A CNG bi-fuel version of the Chevrolet Cavalier has been developed as an OEM (Original Equipment Manufacturer) vehicle. The fuel management system is an automatically switching bi-fuel system which is able to control fuel flow rate, spark timing, EGR, and perform OBD-II (On-Board Diagnostics II). The system consists of a CNG fuel tank, fuel filter, electric and manual fuel shutoff valves, high and low pressure regulators, gas mass sensor, mixture control valve, gas distribution system, CNG fuel gauge, OEM exhaust gas oxygen sensor, digital engine control unit (ECU), OEM powertrain control module (PCM) and unique wiring harness. This paper discusses the components, operation, and calibration of the CNG bi-fuel management system.
A computer engine simulation model able to predict engine performance, fuel economy, and exhaust emissions, was used to assist spark, fuel, and EGR table mapping. The use of computer modeling reduced system calibration turn-around time as well as the cost of testing. Regulated exhaust emissions of non-methane organic gases (NMOG), carbon monoxide (CO), and Oxides of Nitrogen (NOx) were measured on the FTP Urban Cycle. It was observed that NMOG, CO, and NOx emissions from the 1998 Cavalier operating on CNG were significantly lower than those on gasoline. The fuel economy difference on an energy equivalence basis was found to be small.