The most common forms are all based on a single engine block and crank case that are milled from a single piece of metal, with the pistons running in a row. These types of engines are all inline engine designs, which can be further broken down by the shape of the line itself. In the straight design all of the pistons are placed in a single row, whereas in the vee configuration they are split into two engine blocks sharing a single crank case. The horizontally opposed (also called flat) design is essentially a V-block with the cylinder banks at 180 degrees from each other, as opposed to 60 or so for the V. H, W and delta designs with more than one bank and crankcase have also seen limited use.
Another grouping, used almost entirely in aircraft, are the radial and rotary designs. In both cases the pistons are placed in individual cylinders arranged around a central crankcase like spokes on a wheel. This configurations keeps all of the cylinders at the front of the engine, which makes it easy to cool with airflow alone.
A final common configuration actually refers to the engine as a whole, in the Wankel engine where the "piston" spins instead of reciprocating. Although ostensibly not an engine configuration, it is often mentioned in this context anyway.
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