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Exhaust Gas Recirculation EGR Systems

. . Chapter 27. Exhaust Gas Recirculation (EGR) Systems. Automotive Fuel and Emission Control Systems. Exhaust Gas Recirculation (EGR) Systems Exhaust gas recirculation (EGR) is an emission control that lowers the amount of nitrogen oxides (NOx) formed during combustion. In the presence of sunlight, NOx reacts with hydrocarbons in the atmosphere to form ozone (O3) or photochemical smog, an air pollutant..

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Exhaust Gas Recirculation EGR Systems

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    4. CONTROLLING NOX The amounts of NOX formed at temperatures below 2,500°F (1,370°C) can be controlled in the exhaust by a catalyst. To handle the amounts generated above 2,500°F (1,370°C), the following are some methods that have been used to lower NOX formation: Enrich the air-fuel mixture. Lower the compression ratio. Dilute the air-fuel mixture.

    7. CONTROLLING NOX

    8. EGR SYSTEM OPERATION Since small amounts of exhaust are all that is needed to lower peak combustion temperatures, the orifice that the exhaust passes through is small.

    9. POSITIVE AND NEGATIVE BACK PRESSURE EGR VALVES Many EGR valves are designed with a small valve inside that bleeds off any applied vacuum and prevents the valve from opening. Some EGR valves require a positive back pressure in the exhaust system. This is called a positive back pressure EGR valve. On each exhaust stroke, the engine emits an exhaust “pulse.” Each pulse represents a positive pressure. Behind each pulse is a small area of low pressure. Some EGR valves react to this low pressure area by closing a small internal valve, which allows the EGR valve to be opened by vacuum. This type of EGR valve is called a negative back pressure EGR valve

    10. POSITIVE AND NEGATIVE BACK PRESSURE EGR VALVES

    11. COMPUTER-CONTROLLED EGR Many computer-controlled EGR systems have one or more solenoids controlling the EGR vacuum. The computer controls a solenoid to shut off vacuum to the EGR valve at cold engine temperatures, idle speed, and wide-open throttle operation.

    12. EGR VALVE POSITION SENSORS Late model computer-controlled EGR systems use a sensor to indicate EGR operation. On-Board Diagnostics Generation-II (OBD-II) EGR system monitors require an EGR to determine EGR gas flow. A linear potentiometer on the top of the EGR valve stem indicates valve position for the computer. This is called an EGR valve position (EVP) sensor.

    13. EGR VALVE POSITION SENSORS

    14. EGR VALVE POSITION SENSORS Digital EGR Valves Unlike the previously mentioned vacuum-operated EGR valves, the digital EGR valve consists of three solenoids controlled by the PCM.

    15. EGR VALVE POSITION SENSORS Linear EGR Most General Motors and several others vehicles use a linear EGR that contains a pulse-width modulated solenoid to precisely regulate exhaust gas flow and a feedback potentiometer that signals to the computer the actual position of the valve.

    17. OBD-II EGR MONITORING STRATEGIES

    18. OBD-II EGR MONITORING STRATEGIES

    19. DIAGNOSING A DEFECTIVE EGR SYSTEM If the EGR valve is not opening or the flow of the exhaust gas is restricted, then the following symptoms are likely: Ping (spark knock or detonation) during acceleration or during cruise (steady-speed driving) Excessive oxides of nitrogen (NOX) exhaust emissions If the EGR valve is stuck open or partially open, then the following symptoms are likely: Rough idle or frequent stalling Poor performance/low power, especially at low engine speed To check for proper EGR flow, the engine vacuum should drop 6 to 8 in. Hg when the EGR is commanded on by a scan tool when the engine is at idle speed.

    20. BASIC EGR SYSTEM (5-8)

    21. EGR VALVE

    22. EGR VACUUM MODULATOR (5-12)

    23. CUT-OFF CONTROL EGR SYSTEM

    24. EGR CUT-OFF

    25. EGR VALVE POSITION SENSOR

    26. EGR DEFECTS Restricted or Blocked Passageways Vacuum Supply PCM Control Malfunction Mechanical Malfunction Back Pressure Transducer Malfunction Insufficient Exhaust Back Pressure Check service manual information for specifications useful for determining sufficient EGR flow. For example, some manufacturer’s will specify an engine speed drop when a prescribed amount of vacuum is applied to the EGR valve. Check vacuum supply for obstructions or leaks. A seemingly defect free hose can sometimes be porous enough to leak off a small amount of vacuum affecting valve operation. A spare hose is helpful for diagnosing such problems A binding EGR valve can result in insufficient EGR flow Insufficient exhaust system backpressure can reduce the efficiency of backpressure controlled EGR valves. Look for modifications that increase exhaust flow.Check service manual information for specifications useful for determining sufficient EGR flow. For example, some manufacturer’s will specify an engine speed drop when a prescribed amount of vacuum is applied to the EGR valve. Check vacuum supply for obstructions or leaks. A seemingly defect free hose can sometimes be porous enough to leak off a small amount of vacuum affecting valve operation. A spare hose is helpful for diagnosing such problems A binding EGR valve can result in insufficient EGR flow Insufficient exhaust system backpressure can reduce the efficiency of backpressure controlled EGR valves. Look for modifications that increase exhaust flow.

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