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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.

Liquefied Petroleum Gas (LPG) and Compressed Natural Gas (CNG) have long been recognized as ideal clean and potentially available fuels for motor vehicles. However, some disadvantages of the two gaseous fuels such as lower energy density (kW/l) compared to gasoline, a loss of power in unmodified converted engines relative to gasoline and the current shortage of refueling stations hamper their widespread use. Additionally, engines with throttle-body gaseous fuel fumigation need longer cranking time and exhibit relatively slow response during transient engine operation. To alleviate the above shortcomings, a low cost dual-fuel system was developed. The prototype system uses a mixture of gaseous fuel (Liquefied Petroleum Gas) and gasoline to enhance fuel economy, drivability, and lower emissions. The two fuels can be mixed at any percentage depending on the customer's need.

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.

Adapting the current liquid fuel injection technology to gaseous fuel presents some major technical issues. IMPCO Technologies, Inc. has committed to the development and commercialization of propane and natural gas fuel injection systems that use a unique injector design to precisely control the flow of fuel vapor. Considerable effort was focused on product validation tests including establishing gas flow test stand procedures. The gaseous injector that was developed can be used in propane or compressed natural gas applications. Operating inlet pressures are 18 psig and 60 psig for propane and CNG applications respectively. These injectors have distinct features that allow them to operate reliably over the life of the engine without lubrication from the fuel media.

A group of GM C-2500 heavy-duty and GM C-1500 light-duty trucks were modified to bi-fuel compressed natural gas (CNG)/gasoline and liquefied petroleum gas (LPG)/gasoline vehicles. The fuel management systems used for different model year vehicles were introduced. The Alternative Fuel Technology (AFT) System used for 1997 GM C-2500 trucks is discussed in detail. AFT is an automatic switching bi-fuel system which is able to control fuel flow rate, spark timing, and EGR, and perform On-Board Diagnostics (OBD-II). The exhaust emissions of carbon monoxide (CO), total hydrocarbon (HC) and oxides of nitrogen (NOx) were measured using dilute sampling techniques. The vehicles tested were operated on a chassis dynamometer and run on the Federal Test Procedure (FTP) Urban Schedule.