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Exhaust gas Re circulation Systems
The document discusses exhaust gas recirculation (EGR) and its use in internal combustion engines to reduce NOx emissions. It first defines NOx as oxides of nitrogen produced during combustion and describes the mechanisms of NO and NO2 formation. It then discusses factors that affect NOx production and methods to reduce it, including EGR. The document explains the components and working of an EGR system, including different types of EGR control valves. It outlines the advantages of EGR in reducing temperatures and NOx, as well as potential disadvantages in power reduction. Finally, it briefly discusses EGR feedback systems used by engine control modules.
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Exhaust gas Re circulation Systems
- 1. Muhammad Saqlain Haider2011-ME-330 Exhaust gas recirculation and its utilization in IC engines 1/6/20181 University Of Engineering & Technology Lahore, KSK Campus.
- 2. 1/6/20182 Contents Introduction. Whatare NOx?? . Mechanism of NO and NO2 formation. Factors affecting NO emissions. How can NOx be reduced?. Components of EGR system. Working of EGR system. EGR operation schedule. Types of EGR control valves. Advantages and disadvantages of EGR. EGR feed back system.
- 3. 1/6/20184 What are 𝑁𝑂𝑥? Nitrogen N2 and oxygen O2 molecules are separated into individual atoms of nitrogen and oxygen during the combustion process. Formation of NO and NO2. NO and NO2 collectively known as NOx. NO is the predominant oxide of nitrogen produced inside the engine cylinder. Diesel engines contain comparable amount of nitrogen then petrol engines.
- 4. Mechanism of NOformation 1/6/20185 NO forms in both flame front and post flame gases. NO formation in post flame gases always dominates. Thermal NO: oxidation of molecular nitrogen in the post flame zone. Prompt NO: formation of NO in the flame zone (Fennimore mechanism). Fuel NO: oxidation of nitrogen-containing compounds in the fuel. O+ N2=NO +N N+O2=NO+O Zeldovich mechanism N+OH=NO+H
- 5. Mechanism of 𝐍𝐎𝟐 formation 1/6/20186 NO formed in the flame zone rapidly converted to NO2. NO+ HO2=NO2 +OH NO2+O=NO+O2 Highest NO2/NO ratio occur at light load in diesel engine because of large cooler region.
- 6. Factor affecting NOemission 1/6/20187 Fuel to air equivalence ratio Spark timing Burned gas fraction Residual gas fraction Recycled exhaust gas Change in time history of temperature Change in oxygen concentration
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- 8. 1/6/20189 NO variation withspark timing
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- 10. How can NOxbe reduced? 1/6/201811 NOx formation is depends upon temperature When combustion flame front temperatures exceed 2500°F (1370°C), NOx formation increases dramatically. The amounts of NOx formed at temperatures below 2500°F (1370°C) can be controlled in the exhaust by a catalyst. To handle the amounts generated above 2500°F (1370°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 (EGR).
- 11. NO vs temperatureand ф 1/6/201812 High temperature and High oxygen concentrations result in high NO formation rate.
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- 15. Ported EGR valve 1/6/201816 Consistsof Vacuum diaphragm Stem control valve Small pipe from the exhaust manifold or an internal crossover passage in the cylinder head Parted vacuum switch or a computer-controlled solenoid
- 16. Positive pressure EGRvalve 1/6/201817 It use exhaust backpressure to vary the point at which they open and their flow rates. Inside the valve there is a second diaphragm that reacts to backpressure in the exhaust system. Second diaphragm opens and closes bleed hole which control main EGR vacuum.
- 17. Negative pressure EGRvalve 1/6/201818 It reacts to negative or decreasing pressure changes in the exhaust system to regulate EGR action. A drop in backpressure occurs when there is less load on the engine. This causes the backpressure diaphragm to open a bleed hole and reduce EGR flow. It has the same principle as with the positive type except that the control function occurs when backpressure goes down instead of up
- 18. Pulse width modulatedEGR valve 1/6/201819 Powertrain control module (PCM) cycles the EGR vacuum control solenoid rapidly on and off. The computer controls the flow rate by sending electrical signals to a solenoid vacuum valve between the PCM and the EGR valve. The amount of "on" time versus "off" time for the EGR solenoid ranges from 0 to 100 percent, and the average amount of "on" time versus "off" time at any given instant determines how much EGR flow occurs.
- 19. Digital electronic EGRvalve 1/6/201820 This type of valve also uses vacuum to open the valve but regulates EGR flow according to computer control. It has three metering orifices that are opened and closed by solenoids. To match EGR to the engine's requirements various combinations of these three solenoids can be achieved.
- 20. Linear controlled EGRvalve 1/6/201821 It is electrically operated and uses a small computer-controlled stepper motor to open and close the EGR valve instead of vacuum. It has a control solenoid and EGR valve position (EVP) sensor. The sensor works on the same principle as a throttle position sensor. voltage signal ranges from 0.3 volts to 5.0 volts. The PCM controls EGR flow by pulsing the signal to the EGR solenoid.
- 21. Reduced NOx Improved engine life through reduced cylinder temperatures (particularly exhaust valve life) The exhaust gas, added to the fuel, oxygen, and combustion products, increases the specific heat capacity of the cylinder contents, which lowers the adiabatic flame temperature. Reduced throttling losses in SI engine. Reduced heat rejection. Reduced chemical dissociation. 1/6/201822 Advantages of EGR
- 22. It reducesthe available oxygen in the cylinder. Reduce the peak power available from the engine. The EGR valve can not respond instantly to changes in demand. Can not achieve perfect mixing in each cylinder. Combustion contamination. Control system stability. Materials and durability 1/6/201823 Disadvantages of EGR
- 23. The PCM needsto know when and how much EGR flow is present when commanded and not commanded. It have sensing systems called Feedback, which is simply some sensor used to determine presence of flow and amount. Types: PFE DPFE EVP 1/6/201824 EGR Feed back
- 24. • PFE: PressureFeedback EGR Components: 1. PCM 2. EVR 3. EGR Valve 4. Intake 5. Exhaust 6. EGR pressure measurement 7. PFE sensor 1/6/201825 PFE
- 25. • DPFE: DeltaPressure Feedback EGR Components: 1. PCM 2. EVR 3. EGR Valve 4. Intake 5. Exhaust 6. EGR side signal 7. Exhaust side signal 8. DPFE sensor 1/6/201826 DPFE
- 26. EVP: EGR ValvePosition Components: 1.PCM 2.EVR 3.EVP 4.Intake 5.Exhaust 1/6/201827 EVP
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Editor's Notes
- #6 In most practical combustion devices the thermal NO is the main source The contribution of reaction 3 is small for lean mixtures, but for rich mixtures it should be considered. Forward reaction 1 controls the system, but it is slow at low temperatures (high activation energy). Thus it is effective in post-flame zone where temperature is high and the time is available.
- #15 This causes a correspondingly lower heat release and peak cylinder temperature, and reduces the formation of NOx.
- #16 At idle, the throttle plate in the carburetor or fuel injection throttle body is closed. This blocks off engine vacuum so it can’t act on the EGR valve. The EGR spring holds the valve shut and exhaust gases do NOT enter the intake manifold. When the throttle plate is swung open to increase speed, engine vacuum is applied to the EGR hose. Vacuum pulls the EGR diaphragm up. In turn, the diaphragm pulls the valve open. At higher engine speeds, there is enough air flowing into the engine that the air-fuel mixture is not upset by the open EGR valve.
- #17 The typical ported vacuum EGR valve consists of a vacuum diaphragm connected to a poppet or tapered stem flow control valve. The EGR valve itself is usually mounted either on a spacer under the carburetor or on the intake manifold. A small pipe from the exhaust manifold or an internal crossover passage in the cylinder head and intake manifold routes exhaust to the valve. When vacuum is applied to the EGR valve, it opens. This allows intake vacuum to suck exhaust into the engine. To prevent the EGR valve from opening when the engine is cold, the vacuum line to the EGR valve may be connected to a parted vacuum switch or a computer-controlled solenoid. Vacuum is not allowed to pass to the valve until the engine is warm. EGR isn't needed when the engine is cold, only when it is warm and under load
- #18 The diaphragm opens and closes a small bleed hole in the main EGR vacuum circuit or diaphragm chamber. Opening the bleed hole reduces vacuum to the main diaphragm and prevents the valve from opening fully. Closing the bleed hole allows full vacuum to reach the main diaphragm so the valve can open wide and allow maximum EGR flow. With positive backpressure EGR valves, any increase in exhaust backpressure causes the EGR valve to open. This reduces backpressure somewhat, allowing the backpressure diaphragm to bleed off some control vacuum. The EGR valve begins to close and exhaust pressure rises again. The EGR valve oscillates open and closed with changing exhaust pressure to maintain a sort of balanced flow
- #20 The pulse-width modulated EGR system is controlled entirely by the powertrain control module (PCM). The computer controls the flow rate by sending electrical signals to a solenoid vacuum valve between the PCM and the EGR valve. The solenoid pulses up to 32 times per second. To determine the pulse width, the PCM relies on a ported vacuum signal.
- #22 The advantage of this approach is that the EGR valve operates totally independent of engine vacuum. It can be opened in various increments depending on what the engine control module determines the engine needs at any given moment in time. This provides better regulation of EGR flow than with conventional vacuum controlled EGR valves.
- #24 For this reason the EGR is usually shut off when full power is demanded, so the EGR approach to controlling NOx fails in this situation. the production of particulates (fuel which has only partially combusted) is increased when EGR is applied.
- #25 For this reason the EGR is usually shut off when full power is demanded, so the EGR approach to controlling NOx fails in this situation. the production of particulates (fuel which has only partially combusted) is increased when EGR is applied.