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Ways to Boost Fuel Economy

Ways to Boost Fuel Economy

By Jeff Youngs, February 24, 2012
American motorists, along with the manufacturers that serve them, have been "thinking big" for so long that it's difficult to grasp the urgency of recently skyrocketing gasoline prices. It's a simple equation: big cars with big engines translate to unappetizing gas mileage. Many drivers and industry leaders appear to have forgotten the lessons of the past, when the 1973-74 OPEC oil embargo-and a second crisis at the end of that decade-triggered a wave of downsizing and stimulated interest in smaller cars.

Concern about fuel efficiency dissipated a few years after the twin crises. Asian automakers continued to offer smaller cars with tinier engines, but American manufacturers concentrated more on power and performance-just as they had before the OPEC embargo. Not until fuel prices passed the $2 and $3 marks, heading toward and past $4, did appreciable numbers of motorists begin to realize that steps might have to be taken to counteract the long-term infatuation with big guzzlers.

Fuel-efficiency improvement demands multiple steps
To improve fuel economy of an existing model, or one under development, engineers might concentrate on any or all of the following:

  • Cutting down on engine weight;
  • Modifications to reduce internal engine losses and boost efficiency;
  • Shrinking engine size (displacement);
  • Reducing the number of cylinders;
  • Turbocharging, to take greater advantage of a smaller engine and obtain performance comparable to a larger one, while at the same time increasing fuel efficiency.
Most important, efficiency increases demand a variety of partial solutions.

While hybrids and electric cars gradually gain favor and hydrogen-fueled vehicles wait for their day in the mass market to come, developers need to focus on vehicles that use liquid fuels that remain readily available-at least until skyrocketing prices metamorphose into actual shortages. For the present, this means concentrating on existing internal-combustion engines.

Turning toward diesel power also will help, because diesel produces greater energy than gasoline. Now that low-sulfur fuel is available, a few manufacturers are turning out impressively quiet and refined engines. If equipped with BlueTEC 6-cylinder engines, for example, 2009 Mercedes-Benz SUVs will be sold in all 50 states.

Internal engine modifications
Through most of automotive history, engines-like many other mechanical components-have operated on a principle of compromise. Valves, for example, were set up to open and close in a way that yielded efficient running at some speeds, some of the time. Outside that engine-speed range, or under diverse conditions, efficiency dropped off substantially.

When gasoline was cheap, not many motorists fretted about reduced-efficiency powertrains. Neither did the engineers who developed them. As long as the engine and transmission operated suitably under most conditions, that was deemed enough. Even if they knew about methods to boost efficiency through a broader speed range, engineers would probably deem those techniques too costly to implement on mass-production powertrains.

Variable valve timing is one method that's yielded substantial improvements in efficiency, but not all engines employ this technology. Not every planner wants to recommend adding complexity to an engine in the interest of efficiency. Electronic cam phasing is another step toward valvetrain efficiency.

Soon, we'll be hearing more about HCCI (homogeneous charge compression ignition). Actually an old idea, undergoing extensive experimentation by automakers and labs, HCCI combines the benefits of a spark-ignition (gasoline) engine with a diesel engine (which uses compression ignition). Combustion occurs without flames, maintaining low temperatures within each engine cylinder and burning the entire fuel/air mixture in one swoop.

Direct gasoline injection
Direct injection is already turning up in various engines. Ford's turbocharged EcoBoost, for instance, will be optional for the new Lincoln MKS in 2009. Because a direct-injected engine can be smaller while yielding comparable output, Lincoln can install an EcoBoost V-6 engine, not a V-8. Within five years, Ford plans to offer half a million vehicles per year with gasoline turbocharged direct injection (GTDI). Instead of mixing gasoline and air at a central location and sending the mixture into each cylinder, the GTDI system allows air and fuel to enter the cylinder separately, which allows more precise metering.

Bosch claims a 15-percent fuel-economy improvement with direct injection; ExxonMobil predicts 30 percent. While direct injection is making inroads, cost factors come into play-an inevitable obstacle.

Cutting down on idling losses
Every moment that an engine idles, it's wasting fuel. Shutting an engine off every time the car comes to a stop isn't always practical. Still, that's a primary cause for the high gas mileage achieved by hybrid (gasoline/electric) powertrains. And it's one that can be applied to conventional engines, too. Years ago, when starter motors engaged the engine's flywheel gear in a brutal manner, start-stop operation wasn't feasible. Today's starters can gently nudge a properly-prepared engine into life, so automatic shutoff and startup no longer takes such a toll on the components involved.

Cylinder deactivation already is used in several engines, especially for full-size trucks. Under gentle conditions, half of the cylinders can remain idle without impairing roadability.

Accessories that are driven by the engine contribute to reduced fuel economy. Engineers have already eliminated many, turning to such units as electric power steering, rather than the belt-driven pumps of the past.

Less weight equals greater gas mileage
Weight is a primary issue, in the engine, chassis, and beyond. For some years now, engines have gradually been incorporating lightweight plastic and composite parts, and engineers need to keep moving in that direction.

Some modern electronic accessories weigh nearly nothing, and draw virtually no energy; but others add substantially to the weight of the car. If weights cannot be reduced significantly, engineers may have to search for alternative methods of producing the same effect. Or, marketers and dealers might have to start discouraging frugality-minded customers from taking every comfort/convenience feature.

Smaller engines make a big difference
Downsized engines make a big difference in fuel economy. If a 6-cylinder engine can replace a V-8, or 4 cylinders take the place of 6, gas mileage is sure to rise appreciably. American automakers may need to look no further than their European subsidiaries for insights here. Because high fuel prices are nothing new in Europe, fuel efficiency has been the rule there for decades. Europeans were paying $4 or more per gallon when American gasoline hovered around a dollar.

Smaller cars are far more common in Europe than in America. So are diesel engines, which promise considerably greater economy. In the near future, not only engines but cars themselves may have to shrink in size, following the European model.

Looking beyond the engine, developers have a long list of potential modifications that may be applied to improve the efficiency of automatic transmissions. Tires with lower rolling resistance can make considerable difference. Though the merits of 4-wheel drive are well known to motorists in Northern states, who face ice and snow for months each year, that feature adds weight to a vehicle and hampers fuel economy.

Despite the best engineering efforts, automobiles remain notoriously inefficient, so there's plenty of room for further improvement. Dean Harlow, president of Ricardo, Inc., wrote in GreenCar.com that as much as 75 percent of the energy in gasoline is lost before it gets to the car's wheels.
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