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 Engines SAN 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.
