 4 strokes.
Petrol Engine (2- Stroke)
The operation of the petrol engine can be clearly described as, at each time when the piston moves up and down, one cycle of rotation is completed. So the word 2- stroke means the operation that takes place in the engine will be in two processes. At first, in order to begin a fire, the necessary things needed are fuel, heat and oxygen. These things make the burning process possible. At the time of starting in a 2-stroke petrol engine, the area below the piston is filled with the fuel-air mixture by the inlet valve. So, during the first stroke, the piston moves down and the intake process takes place. Due to the increase in pressure at the bottom chamber, the inlet valve begins to close; thereby an enriched combination of fuel-air mixture is pushed from the storage towards the combustion chamber. Now due to the flywheel connected to the crankshaft, the piston moves up and the compression of the mixture begins. Now the inlet port opens due to the pressure drop and an air-fuel mixture is again pumped into the bottom chamber. During the compression process, the temperature of the air-fuel mixture increases. Now a little spark is introduced to the mixture; so that it gets ignited and a blast occurs. By next, the second stroke starts, which makes the piston to be forced down again by the force exerted by the explosion. Now the stored air-fuel mixture in the bottom chamber is pumped up due to the increase in pressure. This process pumps the new mixture into the combustion chamber and also forces the burnt gas out through the exhaust valve. So again the compression occurs and the mixture is ignited again.
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2-Stroke Petrol Engine Working |
Petrol Engine (4-stroke)
A 4-stroke petrol engine is similar to a 2-stroke petrol engine, but unlike a 2-stroke engine which performs an engine cycle in 2 processes, the 4-stroke engine performs it in 4 processes. The 4 stroke process is given below.
In 4 stroke engine, the inlet valve and the outlet valve are located at the top of the combustion chamber, whereas in a two-stroke engine, the inlet valve is placed down and the output valve is placed at the top. During the first stroke, the inlet valve opens and a fresh air-fuel mixture is drawn inside. Due to the pressure of the mixture, the piston is forced down. When the piston gets to its maximum position, the inlet valve gets closed thereby allowing no air to get escaped during the combustion operation. This is the end of the first stroke. And during the second stroke, due to the flywheel arrangement, the piston gets moved up, thereby compressing the mixture; so the temperature of the mixture increases. This is the end of the second stroke. During the third stroke, a little spark is introduced by the spark plug into the mixture, which causes an explosion at the top of the piston. This explosion causes an immense pressure on the piston due to which the piston moves down. This is the end of the third stroke. And at the last stroke, the piston again moves up due to the flywheel and forces the burnt gas out through the exhaust valve. Due to these four strokes, one complete engine cycle is made. Again the inlet valve opens and the air-fuel mixture is forced in, and the cycle continues. In this way, we can achieve a continuous rotation.
Diesel Engine (2- stroke)
A 2-stroke diesel engine is similar to a 2-stroke petrol engine. In a petrol engine, the petrol is already mixed with the air and is sent to the combustion chamber. But in a diesel engine, the fuel is not mixed with the air at starting but rather it is mixed with the air during the compression process. Also, a spark plug is not used in diesel engines. Due to the high temperature of the air during the compression, the fuel is injected at that time. So it gets ignited easily. At first, fresh air is pumped into the bottom area of the piston. During the first stroke, the piston is moved down and pure air is pumped into the combustion chamber from the bottom area. During this condition, the inlet valve stays closed. Due to the flywheel, the piston gets moved up and the air gets compressed. Now the inlet valve opens, and the air is again pumped into the bottom area, for further use. At the maximum compression of the air at the combustion area, the diesel is sprayed. Due to this high temperature, the diesel gets ignited. This is the end of the first stroke. During the second stroke, the piston begins to move down due to the high pressure exerted by the combustion process. Now the fresh air rushes into the combustion chamber and pushes the burnt gas out through the exhaust valve.
Diesel Engine (4-stroke)
A 4- stroke diesel engine is also similar to a 4-stroke petrol engine, but it does not use a spark plug. During the first stroke, the inlet valve opens and the air is pumped in, due to which the piston moves down. And during the second stroke, the piston moves up due to the flywheel, due to which the air gets compressed and the temperature of the air gets increased. At this time, the diesel is sprayed in the high-temperature air, by which the mixture gets ignited. And at the third stroke, the immense pressure created by the combustion process makes the piston to move down again. And during the fourth stroke, the exhaust valve opens and the piston moves up and pumps the burnt gas out.
As the combustion takes place only during the compression time, diesel engines are prone to be more vibrant than petrol engines. So diesel engines are always constructed in a rugged structure. The opening and closing of the inlet and the outlet valves are also controlled by the rotation of the engine. To precisely control the movement of valves with respect to the combustion process, the valves are made to rotate at a speed that is half to the speed of the engine.
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