Dual-Fuel Partially Premixed Combustion: Gasoline + Diesel = Cleaner, 20% More Efficient Engines

gasoline and diesel fuel photo

Could be 20% More Fuel Efficient Than Diesel Alone
Gasoline and diesel have different combustion characteristics that make one or the other preferable depending on the situation. But what if you could get the best of both world? That's what researchers at the University of Wisconsin are trying to do; they are working on an engine that can blend the two fuels on the fly inside the combustion chamber, with ratios varying depending on driving conditions. Preliminarly results show that this could lead to engines that are cleaner and more efficient than regular diesel engines (which already have better thermal efficiency than gasoline engines).
gas and diesel chart

Ultra-Low Emissions & 53% Thermal Efficiency
"Fast-response fuel blending" works by using an onboard computer to determines the best fuel mix at any time, and port fuel injection is used for gasoline and direct injection for diesel, with the fuel mixing inside the cylinder. Of course, that would also require two fuel tanks, but that's a small price to pay for a 20% improvement in fuel efficiency and cleaner emissions.

Science Daily writes:

Under heavy-load operating conditions for a diesel truck, the fuel mix in Reitz' fueling strategy might be as high as 85 percent gasoline to 15 percent diesel; under lighter loads, the percentage of diesel would increase to a roughly 50-50 mix. Normally this type of blend wouldn't ignite in a diesel engine, because gasoline is less reactive than diesel and burns less easily. But in Reitz' strategy, just the right amount of diesel fuel injections provide the kick-start for ignition. [...]

Two remarkable things happen in the gasoline-diesel mix, Reitz says. First, the engine operates at much lower combustion temperatures because of the improved control -- as much as 40 percent lower than conventional engines -- which leads to far less energy loss from the engine through heat transfer. Second, the customized fuel preparation controls the chemistry for optimal combustion. That translates into less unburned fuel energy lost in the exhaust, and also fewer pollutant emissions being produced by the combustion process. In addition, the system can use relatively inexpensive low-pressure fuel injection (commonly used in gasoline engines), instead of the high-pressure injection required by conventional diesel engines.

Here's an interesting quote by Dr. Rolf Reitz of the University of Wisconsin: "The US consumes about 13.5 million barrels of oil per day in transportation. Hypothetically, if the such dual-fuel engines with 53% thermal efficiency could be applied across the entire fleet, the US could reduce its oil consumption by 4 million barrels per day--about one-third of all oil destined for transportation."

But this still leaves 9 million barrels of oil per day.

Maybe the real benefit of this technology will be in series plug-in hybrids. You'd have an all electric range for most of your daily driving, and when you need more, a diesel-gasoline generator with a thermal efficiency of about 53% could recharge the batteries (preferably using carbon neutral biofuels like cellulosic ethanol and algae biodiesel).

Via Green Car Congress, Science Daily
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