Cleaning up diesel emissions

Oct 28, 2005

A Kettering University researcher is working to clean up diesel emissions as part of the effort to reduce U.S. dependence on foreign oil.

Running numbers usually involves betting on a horse, but not for Dr. Bassem Ramadan, associate professor of Mechanical Engineering at Kettering University. He is running the numbers - performing numerical simulations - to test Engineering theories aimed at improving diesel fuel efficiency while reducing harmful emissions.

His research, a collaborative effort with researchers at Michigan State University (MSU), is funded by an Environmental Protection Agency (EPA) grant totaling $800,000 over next three years. Ramadan and Kettering graduate student Amit Khamkar, will complete numerical simulations and Dr. Harold Schock, professor of Mechanical Engineering at MSU, will perform physical testing. "MSU has good experimental capability in terms of performing in-cylinder flow visualization, LDV measurements, and infrared imagery and we have good simulation capabilities," said Ramadan.

"The focus is on diesel engines because the direct injection diesel engine has proven to be the most efficient plant of the last century. The automotive manufacturers in the U.S. have recognized they are behind compared to the Europeans in terms of the dieselization of light duty vehicles. Now they are considering diesel engines as an option for meeting the new emission standards," Ramadan said.

"In Europe diesel engines are no longer noisy, dirty, or slow to accelerate, so more customers are accepting them," he said, "it is estimated that half of the cars in Europe would be equipped with diesel engines by 2006," said Ramadan.

An incentive to improve diesel engines is that they operate at higher efficiency because they are designed to have higher compression ratios than gasoline engines, according to Ramadan. In the U.S. where there are a lot of personal trucks and light duty trucks on the road, "it makes sense to put diesel engines in them," he said.

The hope of the EPA is that improved diesel fuel efficiency will reduce dependency on imported oil and reduce emissions.

The major problem associated with diesel combustion compared to gasoline combustion is how to simultaneously reduce NOx and particulate matter emissions.

Soot vs. NOx
"Particulate matter is unhealthy," said Ramadan, "some diesel particulates are small in size so your nose does not filter them, but big enough to deposit on your lungs. Over time they can cause health problems."

Simply explained, soot is un-oxidized carbon particles that result when hydrocarbon fuels are burned with air in automobile engines. Due to combustion in air the hydrogen and the carbon in the fuel are oxidized. Without enough air, the carbon doesn't get oxidized properly and the result is soot, or un-oxidized carbon particles.

In addition, NOx particulates are a problematic byproduct of any engine that uses air, and diesel or gasoline. Because air has nitrogen and oxygen in it, when air is used as an oxidizer in a combustion system the nitrogen and oxygen, at high temperatures, react in the engine and form NOx .

"The immediate solution the way the EPA sees it," said Ramadan, "is to clean up diesel emissions by controlling or reducing NOx and particulates from diesel combustion engines. The challenge is that these two compete, if you try and reduce soot by using stoichiometric air your NOx goes up, if you try to reduce NOx by using less air then soot will go up," he said.

"In order to reduce soot there needs to be enough air mixed with the fuel because if there isn't enough air then the carbon doesn't get oxidized properly and soot results. However, when the stoichiometric amount of air is used more NOx results. This happens in diesel engines because the fuel is injected directly into the cylinder after the piston compresses the air. The fuel does not disperse properly and does not have enough time to evaporate and mix with the air within the cylinder, and hence, some regions will be fuel rich which result in soot, and some regions will be perfectly mixed which result in hot spots leading to high NOx.



The stoichiometric condition produces the highest flame temperature. In a cylinder, the temp is not uniform, so there are hot spots and cold spots. The hot spots produce NOx and cold spots produce soot. How to solve this problem is Ramadan's focus.

"Part of our job out of this grant with the EPA is to see how the air flow and the injection process affect the mixing process," Ramadan said.

"We can change the piston cup," he said, "the shape of the cup influences the air motion as does the design of the intake system. The design of the fuel injector can also have a significant influence on fuel atomization and dispersion. When the proper design of piston-cup, fuel injector, and intake system is achieved, regions within the combustion chamber where soot or NOx are produced can be eliminated," Ramadan said.

Another theory involves the intake system and creating turbulence. "There always needs to be some sort of a swirling motion in the cylinder to help the fuel and air mix, Ramadan said, "traditionally, that is considered wasteful because energy is being wasted by spinning the air. In addition, the combusted gases lose more heat through the cylinder walls because turbulence enhances convective heat transfer, but without swirl, the fuel and air will not mix very well, and this is one of the challenges too."

"We are trying to address the problem from within the cylinder," said Ramadan. He is simulating a number of scenarios, including: adding a pre-chamber and using two injectors, multiple injection events with the same injector and creating more turbulence in the cylinder.

The pre-chamber is not a new idea, what is new is using two injectors, one to inject in the pre-chamber and one to inject in the main chamber or piston bowl, two injections one after another. As a piston is moving up, air is being compressed and forced to flow into the pre-chamber creating a swirling motion inside, and as a result of fuel injection and combustion in the pre-chamber, a jet of air flows into the main chamber creating turbulence in the main chamber. It is hoped that turbulence can be created before fuel is injected into the main chamber to improve the mixing of fuel and air before combustion occurs.

Using multiple injection events with the same injector, injections can be made early in the compression stroke and at millisecond intervals throughout the stroke to mix properly. "When the fuel is injected early not all of it is injected, and there is not enough fuel to initiate combustion. Additional injections will bring the total amount of fuel to the minimum flammability limit. This process allows the fuel to thoroughly mix with air to reduce soot," said Ramadan.

"We have a good understanding of the physics, what we hope to gain through these simulations is additional understanding of the physics and the interactions between the air and the fuel evaporation, mixing, and combustion so that we can come up with strategies to reduce both NOx and soot emissions," Ramadan said.

"These strategies could be one of the above or a combination of them or some new ones that we might develop in the future," he said. "In some cases we know what we need to do, but how to achieve it in an engine cylinder is the challenge."

Written by Dawn Hibbard