Explanation:

Variable speed SI engine refers to an engine where the speed can be varied by controlling the fuel supply or throttle opening. In such engines, the maximum torque occurs at the maximum brake power.

Maximum Torque and Power:

- Torque is the twisting force applied to a shaft or crankshaft by the engine. It is the force that rotates the engine and drives the vehicle's wheels.
- Power is the rate at which work is done. In engines, power is the product of torque and speed. It is measured in units of horsepower or kilowatts.

Effect of Speed on Torque and Power:

- In an SI engine, the torque and power output depend on the speed of the engine. At low speeds, the torque is high but the power output is low. As the speed increases, the torque decreases but the power output increases.
- This relationship between torque and speed is described by the engine's torque curve. The torque curve shows the maximum torque that can be produced at each speed.

Effect of Fuel Supply on Torque and Power:

- The fuel supply or throttle opening controls the amount of air and fuel that enters the engine. This affects the torque and power output of the engine.
- At low speeds, a rich fuel-air mixture is required to produce high torque. As the speed increases, a leaner fuel-air mixture is required to produce high power.

Relation between Torque and Brake Power:

- Brake power is the power output of the engine that is available at the engine's flywheel. It is the power that is used to drive the vehicle's wheels.
- The maximum torque occurs at the speed where the brake power is maximum. This is because the engine is producing the most power output, given the available fuel supply and air intake.
- At speeds above the maximum brake power, the torque decreases even though the power output may continue to increase. This is because the engine is running out of fuel and air supply to produce more torque.

Conclusion:

In a variable speed SI engine, the maximum torque occurs at the maximum brake power. This is because the engine is producing the most power output, given the available fuel supply and air intake. The torque and power output of the engine depend on the speed and fuel supply. The torque curve shows the maximum torque that can be produced at each speed.