Car engine. Computer-aided design (CAD) of a car engine. The engine, an internal combustion engine, is shown as a wire-frame model. The design of an engine has to be perfect to obtain the optimum performance. In a standard engine, fuel and air are mixed in cylinders (six seen, lower frame), compressed, and the mixture ignited. The explosion drives the piston down, which turns the crankshaft which powers the car. The flow of fuel and exhaust gases is controlled by spring-mounted valves (centre). CAD is a tool by which objects can be completely designed in three dimensions, and tested before they are built. This saves the time and money associated with building a prototype.