Hybrid cars are not the only answer to increasing gas mileage. Carmakers are employing an array of other promising — but less ballyhooed — technologies and one may soon be sitting in your garage.

Government fuel-efficiency requirements — often referred to as CAFE, or Corporate Average Fuel Economy, standards — refer to the average fuel economy, expressed in miles per gallon, of a manufacturer’s fleet of passenger cars or light trucks.

Recently, Congress mandated that cars and light trucks combined achieve 35 mpg by 2020. To achieve that number, passenger cars will have to jump from 27.5 mpg to 35.7 mpg and light trucks from the current 23.5 mpg to 28.6 mpg between 2011 and 2015. That’s roughly a 25-percent increase.

To meet this requirement means automakers — particularly those lacking a hefty portfolio of diesel-powered or hybrid vehicles — will have to enhance current internal combustion engine, or ICE, technologies and develop some new ones.

Internal combustion engines produce power by sucking an air-gas mixture into a cylinder, compressing it with a piston and igniting it with a spark. The resulting explosion forces the piston down, which generates power.

Here are some of the technologies we can expect to see in greater use during the next decade:

Cylinder deactivation. GM calls this Displacement on Demand, or DOD, and Chrysler uses Multiple Displacement System. Whatever the terminology, cylinder deactivation rests certain cylinders when power demand is low.

At cruising speed, an ICE is working only at about 30 percent of power and the throttle valve is only partially open, limiting the amount of fuel and air. This makes the engine work harder to draw air — wasting power that could be used to propel the vehicle.

Shutting down four of the eight cylinders at cruising speed opens the throttle valve more fully, enabling the engine to “breathe” easier and reducing piston drag. The change in the number of “firing” cylinders is seamless. This can save as much as 25 percent of fuel, depending on the vehicle and the driver’s habits.

Turbocharging. The best way to boost a gasoline engine’s power is to increase the amount of air and fuel it burns. This used to be done by increasing the number and/or size of the cylinders. However, that’s counterproductive when the goal is to increase fuel efficiency.

Turbocharging captures escaping exhaust, or wasted energy, and forces it to spin a turbine that forces clean air into the engine’s intake manifold rather than using the engine’s own energy to suck it in. This rush of air creates extra torque and horsepower, enabling a small, more fuel-efficient engine to generate the acceleration power of a large, more fuel-thirsty engine. There is as much as a 30 percent to 40 percent net gain in power with turbocharging — not only from the efficiency of the air delivery system, but also through the weight savings of a smaller, lighter engine.

Direct fuel injection. Most fuel-injected engines use indirect fuel injection that premixes the air and fuel in the intake manifold. With direct fuel injection, the air still arrives in the cylinder through the intake manifold, but the fuel is injected directly into the cylinder separately.

Because direct fuel injection uses extremely advanced computer management, it is very precise regarding the amount of fuel injected but also exactly when it is injected. Additionally, direct fuel injection produces a finer mist of fuel than indirect injection, which burns more completely thus reducing waste.

Direct fuel injection is a more expensive system to build than indirect fuel injection, and consequently, its use has not been widespread. Its attraction, however, is that it increases power significantly — 40 percent in the Cadillac CTS — over indirect fuel injection with virtually no loss of fuel economy.

Homogeneous Charge Compression Ignition, or HCCI. Ever wondered why diesel engines get better fuel economy than ICEs? One reason is HCCI. Gasoline engines use a sparkplug to ignite the fuel-air mixture in the cylinder. Diesel engines instead use the heat created by the compression itself to ignite the fuel-air mixture.

GM is currently developing an HCCI system that includes other enabling technologies, such as direct fuel injection and combustion pressure sensors. In the Saturn Aura HCCI concept vehicle, the HCCI uses a traditional spark ignition system. HCCI carries the load at idle and at speeds up to 55 miles per hour, when the traditional spark ignition system kicks in.

GM claims fuel savings of up to 15 percent with HCCI, which is expected to provide fuel economy approaching that of a diesel, but without high levels of nitrogen oxides found in diesel exhaust and the expense of after-treatment systems required to scrub out the nitrogen oxides.