G3408C and G3412C Engines Caterpillar

The engine electronic control group for the air/fuel ratio is a digital control for use with low emission engines that use natural gas with one gas pressure regulator and a carburetor. This system can be configured in the field with the keypad that is on the control module. The settings of the control module and the system diagnostics are designed to control the fuel system in order to maintain operation at reduced emission levels. The system consists of five basic components:

• Control module

• Manifold air pressure sensor

• NO_X Sensor

• Speed sensor (magnetic pickup)

• Fuel valve

Illustration 1	g01386787
(1) Control module (2) Manifold air pressure sensor (3) NOX sensor (4) Speed sensor (magnetic pickup) (5) Fuel valve

Control Module

Illustration 2	g01417215
Control module (6) "POWER" indicator (7) "ALARM" indicator (8) Display (9) Keypad

The inlet manifold air pressure is a measure of engine load. The exhaust NO_X level is a measure of the exhaust emissions. The engine speed is monitored by a speed sensor. The control uses the information from these inputs plus the target NO_X settings in order to determine the desired emissions setpoint. The control uses the information from these inputs in order to maintain engine operation at that setpoint.

The system is separate and independent from all of the other engine control systems. When the system is ON, "POWER" indicator (6) is illuminated. The engine can be run with the system in a manual mode or an automatic mode.

If the system detects a diagnostic condition, "ALARM" indicator (7) is illuminated. If the system is operating in the manual mode, or if a diagnostic condition has been acknowledged, the "ALARM" indicator (7) blinks.

Display (8) informs the operator of the status of the system. The manifold pressure and the level of the NO_X sensor are continuously updated on the display in both the manual mode and the automatic mode. The NO_X sensor must be active and ready.

The mode is selected with keypad (9). The display and the keypad are the user interface for the air/fuel ratio control. The operator uses the display and the keypad for several functions. The display and the keypad are used for adjustment of the system parameters and for calibration of the NO_X sensor.

When the system is in the automatic mode, the control module moves the fuel valve in order to control the emissions. The automatic control is based on the closed loop feedback for the part per million (ppm) of NO_X in the exhaust. The control adjusts the position of the fuel valve in order to maintain the desired exhaust NO_X level. The fuel valve is adjusted according to the engine speed, the engine load, and the target NO_X settings.

When the system is in the manual mode, the fuel valve remains in a fixed position. The engine's exhaust emissions can change as the load, the speed, or the ambient conditions change. The manual mode can be useful for adjusting the system and for investigating diagnostics. The manual mode can also be used in case of a problem with a sensor.

Manifold Air Pressure Sensor

The manifold air pressure sensor is mounted in the engine's air inlet manifold. The sensor measures the engine's absolute inlet manifold air pressure. The control uses the information to calculate the engine load. The three-wire sensor has a pulse width modulated signal (PWM) with a width of 500 HZ. The duty cycle is measured by the control and the signal is displayed in absolute kPa.

NO_X Sensor

The NO_x sensor is mounted above the exhaust elbow. The sensor indicates the concentration of NO_x in the engine's exhaust. The sensor incorporates a heater and a heater control. Power for the sensor is provided by the control module's output switch. The control only provides power to the sensor when engine operation is detected. The output switch limits the current to five amperes. The switch is protected from excessive current with a shutdown mode that is activated by thermal overload.

The control module communicates with the sensor by a CAN data link. The CAN data link reads the concentration of NO_X in parts per million (PPM). The CAN data link is used to activate the sensor when engine operation meets the following conditions:

• The engine speed is greater than 1100 RPM.

• The inlet manifold pressure is greater than 75 kPa.

Note: The above two values are default values that can be adjusted.

If there is a problem with the NO_X sensor, a diagnostic status or an alarm message will display the appropriate diagnostic message.

Speed Sensor

A speed sensor monitors the engine rpm. The signal from the speed sensor is used to determine an engine condition for activating the NO_X sensor. The signal from the speed sensor is also used by the control during start-up to determine an offset value to the NO_X target.

Fuel Valve

The fuel valve is located between the gas pressure regulator and the carburetor. The valve improves on the function of the traditional load control valve. The valve is motorized. The motor uses a range of steps that represent the position of the valve. The range of the steps is 0 to 1700. Zero steps represent the fully open position or the richest position. 1700 steps represent the most restricted position or the leanest position.

The control sends a command to the motor in order to move the fuel valve. The control keeps track of the number of steps that are taken. Rather than using a position sensor, the control's tracking provides information on the valve's position.