Exhaust-gas throttle: with the right control, it can boost engine efficiency.
They are looking at a two-zone combustion system in which air is held in the center of the cylinder and is surrounded by recirculated exhaust gases, which are all but void of oxygen. An intake system that can control the balance between the air and exhaust controls the amount of oxygen in the cylinder, and can replace a throttle.
During intake, the piston's work to reach the end of its stroke increases with the throttling of the airstream. As intake is restricted, resistance to the stroke increases. The pumping takes some of the engine's power.
If throttling results from the balance of the gases, however, their total volume remains consistent, and the engine avoids pumping losses. Fuel economy could improve significantly, according to Bassem Ramadan, an associate professor of mechanical engineering who heads the project at the university in Flint, Mich.
The key is in keeping the gases separate.
"Some results look promising, but further work is needed to determine if the two regions will remain unmixed during the engine operating cycle," he said. While complete stratification would be ideal, he expects that maintaining partial stratification of about 80 percent during the compression stroke is acceptable.
Exhaust gas recirculation, or EGR, has been used for more than three decades to curb the formation of nitrogen oxides in automobile engines. In a cylinder full of air, some of the oxygen reacts with the hydrogen and carbon in the fuel, and there is usually plenty left over to combine with atmospheric nitrogen or with sulfur in the fuel. Oxides of nitrogen and sulfur are culprits in acid rain, which spoils bodies of water and can dissolve the stone of monuments. Various oxides of nitrogen can settle on the hemoglobin in the blood, or form nitric acid in the lungs, which can lead to damage of the breathing passages.
Bringing exhaust gas back to the cylinder reduces the excess supply of oxygen.
To maintain air in the center of the cylinder and exhaust gas in the outer layer, the researchers have air flowing through a centrally located intake valve, while exhaust gas flows through ports on the periphery of the cylinder. They have found that placing EGR ports on opposing sides of the cylinder and implementing a helical intake system triggers a swirl motion that helps sustain stratification of the gases.
Ramadan and a team aided by Environmental Protection Agency engineers are simulating the dynamics of two-zone combustion in KIVA-3V, a computational fluid dynamics computer code developed at Los Alamos National Laboratory in New Mexico. It is "tailored for engine applications," according to David Torres, whose job at the lab includes work on updating the software.
Researchers initiate the simulations at the beginning of the intake stroke. The withdrawal of the piston produces a vacuum in the cylinder, sucking air in through the center valve and exhaust gas through the peripheral ports. The simulations continue through compression. The group varies engine parameters--such as bore, stroke, squish, rpm, valve timing, and port geometry--to determine the best configuration for achieving stratification.
CFD calculations run on an IBM supercomputer. Results are sent to another computer on the network, a Silicon Graphics Octane workstation. There the files are processed by EnSight, visualization software from CEI of Apex, N.C., to create a model from the results. A typical model contains about 500,000 cells.
Researchers have established that two zones in a combustion system can be generated and maintained, but mixing will be difficult to control. Generally, cylinders with four exhaust gas recirculation ports have shown the most promising results.
In one model, air mass fraction contours were plotted at the point where the piston bottoms out at the end of the intake stroke. Results showed a maximum concentration of exhaust gas in the air layer of 30 percent.
The project is funded by the U.S. Environmental Protection Agency National Vehicle and Fuel Emissions Laboratory in Ann Arbor, Mich. Work started in May 2003 and will last for a total of two years.
This article was prepared by staff writers in collaboration with outside contributors.
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|Date:||Jun 1, 2004|
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