1995-08-01

Dual Fuel CNG Retrofit for Mexican Public Transit 951915

An open loop carbureted GM 5.7L engine has been converted to operate either as carburetor gasoline, or as a closed loop controlled compressed natural gas (CNG) fueled system. In the gasoline mode, fuel delivery is made through the stock four barrel carburetor, however, the stock mechanical advance ignition distributor has been replaced with later model utilizing electronically controlled spark advance. In the CNG fuel mode, stoichiometric fuel delivery is achieved with the use of two Servojet HSV injectors mounted in a central distribution center and controlled by a microprocessor. Closed loop control is obtained through the use of a stock GM heated gasoline oxygen sensor.In order to provide proper distribution of the CNG fuel with the intake air with the use of only two injectors, a constant flow multipoint injection system was developed to assure that each cylinder would receive equivalent masses of fuel. In this system, the fuel at 1.4 MPa absolute pressure first passes through the choked flow gas injectors into a central distribution plenum and is reduced to a minimum pressure of 0.7 MPa. The CNG fuel leaves the plenum through eight individual converging nozzles which direct the gas to each respective intake port. Tests demonstrate the systems ability to provide an even distribution of fuel to each intake port over a wide range of flow rates.The gasoline oxygen sensor employed in this study provides a lean biased signal when used to indicate stoichiometry in a CNG produced exhaust mixture. To correct this error on the feed back loop, the proportional-integral control algorithm was modified to correct for the true stoichiometric exhaust air-fuel ratio.

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