Short of running a full gas-electric hybrid system, automakers are exploring many ways to make engines more efficient. We look at 11 new technologies that help modern cars get great gas mileage.
Port fuel-injection systems long ago replaced carburetors in cars because of their efficiency and lower maintenance requirements. But now automakers are moving toward the even more efficient direct injection. With port fuel injection, gasoline is sprayed into the intake manifold, where it mixes with air, and then is sucked down into the cylinders. Direct injection places an injector on each cylinder, spraying gasoline into the cylinder itself.
Because of the multiplication of injectors, direct-injection systems are more expensive, and have also been associated with increased engine noise. But direct injection leads to more-efficient engines, as the gasoline-air mixture burns more completely. The individual injectors also ensure that each cylinder gets the same amount of fuel, and the spray can be more precisely timed.
Audi embraced direct injection early this century, but most automakers are starting to roll out new engines using the technology.
Turbocharging is not exactly a new technology, but it was formerly associated with performance. Now automakers are turning to turbocharging for fuel economy. A turbocharger is a turbine that uses an engine's own exhaust gasses to spin up. At the other end is a turbine that forces air into an engine's intake manifold. That pressurized air running into the cylinders creates a more forceful explosion, leading to more power.
Automakers use turbochargers to decrease engine displacement. Where a car might have previously used a six-cylinder engine, a turbocharged four-cylinder will suffice. During steady-speed cruising, the turbocharger is idle and the engine gets four-cylinder fuel economy. When the driver needs to accelerate, the turbo kicks in to provide the extra power.
In their quest for fuel efficiency, automakers look to every part of the drive cycle to determine where they can make gains. A recent innovation is shutting off the fuel flow to the cylinder when the car is decelerating. Cars traditionally have gone to idle speed when you take your foot off the gas, still feeding fuel to the cylinders to keep the engine turning. But if the car is moving at sufficient speed, its own momentum can keep the engine turning.
Ford started using this technology a few years ago, and GM will deploy it in its newer cars.
The idea of shutting off fuel to select cylinders when not needed has been around for a while. Cadillac started using this technology in the 1980s in one of its V-8 engines. Cylinder deactivation is based on the idea that, when cruising at a steady speed, you don't need your engine's full power output. The original Cadillac version shut down two to four cylinders of the V-8, depending on the operating conditions, thus saving gas.
The technology has been improved substantially from its first version because of the advent of fuel injection and engine control computers. Honda is a big current proponent of the technology, using it in its 3.5-liter V-6. GM also still uses the technology.
Idle stop saves fuel by shutting down the engine when the car is not moving, such as at a stop light. This feature has seen its widest and earliest dispersal in hybrid cars, but is starting to be adopted for standard gasoline engines. When you depress the foot brake for a stop, the engine shuts off. When you let the clutch out or take your foot off the brake, the engine starts up again.
These systems require a robust starter, and often cause a little hesitation in acceleration. Drivers can anticipate the green light, getting the engine running a little ahead of time. Idle-stop systems work poorly in stop-and-go traffic, where they cause engines to turn on and off multiple times within minutes.
Idle-stop systems have not seen much use in the U.S., except in hybrid cars, but are becoming more widespread in Europe. BMW added idle stop to the current model M3 to reduce gas consumption.
This technology, developed and used by BMW, borrows heavily from hybrid cars. When decelerating or braking, the car captures the kinetic energy and converts it to electricity. This electricity is used to charge up the car's 12-volt battery, making it unlikely the engine will need to generate extra electricity to keep the battery charged. BMW claims that this technology can increase fuel economy by 3 percent.
Historically, manual transmissions have offered better fuel efficiency than automatics, as the latter use a torque converter, which wastes engine energy when shifting. A lock-up clutch in an automatic transmission helps keep gear shifts efficient by maintaining a fixed connection between the transmission and drive shaft whenever possible.
Mercedes-Benz has exploited this technology considerably, making its seven-speed transmissions more efficient, and refining it further for performance cars in its AMG division.
Derived from race car technology, a dual-clutch transmission combines the efficiency of a manual transmission with the convenience of an automatic. Sometimes called an automated manual, a dual-clutch transmission uses two computer-controlled clutches to grab gears, one clutch getting the odd-numbered gears and the other taking the even gears. When one clutch has third gear engaged, for example, a computer looks at driver input to help it guess which gear should be engaged next, two or four, pre-positioning the second clutch over that gear.
Dual-clutch transmissions eliminate the lag time required for a human driver to work a manual clutch pedal. Most allow either manual, sequential shifting, or a fully automatic mode in which the computer decides when to shift. Although primarily used on performance cars, dual-clutch transmissions are starting to be used for fuel efficiency.
The Continuously Variable Transmission (CVT) does away with fixed gears in favor of a pulley system, which can constantly adjust the drive ratio to best match the optimum engine speed. And instead of the limited ratios, tied to the number of gears, in a conventional automatic transmission, the CVT offers potentially thousands, depending on the system. Letting the engine run at an optimum speed for as much driving time as possible leads to greater fuel efficiency.
Past CVTs have not been able to handle high engine output, although the technology has improved considerably. And with some CVTs, drivers will find a disconnect between engine sound and vehicle acceleration.
The CVT is a very old concept, and versions of it have been used in cars, light tractors, and motorcycles over the last century. Nissan is currently a big proponent of the technology, while Subaru and Audi have their own offerings.
Past power-steering systems used hydraulics, relying on the engine to build pressure in the system. The hydraulic pump created a drag on the engine that caused it to burn more fuel. Using an electric motor for power-steering boost eliminates this drag. Although the engine may have to generate more electricity to keep the 12-volt battery charged, it only need do this when a driver is turning the wheels. The older hydraulic systems maintained pressure at all times, which used more gas.
Early electric power-steering systems felt numb, with no connection to the road, but the engineering has rapidly improved. Current systems are tuned to produce lower amperage, meaning less boost, at speed and as the driver turns the wheel more. They almost completely replicate the feeling of a hydraulic system.
Almost all automakers are moving to electric power-steering systems, as they improve fuel efficiency and require less maintenance.
As a similar strategy to electric power steering, electric-air-conditioning systems rely on a motor rather than an engine-run pump to provide power. Again, reducing drag on the engine leads to better fuel economy.
But electric air-conditioning systems have not yet become as widespread, because of their greater power needs. Electric air conditioning is most often used in hybrids, which have a greater electricity supply on hand than the typical 12-volt starter battery of a conventional gas-engine car. An advantage of electric air conditioning in a hybrid is that, when the engine shuts off at at a stop light, the air conditioning stays on.
Electric air conditioning would see wider use in gas engine cars if the systems were made more efficient, drawing less energy.