825231A02 Mercury GASKET SET, Engine

Illustration 1 g06471531Reference: For more information, refer to supplemental video "C175 Tier 4 Final Engines for Off-Highway Trucks Dosing Control System Operation" on Caterpillar Channel1.Note: Click or copy the following link into a web browser (a CWS login is required to access Caterpillar Channel1), or scan the following QR code using a QR enabled device.https://channel1.mediaspace.kaltura.com/media/1_6oncmpbb
Illustration 2 g06451134General Operation
Illustration 3 g06177323
Not all component quantities are shown. Aftertreatment system may contain two CEM packages.
(1) Exhaust inlet
(2) Inlet Nitrogen Oxide (NOx) sensor
(3) Diesel Exhaust Fluid (DEF) injector
(4) Catalyst brick
(5) Outlet NOx sensor
(6) Exhaust out
(7) DEF supply line
(8) Coolant return to engine line
(9) DEF pump
(10) Coolant supply from engine line
(11) Coolant diverter valve
(12) DEF tank
(13) Coolant supply connector
(14) Suction connector
(15) Backflow connector
(16) Coolant return connector
(17) DEF suction line
(18) DEF backflow line
(19) Dosing Control Unit (DCU)
(20) Aftertreatment Electronic Control Module (ECM)
(21) DEF backflow valveThe purpose of Diesel Exhaust Fluid (DEF) is to create a Selective Catalytic Reduction (SCR) process to take place in the exhaust system to reduce Nitrogen Oxides (NOx).When the engine is keyed on but not running, DEF is not being injected into the system. When the engine is running, the DEF may be too cold or frozen to be injected. When the ambient or tank temperatures are cold enough, the DEF control unit (DCU) opens the coolant diverter valve to allow engine coolant to flow through the DEF tank to warm the system. The DCU also activates the heater circuits in the lines and pump to prevent DEF from freezing in the lines and pump. The time it takes to thaw the system depends on the tank temperature. Since the system always starts with empty lines, the pump activates to fill the lines with fluid before DEF injection begins.Once the system is thawed and primed, the DEF will be injected into the exhaust system when the DEF tank and SCR temperatures are warm enough. If the system does not heat up after duration of time, a diagnostic code will become active.Note: Typical usage for DEF is 1-3% of diesel fluid. The DEF tank should be refilled at the same time as the fuel tank.The DEF system has three operation modes:1. DEF Priming ModeSince the system always starts with empty lines, the pump activates to fill the lines with fluid before DEF injection begins. Priming will start when engine speed is greater than 450 rpm, DEF tank temp is greater than −6° C (21.2° F), and SCR inlet temperature is greater than 100° C (212° F). Once the system reads the appropriate pressures, priming is complete.2. DEF Injection ModeOnce the system is primed, the DEF will be injected when SCR inlet temperature is greater than 200° C (392° F). The DEF pump draws DEF from the tank and pressurizes the DEF to 900 kPa (130.5 psi). This pressure is constant throughout the operating cycle. The DEF injector sprays DEF into the exhaust before the SCR catalyst and continues spraying by pulsing on and off. The DEF injection rate is controlled by changing the duration of the DEF injector on time. If the engine is producing higher NOx, the DEF injector will turn on more frequently. To control the pressure to the DEF injector, a percentage of DEF will flow through the backflow line back to the tank.3. DEF Purge ModeWhen a DEF-related fault code becomes active or the engine is keyed OFF, the DCU enters purge mode. The system is designed to purge all DEF back to the DEF tank to prevent the DEF from freezing in the pump and lines. The DEF injector opens and the DEF Reverting valve in the DEF pump reverses the flow of the DEF inside the pump. This reversal removes the DEF from the pressure line and pumps the DEF back to tank through the suction line. The purge takes approximately 2 minutes to remove DEF from the lines and pump. DO NOT turn off the battery disconnect switch during the purge process. If the purge process does not complete and the temperature of the DEF drops below −11° C (12° F), then the pump and lines are at risk of damage. If the purge process does not complete, a diagnostic code will become active.NOx MonitoringThere are four NOx sensors supplied with the engine. Two NOx sensors are installed in the exhaust inlet from the turbocharger (CEM inlet). The other NOx sensors are installed in the exhaust pipe of the application (CEM out).The amount of DEF needed is based on NOx production. The engine produces NOx as the engine operates at a specific target to meet emission requirements. The engine out NOx sensor measures the amount and communicates that value to the 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 DEF will adjust to meet the NOx allowed per regulations.NOx Sensors
Illustration 4 g03364155
(21) Electrical connector
(22) Sensor electronic control unit
(23) Cable (black for engine out, gray for CEM out)
(24) Sensing unitThe 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 Ammonia (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