Variable Speed SI Engine

In a variable speed SI (spark ignition) engine, the torque and power characteristics vary with the engine speed. The torque is the rotational force produced by the engine, while power is the rate at which work is done. Both torque and power are important parameters in determining the performance of an engine.

a) Both the torque and power are maximum at the same speed:
This statement is incorrect. In a variable speed SI engine, the torque and power curves do not necessarily peak at the same speed. The torque curve represents the engine's ability to produce rotational force, while the power curve represents the engine's ability to do work over time. These two curves usually have different shapes and peak at different speeds.

b) Maximum torque occurs at a speed higher than that at which maximum power occurs:
This statement is incorrect. In a variable speed SI engine, the maximum torque occurs at a speed lower than that at which maximum power occurs. The torque curve typically peaks at lower engine speeds, while the power curve peaks at higher engine speeds. This is due to the engine's ability to generate more torque at lower speeds, while power is a function of both torque and speed.

c) Maximum torque occurs at a speed lower than that at which maximum power occurs:
This statement is correct. In a variable speed SI engine, the maximum torque occurs at a speed lower than that at which maximum power occurs. The torque curve usually rises rapidly at lower speeds and then levels off or starts to decline as the engine speed increases. On the other hand, the power curve continues to increase with speed until it reaches its peak. This is because power is the product of torque and speed, and as the speed increases, the power output of the engine also increases.

d) Power goes on increasing monotonically with speed:
This statement is incorrect. In a variable speed SI engine, power does not go on increasing monotonically with speed. The power curve usually rises with engine speed until it reaches its peak, beyond which it starts to decline. This is because at higher speeds, the engine may not be able to sustain the same level of torque output due to limitations such as valve timing, fuel-air mixture, and mechanical constraints. Therefore, the power output eventually reaches a maximum and then decreases as the engine speed increases further.

Conclusion:
In summary, in a variable speed SI engine, the maximum torque occurs at a speed lower than that at which maximum power occurs. The torque curve usually peaks at lower speeds, while the power curve continues to rise until it reaches its peak and then starts to decline. Understanding the torque and power characteristics of an engine is crucial for optimizing its performance and efficiency in various operating conditions.