EURO 4-5 Diesel Exhaust Pollutant. After-Threatment

Transcription

1 EURO4-5 Common Rail EURO 4-5 Diesel Exhaust Pollutant After-Threatment 1

2 Exhaust gas recirculation EGR fundamentals: AFR: Air to Fuel Ratio. This parameter is used to define the ratio between fuel (petrol, diesel LPG, CNG etc) and air during a combuston process Lambda: λ is defined as the ratio between the intake air quantity and the optimal air quantity necessary to reach a stoichometric combustion λ = Airintake / Airideal Lambda is an adimensional parameter and is independent from the fuel used in the combustion process Positive ignition engines work with a lambda value always close to 1 Direct injection diesel engines work work with a variable lambda always > 1 Almost all of the positive ignition engines on the market have a compression ratio (10/1) < CR < (12/1) Almost all of the diesel engines on the market have a compression ratio (16,5/1) < CR < (17,5/1) Due to this higher compression ratio, in the diesel engines combustion chamber there's a continuous oxidation of nitrogen gas, especially during idle and partial load operation (when λ >>1) As several differenr Nitrogen oxides are generated during combustion, it is assumed to name the whole composites as NOx. Diesel engines produce therefore high quantity of NOx emissions Since the end of the '90s the first and main solution adopted was to avoid to fulfill the cylinder of free air: so variable quantities of exhaust gases are recirculated into the combustion chamber 2

3 Exhaust gas recirculation EGR fundamentals: EURO4-5 applications have an high pressure EGR only, with position feedback analog signal EURO5-6 applications have an high pressure EGR and a low pressure EGR with position feedback In order to optimize the EGR flow through the intake manifold, an EGR throttle is also implemented In order to reduce the exhaust gas temperature, an EGR cooler with a bypass valve is also present EGR control is feeded-back by the intake mass air flow information. A target map into the software is used to determine the optimal air value in all the conditions 3

4 Exhaust gas recirculation EGR control map: values are expressed in (mg/strk) 4

5 Exhaust gas recirculation EGR system valves on the engine 5

6 Exhaust gas recirculation EGR system valves on the engine 6

7 Exhaust gas recirculation Manifold Throttle valve Latest NOx reduction techniques implement the modulation of the intake air into the manifold A throttle valve upstream the EGR valve reduces the amount of fresh air into the manifold In some operating conditions throttle valve determines a pressure gap suitable to improve the exhaust gas recirculation As all the other ECU controlled valves on the engine, throttle body has an embedded position sensor Manifold Throttle valve pin-out Throttle valve power supply (12V) Throttle valve PWM control (GND control) Feedback sensor power supply (5V) Feedback sensor analog signal (0,5V- 4,5V) Feedback sensor GND 7

8 High Pressure EGR and Low Pressure EGR High pressure EGR (EURO 3 / 4 implementation) Low pressure EGR (EURO 5 / 6 implementation) 8

9 High Pressure EGR and Low Pressure EGR High pressure EGR (EURO 3 / 4 implementation) The HP EGR system fetches the exhaust gas upstream of the turbocharger and enters them again in the system downstream of it. This system minimizes the pressure drop of the exhaust gases along the path of re-entry. This also prevents the passage of exhaust gases through the compressor which could lead to a rapid deterioration of this organ. Various strategies have been used to increase the pressure difference between the intake and exhaust ducts to facilitate a greater flow of EGR to the engine. We've already examinated the throttle valve implementation. Unfortunately, the HP EGR system reduces the flow of gas through the turbocharger. If we increase the degree of EGR (increasing the passage section of the EGR valve) the flow rate processed by the turbine decreases: so at equal expansion ratio, the speed of the turbocharger increases. Also decreases the power transferred to the turbine and then to the compressor which consequently develops a lower flow rate and moves the operating point toward the surge line. Consequently, it determines a reduction of power output from the engine while increases the specific fuel consumption. 9

10 High Pressure EGR and Low Pressure EGR Low pressure EGR (EURO 5 / 6 implementation) The LP Loop is a well-known system that offers an alternative to overcome the limitations of the high pressure system: first of all the introduction of a substantial percentage of EGR during engine high loads operation. The exhaust gases are taken downstream of the turbine and introduced upstream of the compressor. Prior to the introduction into the market of the diesel particulate filter (DPF), the entire intake system, including the compressor, would be subject to rapid corrosion. This was one of the reasons that has limited the application of such a system. Introducing the DPF all the exhaust gases can pass through the turbine while preserving the performance and durability of the turbo. 10

11 High Pressure EGR Vs. Low Pressure EGR LP EGR advantages Potential lower specific fuel consumption as a result of the improved performance of the turbine and of a greater flexibility in the management of the turbine, no longer tied to the operation of the EGR system. Faster dynamic response of the engine system: a change of the operating point of the engine causes smaller changes of the speed of the turbine for the most constancy in the capacity of both the exhaust gases passing through the turbine and the mixture that passes through the compressor. The beginning of the transient phase decreases the fraction of EGR (the EGR flow is roughly constant) while increasing the content of fresh air in the mixture sucked by the compressor. Possibility to avoid the throttle valve on the suction. That component is required in the HP EGR to reduce the amount of air introduced at medium loads (high flow rate through the turbine), in order to maintain an high EGR rate. Lower fluid losses and simplification of the control system. The compressor provides a better mixing of the fresh air charge with the EGR that is distributed in the same proportions in each cylinder. Improved durability of the components downstream of the compressor and the lubricants for the reduced content of soot present in the exhaust gases that are going to mix with the fresh charge intake Reduced obstruction of the flow of EGR to the presence of soot deposits. The path followed by the recirculated exhaust gases to return to the engine is longer, and then determines a greater heat release before going into the engine. Lower temperatures while maintaining the operating conditions of the chiller. 11

12 High Pressure EGR Vs. Low Pressure EGR LP EGR drawbacks The increased volume and the greater length of the recirculation branch determine a delay in the achievement of the desired percentage of EGR in the combustion chambers. An unwanted increase of NOx in transient conditions The particulate filters, even the most efficient, let pass a small portion of the soot that can corrode long time the blades of the compressor particularly at high rotation speed. In the presence of a heat exchanger upstream of the compressor is possible condensation of water droplets which affect the turbocharger components. Increase of the losses due to the greater length of the path traveled by gas recirculation Hybrid HP +LP EGR Loop (VAG Euro5 implementation) The LP EGR gasses are filtered before being sent through the cooler and back into the intake. The HP gases go straight from the exhaust manifold to the EGR valve and into the intake. The LP EGR is used more at higher engine rpm and load. 12

13 Diesel Particulate Reduction Diesel engines up to now have been realized using the direct injection tecnique: this way the AFR changes dramatically with any load variation. Therefore particulate soot emissions were considered an unavoidable drawback of this generation of engines. In order to comply with emission regulations two different approaches have been developed: Diesel Particulate Filters with additive Diesel Particulate Filters without additive 13

14 Diesel Particulate Filter system with additive General architecture DPF system have been introduced with EURO4 regulations, when limitations imposed PM not go over 0,5 g/km. As the same engine will produce different soot quantities, depending on the car, the use of DPF began to be mandatory. 1 Instrument Cluster 9 Turbocharger 2 Engine Control Unit 10 Oxygen sensor G39 3 Additive tank 11 Oxydation catalyst 4 Additive Sensor G Upstream FAP temperature sensor 5 Additive pump V FAP 6 Fuel tank 7 Engine 14 Differential pressure sensor 8 Upstream turbocharger temperature sensor 15 Silencer 16 Mass air flow meters 14

15 Diesel Particulate Filter system with additive 15

16 Diesel Particulate Filter system without additive General architecture DPF system have been introduced with EURO4 regulations, when limitations imposed PM not go over 0,5 g/km. As the same engine will produce different soot quantities, depending on the car, the use of DPF began to be mandatory. 16

17 Diesel Particulate Filter system without additive DPF General architecture DPF system needs always three thermocouples, an oxygen sensor and a differential pressure sensor Upstream the DPF filter is placed the oxidation catalyst: besides oxidation of CO and HC gases, it keeps stable the DPF temperature in all the engine operating conditions (i.e. during cut-off) 17

18 Diesel Particulate Filter system without additive DPF Euro5 architecture with NOx filtering (no SCR) The VW Jetta DPF system is shown below. The Golf DPF and Audi A3 TDI exhaust are basically the same. In this exhaust system implementation there's also an exhaust valve put to increase backpressure for proper low pressure EGR flow during a NOx regeneration Because there is no Adblue, it also uses an H2S converter. 18

19 Diesel Particulate Filter system without additive DPF regeneration Saturation degree is calculated by the ECU according two mathematical models The first model uses the driving style and the information coming from the DPF thermocouples The second model uses differential pressure signal, MAF sensor and DPF thermocouple signals 19

20 Diesel Particulate Filter system Differential pressure signal, MAF sensor and DPF thermocouple model Differential pressure sensor gives the analog information of the pressure gap upstream and downstream the filter Exhaust volumetric flow corresponds roughly to the mass air flow measured from the MAF sensors. Exhaust gases volume depends also from its temperature measured upstream the filter. Combining mass air flow and temperature, ECU is able to calculate the istantaneous volume of exhaust coming into the particulate filter Combining differential pressure and exhaust volumetric flow, ECU can measure the degree of loading of the particulate filter 20

21 Diesel Particulate Filter system DPF regeneration sequence ECU evaluates the actual operating conditions (V > Vmin i.e. 80Km/h, T > Tmin i.e. 80 C, RPM > RPMmin i.e. 2000RPM) and activates the oxidation process into the catalyst to raise the DPF filter temperature. A closed loop temperature control is implemented. Maximum closed loop correction is limited by software (±4mm3/strk). Here it is the standard DPF regeneration sequence: 1. The throttle valve is activated, in order to maximize the oxygen intake 2. EGR system is de-activated to maximize the oxygen content into the exhaust gases 3. Post injections are activated to increase the combustion temperature 4. Post-injected fuel doesn't burn into the combustion chamber, but evaporate out with the exhaust gases 5. Residual HC burn into the oxidation catalyst, just upstream the DPF filter. DPF Upstream temperature increases up to 620 C 6. Upstream DPF temperature thermocouple is used as feedback signal for the post-injected fuel quantity correction 7. Intake manifold pressure is adapted in order to keep stable the engine torque. As a result of this operation, the driver does not feel any change in the engine behaviour 8. Regeneration procedure continues until the calculated soot mass goes below the fixed treshold (usually 5 g ). A regeneration completed flag is set into the EEPROM ECU if the process completes correctly. 21

22 Diesel Particulate Filter system DPF Filter loading: Tiguan 2.0 TDI case study Green line: 0 g < soot mass < 18 g Regenerated DPF Filter Passive regeneration Blue line: 18 g < soot mass < 24 g DPF loaded Active regeneration Black line: 24 g < soot mass < 40 g DPF clogged induced regeneration Red line: 40 g < soot mass < 45 g DPF completely clogged forced regeneration (diag) Red line: 45 g < soot mass DPF substitution system re-initialization (diag) 22

23 NOx reduction for EURO 5 applications: SCR System Selective Reduction Catalyst In order to comply with latest EURO5 NOx limits for passenger car and, mainly, for heavy duty application a new technical approach has been introduced. Up to now this very efficient solution ( > 90 %) has been widespread only on heavy duty application because the NOx catalyst is not compact as DPF: it is approximately 5 times the engine displacement. SCR post threatment is a specific reduction catalyst able to reduce NOx into N2 and H2O by means the injection of an urea mixture (AD Blue its commercial name) upstream the NOx catalyst. 23

24 NOx reduction for EURO 5 applications: SCR System Selective Reduction Catalyst Operating Principle into the hydrolysis section the reducing agent AdBlue is decomposed into ammonia (NH3) and carbon dioxide (CO2) In the reduction catalyst, divided into two parts, the ammonia reacts with nitrogen oxides to form nitrogen and water 24

25 NOx reduction for EURO 5 applications: SCR System Selective Reduction Catalyst VAG Implementation SCR system has been implemented first time on 3.0 TDI engines in order to comply with more tight US regulations 25

26 NOx reduction for EURO 5 applications: SCR System Selective Reduction Catalyst VAG Implementation SCR system has been implemented first time on 3.0 TDI engines in order to comply with more tight US regulations. This is a Touareg 3.0 TDI exhaust. 26

27 SCR System input signals ECU calculates the amount of ADBlue to be injected according to: Inlet air mass flow Percentage of EGR opening Exhaust gas temperature 27

28 SCR System components Selective Reduction Catalyst VAG Implementation SCR system has been implemented first time on 3.0 TDI engines in order to comply with more tight US regulations 28

29 SCR System: VAG implementation More control units are involved into NOx reduction Engine Control Unit ADBlue Heater Unit NOx sensor Control Unit 29

30 ADBlue characteristics ADBlue It is water solution of urea (32,5%) Freeze point -11 C Evaporation point ~ 80 C It has an high penetration capacity 30

31 SCR System components ADBlue injector and mixer ADBlue injector with PWM control 31

32 SCR System components NOx Sensor is used to evaluate the NOx trap efficiency 32

33 SCR System components ADBlue tank 33

34 SCR System components ADBlue pump with pressure sensor, level sensor and reversing valve 34

35 SCR System components ADBlue pump with Hall effect encoder 35

36 SCR System components ADBlue pressure sensor for pump speed regulation 36

37 SCR System components ADBlue flow reversing valve 37

38 SCR System components ADBlue level and quality sensors 38

39 SCR System components ADBlue level measurement principle 39

40 SCR System: instrument cluster indications ADBlue empty in km Graphic indication signal activated in Instrument Cluster 40

41 SCR System: instrument cluster indications ADBlue empty in km Acoustic signal activated in Instrument Cluster 41

42 SCR System: instrument cluster indications ADBlue tank empty Recovery mode activated: engine does not start; loud acoustic signal activated 42