Abstract: An engine for driving the compressor of an air conditioner system which is powered by the heat of the exhaust manifold of a vehicle internal combustion engine. The engine of the invention includes a closed cylinder with a piston dividing the cylinder into two compartments, and with the piston rod extending beyond one end of the cylinder to power both the air conditioner compressor pump and a liquid pump that supplies the engine with pressurized liquid. The inlet port of the engine is located on a cylinder wall midway between the cylinder ends, with bores in the piston linking the inlet port with each cylinder compartment at the position of the cylinder that reduces the particular compartment to its minimum volume. Each end of the cylinder is surrounded by a heated exhaust jacket so that the liquid fed into the cylinder compartment is vaporized to expand the cylinder compartment and drive the piston.