Explanation:

When considering the flame propagation in an internal combustion engine, the crank angle refers to the angle made by the crankshaft with respect to the top dead center (TDC) position. The crank angle is an important parameter that affects the combustion process and the efficiency of the engine.

Effect of Speed on Crank Angle:

1. Flame Speed:
The flame speed refers to the rate at which the flame front propagates through the air-fuel mixture in the combustion chamber. It depends on various factors such as the fuel properties, air-fuel ratio, turbulence, and temperature.

2. Piston Speed:
The piston speed is directly proportional to the engine speed. As the engine speed increases, the piston moves faster, resulting in a shorter time for the combustion process to occur.

3. Flame Propagation:
When the engine speed increases, the time available for the flame front to propagate from the spark plug to the farthest end of the combustion chamber decreases. However, the flame speed remains relatively constant.

4. Crank Angle:
The crank angle required for flame propagation is the angle at which the flame front reaches the farthest end of the combustion chamber. As the engine speed increases, the crank angle required for flame propagation remains relatively constant.

Explanation of the Correct Answer:

The correct answer is option 'D' - not affected. The crank angle required for flame propagation is not affected by the increase in speed. This is because the flame speed remains relatively constant, and the time available for flame propagation decreases with increasing speed. As a result, the crank angle required for the flame front to reach the farthest end of the combustion chamber remains unchanged.

In summary, the increase in speed does not affect the crank angle required for flame propagation in an internal combustion engine.