Going with the flow
Flow visualization helps Kettering researcher Dr. Bassem Ramadan improve catalytic converter emissions.
Designing a better car has nothing to do with the proverbial drawing board anymore, and everything to do with the computer monitor. Right down to the smallest detail on individual parts, computer modeling is taking researchers and engineers places their predecessors never dreamed of being able to go.
Dr. Bassem Ramadan, associate professor of Mechanical Engineering, is working on just such a project with engineers at Delphi. They are researching a way to increase emissions efficiency in catalytic converters using flow visualization. Working in a virtual world, he uses specialized software to model the flow of gases through various new designs of catalytic converters.
Catalytic converters are pollutant-reducing devices currently located in vehicles between the engine and the tailpipe. Hot exhaust gas from the engine flows through the catalytic converter, which contains a catalyst that causes a chemical reaction needed to burn off unburned hydrocarbons and carbon monoxides, turning them into water vapor, carbon dioxide and other less toxic gases.
"The catalytic converter is like a honeycomb," said Ramadan, "there are hundreds of tiny channels clustered like a bunch of straws glued together. This is called the catalyst substrate.
Gases flow through each straw separately. The walls of the substrate are coated with a catalyst, typically a noble metal like platinum, rhodium or palladium, that is needed in order for a chemical reaction to take place," he said.
This reaction is where carbon monoxide is converted into carbon dioxide, nitrogen oxide is converted into nitrogen and oxygen, and unburned hydrocarbons are converted into water vapor:
The catalyst substrate needs to be at a certain temperature for these chemical reactions totake place. "Typically, when a vehicle engine is first started, it will take a few minutes for it to warm up enough to cause the chemical reactions that reduce harmful emissions," Ramadan said.
"During that time the catalytic converter is doing nothing, so about 80 percent of harmful emissions occur within the first two or three minutes after cold start in the FTP-75 test cycle," he said. The figure below shows the conversion efficiency as a function of temperature.
In order to reduce the warm-up time of the catalyst, one method is to put the converter very close to the engine. In most vehicles, the catalytic converter is underneath the passenger area, as shown below:
"Since the engine is the warmest part on an automobile, the converter warms up faster, and when the engine is turned off the catalytic converter remains warmer for a longer period of time," said Ramadan. "Some manufacturers are already doing that, but not many because it is more expensive to do that and there are some issues that still need to be resolved, which is part of the focus of this research project."
"In order to design "close-coupled" converters, the designer may have to resort to truncated inlet and outlet cones, or distorted inlet pipes due to space limitations. Hence, it is very difficult to achieve good mixing of the exhaust gas, and a good flow distribution at the inlet cross section of the substrate.
The results for the catalytic converter design shown below, showed that the gases from Runners 1 and 2 didn't flow as well as the gases from Runners 2 and 3 that are less angled.
"Gases lose energy as they flow," said Ramadan, "and so when they encounter sharp changes in the flow geometry, they don't flow through as well, and you get less flow through these areas of the honeycomb and it doesn't heat up as much. Ideally, it is desirable to achieve a uniform flow through the substrate. This results in uniform heating, and complete utilization of the substrate volume. When the flow is non-uniform through the substrate, then over the long term, parts of the substrate will wear out a lot faster than other areas so reliability and durability are compromised," he added.
Having the catalytic converter close to the engine means the converter will experience a pulsating flow. "Gases from the different cylinders flow through the converter at different times because the flow is controlled by the opening and closing of the exhaust valves as the engine is running," he said.
As a supplier to a number of automakers, Delphi is interested in learning how different catalytic converter designs will impact the flow supply to the catalyst.
"Computer modeling allows us to answer a multitude of questions. Because it is difficult and expensive for manufacturers to build prototypes and test them, computer modeling saves money and time," said Ramadan.
Helping with the research is graduate student Arvind Jujare, of India.
For more information about Ramadan's research on flow visualization in catalytic converters, contact him at firstname.lastname@example.org.
Written by Dawn Hibbard