HEAD 4N0351 - Caterpillar

Clean Emissions Module (CEM)
Illustration 1 g06239572
(1) Diesel Exhaust Fluid (DEF) injector
(2) Selective Catalytic Reduction (SCR) system
(3) SCR mixing tube
(4) Diesel Oxidation Catalyst (DOC)
(5) Diesel Particulate Filter (DPF)
(6) Exhaust intake connection
(7) Exhaust outlet connectionClean Emissions Module Basics
A flexible exhaust pipe connects the engine to the Clean Emissions Module (CEM). The CEM contains the Diesel Oxidation Catalyst (DOC), Diesel Particulate Filter (DPF) and the Selective Catalytic Reduction (SCR) systems. The CEM contains several mechanical and electrical components that reduce various exhaust emissions emitted from the engine. All the systems on the CEM are controlled by the engine ECM. Exhaust gases entering the CEM flow through the DOC first, then the DPF and SCR.
Illustration 2 g06239584
(1) SCR Outlet Temperature
(2) Exhaust Differential Inlet Port
(3) Exhaust Differential Outlet Port
Illustration 3 g06258346
(4) DOC Inlet Pressure Port
(5) DOC Outlet Pressure Port
(6) DPF Inlet Temperature
(7) DOC Inlet Temperature
Illustration 4 g06239590
Air Flow through CEM Package
The first filter media that exhaust gas is exposed to is the DOC, a flow through style filter. The DOC oxidizes hydrocarbons, carbon monoxide, and soluble organic fractions as the exhaust gas flows through the filter. There are two temperature sensors, one at the inlet of the DOC canister, and one at the outlet. These two sensors are used to make sure DOC is hot enough to oxidize efficiently. The DOC outlet temperature is also used by ECM to know that the exhaust leaving the DOC, and entering the DPF, is hot enough for regeneration to begin. There also two ports on the DOC that can be used to hook up a delta pressure sensor for troubleshooting.Once the exhaust gas flows through the DOC, the exhaust gas enters the SCR mixing tube where Diesel Exhaust Fluid (DEF) is injected for the SCR. Before the SCR, exhaust gas enters the DPF. The DPF is a catalyzed ceramic filter which traps particulate matter (soot). The trapped soot is cleaned from the DPF through a catalytic reaction by heating the filter through a process called regeneration. The inlet section of the DPF cannister contains the DPF inlet pressure sensor, and DPF delta pressure sensor. The DPF Inlet Pressure sensor is used to measure the backpressure being generated by the DPF. The delta pressure sensor is used to measure soot collected in the DPF. The delta pressure sensor is measuring the pressure drop across the DPF. Since the delta pressure sensor is measuring flow resistance across the DPF, the sensor will also detect ash loading.After the exhaust gas flows through the DPF, the gas enters the SCR. The DEF injected prior to the DPF, mixes with exhaust gas in SCR to reduce Nitrogen Oxides (NOX). DEF contains deionized water and urea, which turns to ammonia when heated. The ammonia reacts with the SCR catalyst to convert NOX into harmless nitrogen and water vapor. At the inlet of the SCR cannister is the SCR inlet temperate sensor. This sensor is used to make sure that the temperature is hot enough in the catalyst for DEF dosing to occur.Engine Throttle Valve and HC