Exhaust Gas Recirculation (EGR) System Description

Engine Controls and Fuel - 3.0L LM2 Diesel | Description and Operation | Document ID: 5190633 The exhaust gas recirculation (EGR) system consists of a high pressure EGR valve, a low pressure EGR valve, a low pressure EGR cooler, an exhaust flow control valve, a low pressure EGR differential pressure sensor, and two EGR temperature sensors. The high pressure EGR valve directs exhaust gas from the exhaust manifold to the intake duct upstream of the intake manifold. The low pressure EGR valve directs cooled exhaust gas from the exhaust pipe to the intake duct upstream of the turbocharger. The exhaust flow control valve partially closes to increase exhaust flow to the low pressure EGR valve. The Engine Control Module (ECM) supplies the high pressure and low pressure Exhaust Gas Recirculation (EGR) valves and the exhaust flow control valve with a 5 V reference circuit, a low reference circuit, an H-bridge motor directional control circuit, and a signal circuit. The EGR valve position sensor and exhaust flow control valve position sensor information is transmitted between the valve and the ECM on the signal circuit using serial data. The EGR system is used to reduce the amount of nitrogen oxide (NOx) emission levels caused by high combustion temperatures. At temperatures above 1,371° C (2,500° F) oxygen and nitrogen combine to form NOx. Introducing small amounts of exhaust gas back into the combustion chamber displaces the amount of oxygen entering the engine. With less oxygen in the air/fuel mixture, the combustion pressures are reduced, and as a result, combustion temperatures are decreased, restricting the formation of NOx. Exhaust Gas Recirculation (EGR) System Operation Intake air is mixed with cooled exhaust gas coming from the exhaust gas recirculation EGR valve. The exhaust gas mixes with the intake air to satisfy emissions, performance, fuel economy and cool the combustion process. The Mass Air Flow (MAF) sensor signal is used by the ECM to detect the proper amount of EGR flow. The ECM will close the EGR valve, then open the EGR valve to 100%. The ECM will then calculate the MAF difference and determine if the proper EGR flow has been detected. The recirculate gas also prevents the formation of NOx related gases, refer to as NOx emissions, which are a common cause for failing emissions testing. When a failure occurs both the air intake system and exhaust system should be checked in order to isolate the root cause.