Four Stroke Engine MCQs with Detailed Explanation

Correct Answer: A. 1000

• In a four-stroke engine, one complete cycle is completed in two revolutions of the crankshaft.
• During one complete cycle, each valve opens once and closes once.
• So, if the engine speed is 2000 rpm, the number of complete cycles per minute is 2000 ÷ 2 = 1000.
• Therefore, each valve opens and closes 1000 times per minute.

Correct Answer: A. Parallel to the crankshaft

• The camshaft is an important part of the valve operating mechanism.
• It operates the inlet and exhaust valves at the correct time.
• For a simple and compact drive arrangement, the camshaft is mounted parallel to the crankshaft.
• This arrangement helps to transmit motion easily through timing gears, chain or belt.

Correct Answer: C. Piston rings

• Piston rings are metallic split rings fitted in grooves on the outer surface of the piston.
• They provide proper sealing between the piston and cylinder wall.
• This sealing reduces leakage of combustion gases from the combustion chamber to the crankcase.
• If leakage occurs, pressure drops and the piston receives less force during the power stroke.
• Piston rings also help to maintain compression ratio and prevent leakage of air-fuel mixture during compression stroke.

Correct Answer: B. Two revolutions of crankshaft

• A four-stroke engine has suction, compression, power and exhaust strokes.
• These four strokes are completed in two revolutions of the crankshaft.
• That is why valve opening frequency is half of the crankshaft rpm.
• This point is very important for AMVI and automobile engineering MCQs.

Correct Answer: A. Compression rings

• Compression rings are used mainly for sealing the combustion chamber.
• They prevent high-pressure combustion gases from leaking into the crankcase.
• Good sealing improves engine power output and efficiency.
• It also helps the engine to maintain proper compression pressure.