In a four-stroke CI (Compression Ignition) engine, the inlet valve opens and closes at specific crankshaft positions to allow the entry of air into the combustion chamber. The correct option, option 'A', states that the inlet valve opens at 20° before top dead centre (TDC) and closes at 40° after bottom dead centre (BDC). Let's understand why this is the correct answer.

Four-Stroke CI Engine:
A four-stroke CI engine follows a specific cycle, known as the Otto cycle, consisting of four strokes: intake, compression, power, and exhaust. During the intake stroke, the piston moves from top dead centre (TDC) to bottom dead centre (BDC), creating a partial vacuum in the cylinder. This allows fresh air to enter the combustion chamber through the open inlet valve.

Explanation of the Correct Option (Option 'A'):

Inlet Valve Opening at 20° Before TDC:
The inlet valve opens at 20° before top dead centre (TDC). This means that the valve starts to open before the piston reaches its highest point in the cylinder. Opening the inlet valve before TDC helps in creating a pressure difference between the atmosphere and the cylinder, facilitating the entry of fresh air.

Inlet Valve Closing at 40° After BDC:
The inlet valve closes at 40° after bottom dead centre (BDC). This means that the valve remains open for some time after the piston starts moving upwards from its lowest position. Closing the inlet valve after BDC ensures that the maximum amount of air is drawn into the cylinder during the intake stroke.

Significance of the Correct Valve Timing:
The specific timing of the inlet valve opening and closing is crucial for the engine's performance. Opening the valve before TDC and closing it after BDC ensures efficient filling of the combustion chamber with fresh air. This allows for better air-fuel mixing, leading to improved combustion and power output.

Other Options:

Option 'B': Opening at 20° after TDC and closing at 40° before BDC would result in a shorter intake stroke, limiting the amount of air entering the cylinder.

Option 'C': Opening at TDC and closing at BDC would mean that the inlet valve is open throughout the engine cycle, which is incorrect and would prevent compression and power strokes from occurring.

Option 'D': Opening at BDC and closing at TDC would also prevent the intake stroke from occurring, resulting in no air entering the cylinder.

Therefore, the correct option is 'A' as it aligns with the standard timing for the inlet valve in a four-stroke CI engine, maximizing air intake and optimizing engine performance.