I'd really like to increase efficiency. More work for less fuel. And less money. More for less. But you can only do so much.
Physicists have proof that there is a maximum level of efficiency possible, due to the laws of thermodynamics and entropy. Any more and you're getting a free lunch, as prohibited by the rules. You will have less than 100% efficiency, and you will have entropy, and there's just no getting around that.
For car engines, for instance, the French physicist Carnot proved about 180 years ago that the most efficient possible engine using a combustable fuel would depend on the temperature (Thanks wikipedia):
"Tc" being the temperature, expressed in the absolute "Kelvin" scale, of the cold area around the engine, and "Th" being the temperature of the exploding gasoline in the engine, again in Kelvins. Siince most of us don't operate cars in absolute zero, or even Arctic conditions, efficiency isn't very good. We're also limited by the heat tolerence of the engine. Make it too hot and it just kind of melts. Or malfunctions in some other way. You can expect maybe 20% efficiency.
Of course, one way around this is to not use a heat cycle in your engine, using some other means to generate the force. Like electricity. Electrical engines are 90% efficient. But we don't use them because we can't store it well enough. Cars use huge amounts of energy, and electrical cars as exist now have tons and tons of batteries and still have a really tiny range before the batteries are all depleted.
This gives me an idea, I'll discuss it tomorrow.