C3.4B Engines for Caterpillar Built Machines Caterpillar

Illustration 1	g03701426
Typical example (1) Exhaust inlet (2) Selective Catalytic Reduction (SCR) system (3) Exhaust out (4) Ammonia sensor (5) Nitrogen Oxide (NOx) sensor (6) Diesel Exhaust Fluid (DEF) injector (7) Diesel Exhaust Fluid (DEF) supply line (8) Diesel Exhaust Fluid (DEF) pump (9) Coolant supply from engine line (10) Coolant diverter valve (11) Diesel Exhaust Fluid (DEF) tank (12) Coolant supply connector (13) Suction connector (14) Backflow connector (15) Coolant return connector (16) DEF suction line (17) DEF backflow line (18) Heater Control Unit (HCU) (19) Electronic Control Module (ECM)

The Diesel Exhaust Fluid (DEF) Dosing Control System consists of the following components.

• DEF pump

• DEF injector

• DEF header

• Coolant diverter valve

• DEF system heated lines

Diesel Exhaust Fluid (DEF) Pump

Illustration 2	g03701378
Typical example (1) Backflow (2) Outlet (3) Inlet (4) Electrical connection (5) DEF filter

The pump supplies filtered DEF fluid to the DEF injector. The pump consists of inlet port (3) which is the suction side of the pump. The pump then pressurizes the fluid up to 500 kPa (73 psi) and supplies the fluid though outlet port (2). There is a fixed orifice within the pump that purges the pressure off the pressurized circuit. This pressure relief will stabilize the pressure in the line when the DEF injector is opening and closing.

Once the engine is shut down, the pump will start to go into a purge mode. The DEF Injector will open and the reverting valve within the pump will reverse the flow of the pump and purge the pump and pressure line. This reversed flow will route any remaining DEF fluid back to the tank.

Refer to Operation and Maintenance Manual for the correct procedure to replace the DEF filter.

Electric Components of the DEF System

The electric components of the DEF system consist of the following components:

Coolant Diverter Valve

Controls coolant flow to the DEF tank.

Heated Lines

The heated lines are electrically heated lines that will turn on any time the ambient temperature is cold. These lines will thaw frozen DEF after starting in cold ambient temperatures. The heated lines will also stay heated during operation to prevent any freezing during operation.

DEF Level Sensor

The level sensor will measure the amount of DEF in the tank. The system uses an ultrasonic level sensor to determine the amount of DEF in the DEF tank. To measure the level of DEF, the sensor measures the distance to the surface of the fluid. The sensor provides a signal to the DCU that will be converted into a "DEF Level Percentage".

DEF Tank Temperature Sensor

This sensor is at the bottom of the tank and is integrated within the DEF Level Sensor.

DEF Tank

The DEF tank stores the DEF. The size of the DEF tank will depend on the application.

The DEF solution is made of 32.5 percent urea solution and 67.5 percent deionized water. Urea is a compound of nitrogen that turns to ammonia when heated.

Ensure that the correct specification of DEF is used. Refer to Operation and Maintenance Manual for more information.

DEF does degrade over time at elevated temperatures. Due to freezing risk, the tank, pump, and lines must be heated and designed to contain frozen DEF.

DEF Tank Header

Illustration 3	g03708818
Typical example (1) Coolant tubes (2) DEF pickup tube (3) DEF tank header filter (4) DEF tank temperature sensor, level sensor, and quality sensor

The DEF tank header is located in the DEF tank. The tank header consists of several parts and performs the following functions:

Coolant Tubes

The coolant supplied by the engine will flow through the tubes when the coolant diverter valve opens. Coolant runs along the pickup tube to the bottom of the tank and then spirals around the level sensor. The heat from the coolant will thaw any frozen DEF in the tank or header. The heat will also prevent the DEF from freezing around the pickup tube.

DEF Pickup Tube

The DEF is pulled from the bottom of the tank through a screen.

DEF Tank Header Filter

At the end of the DEF Pickup tube is a 100 micron filter to filter debris from entering the system.

DEF Return

The DEF is returned back to the tank from the DEF injector.

DEF Tank Temperature Sensor

At the bottom of the tank header near the DEF tank header filter is a tank temperature sensor. The temperature sensor is a passive thermistor. The resistance of the thermistor varies with temperature. This sensor monitors the temperature of the DEF in the tank. This temperature is used to determine when to turn on the coolant diverter valve.

DEF Quality Sensor

At the bottom of the tank header next to the DEF tank header filter is the DEF quality sensor. The ultrasonic DEF quality sensor uses the speed of sound within a fluid to identify concentration of the fluid. This is achieved by measuring the time it takes for the emitted ultrasonic signal to be reflected off a known fixed reference point. Based on the chemical properties and temperature of the fluid, the sensor is able to detect urea concentration with an accuracy of up to 2 percent. If the sensor detects the quality of DEF is not within specifications, a fault code will trip indicating the DEF concentration is not correct. The engine may derate.

Coolant Diverter Valve

Illustration 4	g03690330
Typical example (1) Outlet connection (2) Electrical connection (3) Inlet connection (4) Flow direction indicator

The coolant diverter valve is mounted to the application. The coolant diverter valve is a normally closed valve. The valve is a unidirectional flow valve. The system is used to thaw the frozen DEF fluid in the DEF tank before dosing occurs. Warming the system is achieved by taking warm coolant from the engine and routing the coolant through the DEF tank. The coolant diverter valve will open when the tank temperature needs to be increased. With the coolant diverter valve fully opened, engine coolant will flow to the DEF tank header and then back to the engine. Once the DEF tank temp is warm enough, the system will start dosing. The coolant diverter valve will close to prevent any further heat transfer to the DEF tank. This avoids DEF in the DEF tank overheating. The usable temperature of the DEF is between −10° to 55°C (14° to 99°F).

After the keyswitch is moved to the OFF position, the ECM will cycle the coolant diverter valve open and then closed. The coolant diverter valve is cycled open and then closed to exercise the coolant diverter valve so that the coolant diverter valve does not seize up over time.

DEF Injector

Illustration 5	g03749264
Typical example (1) Coolant return port (2) Electrical Connector (3) DEF Supply line (4) Coolant supply port

The DEF injector is mounted to the Clean Emissions Module (CEM). The DEF injector is a valve that injects DEF as a fine spray into the exhaust gases as an atomized mist after the Diesel Oxidation Catalyst (DOC). The spray pattern being conical for good mixing with the exhaust is critical.

The tip of the injector, that is located in the exhaust flow, is cooled by coolant that flows through an internal coolant passage from the engine coolant supply.

Nitrogen Oxide (NOx) Sensors

There are two NOx sensors supplied with the engine. One NOx sensor is installed before the Diesel Oxidation Catalyst (DOC). The other NOx sensor is installed in the tube assembly after the SCR canister.

The engine produces NOx as the engine operates. The engine out NOx sensor measures the amount and communicates that value to the Diesel Exhaust Fluid Control Unit (DCU). The tailpipe out NOx sensor monitors the NOx level out of the catalyst and communicates that value to the DCU. Based on the engine out and the tail pipe out NOx sensor values, the flow rate of the Diesel Exhaust Fluid (DEF) will adjust to meet the desired tail pipe out target.

Illustration 6	g03403971
Typical example (1) Sensing unit (2) Cable (black for engine out, gray for CEM out) (3) Sensor electronic control unit (4) Electrical connector

The NOx sensor contains a sensing element, a harness, and an electronic control unit. The sensing element is composed of two chambers and a heater. The first chamber measures the amount of oxygen and the second chamber measure the amount of NOx and NH₃. Using electrochemistry, a ceramic material attracts ions at approximately 800° C (1472° F) and an electrode on the chamber wall measures the electrical charge in voltage or current. The voltage or current signal is then received by the NOx sensors electronic control unit and interpreted into a NOx concentration. The heater is used to maintain the sensing element temperature, as sensing element is sensitive to moisture. If the sensor sees any moisture while at 800° C (1472° F), then the moisture could quickly cool the element which would cause the element to crack. At engine start-up there is always some moisture within the exhaust pipes. So the sensors will not start working until the exhaust temperature around the sensors is greater than 100° C (212° F) for approximately two and a half minutes.

DEF System Heated Lines

The DEF flows in the heated lines from the DEF tank to the DEF pump. The DEF then travels through another heated line from the DEF pump to the DEF injector. The heated lines are heated by an electrical resistance.

Even though the system has been purged of DEF there are small quantities of DEF that can be left behind. The DEF supply line, suction, and the back flow line are heated to thaw any remaining DEF in the lines. The heated lines will prevent restrictions in the pump and injector once the system begins to dose.

The heated lines have the following characteristics.

• Thermoplastic core tube with fabric reinforcement

• Stainless steel heating wire

• Extruded thermoplastic jacket

• Heat/abrasion shield

• Quick disconnect connector

Heater Control Unit (HCU)

The Heater Control Unit (HCU) is used for control and monitoring of DEF tank and DEF heated lines.

The HCU contains a CAN interface for communication with the Electronic Control Module (ECM).

The power stages inside the HCU are activated based on command from the ECM.

The HCU provides information about diagnostic, currents, and voltages via CAN interface to the ECM.

Cold Weather Operation

Since DEF freezes at −11.5° C (11.3° F), the system can thaw the DEF before dosing.

Engine coolant is supplied to the DEF tank. The coolant diverter valve will turn on anytime the DEF Tank temperature is less the 15° C (59° F) at start-up. During operation, if the DEF tank temperature drops below 20° C (68° F) or ambient temperature falls below 0° C (32° F) then the coolant diverter valve will activate to keep the DEF from freezing. The tubes running through the tank will thaw the DEF to a useable temperature. Then DEF dosing will occur once the DEF has been thawed and the DEF tank temperature is greater than −8° C (18° F). The DEF dosing will not wait for the entire tank to thawed. The DEF pump is heated by an internal heating element. The coolant will then return to the engine water pump inlet. The DEF supply lines, suction lines, and backflow lines are heated electrically.

The DEF dosing system must be fully functional within 70 minutes following initial start-up.

Note: If the DEF has not been removed from the lines and pump during cold-weather operation, the DEF will expand as the fluid freezes and damage the pump. For this reason the purge process must be followed in cold weather. The purge process will take 2 minutes of run time. Do NOT turn off battery power until this pump has completed the purge cycle.