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Difficulty: HardEngine Principles and Internal Components

During a diagnostic inspection of a four-stroke internal combustion engine, a technician notes that the crankshaft has completed exactly 180∘180^\circ of rotation since the start of the intake stroke. As the piston now begins moving upward toward Top Dead Center (TDC), which operational state and cylinder event are occurring?

  1. A
    The exhaust valve is open while upward piston travel expels spent combustion gases into the exhaust manifold.
  2. Both the intake and exhaust valves are closed while upward piston travel compresses the trapped air-fuel mixture.Answer
  3. C
    The intake valve remains open while upward piston travel creates high vacuum to draw in additional fuel mixture.
  4. D
    Both valves are closed while high-pressure burning gases force the piston downward to generate crankshaft torque.

Answer

Both the intake and exhaust valves are closed while upward piston travel compresses the trapped air-fuel mixture.
In a four-stroke internal combustion engine, each complete stroke corresponds to 180∘180^\circ of crankshaft rotation. The intake stroke occurs from 0∘0^\circ to 180∘180^\circ (TDC to BDC). From 180∘180^\circ to 360∘360^\circ, the engine executes the compression stroke, during which the piston travels upward from BDC to TDC with both the intake and exhaust valves completely sealed to compress the air-fuel mixture.

Step-by-Step Solution

1
Determine the cycle phase corresponding to the crankshaft degree position.
The intake stroke spans 0∘0^\circ to 180∘180^\circ (piston moving downward from TDC to BDC). The next 180∘180^\circ (180∘180^\circ to 360∘360^\circ) represents the compression stroke, where the piston travels upward from BDC to TDC.
Each stroke in a four-stroke internal combustion engine corresponds to 180∘180^\circ of crankshaft rotation (720∘720^\circ total for all four strokes).
2
Identify the required valve positions and internal cylinder state during the compression stroke.
Both the intake valve and exhaust valve must be closed and fully seated against their valve seats.
Sealing the cylinder ensures the upward movement of the piston compresses the trapped air-fuel charge into the combustion chamber without leakage.

Key Concept

Four-stroke cycle timing and internal valve/piston kinematics
Estimated Time:1m 15s
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Topics

Engine Principles and Internal Components

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