New Technology Solves Problems of Misfire in Lean-Burn Natural Gas Engines
6 May 1999
New Technology Solves Problems of Misfire in Lean-Burn Natural Gas EnginesSAN ANTONIO, May 6 -- A patent issued to engineers in SwRI's Engine and Vehicle Research Division provides technology that will help manufacturers and operators of lean-burn, natural gas engines solve the problem of misfire, which affects engine performance in regions or seasons of high humidity (defined as dew point temperatures above 15 degrees C). Most natural gas engines operate close to their "lean" limit to maximize engine efficiency and reduce emissions. High humidity, however, reduces the rate of combustion in these highly boosted engines and increases the likelihood of misfire, which raises emission levels of unburned hydrocarbons and carbon monoxide as well as degrades engine driveability and efficiency. Research leading to the patent 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 cooperation with the Deere Power Systems Group, the Blue Bird Corporation, and the CNG Cylinder Company. "As a result of the extensive engine mapping necessary to determine the successful limits of operation for the school bus engine, we were able to identify a relationship between atmospheric humidity levels and engine performance and emissions characteristics," says Principal Engineer John Kubesh. "There was some understanding of these problems before testing," he continues. "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 establish that a natural gas engine that has been calibrated for operation in a dry winter climate such as the southwestern United States would have trouble functioning efficiently on the Gulf Coast during the summer months. 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 trigger a shift to a leaner mode which increased misfire. An algorithm solves these problems by dictating a series of adjustments to the oxygen sensor calibration and the engine's fueling, spark timing, and boost pressure setpoint tables based on the humidity of the ambient air. The team found that implementing these solutions alleviated many of the misfire problems and, with proper adjustments, could minimize the emissions levels. "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 U.S. Patent No. 5,735,245, "Method and Apparatus for Controlling Air/Fuel Mixture in a Lean-Burn Engine," are Staff Engineer Lee G. Dodge and Principal Engineer Daniel J. Podnar.