Stirling engines have been around since Stirling invented his in 1816. If a Stirling were a useful alternative to an internal combustion engine, it would have made its mark in the market long ago. Unlike an internal combustion engine that actually converts potential energy (in the form of fuel) into kinetic energy, a Stirling engine only converts kinetic energy (thermal, or heat) to kinetic energy (rotary motion). It is able to achieve a high degree of efficiency in doing so, but is not well suited for applications that require uneven applications of torque, like driving a car, because its output doesn't respond very quickly to changes in input, ie, it doesn't "throttle" very well.