Five years ago we first heard about a Caterpillar diesel engine located at the University of Wisconsin-Madison, that had been modified to run on an unlikely-sounding mixture of diesel and gasoline. Not only did the one-cylinder engine work, but it was more efficient than pure-diesel or pure-gas engines at converting the chemical energy of fuel into motion. Sitting in a basement lab, however, isn't the same as experiencing use in the real world. That's why students at UW-Madison, led by Prof. Rolf Reitz, have now put another diesel/gas engine into a 2009 Saturn.

The original Cat engine utilizes a system known as reactivity controlled compression ignition, or RCCI. It uses sensors and a computer system to continuously and instantaneously adjust the ratio of the two fuel types. Diesel is injected to start the engine, as it has a lower ignition temperature than gas. From there, the fuel blend varies depending on real-time operating conditions, with the goal of "exploiting each fuel’s strong points."

As a result, the lab-based engine currently has an efficiency of 59.5 percent – regular diesel truck engines manage no more than 52 percent, and the theoretical maximum is 64 percent.

Assistant professor Sage Kokjohn (left) and Rolf Reitz with the original Cat engine

The engine's high efficiency is due largely to the fact that it runs much cooler than a pure gas or diesel engine, so less heat is wasted by being absorbed into the engine block or radiator. That lower operating temperature also allows it to run cleaner, minimizing its production of soot and noxious nitrogen oxides. Additionally, the injected diesel doesn't need to be pressurized nearly as high as it does in a conventional diesel engine, thus saving costs and lessening complexity.

The new RCCI engine replaces the existing combustion engine in a 2009 Saturn Aura hybrid sedan, and runs on conventional gasoline and diesel fuel.

"The engine has a lot of controls, so when you put your foot on the gas, we automatically change the amount of diesel and gasoline to optimize the combustion process," said Reitz. "We can blend the correct dosage on a cycle-by-cycle basis."

Road tests have just begun, so we look forward to hearing about how it performs and what sort of mileage it gets.

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