Some current aftermarket natural gas closed loop carburetion systems use an integral control strategy to maintain a fuel-air equivalence ratio centered in the peak conversion window of a three-way catalytic converter. Fuel control system performance under steady-state engine operating conditions can be characterized by the time-averaged value of the fuel-air equivalence ratio, the rich and lean excursion limits, and a skewness parameter that represents the non-symmetry of the time varying fuel-air equivalence ratio about the control value (ϕaverage). Using a representative aftermarket feedback control system, the effect of these parameters on the exhaust emissions of a natural-gas fueled four-cylinder engine has been investigated. In addition, the effect of EGO sensor characteristics on control system performance has been examined.
It is shown that the average CO levels in the engine exhaust are a function of the rich excursion limit, the average exhaust O2 levels depend on the lean excusion limit, and that average NOx emissions are affected by the amplitude, and to a lesser extent the skewness, of the time variation in fuel-air equivalence ratio. Thus, increasing the amplitude of the fluctuations results in an increase in carbon monoxide emissions, an increase in the exhaust oxygen concentration, and a decrease in NOx emissions from the engine. FID total hydrocarbon emissions were the same as those produced during open loop operation at the same ϕaverage.
A limited range of adjustability of the average fuel-air equivalence ratio was achieved by applying an exponential gain function to the EGO sensor output. The zirconia EGO sensor tested provided a wider range of adjustability than the titania EGO sensor tested.