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May 05, 2011

Combining gas and diesel engines could yield best of both worlds

Steve Ciatti, a mechanical engineer at Argonne National Labs, is heading a team to explore the possibilities of a gasoline-diesel engine. The result, so far, is cleaner than a diesel engine and almost twice as efficient as a typical gasoline-powered engine. What's the catch? This approach results in better efficiency and cleaner emissions, but sacrifices some of the power density. That is, at peak power—when you push the accelerator pedal to the floor—the engine won't provide quite as much power: about 75 percent at present. "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.





In a typical engine, pistons turn the wheels of the car. Each piston is moved by the explosive force of hot air when fuel is ignited above it in a cylinder.

Both spark-ignited gas engines and diesel engines do this, but they go about it differently. A gasoline engine first mixes air with the fuel, then compresses the mixture, and finally ignites it with a spark plug. In a diesel engine, the air is first compressed and then the fuel is injected; compressing the air makes it hot enough to ignite the fuel without a spark. This is what makes diesel more efficient—and also dirtier.

On one hand, diesel engines are more efficient because they do not control power with a throttle, which restricts air to the chamber. This means the fuel mixes more evenly with air, so more of it burns. Lack of a throttle also eliminates "engine knock" —caused by fuel igniting prematurely in the engine—because fuel is introduced only in the combustion chamber.

On the other hand, the introduction of fuel so late in the cycle creates a problem: emissions. Since fuel burns more easily when the droplets are smaller, the fuel is sprayed into the chamber as a fine mist. But diesel fuel is so easy to auto-ignite that it begins to react almost immediately—long before all of the fuel is in the chamber. Intentionally, the fuel isn't mixed perfectly with the air because diffusion controls the combustion; but diffusion also means some air and fuel are converted into nitrous oxides and soot.

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," Ciatti said. "So we lose the throttle and spark plugs, because those create inefficiencies. We start with a diesel engine and inject gasoline instead.

Because gasoline doesn't ignite immediately the way that diesel would, we can actually inject several times before the fuel ignites. That way, we can make sure that the most or all of the fuel is mixed with the air, significantly decreasing NOX and soot."

The engine's performance is close to diesel efficiency, and roughly double that of today's automotive engines at low speeds and loads.

What's the catch? This approach results in better efficiency and cleaner emissions, but sacrifices some of the power density. That is, at peak power—when you push the accelerator pedal to the floor—the engine won't provide quite as much power: about 75 percent at present.

"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.

Ciatti and his colleagues are working to make the system predictable and reliable enough to be successful in a commercial vehicle. Argonne is collaborating with General Motors on this project.
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