**2. Design parameterization**

One of the common approaches for searching for design alternatives is to vary the part size or shape of the mechanical system. In order to vary part size or shape for exploring better design alternatives, the parts and assembly must be adequately parameterized to capture design intents.

At the parts level, design parameterization implies creating solid features and relating dimensions so that when a dimension value is changed the part can be rebuilt properly and the rebuilt part revealed design intents. At the assembly level, design parameterization involves defining assembly mates and relating dimensions across parts. When an assembly is fully parameterized, a change in dimension value can be automatically propagated to all parts affected. Parts affected must be rebuilt successfully; and at the same time, they will have to maintain proper position and orientation with respect to one another without violating any assembly mates or revealing part penetration or excessive gaps. For example, in a single-piston engine shown in Fig. 1 (Silva & Chang, 2002), a change in the bore diameter of the engine case will alter not only the geometry of the case itself, but also all other parts affected, such as piston, piston sleeve, and even crankshaft. Moreover, they all have to be rebuilt properly and the entire assembly must stay intact through assembly mates, and faithfully reveal design intents.

Fig. 1. A single-piston engineexploded view, (a) bore diameter 1.2", and (b) bore diameter 1.6"
