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Difficulty: MediumIgnition System

In a four-stroke internal combustion gasoline engine, the ignition system must deliver high-voltage current to ignite the air-fuel mixture at the optimal moment. Under normal operating conditions, at which point in the engine operating cycle does the spark plug fire?

  1. A
    At bottom dead center as the intake stroke finishes and the cylinder fills with air and fuel
  2. Slightly before top dead center near the end of the compression strokeAnswer
  3. C
    At the exact instant the alternator delivers maximum charging voltage to the vehicle battery
  4. D
    During the middle of the power stroke when the expanding gases produce a lean mixture condition

Answer

Slightly before top dead center near the end of the compression stroke
In a gasoline engine, ignition timing is set to fire the spark plug several degrees before top dead center (BTDC) near the end of the compression stroke. Because the air-fuel mixture requires a brief moment to burn and expand, initiating the spark just before the piston reaches the top of its compression travel ensures that maximum cylinder pressure pushes down on the piston at the start of the power stroke.

Step-by-Step Solution

1
Identify the purpose of the ignition spark in a four-stroke internal combustion cycle.
The spark must ignite the tightly compressed air-fuel charge to create high pressure that drives the piston downward.
Maximum thermal and mechanical efficiency requires peak cylinder pressure to occur just after the piston begins descending on the power stroke.
2
Account for flame propagation time.
Combustion is not instantaneous; the flame front takes several milliseconds to spread across the combustion chamber.
To achieve peak combustion pressure at the beginning of the power stroke, the ignition event must be initiated slightly before the piston reaches Top Dead Center (TDC) on the compression stroke.

Key Concept

Ignition Timing and Four-Stroke Cycle Synchronization
Estimated Time:1m 0s
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Topics

Ignition System

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