Diesel used as gasoline 'spark plug' improves economy and emissions

The two engine technologies tend to be regarded as completely separate, so we rarely contemplate how gasoline and diesel can work together. But, in a series of tests conducted at the University of Wisconsin, scientists have used an engine’s fuel injection to produce the optimal diesel-gas mix for any given moment. The results are impressive: an average 20% greater fuel efficiency; combustion temperatures reduced by up to 40%; and effortless meeting of the stringent EPA 2010 emission regulations. Plus, the researchers believe that if their findings were implemented into every gasoline and diesel engine in the US, the savings could be as great as 4 million barrels of oil daily.

Because it’s less reactive and won’t burn so easily, gasoline could normally never fuel a diesel engine. However, the fuel-injected diesel becomes a kind of liquid spark plug, providing a kick-start for ignition. The fuel mix varies depending on circumstance: a heavily-laden truck might require a mix as high as 85% gasoline to 15% diesel, while a light load would require a roughly 50-50 blend. Fast-response fuel blending, in which an engine's fuel injection is programmed to produce the optimal gasoline-diesel mix based on real-time operating conditions, determines the correct mix.

While the theory was initially computer-modeled, researchers put it to the test using a heavy-duty Caterpillar diesel engine. The test confirmed two of the greatest benefits of blended fuel combustion.

First, the combustion temperatures were reduced by as much as 40%, meaning far less energy was lost from the engine through heat transfer. Second, the customized fuel mix optimized combustion, with less unburned fuel lost in the exhaust and fewer emissions. (In fact, the process easily achieves the EPA’s requirement that 90% of soot and 80% of nitrogen oxide be eliminated from diesel emissions by 2010.)

In combination, these helped the test engine achieve a best result of 53% thermal efficiency. Thermal efficiency, basically, measures the percentage of fuel converted into power, and not lost in heat transfer or exhaust. The figure of 53% mightn’t seem like a lot, but so far the most fuel-efficient diesel engine in the world can only achieve a best figure of 50%.

The very good news is that this idea is, relatively speaking, fairly easy to implement. The scientists believe it will work just as well with the low-pressure fuel injection of gasoline engines as with diesel’s high-pressure valves. And, because gasoline engines average only 25% thermal efficiency, the potential for fuel economy is even greater.

The research group estimates that, if every gasoline and diesel engine in the U.S. converted to this blended fuel process – and achieved an overall thermal efficiency of 53% - oil consumption would reduce by about 4 million barrels a day, a little under a third of today's current consumption. Little wonder the Department of Energy has its funding fingers in this pie.