Southwest Research Institute (SwRI) News

Southwest Research Institute® technology solves problems of misfire in lean-burn natural gas engines

San Antonio, Texas -- April 9, 1999 -- A patent recently issued to engineers in Southwest Research Institute's (SwRI^®) Engine and Vehicle Research Division provides technology that will help manufacturers and operators of lean-burn, natural gas engines solve the problem of misfire that can affect engine performance in regions or seasons of high humidity (defined as dewpoint temperatures above 15°C).

Most natural gas engines are designed to operate close to their "lean" limit to maximize engine efficiency and reduce emissions. Higher humidity levels, however, reduce the rate of combustion in these highly boosted engines and increase likelihood of misfire thereby raising emission levels of unburned hydrocarbons (HC) and carbon monoxide (CO) as well as degraded engine driveability and efficiency.

U.S. Patent No. 5,735,245, titled "Method and Apparatus for Controlling Air/Fuel Mixture in a Lean-Burn Engine," describes a series of practical and effective humidity compensation and engine control methods to address these problems.

Research leading to the patent award evolved from a project to design a prototype, low-emissions, school bus engine powered by natural gas. The project was sponsored by the National Renewable Energy Laboratory in Golden, Colorado, in cooperation with the Deere Power Systems Group, Waterloo, Iowa, a leader in diesel engines; the Blue Bird Corporation, Fort Valley, Georgia, a leading manufacturer of school buses; and the CNG Cylinder Company of Long Beach, California.

"As a result of the extensive and necessary engine mapping to determine the successful limits of operation for the school bus engine," says Principal Engineer John Kubesh who was project manager, "we were able to identify a clear relationship between atmospheric humidity levels and engine performance, and emissions characteristics."

"There was some understanding of these problems before our tests," continued Kubesh. "For example, both the Society of Automotive Engineers and the Environmental Protection Agency use correction factors that take into account the effect of humidity on gasoline and diesel engine performance and emissions measurements, but these corrections are relatively minor in terms of overall performance measurements. The effects of humidity on the performance of natural gas engines are far more profound."

A water injection and atomizer system was constructed in a test cell and experiments were conducted to measure the effects of different degrees of humidity on the engine, including its lean misfire limit and rate of combustion. As a result, the SwRI team was able to clearly establish that a natural gas engine that has been calibrated for operation in a dry winter climate such as the southwestern United States would clearly have trouble functioning efficiently during summer on the Gulf Coast.

An unexpected discovery was that the output of the oxygen sensor was affected by varying humidity levels. In particular, increased humidity caused the electronic control module to "think" the engine was running too rich and therefore triggered a shift to a leaner mode which increased misfire.

To solve these problems, the patent describes an algorithm that dictates a series of adjustments to the oxygen sensor calibration and the engine's fueling, spark timing, and boost pressure setpoint tables based on the specific humidity of the ambient air. The team found that implementing these solutions answered many of the misfire problems and with proper adjustments the emissions levels can be minimized.

"Geographical and seasonal variations in ambient moisture should be taken into account by engine developers when considering a specific engine application," adds Kubesh. "This is a useful piece of information for us as developers and for the natural gas engine industry in general."

Other inventors named in the patent award were Staff Engineer Lee G. Dodge and Principal Engineer Daniel J. Podnar, both of SwRI's Engine and Vehicle Research Division.

Download LEANMIS.TIF as a tif file suitable for publication quality printing (1.8MB).

For more information about lean-burn, natural gas engines, contact Deborah Deffenbaugh, Communications Department, Southwest Research Institute, P.O. Drawer 28510, San Antonio, Texas 78228-0510, Phone (210) 522-2046, Fax (210) 522-3547.

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