Demand for automobiles is rising worldwide. So is concern about greenhouse gas emissions. In response, scientists and engineers are working diligently to perfect new power plants for future vehicles, including battery and hydrogen fuel-cell electric cars. Although these and other alternatives show great promise for the long term, perhaps the single greatest way to reduce fossil-fuel consumption in the near term is to further improve today’s dominant transportation power plant: the gasoline internal-combustion (IC) engine.
Fortunately, efficiency can be raised in a number of ways, notably, better control over the air-fuel mixture entering the combustion chamber, over the way gasoline is ignited there, and over the mechanical systems that harness that energy. These can improve traditional automobiles as well as gasoline-electric hybrid models.
Rapidly rising fuel prices in the latter half of 2008 began steering many consumers toward vehicles offering the best fuel efficiency, but recent price declines have hurt demand for them. Strict new fuel economy and greenhouse gas emissions regulations, about to go into force, should reverse this trend, however, and drive even more significant advancements to the technology that will be under the hood of your next new car.
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The modern IC engine powers all but a handful of the world’s automobiles, trucks, motorcycles and motorboats. Its greatest advantage is its use of a fuel—gasoline—that is still relatively abundant, inexpensive and energy-dense. Its greatest drawback is its mediocre efficiency. The most efficient gasoline spark-ignition engines in mass-produced automobiles today convert only 20 to 25 percent of the fuel’s chemical energy into work. A modern diesel or gasoline-electric hybrid power train can reach 25 to 35 percent, but at substantially higher cost. In contrast, hydrogen fuel-cell electric cars—such as Honda’s FCX Clarity, now in limited production—convert about 60 percent of the energy in gaseous hydrogen into motive power.
Despite the IC engine’s reputation as old and outmoded technology, however, it continues to improve. A recent Environmental Protection Agency study showed that the fuel efficiency of engines in U.S. automobiles rose by roughly 1.4 percent a year from 1987 to 2006. The increases came through incremental gains in combustion (thermal) efficiency, reductions in engine and drivetrain friction, more advanced transmissions and reduced losses in accessory systems. Most of these gains, however, did not help drivers consume less gasoline. Instead they went to meet market demand for larger, more powerful and better-equipped vehicles.
New Rules Prompt Gains
Impending regulations will help ensure that future power train efficiency gains go primarily toward actual fuel economy. The EPA is finalizing stringent new greenhouse gas standards for automobiles, and the Department of Transportation is finishing tougher corporate average fuel economy (CAFE) standards. The agencies must issue a final ruling that incorporates both sets of requirements by April 1. In directing the DOT, Congress mandated “maximum feasible” increases in the average fuel economy of the U.S. car and light truck fleet between 2011 and 2030. Based on current proposals, the first phase of the standards will raise the fuel economy of most cars, SUVs, pickups and minivans by 4.4 percent each year from 2012 through 2016, reaching about 35.5 miles per gallon for many cars. And indications are that the bar will continue to be raised aggressively through 2020 or even 2030.
The rules will also change how greenhouse gas and fuel economy targets are calculated, which will affect how automakers will respond. Instead of setting a single standard for all cars or all light trucks in an automaker’s U.S. fleet, as current standards do, the new fuel economy targets will be based on a vehicle’s footprint—the rectangular area defined by the vehicle’s four wheels. Each manufacturer will also have a unique target based on the sales-weighted average footprint of its combined car and light truck fleets.
This approach means automobile companies will no longer find any advantage in building
a greater number of small cars, because the resulting mix of vehicles will simply have to meet higher fuel economy and lower emissions targets for carbon dioxide. Nor will there be any real disadvantage to automakers whose fleets contain a higher percentage of larger cars and light trucks. The goal of the new standards is not to encourage or discourage the production or purchase of any class of vehicle but to make every class as fuel-efficient as possible, within the bounds of what regulators deem to be both economically practical and technically feasible.
Cutting Losses in Many Ways
The new regulations present automakers with a daunting challenge: how to best invest limited engineering resources to dramatically raise IC engine efficiency in a very short time, while continuing to meet consumers’ demands for performance, safety, utility and comfort.
The most compelling options would reduce the major sources of energy loss. About 60 percent of the energy in combusted gasoline is lost to heat—roughly half of that through the engine and half through the exhaust. Another 15 to 25 percent is lost to engine friction and to fuel consumed when the engine is idling or the car is decelerating—when no usable work is being done. Engine friction includes so-called pumping losses created by the process of pulling air past a partially closed throttle valve into the cylinders, to burn with fuel.
The remaining energy is engine output. Half to two thirds of it (10 to 15 percent of the gasoline’s total energy) is used to overcome the vehicle’s tractive hurdles: inertia (reflecting the car’s weight), aerodynamic drag and rolling resistance (friction between tires and the road). The balance (5 to 10 percent of the total) is consumed by the drivetrain (transmission and drive shafts to the axles) and by accessories such as power steering, air conditioning and the alternator that creates electricity for such equipment.
Efficiency can be improved in every one of these areas, and even minor advances can yield substantial benefits. For example, a 1 percent reduction in tractive losses translates into a 4 to 5 percent improvement in fuel economy. The challenge is to implement a set of technologies that offers the highest efficiency gains at the lowest cost. Because each automaker has a unique fleet and particular technological strengths and weaknesses, each company will likely choose a different mix of enhancements.
An exhaustive list of advancements is beyond the scope of this article. For example, reducing engine friction involves materials, geometry of the moving parts, lubricants and parts design; dozens of minor changes can be combined to improve efficiency by a few percent. Nevertheless, most automotive engineers would probably agree on a short list of approaches that are very promising and widely applicable within the next decade.
Superengine
As we look even further ahead, additional gains in IC engine efficiency will rely heavily on system optimization. Combinations of hardware and software are virtually limitless. One kind of future “superengine” would employ several of the advances depicted here: direct injection of gasoline, with continuously variable timing of camless piston valves, combined with a hybrid-electric motor and a turbocharger (which boosts power by harnessing waste gases streaming through the exhaust system).
In this hypothetical system, the batteries alone would power driving at low speeds and loads. When the engine comes on, to maximize efficiency it would switch among various modes, or operating cycles, that are common to combustion engines, such as the Atkinson cycle and the Otto cycle (a conventional engine can operate only in one mode). The hybrid motor and turbocharger would provide instant power during acceleration. And exhaust gas—a free source of energy—would be tapped to generate electricity that recharges the batteries.
For a car optimized in this way, the engine could be one half to one third of the size of current engines, reducing friction losses and cutting weight to boot. Such a system would offer major efficiency benefits, but it would also be extremely complex and costly. One important task would be implementing software that could determine the best operating strategy for every speed and load condition and control the engine as it switched between modes.
In the long run, and in the face of an inevitable downturn in oil supplies, the world needs as many practical alternatives to gasoline as science and engineering can muster. But the allure of advanced vehicles should not stall the progress industry can and should make to improve IC engine efficiency right now. No single technology or energy source can satisfy the world’s growing transportation energy demands. But substantial gains in IC engine efficiency, along with expanded use of hybrid technology, will help smooth the transition from petroleum to more renewable fuel options. In this context, the gasoline IC engine can be viewed not as the enemy to progress but as a weapon in the battle to reduce greenhouse gas emissions and bring about a cleaner and more sustainable future.