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Heavy Duty Equipment Technician Red Seal Exam Prep · Question

In the power stroke of a 4-stroke diesel engine, after combustion, the high-pressure gases push the piston downwards. What is the state of the engine valves during this stroke?

During the power stroke, both the intake and exhaust valves must remain closed to contain the expanding high-pressure gases. This allows the maximum force to be

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Question: In the power stroke of a 4-stroke diesel engine, after combustion, the high-pressure gases push the piston downwards. What is the state of the engine valves during this stroke?

Answer options: ✅ Both the intake and exhaust valves are closed.

  • The intake valve is open, and the exhaust valve is closed.
  • The exhaust valve is open, and the intake valve is closed.
  • Both the intake and exhaust valves are open.

Correct answer: Both the intake and exhaust valves are closed.

Explanation: During the power stroke, both the intake and exhaust valves must remain closed to contain the expanding high-pressure gases. This allows the maximum force to be exerted on the piston, converting the chemical energy of the fuel into useful mechanical work. If either valve were open, pressure would escape, reducing efficiency.

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