Development of the GM 5.7L CNG Bi-Fuel Pickup Trucks 980817
A CNG bi-fuel version of the GMC and Chevrolet C2500 pick up trucks was developed as an OEM (Original Equipment Manufacturer) vehicle. The converted bi-fuel vehicles use the same engine with modifications for use with gaseous fuels, and the same catalytic converters as the gasoline baseline. The fuel management system is an automatic switching bi-fuel system which is able to control fuel flow rate, spark timing, EGR, and perform OBD-II (On-Board Diagnostics II) on both gasoline and CNG. This system has been extensively tested and validated for durability, electro magnetic compatibility requirements, crash integrity, corrosion resistance, emissions performance and powertrain performance under extreme environmental conditions and different CNG fuel compositions. The CNG fuel storage technology of the bi-fuel system was validated to the most stringent safety and durability requirements in the industry.
A computer engine simulation model, which is able to predict engine performance, fuel economy, and exhaust emissions, was used to assist mapping and calibration of spark, fuel, and EGR tables, and control of catalyst temperature. Calibration and control strategies for the CNG bi-fuel vehicle are also discussed in this paper. Regulated exhaust emissions of both converted and baseline vehicles operating on FTP (Federal Test Procedures) urban cycle are presented. It was found that the NMOG HC and CO emissions of the vehicle operating on CNG are significantly lower than those operating on gasoline, while the NOx emissions were higher than those operating on gasoline. The CNG fuel economy on a gasoline equivalent energy basis is slightly better than that of the gasoline baseline.