5 Fuel-Saving Technologies

In the long run, the internal combustion engine (ICE) is on the way out and electric motors are on the way in, but ICEs have been around for so loooong that we should be careful about announcing their demise. They're going to stick around a while longer, and so it's very important to make them as efficient and clean as possible.

Clean Diesel

Various advances such as the availability of ultra low sulfur diesel fuel, better catalysts and particulate matter traps, better control over combustion are making diesel engines cleaner, so you can expect a new wave of diesel passenger vehicles to come to the US in the next few years.
Diesel engines are certainly far from perfect, but they have inherently better thermal efficiency than gasoline engines, and they are usually more durable (if also more expensive and heavier). Another benefit is that they can run on biodiesel, which if you can find fuel made from waste cooking oil or (in the next few years) from algae can be very green.

Direct Injection

Before direct injection, the fuel was mixed with air in the car's intake manifold. Now, with direct injection, the fuel is mixed with air inside the cylinder, allowing for better control over the amount of fuel used, and variations depending on demand (acceleration vs. cruising). This makes the engine more fuel efficient.

Cylinder Deactivation

The name says it all. ICEs with this feature can simply deactivate some cylinders when less power is required, temporarily reducing the total volume of the engine cylinders and so burning less fuel. This feature is found on V6 and V8 engines.

Turbochargers

Turbochargers increase the pressure inside cylinders, cramming more air and allowing combustion to generate more power. This doesn't make the engine more economical in itself, but since a smaller displacement engine can generate more peak power, you can more easily downsize and save there.

Variable Valve Timing and Lift

Valves open and close to allow air and fuel to enter cylinders and for the products of combustion to exit. Different valve timings produce different results (more power, better fuel economy). Traditionally, you couldn't vary that timing, so the choice had to be made once when the engine was designed. But many modern engines can vary valve timing, allowing for example the default low RPM range of the engine to have more economical timing, and the higher RPM range to go for max power. This allows a smaller displacement engine to produce more peak power, so it allows for downsizing and fuel savings.