Gas-powered diesel engine may double fuel efficiency

Government researchers have developed an engine prototype that's cleaner than a diesel engine and almost twice as efficient as a typical gasoline-powered engine.

Recently, it seems like car engine innovation has become about moving away from the internal combustion engine and toward more environmentally-friendly alternatives like electric motors . But the truth is gasoline engines, which have been with us for well over a century, aren't headed for the junk yard any time soon.

With that in mind, engineers at the U.S. Department of Energy's Argonne National Laboratory have been hard at work on a project to improve the fuel efficiency of gas-powered engines . Their approach has been to figure out a way to take advantage of the highly fuel efficient technologies found in diesel engine while also keeping harmful emissions to a minimum.

So far, the result is a gas-powered prototype that's cleaner than a diesel engine and almost twice as efficient as a typical gasoline-powered engine.

The major difference between gas and diesel engines is the technology used to ignite the fuel. Gasoline engines are designed to mix air with the fuel prior to compressing and igniting the mixture. But in a diesel engine, the air is compressed first and then the fuel is injected. This makes it so the air is hot enough to ignite the fuel without spark plugs or the use of an air-restricting throttle, which allows the fuel to mix more evenly with air so that more of can be burned.

The drawback is that the process produces unacceptably high levels of nitrous oxides and soot. This is because diesel fuel is so easy to auto-ignite that it begins to react the moment it's introduced—long before all of the fuel is in the chamber.

Nitrous oxides are created when the flame jet created by the diesel injection burns so hot that nearby nitrogen and oxygen molecules in the air start to break apart and react. Meanwhile, soot is created inside the hot jet because the fuel doesn't have enough oxygen to fully burn, creating soot instead.

"What we want to do is combine the efficiency of diesel with the cleanliness of gas," said Steve Ciatti, an engineer working on the project. "So we lose the throttle and spark plugs, because those create inefficiencies. We start with a diesel engine and inject gasoline instead."

And since gasoline doesn't ignite immediately, the fuel can be injected several times to ensure that most or all of the fuel is mixed with the air before it is ignited, a process that significantly decreases NOX and soot emissions, he explains.

The trade-off, however, is that a combination engine has less power density than either. That is, at peak power—when you push the accelerator pedal to the floor—the engine provides about 75 percent the horsepower of standard cars you see on the road.

"But if you don't drive pedal to the metal, however," Ciatti said, "this won't affect the car's performance. It's excellent in the power range where most people actually drive."

So what's the next step for Ciatti's team? Improvements that would make the system predictable and reliable enough to be successful in a commercial vehicle.

(via Argonne National Lab)

Image: Argonne National Lab

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