Better than Hybrids
A proposed engine design approaches the efficiency of gas-electric hybrids, but could be far cheaper.
Consumers hoping to cut gasoline spending, with average gas prices nearing $3 a gallon, could opt for hybrids. But even with gas prices high, the added cost of hybrid cars can cancel money saved at the pump, suggesting the need for lower-cost alternatives.
A new type of ethanol-boosted, turbocharged gasoline engine could be the answer. The engine would be almost as efficient as gas-electric hybrids, but cost much less, according to its MIT inventors – Leslie Bromberg and Daniel Cohn, plasma science and fusion center researchers, and John Heywood, professor of mechanical engineering.
The new engine would improve efficiency in two ways. The first is to decrease the size of the engine, which reduces friction, thus saving fuel at light engine loads, such as during city driving. When more power is needed, a turbocharger kicks in. It uses exhaust flow to compress air, making it possible to combust more air and fuel in a smaller space.
The second approach is to engineer the engine to have a higher compression ratio – the ratio of the volume of air and fuel before and after it is compressed in an engine. A higher compression ratio “makes the engine more efficient, because you expand the burned gases more and extract more energy out of them,” Heywood says.
Neither of these are new ideas. But in the past, such efforts have been limited by a phenomenon called knock: high compression ratios and extreme turbocharging cause gasoline to spontaneously combust when the engine is under heavy loads, such as during acceleration or at high speeds, potentially causing serious damage. The MIT researchers have found a way to prevent knock, allowing them to crank up the turbocharger and increase the compression ratio – and thereby increase the power of an engine by 250 percent.
If this increase in power is taken advantage of to reduce the size of the engine – which would go against long-time trends emphasizing performance over fuel economy – it could save gas. “This allows very large pressure turbocharging, very large downsizing of the engine, and makes it possible to have a small engine with much higher efficiency,” Cohn says.
The researchers solved the knocking problem by injecting into combustion chambers precisely controlled amounts of ethanol at moments when the engine is working hard enough to cause knock. Compared with gasoline, ethanol has higher octane, a rating of how much a fuel can be compressed before it combusts spontaneously, that is, before it causes knocking. The injected ethanol also cools the mixture, so it effectively increases the octane rating of the fuel mix to about 130 – as good as high-performance racing fuels, Cohn says.
The system would use relatively little ethanol, about 1 gallon per 20 gallons of gasoline, so Cohn estimates the separate ethanol tank would have to be refilled about as often as an oil change. Furthermore, since it would require relatively minor modifications to existing technologies, Cohn says the design could be in production vehicles as soon as 2011 – with the help of a recent collaboration between their startup, Ethanol Boosting Systems (EBS), Cambridge, MA, and Ford Motor Company, Dearborn, MI.
The MIT researchers estimate their engine would add only $500-1000 to the cost of a vehicle, which includes the added costs of the high-end turbocharger, a direct-injection system, and a stronger, smaller engine. This modest premium compares favorably to that of hybrid cars. According to a review in Consumer Reports (April 2006), some hybrid vehicles failed to pay for themselves over the course of five years, even when factoring in federal tax credits and gas prices that rise to $4 a gallon. In contrast, Cohn says, their engine would pay for itself in two to three years.*
This modest premium compares favorably to that of hybrid cars. According to a review in Consumer Reports (April 2006), some hybrid vehicles failed to pay for themselves over the course of five years, even when factoring in federal tax credits and gas prices that rise to $4 a gallon. In contrast, Cohn says, their engine would pay for itself in two to three years.
The new engine should be 30 percent more efficient than conventional engines, based on a computer model the researchers say accurately reproduces the behavior of internal-combustion gasoline engines. In comparison, a Toyota Prius gets about 30-35 percent better fuel economy than a comparable vehicle, according to tests by Consumer Reports. In the same review, the magazine showed a $5,700 price premium for the Toyota Prius over a conventional vehicle.
Rodney Tabaczynski, former director of powertrain research at Ford (who is not involved with EBS), says the ethanol “will definitely help the octane problem” and existing electronic controls and feedback systems should make the controlled injection feasible.
The challenges EBS is likely to encounter he says, have more to do with logistics – two fuel tanks in a vehicle can be hard to implement, and there’s the challenge of making sure ethanol is available at the corner gas station. Also, the engine will need a system that ensures it isn’t damaged if the driver forgets to fill the ethanol tank.
Tabaczynski also cautions that real fuel savings will depend on an individual’s driving habits. As with hybrids, cars with these engines will get their best mileage when driven in a city, not at 70-75 miles per hour on the highway with the throttle wide open.
*Correction: The original version of this story quoted Consumer Reports as saying no hybrid made up for its cost premium over five years. CR has corrected that statement: two hybrids, the Toyota Prius and Honda Civic Hybrid save about $400 and $300, respectively, they say, albeit only after a sizable tax credit.
Home page image is an illustration of direct fuel injection, an aspect of the new engine. Image courtesy of the U.S. Department of Energy.
Keep up with the latest in sustainable energy at EmTech MIT.
Discover where tech, business, and culture converge.
September 17-19, 2019
MIT Media Lab