Once a market need has been identified the first step in designing a new engine is to determine its required displacement. In this context the engine should be viewed as a positive displacement air pump. In order to produce the required work one must burn sufficient fuel. In order to completely react the fuel to products one must supply sufficient air. Determining the displacement required for a given engine is thus a matter of working backwards from the desired work output (or power at a given shaft speed), as further depicted in Fig. 5.1. Each of the steps in this process will now be covered in greater detail.