MIT scientists are exploring the use of ethanol direct injection (DI) to support the use of small, highly turbocharged engines with substantially increased efficiency as a downsizing strategy to reduce fuel consumption and emissions.
The researchers project that ethanol DI could result in a part-load efficiency increase of 30% relative to conventional port-fueled injection engines. The proposed direct injection approach could thus potentially provide a more cost-effective alternative to current generation gasoline-electric hybrids and turbodiesels.
The foundation of the approach is the enhanced knock suppression resulting from such a use of ethanol, which could allow for more than a factor of two increase in manifold pressure relative to conventional, while also supporting an increase in compression ratio.
Knock refers to the autoiginition of unburned gas in the cylinder. There are a number of factors that contribute to knock, but two of the main ones are cylinder pressure, temperature and fuel octane.
Turbocharged boosting of an engine can contribute to engine efficiency, and thereby support the use of a smaller engine. The application of turbocharging, however, is limited by the occurrence of knock under higher cylinder pressures.
The ethanol direct-injection concept uses the high octane rating of ethanol coupled with the evaporative cooling from direct injection to support the higher-pressure, more efficient engines. For example, a 3.0-liter engine could potentially be replaced by an engine of about half its size, resulting in a 30% increase in fuel efficiency over a typical driving cycle, according to the researchers.
The ethanol direct injection system is controlled separately form the gasoline injection system, and the ethanol is stored in a separate tank. The gasoline system can continue to use conventional port-injection.
The ethanol injection is carried out so as to maximize evaporative cooling which occurs when it is directly injected into the engine cylinders. The resulting reduction in temperature of the fuel/air charge from the ethanol evaporative cooling is the major factor in enhancing the fuel octane rating and suppressing knock.
The concept would operate the engine 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, the 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.
Only a small amount of ethanol—less than one gallon of ethanol for every twenty gallons of gasoline—may be required to achieve the large increase in efficiency.