Feel free to include some other technologies (even if in development or theory stage) or discuss the ones mentioned here.
* Forced induction turbocharging/supercharging.
When feeding the engine with compressed air you get more power.
Of course the engine must produce more power to compensate for energy wasted compressing air.
Detonation limits power increases.
Supercharger - Wikipedia, the free encyclopedia
Turbocharger - Wikipedia, the free encyclopedia
* Cylinder deactivation
Deactivated cylinders don't use gas but they cause a bit of drag, the transition isn't that smooth and it's meachanically complicated. Also cylinders wear unevenly.
Variable displacement - Wikipedia, the free encyclopedia
* Variable compression
IMO this is the future as it not only varies the engine compression but it also reduces the combustion chamber size. Of course a smaller chamber uses less fuel.
Variable compression ratio - Wikipedia, the free encyclopedia
Saab Variable Compression engine - Wikipedia, the free encyclopedia
* Variable valve timing
You get the best of both worlds. An econobox-type engine with its low end power, smooth idling and low emissions for traffic jams and a sports-car like camshaft profile when hi power is needed.
Variable valve timing - Wikipedia, the free encyclopedia
* Electric hybrids
They use electric only for traffic jams, and gas engine for higher speeds.
The downsides? they are too complicated and require exotic materials like rare earth and Nickel. It is debated whether production of hydrids like the Prius pollutes more than the fuel saved.
Hybrid electric vehicle - Wikipedia, the free encyclopedia
Toyota Prius - Wikipedia, the free encyclopedia
* Kinematic energy recovery systems
They store energy usually wasted as heat braking the vehicle for later use.
There are many ways how they can store the energy. Electricity (either with batteries or supercapacitors), flywheels, even compressed air.
Then the stored energy can be used to move the vehicle directly (i.e. with electric motors), or to make the engine have more power like 2014 proposed F1 turbo tech.
Kinetic energy recovery system - Wikipedia, the free encyclopedia
Formula One engines - Wikipedia, the free encyclopedia
IMO the electricity recovered could also be used to electrolyse tap water and store Hydrogen, Oxygen or both to increase engine power output when needed.
* Lean burn
They intend to use less fuel relative to the air present in the combustion chamber volume.
Lean burn - Wikipedia, the free encyclopedia