The first question you’re probably asking is, “Why in the world do you want to do that?” Well, I got the idea after talking to some colleagues who have been working on similar concepts.

We’re working on a combustion system that’s not traditional diesel combustion but not spark-ignition combustion either. Most researchers call this approach low temperature combustion, or LTC. Several types have been explored, such as HCCI (homogeneous charge compression ignition), M-K (modulated kinetics or smokeless rich) and UNIBUS (uniform bulky stratified) combustion.

Why, you ask, would we explore this alphabet soup of technologies when when traditional diesel and spark ignition have served us well for more than a century? Because traditional diesel combustion spews a lot of particulate matter and oxides of nitrogen (NOx). And spark-ignition gasoline combustion has a significant efficiency problem due to the throttle, which is needed to control power output. Because of the nature of these two systems, there’s really no significant improvement that can be achieved with either one.

We decided to look for something that was a cross between the two.

This new system is more like traditional diesel combustion than spark ignition but uses a fuel and combustion approach that minimizes the emissions problems associated with diesels. It cuts NOx more than particulate matter but has advantages for both.

We wanted to eliminate the throttle so we could retain efficiency while avoiding what we call “mixing controlled” combustion. Mixing controlled combustion occurs when diesel jets squirt fuel into the engine, resulting in almost immediate ignition. It requires diffusing fuel from the center of the jet to the reaction zone and the diffusion of air from the outside into the reaction zone as combustion progresses.  This diffusion is what produces particulates and NOx.

Our approach is to use a fuel injector — in this case a diesel fuel injector — but use fuel that is difficult to auto-ignite. In this case, gasoline.  The advantage is we can inject fuel rather early in the compression stroke without it igniting immediately.  In fact, we can inject fuel two or three times during the compression stroke and it won’t ignite until the piston is very close to the top of the cylinder, a position called top dead center, or TDC.

This approach also lets us place ignition exactly where we want it because we can control the precise timing of injection events. Using a fuel that is difficult to auto-ignite allows a long delay between injection and ignition, providing the opportunity to get all the fuel into the combustion chamber before ignition. This is important because this is how we can avoid particulate and NOx production. There is no liquid fuel to “coke up” because all of it is somewhat mixed with air before ignition. There also is little air present that does not have fuel near it, so the availability of air to heat up and dissociate into NOx is drastically reduced. Using exhaust gas recirculation lets us reduce NOx further.

The next question you may ask is, “OK, this sounds great. Now what do you need to give up to get high efficiency and clean emissions?” The answer is power density.

Because we are reducing the “violence” with which the combustion reactions occur, peak power will drop roughly 25 percent. However, the standard operation of vehicles in the United States rarely requires engines be operated at peak power. How often do you have the accelerator pedal mashed to the floor? If your answer is “often,” I’d ask if you drive in NASCAR. Besides — the torque profile of the new system is essentially the same as a conventional diesel and it provides excellent performance in the powerband where most people actually drive. The impact on drivers will be negligible.

One thing we’re doing differently than others who’ve explored this area is we’re using gasoline with a slightly lower octane than pump gasoline. We’re burning fuel in the 80 to 85 RON, or research octane number, range. It’s a little easier to auto-ignite than pump gasoline, yet provides energy companies with an easier target to hit when refining a barrel of petroleum. This is one reason energy companies like BP and ConocoPhillips are monitoring our work and providing some advice.

Regardless of fuel, the real trick to this approach is the careful control of fuel injection into the cylinder — the number of injections, the fuel pressure, the timing of each injection and so forth. The system is not as robust as traditional spark ignition or diesel combustion, but it is much more robust than some other systems that have been tried. We’re working with General Motors to explore this combustion system see what the possibilities are.

Steve Ciatti is a mechanical engineer at the Transportation Technology Center at Argonne National Laboratory.

Photo: Argonne National Laboratory. Steve Ciatti, working on a diesel engine that burns gasoline.

Authors: Steve Ciatti, Argonne National Laboratory

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