An engine configuration that places multiple pistons on one or two oscillating discs. 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. 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 is especially suited as a driver for electrical power generation as it delivers high torque at low engine speeds, among other uses. Its compact design results in high power to weight ratios.