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Spark plug CAD. Computer aided design (CAD) of a spark plug. Half the spark plug is shown as a wire-frame model, while the other half is shown solid. Spark plugs are used in petrol engines to ignite the petrol/air mixture, which explodes and drives the engine. CAD is a tool by which objects can be completely designed in three dimensions, and tested before they are built. It saves time and money, as incorrect designs can be corrected quickly and easily. The data from the computer can be fed into a mechanical production line, and the object can be automatically built to specifications.