Four stroke petrol engine
4 STROKE ENGINE1234.docx
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What is an Engine?
The word Engine comes from the Latin word ingenium which means 'ability'. Engine is machinery. Generally a combustion engine is defined as a machine that produces power(mechanical) by the combustion of substance like water or a fuel.
Engines are classified into various categories based on the type of cycle they use, the layout, the energy source used, the cooling mechanism employed or its use.
Based on the layout, there are two types of Engines
•Internal Combustion Engines
An engine is called an Internal Combustion Engine, If the combustion of a fuel like petrol takes place inside it, in a chamber. The oxidizer used for combustion is usually air. The gases produced as a result of combustion are high temperature and pressure. These gases exert pressure on components such as a piston which moves over and produces energy (mechanical). E.g. Petrol Engine
•External Combustion Engines
An External Combustion engine is one in which the energy is externally applied on a non combustible fluid like pressurized/hot water, liquid sodium etc. Here these fluids are heated in an external chamber like boiler and steam is used to drive the engine. E.g. Steam Engine
Working of a Four Stroke Petrol Engine
A stroke is the movement of the piston from the top, to the bottom of the cylinder.
As the name suggest the Four Stroke Petrol Engine uses a cycle of four strokes and petrol as the fuel. Each cycle includes 2 rotations of the crankshaft and four strokes, namely:
1.An Intake Stroke
2.A Compression Stroke
3.A Combustion Stroke also called Power Stroke
4.An Exhaust Stroke
As the name suggests in this stroke the intake of fuel takes place. When the engine starts, the piston descends to the cylinder's bottom from the top. Thus the pressure inside the cylinder reduces. Now the intake valve opens and the fuel and air mixture enters the cylinder. The valve then closes.
This stroke is known as compression stroke because the compression of the fuel mixture takes place at this stage. When the intake valve closes (exhaust valve is already closed), the piston forced back to the top of the cylinder and the fuel mixture gets compressed. The compression is around 1/8th of the original volume. An engine is considered more efficient if its compression ratio is higher.
Now in case of petrol engine when the fuel mixture compresses to the maximum value the spark plug produces spark which ignites the fuel mixture. The combustion leads to the production of high pressure gases. Due to this tremendous force the piston is driven back to the bottom of the cylinder. As the piston moves downwards, the crankshaft rotates which rotates the wheels of the vehicle
Exhaust and Inlet Valve Overlap
Exhaust and inlet valve overlap is the transition between the exhaust and inlet strokes and is a practical necessity for the efficient running of any internal combustion engine. Given the constraints imposed by the operation of mechanical valves and the inertia of the air in the inlet manifold, it is necessary to begin opening the inlet valve before the piston reaches Top Dead Centre (TDC) on the exhaust stroke. Likewise, in order to effectively remove all of the combustion gases, the exhaust valve remains open until after TDC. Thus, there is a point in each full cycle when both exhaust and inlet valves are open. The number of degrees over which this occurs and the proportional split across TDC is very much dependent on the engine design and the speed at which it operates.
How the modern engine uses energy to make the wheels turn
Air enters the engine through the air cleaner and proceeds to the throttle plate. You control the amount of air that passes through the throttle plate and into the engine with the gas pedal. It is then distributed through a series of passages called the intake manifold, to each cylinder. At some point after the air cleaner, depending on the engine, fuel is added to the air-stream by either a fuel injection system or, in older vehicles, by the carburettor. Once the fuel is vaporized into the air stream, the mixture is drawn into each cylinder as that cylinder begins its intake stroke. When the piston reaches the bottom of the cylinder, the intake valve closes and the piston begins moving up in the cylinder compressing the charge. When the piston reaches the top, the spark plug ignites the fuel-air mixture causing a powerful expansion of the gas, which pushes the piston back down with great force against the crankshaft, just like a bicycle rider pushing against the pedals to make the bike go.