0398340 JOHNSON CARBURETOR AY

Illustration 1 g06258865
Diagram of the air/fuel ratio control
Required Inputs from the Customer
to control the air/fuel ratio, the control system depends on these inputs from the customer:
Fuel QualityFuel Quality
Before the initial start-up, a current gas analysis is required. Periodic gas analyses are also recommended.Data from the gas analysis must be entered into Caterpillar Software , "Gas Engine Rating Pro (GERP)". The results are programmed into the ECM.Note: Is important to use the Caterpillar Software , "Gas Engine Rating Pro (GERP)". Use of only the data from the gas analysis can result in incorrect settings.The ECM must have values for the following parameters. The values are entered via the "Configuration" screen of Cat ET:
"Fuel Quality"
"Gas Specific Gravity"
"Fuel Specific Heat Ratio"A fixed LHV will be entered in the "Fuel Quality" parameter for engines that run on a consistent fuel supply.The ECM uses the LHV to help determine the fuel command to the fuel metering valve.The fuel command is sent to the fuel metering valve via the CAN data link.The ECM provides the following information to the fuel metering valve via the CAN data link:
"Fuel Specific Heat Ratio"
"Gas Specific Gravity"Note: For more details on these parameters, refer to Systems Operation/Testing and Adjusting, "Electronic Control System Parameters".Mode of Operation
The air/fuel ratio control has two basic modes of operation:
Open loop
Emissions feedbackThe mode of operation is determined by the "Emissions Feedback Mode Configuration" parameter. Cat ET is used to change the mode of operation.Select "Disable" to activate the open loop mode.Select "Enable" to activate the feedback mode.Open Loop Mode
The open loop mode of operation uses no feedback.During operation in this mode, the ECM controls the air/fuel ratio with maps and with calculations for the desired air/fuel ratio.The Air/Fuel ratio is controlled in the open loop mode from startup until the catalyst inlet temperature reaches 180C. The system will remain in closed loop mode until the engine is shutdown.The ECM uses a Fuel Correction Factor (FCF) to help determine the fuel command to the fuel metering valve. During operation in the open loop mode, the FCF is always 100 percent.The ECM uses the LHV to help determine the fuel command to the fuel metering valve.The LHV can be varied in the customer programmable "Fuel Quality" parameter to affect the air/fuel ratio.Refer to Systems Operation/Testing and Adjusting, "Air/Fuel Ratio Control - Adjust" .O2 Feedback
During operation in the feedback mode, the ECM continues to calculate the desired air/fuel ratio. The result is still used to calculate the fuel command.The ECM uses the following to calculate the desired O2 factor.
"Desired Emissions Gain Adjustment Percentage"
Engine load
Engine speed
The measured 02 level in the exhaust.Exhaust 02 concentration is communicated to the ECM from the 02 sensor via the CAN data link.The customer programmable "Desired Emissions Gain Adjustment Percentage" can be varied to affect the air/fuel ratio.The calculation for the desired 02 level is used to adjust the FCF.Because some of the factors for calculating the desired 02 level will vary during operation, the FCF will also vary during operation.The ECM determines the fuel command to the fuel metering valve. The command is sent to the fuel metering valve via the CAN data link.