The Incredible Shrinking Engine
A new engine design could significantly improve fuel efficiency for cars and SUVs, at a fraction of the cost of today’s hybrid technology.
The following article appears in the March/April 2007 issue of Technology Review.
For Daniel Cohn, a senior research scientist at MIT’s Plasma Science and Fusion Center, the century-old internal-combustion engine is still a source of inspiration. As he strides past the machinery and test equipment in the MIT Sloan Automotive Laboratory, his usually reserved demeanor drops away. “An engine this size,” he says, pointing out an ordinary-looking 2.4-liter midsize gasoline engine, “would be a rocket with our technology.”
By way of explaining that technology, he shows off a turbocharger that could be bolted to the 2.4-liter engine; the engine, he adds, uses direct fuel injection rather than the port injection currently found in most cars. Both turbocharging and direct injection are preëxisting technologies, and neither looks particularly impressive. Indeed, used separately, they would lead to only marginal improvements in the performance of an internal-combustion engine. But by combining them, and augmenting them with a novel way to use a small amount of ethanol, Cohn and his colleagues have created a design that they believe could triple the power of a test engine, an advance that could allow automakers to convert small engines designed for economy cars into muscular engines with more than enough power for SUVs or sports cars. By extracting better performance from smaller, more efficient engines, the technology could lead to vehicles whose fuel economy rivals that of hybrids, which use both an electric motor and a gasoline engine. And that fuel efficiency could come at a fraction of the cost.
Cohn says that his colleagues–Leslie Bromberg, a principal research scientist at the Plasma Science and Fusion Center, and John Heywood, a professor of mechanical engineering and director of the Sloan Auto Lab–considered many ways to make internal-combustion engines more efficient. “And then, after a lot of discussion, it just sort of hit us one day,” Cohn recalls. The key to the MIT researchers’ system, he explains, was overcoming a problem called “knock,” which has severely limited efforts to increase engine torque and power.
In gas engines, a piston moves into a cylinder, compressing a mixture of air and fuel that is then ignited by a spark. The explosion forces the piston out again. One way to get more power out of an engine is to design the piston to travel farther with each stroke. The farther it travels, the more it compresses the air-fuel mixture, and the more mechanical energy it harvests from the explosion as it retreats. Overall, higher compression will lead to a more efficient engine and more power per stroke. But increasing the pressure too much causes the fuel to heat up and explode independently of the spark, leading to poorly timed ignition. That’s knock, and it can damage the engine.
To avoid knock, engine designers must limit the extent to which the piston compresses the fuel and air in the cylinder. They also have to limit the use of turbocharging, in which an exhaust-driven turbine compresses the air before it enters the combustion chamber, increasing the amount of oxygen in the chamber so that more fuel can be burned per stroke. Turning on a car’s turbocharger will provide an added boost when the car is accelerating or climbing hills. But too much turbocharging, like too much compression, leads to knock.
An alternative way to prevent knock is to use a fuel other than gasoline; although gasoline packs a large amount of energy into a small volume, other fuels, such as ethanol, resist knock far better. But a vehicle using ethanol gets fewer miles per gallon than one using gasoline, because its fuel has a lower energy density. Cohn and his colleagues say they’ve found a way to use both fuels that takes advantage of each one’s strengths while avoiding its weaknesses.
The MIT researchers focused on a key property of ethanol: when it vaporizes, it has a pronounced cooling effect, much like rubbing alcohol evaporating from skin. Increased turbocharging and cylinder compression raise the temperature in the cylinder, which is why they lead to knock. But Cohn and his colleagues found that if ethanol is introduced into the combustion chamber at just the right moment through the relatively new technology of direct injection, it keeps the temperature down, preventing spontaneous combustion. Similar approaches, some of which used water to cool the cylinder, had been tried before. But the combination of direct injection and ethanol, Cohn says, had much more dramatic results.
The researchers devised a system in which gasoline would be injected into the combustion chamber by conventional means. Ethanol would be stored in its own tank or compartment and would be introduced by a separate direct-injection system. The ethanol would have to be replenished only once every few months, roughly as often as the oil is changed. A vehicle that used this approach would operate around 25 percent more efficiently than a vehicle with a conventional engine.
A turbocharger and a direct-injection system would add to the cost of an engine, as would strengthening its walls to allow for a higher level of turbocharging. The added equipment costs, however, would be partially offset by the reduced expense of manufacturing a smaller engine. In total, an engine equipped with the new technology would cost about $1,000 to $1,500 more than a conventional engine. Hybrid systems, which are expensive because they require both an internal-combustion engine and an electric motor powered by batteries, add $3,000 to $5,000 to the cost of a small to midsize vehicle–and even more to the cost of a larger vehicle.
When the MIT group first hatched its idea, Bromberg created a detailed computer model to estimate the effect of using ethanol to enable more turbocharging and cylinder compression. The model showed that the technique could greatly increase the knock-free engine’s torque and horsepower. Subsequent tests by Ford have shown results consistent with the MIT computer model’s predictions. And since the new system would require relatively minor modifications to existing technologies, it could be ready soon. Ethanol Boosting Systems, a company the researchers have started in Cambridge, MA, is working to commercialize the technology. Cohn says that with an aggressive development program, the design could be in production vehicles as early as 2011.
While Cohn applauds the benefits of hybrids and says his technology could be used to improve them, too, he notes that the popularity of hybrid technology is still limited by its cost. Cheaper technology will be adopted faster, he suggests, and will thus reduce gasoline consumption more rapidly. “It’s a lot more useful,” he says, “to have an engine that a lot of people will buy.”