Direct-injection ethanol provides low-cost fuel efficiency

MIT scientists and engineers earlier this year founded a company - Ethanol Boosting Systems - to commercialize their work on direct-injection ethanol boosting combined with aggressive turbo charging in a gasoline engine. The result is a gasoline engine with the fuel efficiency of current hybrids or turbo diesels - up to 30% better than a conventional gasoline engine - but at a much lower cost.

EBS has a collaborative R&D agreement with Ford Motors, and anticipates engine tests in 2007 with subsequent licensing to Ford and other automakers. If all goes as expected, vehicles with the new engine could be on the road by 2011.

The foundation of the approach is the enhanced knock suppression resulting from the separate, direct injection of small amounts of ethanol into the cylinder in addition to the main gasoline fuel charge.

Efforts to improve the efficiency of the conventional spark-ignition (SI) gasoline engine have been stymied by a barrier known as the knock limit. Changes that would have made the engine far more efficient would have caused knock (spontaneous combustion).

The injection of a small amount of ethanol into the hot combustion chamber cools the fuel charge and makes spontaneous combustion much less likely. According to a simulation developed by the MIT group, with ethanol injection the engine won’t knock even when the pressure inside the cylinder is three times higher than that in a conventional SI engine. Engine tests by collaborators at Ford Motor Company produced results consistent with the model’s predictions.

With knock essentially eliminated, the researchers could incorporate into their engine two operating techniques that help make today’s diesel engines so efficient: a high degree of turbo charging and the use of a higher compression ratio.

The engine would operate with a wide range of ethanol consumption from a minimum of less than 5% up to 100%. A knock sensor would determine when ethanol is needed to prevent knock. During the brief periods of high-torque operation, fractions of up to 100% ethanol could be used. For much of the drive cycle, vehicles are operated at low torque and there is no need for the use of ethanol.

The combined changes could increase the power of a given-sized engine by more than a factor of two. But rather than seeking higher vehicle performance, the MIT researchers cut their engine size in half. Using well-established computer models, they determined that their small, turbocharged, high-compression-ratio engine will provide the same peak power as the full-scale SI version but will be 20 to 30% more fuel efficient.

The ethanol-boosted engine could provide efficiency gains comparable to those of today’s hybrid engine systems for less extra investment: about $1,000 as opposed to $3,000 to $5,000. The engine should use less than five gallons of ethanol for every 100 gallons of gasoline, so drivers would need to fill their ethanol tank only every one to three months. The ethanol used could be E85.

Given the short fuel-savings payback time - three to four years at present US gasoline prices - the MIT researchers believe that their ethanol-boosted turbo engine has real potential for widespread adoption.

The environmental benefits of a more efficient engine can be seen in reduced emissions of greenhouse gases and smog-forming pollutants. The use of ethanol can be considered to reduce greenhouse gas emissions depending on the efficiency of production, as carbon is absorbed during the growth of crops used to produce the fuel.

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