An improved flex-fuel engine will allow vehicles to get better fuel efficiency from a mix of ethanol and gasoline. The pressure within the engine changes to adapt to different fuel blends. This, together with the use of turbochargers, makes it almost as efficient as a diesel engine, according to its developers.
So-called flex-fuel engines use a mixture of fuels, the most common combination being around 15 percent gasoline and 85 percent ethanol. But these engines tend to pay a 30 percent efficiency penalty because they are optimized to use gasoline, says Rod Beazley, product group director for gasoline at the engineering-research firm Ricardo, based in Detroit. Therefore, even though commercial ethanol-fuel mixes such as E85 cost less than gas at the pump, it still works out cheaper to operate a regular gasoline engine, he says.
Ricardo’s new Ethanol Boosted Direct Injection (EBDI) engine is designed to take full advantage of the favorable properties of ethanol to improve performance and reduce consumption. “[Ethanol] has a very high octane rating compared to other fuels, and a higher heat of vaporization,” says Luke Cruff, chief engineer for the EBDI program at Ricardo.
A higher octane rating means that a fuel is less prone to unwanted detonation, or “knocking.” A higher latent heat of vaporization means that ethanol can help control gas conditions in the combustion chamber by lowering temperatures. By modifyingthe pressure and temperature, fuel injected into the engine will burn more efficiently and reduce the formation of nitrous-oxide gases.
Regular flex-fuel engines are unable to exploit these properties because they are optimized for gasoline and run at lower cylinder pressures, says Cruff. The EBDI engine continually monitors the fuel blend using sensors. It then modifies cylinder pressure, fuel injection, valve timing, and other factors to ensure that the conditions get the most out of the fuel mix. One way that the engine modifies cylinder pressure is by using turbocharging.
“We have designed this engine to be optimized for ethanol and gas, and any blend of the two,” says Beazley. “We’re using all these control mechanisms to effectively change the compression characteristics of the engine.”
As a result, the engine is able to close the gap between spark-ignited engines and diesel, says Cruff. “We can get equivalent performance out of the vehicle and, in some drive cycles, equivalent fuel consumption,” adds Beazley. Besides boosting the performance of ethanol-gasoline blends, these modifications also deliver improvements with gasoline, although the benefits aren’t as great. The turbocharging, for example, allows the use of smaller, more efficient engines without sacrificing power.
This is a natural progression, says Hua Zhao, director of Centre for Advanced Powertrain and Fuels Research at Brunel University, in London, U.K. “Various companies are trying similar approaches,” he says. “It’s a good idea, and part of a trend to downsize engines by replacing big engines with smaller ones.”
The technology is different from another approach to boosting engine performance with ethanol. In that approach, the engine is optimized to run on ethanol, and when gasoline is used, small amounts of ethanol are injected from a separate fuel tank into the engine to prevent knocking. (See “The Incredible Shrinking Engine.”)
Currently, Ricardo’s concept engine has only been tested under laboratory conditions. But the company is installing a prototype in a GMC Sierra 3500 HD pickup truck. By replacing the large 6.6-liter, V8 diesel engine with Ricardo’s smaller 3.2-liter, V6 EBDI engine, it should be possible to get the same sort of fuel economy and performance from the vehicle. The technology is also very scalable, says Beazley. Besides replacing engines in passenger cars, it can be used for engines in large agricultural equipment, he says.
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