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In a four-stroke piston engine, the piston travels upward from Bottom Dead Centre (BDC) toward Top Dead Centre (TDC) during which two of the four strokes?

  • A

    Compression and power strokes

  • B

    Compression and exhaust strokes

  • C

    Induction and compression strokes

  • D

    Exhaust and power strokes

Refer to figure.
A four-stroke piston engine completes one full operating cycle over two complete crankshaft revolutions, consisting of four distinct strokes. 

The four strokes in order are: induction, compression, power, and exhaust.

During induction, the piston moves DOWNWARD from TDC to BDC, drawing the air-fuel charge into the cylinder through the open intake valve. 

During compression, the piston moves UPWARD from BDC to TDC, compressing the charge with both valves closed. The spark fires near TDC, igniting the compressed charge. 

During the power stroke, combustion pressure forces the piston DOWNWARD from TDC to BDC, delivering energy to the crankshaft. 

During the exhaust stroke, the piston moves UPWARD from BDC to TDC, expelling the burned gases through the open exhaust valve. 

Compression and power strokes → INCORRECT. The compression stroke is correctly identified as an upward stroke (BDC to TDC). However the power stroke is a DOWNWARD stroke in which combustion pressure drives the piston from TDC to BDC to deliver work to the crankshaft. 

Compression and exhaust strokes → CORRECT. Both the compression stroke and the exhaust stroke involve the piston moving UPWARD from BDC toward TDC. During compression the piston rises to compress the air-fuel charge with both valves closed. During the exhaust stroke the piston rises again to push the burned exhaust gases out of the cylinder through the open exhaust valve. These are the two upward strokes of the four-stroke cycle and this is the correct answer.

Induction and compression strokes → INCORRECT. The compression stroke is correctly identified as an upward stroke. However the induction stroke is a DOWNWARD stroke in which the piston moves from TDC to BDC to draw the intake charge into the cylinder through the open intake valve. The induction stroke involves downward piston movement and should not be included in the answer to this question.

Exhaust and power strokes → INCORRECT. The exhaust stroke is correctly identified as an upward stroke (BDC to TDC). However the power stroke is a DOWNWARD stroke where combustion pressure pushes the piston from TDC to BDC. 

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