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Difficulty: Very hardEngine Principles and Internal Components

A technician is diagnosing a four-stroke internal combustion engine. For a specific cylinder, the piston is currently positioned at bottom dead center (BDC) exactly at the completion of the intake stroke. The technician then manually rotates the engine such that the camshaft turns exactly 225∘225^\circ. Based on this rotation, which of the following accurately describes the current state of that cylinder?

  1. The engine is in the exhaust stroke; the piston is moving upward, the exhaust valve is open, and the intake valve is closed.Answer
  2. B
    The engine is in the power stroke; the piston is moving downward, and both the intake and exhaust valves are closed.
  3. C
    The engine is in the compression stroke; the piston is moving upward, and both the intake and exhaust valves are closed.
  4. D
    The engine is in the exhaust stroke; the piston is moving downward, the exhaust valve is open, and the intake valve is closed.

Answer

The engine is in the exhaust stroke; the piston is moving upward, the exhaust valve is open, and the intake valve is closed.
Because the camshaft turns at half the speed of the crankshaft, a 225∘225^\circ camshaft rotation results in a 450∘450^\circ crankshaft rotation. Starting from 180∘180^\circ (the end of the intake stroke), adding 450∘450^\circ places the engine at 630∘630^\circ into its 720∘720^\circ cycle. This angle is exactly in the middle of the exhaust stroke (540∘540^\circ to 720∘720^\circ), during which the piston physically moves upward to expel combustion gases while the exhaust valve is open and the intake valve remains closed.

Step-by-Step Solution

1
Identify the starting position of the crankshaft.
The end of the intake stroke, with the piston at Bottom Dead Center (BDC), corresponds exactly to 180∘180^\circ of crankshaft rotation from the start of the cycle.
Establishing the baseline angle in the 720∘720^\circ cycle is necessary to calculate the final position after rotation.
2
Convert the given camshaft rotation to crankshaft rotation.
A camshaft rotation of 225∘225^\circ equals a crankshaft rotation of 450∘450^\circ.
In a four-stroke engine, the crankshaft rotates twice for every single rotation of the camshaft (a 2:12:1 ratio).
3
Calculate the final crankshaft position.
180∘+450∘=630∘180^\circ + 450^\circ = 630^\circ.
Adding the rotated amount to the starting position determines the exact phase of the engine.
4
Map the final position (630∘630^\circ) to the corresponding stroke and mechanical states.
The 630∘630^\circ mark falls exactly in the middle of the exhaust stroke (540∘540^\circ to 720∘720^\circ). During this stroke, the piston moves upward, the exhaust valve is open, and the intake valve is closed.
Each 180∘180^\circ segment of the 720∘720^\circ cycle has defined mechanical functions for the piston and valves to intake, compress, ignite, and expel the air-fuel mixture.

Key Concept

Four-Stroke Cycle Timing and Camshaft-to-Crankshaft Ratio
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Topics

Engine Principles and Internal Components

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