Constant volume cycle

In a petrol engine, the combustion of fuel-air mixture takes place at a nearly constant volume. This process is known as the constant volume cycle or the Otto cycle. Let's understand why the correct answer is option 'B'.

1. Theoretical Operation:
The petrol engine operates on the principle of converting chemical energy stored in the fuel into mechanical energy by utilizing the combustion process. The Otto cycle is an idealized model that represents the thermodynamic processes involved in the operation of a four-stroke internal combustion engine.

2. Four-stroke Operation:
A petrol engine follows a four-stroke cycle consisting of intake, compression, power, and exhaust strokes. During the intake stroke, the fuel-air mixture is drawn into the cylinder. In the compression stroke, the mixture is compressed to a smaller volume, increasing its pressure and temperature. The power stroke is where the combustion occurs, resulting in a rapid increase in pressure and temperature. Finally, in the exhaust stroke, the burnt gases are expelled from the cylinder.

3. Constant Volume Combustion:
The power stroke is the crucial phase where the fuel-air mixture is ignited by a spark plug. In a petrol engine, this combustion process is designed to occur at a nearly constant volume. This means that once the fuel-air mixture is ignited, it rapidly burns and releases energy, causing a sudden increase in pressure. This increase in pressure pushes the piston down, converting the energy into mechanical work.

4. Benefits of Constant Volume Cycle:
The constant volume combustion process offers several advantages in a petrol engine:
- Higher efficiency: The work output is maximized when combustion occurs at a constant volume, leading to improved engine efficiency.
- Better power output: The rapid combustion allows for a more significant force to be exerted on the piston, resulting in higher power output.
- Reduced heat loss: As the combustion happens quickly, there is less time for heat to be lost to the cylinder walls, leading to improved thermal efficiency.

5. Practical Considerations:
While the idealized Otto cycle assumes constant volume combustion, it is essential to note that in real engines, there is a slight deviation from this ideal due to factors such as heat transfer, combustion duration, and valve timing. However, the constant volume cycle serves as a useful model for analyzing the thermodynamic behavior of petrol engines.

Therefore, the correct answer is option 'B' - Constant volume cycle, as it accurately represents the theoretical operation of a petrol engine.