An engine configuration that uses multiple opposing piston pairs to form respective expansion chambers for expanding a gas within to move the pistons to drive a main shaft. The engine can be configured to operate as an internal combustion engine that uses diesel fuel, gasoline, or natural gas, or it can be configured as an expander to convert high pressure high temperature gas to rotary power. The pistons may be mounted on the circumference of one or more disks. For any given set of choices of numbers of pistons and sizes of pistons, disks, and gears, there are disclosed dimensional constraints useful for more efficient functioning of the engine. This engine can be provided with a compact design which results in high power to weight ratios.