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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.

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.