Spark plug

Figure 1. Spark plugs from an internal combustion engine. Note the gap on the left side is where the spark actually ignites the gasoline and air mixture.[1]

A spark plug is used as a source of ignition, as the "spark" in its name implies. It is a key component of internal combustion engines and its primary function is to ignite a fuel/air mixture within the combustion chamber of a car, or other system. Figure 1 shows plugs, the spark travels across the gap on the left side of the spark plug.

The voltage in the ignition system must be high enough to create a spark across the spark plug gap (see figure 2), much like a bolt of lightning traveling from the clouds to the Earth. These voltages can be anywhere from 40 kV to 100 kV, in other words high voltage.[2]

Spark plugs are an important part of gasoline engines, but not a part of diesel engines, since diesel is ignited by compression ignition instead. In figure 2 below note that the timing of when the spark plug fires is very important. The combustion needs to happen at a precise time, combusting too early (usually because of ignition from compression) leads to engine knocking.

Figure 2. 4-stroke internal combustion engine. 1:fuel injection, 2:ignition, 3:expansion(work is done), 4:exhaust.[3]

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