The Texas Project: Part 2 - Control System Characteristics of Aftermarket CNG and LNG Conversions for Light-Duty Vehicles

The Texas Project involves the conversion of light-duty vehicles, up to and heavy light-duty trucks, to bi-fueled vehicles using commercially available aftermarket CNG and LPG conversion systems. The test fleet includes 68 dual fueled conversions. Virtually every type of aftermarket conversion technology for CNG and LPG was evaluated: eight different CNG and seven different LPG conversion “kits”, all of which are modern systems incorporating closed-loop control. The kits were installed and calibrated according to the manufacturer's guidelines and recommendations. The emissions when operating on the alternative fuel were compared to those when operating on certification gasoline to determine the “success” of the conversion. Many of these conversions, performed according to the manufacturer's requirements, were not “successful” (worse emissions than for gasoline operation). In almost all cases, the problem was NO_x emissions that were too high when operating on the alternative fuel. To aid understanding of this problem, detailed measurements of the exhaust oxygen sensor output and air/fuel ratio control output signal from the alternative fuel computer (when available) were recorded and analyzed. It was found that some of the kits operate too lean and/or have control systems that are too slow. Another potential problem is too much timing advance. For some of the vehicle/kit combinations for which the data indicated that adjustments might be beneficial, recalibrations were performed. Many vehicle/kit combinations showed a benefit from the recalibrations, but some kits did not work on any installations, even after recalibration. It is concluded that some conversion technologies are simply much better than others. It was also found that some vehicles appear to be inherently more difficult to convert. Potential reasons for this characteristic are discussed.