The intake valve closes at _____ for a low-speed engine and at _____ for high-speed four-stroke petrol engines.

Explanation:

Intake Valve Closing in Low-Speed and High-Speed Four-Stroke Petrol Engines

Definition: In a four-stroke petrol engine, the intake valve controls the entry of the air-fuel mixture into the cylinder during the intake stroke. The timing of the intake valve closing is critical for optimal engine performance, as it impacts the volumetric efficiency, power output, and fuel economy. The closing timing varies depending on whether the engine is operating at low or high speeds.

Correct Option Analysis:

The correct option is:

Option 4: The intake valve closes at 10° after Bottom Dead Center (BDC) for a low-speed engine and at 60° after Bottom Dead Center (BDC) for high-speed four-stroke petrol engines.

Explanation:

The intake valve closing timing is designed to maximize the efficiency of the air-fuel mixture entering the cylinder. Here's why the timings differ for low-speed and high-speed engines:

• Low-Speed Engine: In low-speed engines, the intake valve closes shortly after the piston reaches BDC during the intake stroke. This timing (10° after BDC) ensures that the cylinder is filled adequately with the air-fuel mixture before the piston starts its compression stroke. Since the piston moves relatively slowly, closing the intake valve too late might lead to backflow of the mixture into the intake manifold.
• High-Speed Engine: High-speed engines require more time for the air-fuel mixture to enter the cylinder due to the rapid movement of the piston. To compensate for the inertia of the incoming mixture, the intake valve remains open longer (up to 60° after BDC) even after the piston has started its compression stroke. This delayed closing allows more air-fuel mixture to enter the cylinder, enhancing volumetric efficiency and engine power output at high speeds.

Additional Information:

Importance of Valve Timing:

• Proper valve timing ensures efficient engine operation by optimizing the intake and exhaust processes.
• Incorrect valve timing can lead to reduced engine performance, lower fuel economy, and increased emissions.

Factors Affecting Valve Timing:

• Engine Speed: Higher speeds require adjustments in valve timing to account for the inertia of the air-fuel mixture and the rapid movement of engine components.
• Engine Design: The geometry and design of the intake and exhaust systems influence the ideal valve timing for an engine.
• Desired Performance Characteristics: Valve timing is tuned to achieve specific performance goals, such as maximizing power, improving fuel efficiency, or reducing emissions.