Combining gas and diesel engines could yield best of both worlds

May 5, 2011 By Louise Lerner
Combining gas and diesel engines could yield best of both worlds

It may be hard to believe, but the beloved gasoline engine that powers more than 200 million cars across America every day didn't get its status because it's the most efficient engine. Diesel engines can be more than twice as efficient, but they spew soot and pollutants into the air.

Could researchers at the U.S. Department of Energy's Argonne National Laboratory engineer a union between the two—combining the best of both?

Steve Ciatti, a mechanical engineer at Argonne, is heading a team to explore the possibilities of a gasoline-diesel . The result, so far, is cleaner than a diesel engine and almost twice as efficient as a typical gasoline-powered engine.

The basic designs for both kinds of engines actually date back to the 19th century. German engineer Nikolaus Otto is credited with the gasoline-fueled four-stroke design still used today, but Rudolf Diesel noticed the inefficiency of the engine and came up with a design of his own, in 1893. The problem is that diesel engines are more efficient, but their emissions are noxious—full of soot and smog-forming nitrogen oxides, or NOX gases. Gasoline engines are cleaner, but a typical gas engine is only about 20 percent efficient—that is, only 20 percent of the energy in the actually moves the car, while 80 percent is lost to friction, noise, engine functions or goes out as heat in the exhaust. But many diesel engines reach 40 percent efficiency and higher.

Today the United States has more stringent emissions requirements than anywhere else in the world. "In fact, as of 2007, in some parts of the country the coming out of a car is cleaner than the air that went in," Ciatti said, and engineers just haven't been able to get diesel emissions low enough to meet those standards. Instead, they need to use expensive exhaust aftertreatment devices: usually a catalyst that reduces nitrous oxides from the exhaust pipe by splitting the oxygen away from the nitrogen.

Ciatti and colleagues wanted to clean up diesel's dirty exhaust, but keep the high efficiency and better gas mileage. To do this, they headed to the dynamometer lab at Argonne's Transportation Technology R&D Center.

The dynamometer is a machine built to test engine performance. Essentially, it's just an electric motor to provide resistance to "fool" the engine into thinking it has a car attached. Dynamometer test cells can be heavily instrumented and accurately controlled, significantly improving data quality.

"If you're trying to test out a new engine, the last thing you want to do is put it in a car," Ciatti explained. "An entire car system introduces all sorts of variables, and you can't get a truly accurate comparison between engines. What you really want to start with is a dynamometer."

Combine a dynamometer with the engine you're testing, and you have an engine test cell: an arrangement that lets you control the tiniest variables, so that engineers can tinker with the engine to see if they can improve its performance.

They can simulate how an engine would perform in different cars—a hybrid car, an electric car, a gas-powered car—and can also measure emissions.

With a diesel engine installed in the test cell, Ciatti and his team were ready to explore the possibilities.

In a typical engine, pistons turn the wheels of the . 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 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, 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.

This video is not supported by your browser at this time.

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 ."

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.

Explore further: First images made of hydrogen burning in working internal combustion engine

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not rated yet May 05, 2011
Would someone please tell this poor simpleton just what the above video is supposed to be showing?

5 / 5 (2) May 05, 2011
in internal combustion engine theory, burn duration is specified in units of "ca-deg" (or degca as in the video above), so the video is showing the burn occurring in the cylinder, with annotation added showing the burn duration. you have to assume that this is the hybrid engine. of course, a caption would have been helpful.
1 / 5 (2) May 05, 2011
Yet another example of some helpful tech that we probably wont ever see in use. Why dont they just capture all the excess heat energy, and all the vibrational energy, and slap solar cells all over the car, to end up with something that runs off a 2-cylinder engine and gets 150 mpg. It can be done, but it wont because their goal is to make money!
not rated yet May 05, 2011
Thanks Paul!
5 / 5 (1) May 05, 2011
Modern diesel engines are quite clean:, esp in Europe, certainly there's no perceptible difference between petrol and diesel engines in my experience. Larger vehicles, like trucks and tractors certainly, but *not* cars.
3 / 5 (2) May 05, 2011
a Diesel Particulate Filter DPF now filters the soot for re-burning. Addition of aqueous uric acid solution 'burns' NOx to N2 and H2O. Some even suppose AdBlue injection to increase mileage.
1 / 5 (1) May 05, 2011
Modern diesel engines are quite clean, and low sulfur diesel, combined with modern engine designs are now legal under the new regulations.

The main issue they've had recently is particulate emissions, which are bad for your respiritory health, but are relatively short lived.

Rather than just adjust the regulations in the US for particulates for diesels, they essentially mandated a change in the entire diesel fuel supply and all new engines. Particulates are the ONLY environmental car regulation that the US is stricter than the rest of the world.

That said, I think this is an awesome idea - However, it's not new. The idea of burning gas in a diesel type engine has been worked on at MIT and another company already.
not rated yet May 05, 2011
hematite asked:
Would someone please tell this poor simpleton just what the above video is supposed to be showing?
PaulRC responded
...the video is showing the burn occurring in the cylinder, with annotation added showing the burn duration. you have to assume that this is the hybrid engine. of course, a caption would have been helpful.
Actually, what is illustrated is just the standard Otto cycle (4-stroke, gasoline engine with spark plug), not the proposed hybrid cycle. The numbers at the top left simply state which of the four steps the process is in.

Hey, PhysOrg folks: could you change the animation to show the hybrid cycle instead?!
2.3 / 5 (3) May 05, 2011
The animated cartoon is from the Wikipedia Four-stroke engine article.

5 / 5 (2) May 05, 2011
If the thing doesn't have a lot of power at the high end, don't you just need more gears in your transmission? I've heard of automatics with 8 gears. And couldn't you also compensate by having four cylinders, but using only three most of the time, until you want to pass someone going uphill? These are things that are done in some engines now, I think.
not rated yet May 06, 2011
What would happen if you coated the top of the piston and top portion of the cylinder with platinum like in a catalytic converter. Would that help the combustion be more complete and have less NOX etc???
3 / 5 (2) May 06, 2011
Why dont they just capture all the excess heat energy, and all the vibrational energy, and slap solar cells all over the car, to end up with something that runs off a 2-cylinder engine and gets 150 mpg. It can be done, but it wont because their goal is to make money!

Who is "their" referring to? Anyone that can sell a 150mpg car will make a lot of money.
not rated yet May 07, 2011
So, we're going to go through another generation or two of gasoline/diesel engines before we get sufficiently switched to electrics and hybrids? What, 30-60 years? Hydrocarbons could easily be much too expensive in only a few years to support our general transportation grid. What is the actual need for such research? Is a coal/gas engine soon to be marketed?
3 / 5 (1) May 07, 2011
Hybrid cars are terrible...
The power of electrical cars is the weight reduction and simplification of the car.
The hybrid car only works more efficient 1% of the time when you maintain an impossible driving style. Most of the time it is much less efficient than a optimized combustion engine alone.
not rated yet May 07, 2011
The problem isn't what gets burned or how . . .

. . . the problem is that we're BURNING stuff!
not rated yet May 09, 2011
Sorry to say.. but building an engine that will run on both gas an diesel sounds contrary and idiotic. Put gas in a diesel engine and all sorts of havoc will occur. Put diesel in a gas engine and it simply won't run.
Modern engines are of very high design. They are meant to run on fuels that have very specific qualities. Oil and gas are very dis-similar.
not rated yet May 10, 2011
Diesel is used in prime movers ie trucks, locomotives ect because it produces more power per gallon than gasoiline so it is cheaper. A large bore engine producing the same power as a smaller engine will use less feul because a large bore cylinder requires less pressure to produce the same power. When I use a 10% diesel/90%gasoiline feul mix in my 350 cubic inch gasoiline engine I get about 25% better miles per gallon. Driving habits and where you buy your gas make a big difference too.

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