An engine was developed that once filled with gas it doesn’t need any further refilling. It’s called a Sterling engine. I believe it was never given a chance to be developed to its full potential because some black goo was lying about in Texas and this stuff could burn. To have an engine that won’t call for return business is bad for economics, isn’t it.

This engine works on the principle that gas expands when heated and contracts when cooled and by creating these two environments and connecting them with pistons and a crank, one can create movement.

Combine this with refrigeration as in a gas fridge (pity it’s ammoniac), where you heat a gas at one end and it has a cooling effect on the other end and one is a lot closer to having a more effective Sterling. I’m not sure if a gas exist with an opposite effect that when cooled on one side it will cause heat on the other, if it does then it can be used as a reverse fridge to the other piston. Another sealed unit. Energy won’t be created but a lot can be saved.

The biggest reason I’ve come across why it is not used extensively is because it takes long to start and stop. Why stop it? A gearbox can disengage the engine. Since no explosions take place this engine operates quietly, maybe that is why they use it in submarines.

Solar energy can further be combined to produce heat on the hot side and maybe charge a battery for evening use. A radiator should cool the cold side efficiently. Liquid Nitrogen might just create a sports car.