A new engine concept developed by researchers at the University of Wisconsin-Madison could cut fuel consumption by about 30 percent in cars and by almost 20 percent in heavy trucks. In gas-powered cars, the new design would add little to the cost of the engine. In heavy-duty trucks, it would substantially reduce costs by eliminating the need for expensive after-treatment systems to reduce emissions.
The concept, which has been demonstrated in test engines, involves precisely mixing two different fuels in the combustion chamber, which gives greater control over both the timing and duration of combustion. It could provide a way to meet fuel economy regulations without the more expensive electric motors and batteries found in hybrid vehicles (although for still greater efficiency, the new design could be incorporated into a hybrid vehicle). The new engine concept is made possible by precise electronic fuel injection and advances in computer simulations. “We discovered this process using advanced computer modeling, which allowed us to identify the recipe for optimal mixing of the fuels,” says Rolf Reitz, a professor of mechanical engineering at UW-Madison.
The design has two versions, one for replacing heavy-duty diesel engines and another, to be unveiled this fall, that would replace conventional gasoline engines. Both use the same combustion process that makes diesel engines significantly more efficient than gasoline engines–compressing fuel and air until it reaches pressures and temperatures that cause it to ignite, rather than using a spark to ignite the fuel. The new design improves engine efficiency beyond that of diesel engines by reducing the amount of energy wasted as heat and by improving control over the timing of combustion. It also greatly reduces the emissions associated with diesel engines, particularly important now that new emissions regulations require automakers to employ expensive after-treatment systems.
In the version designed to replace heavy-duty diesel engines, gasoline from one fuel tank is injected into the intake port near the combustion chamber, where it mixes with air before moving into the chamber (this is the conventional form of fuel injection in gasoline vehicles). Then diesel fuel from another tank is injected directly into the chamber using a low-pressure fuel injector. As this mixture is compressed, the diesel ignites first, followed shortly by the gasoline, which is more resistant to combustion. Controlling the ratio of the two fuels determines both the timing of the combustion and how long it lasts. The design requires precise control over the fuel injection, as the ratio and distribution of the two fuels in the chamber needs to change depending on the load placed on the engine. With light loads, the mix is about 50-50, while heavier loads might need as little as 5 percent diesel. The resulting engine is about 55 percent efficient, compared to 40 to 45 percent efficient for conventional heavy-duty diesel engines. Emissions are low enough to eliminate the need for after-treatment systems for exhaust–systems that, in a heavy-duty truck, can cost as much as the engine itself.
In the version designed to replace conventional gasoline engines, the diesel fuel is replaced with gasoline that’s mixed with an additive to make it more reactive, improving ignition of the fuel. Instead of having two fuel tanks, the car needs only one gasoline tank and a small reservoir the size of a window-washing-fluid bottle to hold the additive. Ordinary gas is injected by the port injector, and gas mixed with the additive is injected directly into the chamber. The result is an engine that’s 45 percent efficient, compared to about 30 percent efficient for conventional gasoline engines.
In both systems, the approach reduces engine pressure and temperature, which reduces the formation of smog-forming and other dangerous pollutants. The lower temperatures also reduce the amount of energy that’s lost to heat, making it available to drive the piston. “We extend the combustion event over a controlled period of time to get a gentle heat release that doesn’t lead to violent pressure rise and high temperatures in the combustion chamber,” Reitz says.
Robert Dibble, a professor of mechanical engineering at the University of California at Berkeley, says the new design “is a clever idea.” He says the two-fuel design could be difficult to get automakers and consumers to adopt, but he notes that current diesel after-treatment systems already require drivers to add a separate exhaust treatment fluid when refueling, so this barrier may be lower now. The version of the new design that uses a gasoline additive would only require refilling the additive every oil change, Reitz says, reducing the inconvenience.
Dibble says that another researcher is taking a similar approach to improving engine efficiency, but his approach uses only one fuel. Bengt Johansson, head of the division of combustion engines at Lund University in Sweden, has shown high efficiencies and low emissions by controlling the timing and duration of combustion with multiple fuel injections. But unlike the UW-Madison method, which uses fuels with different combustion properties, Johansson’s approach controls combustion by creating regions within the combustion chamber with varying concentrations of fuel and air. One potential disadvantage is that, because of the high compression levels used, it requires a more expensive engine than those used in conventional gas-powered cars.