Wasted Spark Ignition Systems
In a Wasted Spark ignition systems there are two spark plugs connected to the ends of one coil ( for 2 cylinder engine )( for 4 cylinder engine two coils are used, each driving two spark plugs ). Both spark plugs will therefore fire simultaneously. The one spark plug will be fired in the compression stroke of the first (A) cylinder - (power), and the other in the exhaust stroke of the opposed cylinder (B)- ( wasted ). When the time comes for cylinder “B” to fire in compression the “A” cylinder will fire in the exhaust stroke. Since both spark plugs are fired at the same time from the ends of only one coil they have opposed current flow. That is the spark “flows” from the center to the outside electrode in the one spark plug and from the outside electrode to the center in the second spark plugs. This can be seen with oscilloscope very clear as the “exhaust sparks” have much lower voltage flowing as there is no need for high energy to bridge the spark plug gap. i.e. no compression. As we said before they also are of opposite (negative) direction to the “power sparks” (positive).
The advantage of such system is that no distributor is needed, so there is no mechanical parts, wear or voltage drops. As a result the coil need less energy so can be made smaller. Usually in a 4 cylinder engine we’ll have two coils in one body – coil pack.
The disadvantage is that there is firing in the exhaust strokes which implies that no fuel mixture may enter at that time because of valve overlap or any other reason. Otherwise the fuel mixture will explode in the cylinder when is it at exhaust stroke and coarse damage to the engine. As result it is not always possible to get an optimum ignition timing. The other disadvantage is that those systems are more difficult to diagnose than the other ignition systems.
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