Robert Stirling made his heat-saver in 1816. He used less fuel compared to the existing steam engines. He pumped water out from a quarry. A float moves the enclosed air between the hot and cold ends of the immersed cylinder, in a closed cycle, with external heat.
This innovation prevents the oil used to lubricate the piston from entering the hot space, where it would be carbonized. The piston compresses the closed air at the cold end of the cylinder. The float moves cold air into hot chambers. The piston is driven back through the air that expands at the end.
This engine was perfected by Phillips 140 years later and used on boats; in 1957 the 40 kw yacht, Johan de Witt used it in the Netherlands (photo).
The 1973 oil crisis and the need for submarines to have engines not using atmospheric air led Sweden's Kockums shipyard to use this old innovation. In 1967 she bought the patent and started testing this system, first on large 400Kw off-road trucks, which needed rapid acceleration without polluting the bottom of the mines, even in the open. Until then those trucks used diesel engines and as the first in the world with turbo, for more power from the same engine.
Kockums was thinking of using this engine on submarines, but kept this strategy a secret until it was perfected. Since 1990 it has been supplying submarines to the Swedish navy, Australia and other countries, using the Stirling AIP (Air Independent Propulsion) that leaves them underwater for weeks.
It uses any source of heat, solar, vegetable oil, or biodiesel. The heat passes to the gas that forces the piston to work and thus create mechanical energy. The engine is quiet, does not vibrate nor pollute and since 1990 its efficiency has increased.
This engine would be ideal for trucks and buses in cities such as London, Paris and Lisbon.