1. Explain the working principle of four stroke Otto engine?
Answers
Explanation:
The principle used in a four stroke petrol engine is commonly known as Otto Cycle. It states that there would be one power stroke for every four strokes. Such engines use a spark plug which is used for the ignition of the combustible fuel used in the engine. Most of the cars, bikes and trucks use a 4 stroke engines.
In every Otto cycle there is an adiabatic compression, addition of heat at constant volume, an adiabatic expansion and the release of heat at constant volume. The P-V diagram for a 4 stroke engine is as follows:
1.Intake 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.
2.Compression Stroke:
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.
3.Combustion/Power Stroke:
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.
4.Exhaust Stroke:
As the wheel moves to the bottom the exhaust valve opens up and due to the momentum gained by the wheel the piston is pushed back to the top of the cylinder. The gases due to combustion are hence expelled out of the cylinder into the atmosphere through the exhaust valve.
The exhaust valve closes after the exhaust stroke and again the intake valve opens and the four strokes are repeated.