A fuel and engine management system have been successively applied to a 4.5L spark-ignition gaseous-fueled engine used for stationary power generation applications up to 80 kW. The system operates on low-pressure commercial grade natural gas or liquid propane gas and governs engine speed at either 1500 or 1800 rpm, for generator frequencies of 50 Hz and 60 Hz respectively. The fuel and engine management system consist of an engine control module, an electronically controlled fuel mixer, a fuel pressure sensor, and other supporting sensors/actuators. This new system replaces a legacy of fixed-orifice metering or vacuum-actuated-valve mechanical mixer designs. This new system allows for closed-loop control of stoichiometric combustion that meets both performance and emissions requirements without the need for a fuel pressure regulator within the genset frame. It also allows a single fuel system assembly to be used on all 4.5L engines regardless of fuel type (LP/NG) or nominal speed (50/60Hz), as these are now selected with standard J1939 CAN protocol messages from the genset controller. This paper reviews the development aspects for the new fuel system on the 4.5L engine.
The electronic-actuated fuel mixer system offers a wider range of fuel flow control compared to mechanical-actuated mixers, this results in better air-fuel control due to variations in fuel quality, low fuel supply pressure and changing ambient conditions. The range of fuel control is result of the different mixer valve construction and control. The fuel metering valve is a butterfly type and is controlled from a stepper motor. The valve controls the fuel flow rate just before the fuel mixes with the combustion air passing thought a venturi. Lag in fuel flow, typical of low supply pressure and low restriction / signal venturi, is corrected for using a feedforward control strategy based on genset electrical load. The mixer sizing and flow bench measurements for calibration, will be reviewed.
The ECM uses a speed-density air flow model, along with fuel supply and air filter pressure, to determine the best mixer valve position. The engine air flow model is used for determining venturi flow and the resulting mixer vacuum signal, but a final calibration correction was created with hot wire-anemometer-flow meters on a running engine. A long-term fuel trim is also applied to the mixer position, but unlike a fuel-injection system which corrects volumetric efficiency based on known fuel flow rate and HEGO sensor feedback, this mixer control strategy learns a steady-state mixer correction or offset. The entire control strategy was developed within a Simulink model and auto code generation tools were used to create final ECM C and machine code. The development of the mixer and ECM control strategy will be detailed herein.