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Difficulty: MediumIgnition System

In a waste-spark distributorless ignition system, each ignition coil is connected to two spark plugs and fires them simultaneously. Which of the following explains why the secondary spark does not cause improper combustion in the paired companion cylinder?

  1. A
    The companion cylinder receives low voltage diverted from the alternator circuit rather than high secondary coil voltage.
  2. B
    The companion cylinder is positioned at the start of its intake stroke before fuel injection occurs.
  3. The companion cylinder is on its exhaust stroke containing spent exhaust gases, so no combustible air-fuel mixture is present to ignite.Answer
  4. D
    The companion cylinder contains an extremely lean air-fuel mixture that chemically prevents flame propagation across the chamber.

Answer

The companion cylinder is on its exhaust stroke containing spent exhaust gases, so no combustible air-fuel mixture is present to ignite.
In a waste-spark distributorless ignition system, companion cylinders share a single coil pack. Their pistons move in mechanical tandem, but their combustion cycles are separated by 360 crankshaft degrees. When the ignition coil discharges, one cylinder is at Top Dead Center on its compression stroke with a fresh air-fuel mixture, while the companion cylinder is at Top Dead Center on its exhaust stroke. Because the companion cylinder contains only inert, post-combustion exhaust gases, the spark jumps across the plug gap harmlessly without igniting any fuel.

Step-by-Step Solution

1
Determine the mechanical and cycle relationship between companion cylinders in an engine.
Companion pistons travel up and down in unison, but their four-stroke operating cycles are offset by exactly 360 degrees of crankshaft rotation.
Understanding four-stroke cycle timing is necessary to identify what phase each cylinder is in when a shared coil fires.
2
Evaluate the contents of each cylinder when the shared ignition coil discharges high voltage.
The primary cylinder is at top dead center on compression with a pressurized air-fuel charge, while the companion cylinder is at top dead center on exhaust pushing out inert burned gas.
Since inert exhaust gases cannot burn again, the secondary spark dissipates harmlessly without creating power or causing backfire.

Key Concept

Waste-spark ignition distribution and four-stroke cycle cylinder phasing
Estimated Time:1m 0s
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Topics

Ignition System

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