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On Road Durability and Performance Test of Diesel Particulate Filter with BS III and BS IV Fuel for Indian Market.

INTRODUCTION

In India the share of diesel engine passenger cars is continuously growing. This is mainly due to their superior fuel economy and subsidies for diesel fuel. Diesel engines produce large amounts of particulate matter and Nitrous Oxide (NOx). Diesel particulate matter (DPM) is made of number of components, including acids (such as nitrates and sulfates), organic chemicals, metals, soil or dust particles and allergens (such as fragments of pollen or mold spores). These particles in diesel exhaust are of special concern due to their respirable size. The particles less than 10 microns in diameter pose the greatest problem because they can get deep into human lungs and some may even get into the bloodstream. Larger particles are of less concern, although they can cause irritation to eyes, nose and throat. The upcoming stringent emission regulation (BS V) in India, aiming to address the above concerns will create new challenges to meet the PM limit not only in terms of mass but also in terms of particle numbers (PN) for diesel cars.

The significant PM reductions over the years were achieved with a combination of engine design optimization, availability of refine d fuels, and the use of exhaust after treatment devices. The reduction of PM via combustion optimization is limited by combustion efficiency and fuel economy. Hence, after treatment system is one of the promising technology to control PM from diesel cars. Diesel particulate filter can reduce PM by more than 95% from exhaust gas.

Currently, Mercedes-Benz uses Silicon Carbide (SiC) based catalyzed Diesel Particulate filters (cDPF) exclusively for passenger car applications. The main advantages of the catalytic coating are to reduce the activation energy for soot oxidation and to suppress secondary emissions (CO & HC) during DPF regeneration.

Diesel particulate filter performance is highly influenced by sulfur content in the diesel fuel and driving pattern. During the combustion, sulfur is oxidized to form sulfur dioxide (>95%) primarily and small portion of sulfur trioxide (2 -5 %) both being acid components. S[O.sub.3] is readily hydrolyzed to form sulfuric acid ([H.sub.2]S[O.sub.4]) and it combines with water in the exhaust gas to form a mist which is exhausted as a part of the fine diesel PM.

S[O.sub.3] and sulfuric acid will easily react with platinum (Pt) and palladium (Pd) precious metals which are incorporated within catalytic converter (DOC & DPF) to form sulfates that are not catalytic and not desirable substance within a catalyst (catalytic converter poisoning). The sulfate can be removed (de-poisoning) from the substrate when it gets exposed to temperatures in the range of 650 to 700[degrees]C and in the process can degrade the original physical character and function of catalytic components (Pt & Pd) [2].

The sulfur compounds present in exhaust gas can also combine with the wash coat layer of the catalyst. In the oxidizing atmosphere and in the presence of platinum (intensive oxidation of S[O.sub.2]) large amounts of sulfur are deposited on the [Al.sub.2][O.sub.3] layer following the reaction.

3 S[O.sub.3] + [A1.sub.2][O.sub.3] [left and right arrow] [A1.sub.2][(S[O.sub.4]).sub.3] (1)

This reaction is very intensive at low temperatures and it precedes inversely at temperatures above 700[degrees]C [6].

Driving pattern is another important factor that needs to be considered while developing DPF system because it directly influences the oil dilution. Low average driving speed and longer idling duration are major concerns in developing countries like India. These factors will lead to longer regeneration duration, which means, post injection duration becomes longer and this phenomenon will lead to higher oil dilution. While injecting fuel very late in the combustion cycle the temperature and pressure in the combustion chamber are much lower. This usually results in incomplete fuel vaporization and fuel impingement on the cylinder liner. The result is that the fuel dilutes the engine oil which adversely affects engine wear, due to the lowering of the oil's viscosity. A typical design consideration is to allow oil dilution up to 10%.

In this paper, the impact of fuel quality (sulfur) and driving pattern on DPF regeneration behavior (temperature during regeneration, regeneration interval & duration), oil dilution and product durability are discussed based on durability test data.

ON-ROAD DPF DURABILITY TESTING

The most realistic and straightforward approach to validate DPF performance and robustness is On-road DPF durability testing. Although it is time consuming and expensive, it gives realistic data with respect to fuel quality and driving pattern. Regeneration behavior and thereby oil dilution levels can be accurately monitored with selection of appropriate test cycles.

During the durability testing, the DPF undergoes repeated soot loading and regeneration cycles. DPF loading generally occurs when the rate of soot accumulation exceeds the rate of soot oxidation (e.g.: Passive regeneration). Soot accumulation in the DPF causes an increase of the pressure drop across the DPF filter which subsequently leads to higher pumping losses and fuel consumption. Therefore, the active regeneration must be done when the soot loading reaches a certain level.

Daimler AG has exclusively developed a DPF system for European market which cannot be directly used in India due to the different duty cycles and high sulfur ([less than or equal to] 350ppm) fuel. Hence, it is necessary to do country specific on-road DPF durability testing with respect to commercially available fuel. It would be a more realistic approach to validate DPF performance and robustness.

The objective of the project is to validate the DPF system which is available in the European market with BS III fuel under real Indian driving conditions to ensure the temperatures inside DOC and DPF are well within the safety limits.

Specifications of Test Vehicles and Aftertreatment System

Durability test was carried out in India on 4 test vehicles; all of which were equipped with instrumented DOC & DPF, temperature and pressure measurement module (ES420, M-Thermo & M-Sens), ECU interface module (ES953) & data logger (ES720). Details about the test vehicles are given in table 1.

All test vehicles were equipped with state-of-the-art V6 diesel engines. The engine specifications are given in table 2. These vehicles were equipped with different exhaust after treatment systems. Vehicle A and B had oxidation catalyst and cDPF closely in one canning. In other two vehicles (C and D) diesel oxidation catalyst were located in closed coupled position directly after turbocharger and cDPF was located slightly away from DOC due to the packaging constraints. However, According to test bed results, the difference in exhaust system architecture does not have much deviation in exhaust gas temperature profiles with respect to different duty cycle. Hence, the change in exhaust system architecture does not have much impact on A, B, C and D vehicle test results.

Diesel particulate filter details are given in table 3. A 2.44L catalyzed DPF was used for on vehicle durability test. The filters were made by silicon carbide (SiC) material coated with platinum and palladium catalyst. The main advantages of the catalytic coating are to reduce the activation energy for soot oxidation, to suppress secondary emissions (CO & HC) during filter regeneration. In addition, it supports soot oxidization (Passive regeneration) at around 300[degrees]C exhaust gas temperature with the help of DOC generated N[O.sub.2] and also it provides higher thermal stability.

Instrumentation and Data Acquisition

Test vehicles were equipped with an instrumented DOC and DPF to monitor temperature profile inside DOC & DPF throughout the field trials. All filters were equipped with 8 K- type thermocouples with diameter varying from 0.75 mm to 1.5 mm. The pressure drop across DPF was obtained from vehicle differential pressure sensor as well as from dedicated pressure sensor. Special care has been taken during the installation of the pressure sensor and the thermocouples to avoid early sensor damage due to the harsh operating environment and vibration during the vehicle operation. The typical test setup is shown in figure 1.

Usually the highest temperature during regeneration is observed at the rear side of DPF and the highest radial temperature gradient is observed close to the skin of the DPF. Hence, it is necessary to place more thermocouples at the rear of the filter. It is also very important to monitor the temperatures in the DOC. Sometimes it witnesses highest temperature during the regeneration period. Upstream and downstream of DOC and DPF temperatures were also monitored. The thermocouple locations in the axial and radial directions are shown in figure 1. The temperature information was sent to ETAS data logger (ES720) via Ipetronik M-Thermo (16Ch) module. Apart from the temperatures, pressure signals were also measured before and after DPF to verify the consistency of vehicle [DELTA]p sensor. A snapshot of an instrumented DPF with 8 thermocouples and pressure sensors is shown in figure 2.

Other ECU variables like DPF regeneration duration, regeneration interval, vehicle speed, engine speed, fuel injection parameters (pilot, main, & post), lambda etc. were recorded via ETAS interface module (ES593).

Derivation of Driving Cycles for DPF Validation

Test cycle or driving pattern selection is important for validation of DPF as it has an influence on regeneration interval, regeneration duration and thereby oil dilution. In order to validate the DPF in worst case driving profile with harsh accelerations and stop-and-go bumper to bumper traffic, an appropriate city cycle fulfilling the criteria was selected. Average driving speed and frequent idling are major concerns in city driving cycle. This impacts regeneration duration and DPF temperatures.

The main challenge in selecting city cycle was to cover the target distance within specified period of time and without violating the driving pattern. The average speed was to be maintained below 18kmph, overtaking and rash driving was prohibited. The vehicles were fitted with GPS in order to monitor the route followed by the vehicle and to inform or instruct the driver in case of any deviations.

Extra urban driving cycle is not as critical as city driving cycle since the exhaust temperatures are generally high enough to execute the passive regeneration and the exhaust flow rate is also higher which enables better regeneration performance. However, these profiles create few challenges for active regeneration, such as engine over run conditions with oxygen rich exhaust flow when the vehicle is approaching a traffic jam on a highway or rapid change in engine speed from rated rpm to idling (Drop to Idle). Typical city and extra urban driving profiles are as shown in figure 3 and 4.

VEHICLE OPERATION

Durability testing on all vehicles was conducted at Pune and Bangalore in India. The tests were conducted under different duty cycles to obtain the diesel particulate filter performance data under wide range of practical driving conditions. The duty cycle was defined in such a way so as to represent the typical customer vehicle driving pattern in real world. City duty cycle consisted of 366 cycles and each cycle covered 95 km. Extra urban driving cycle consisted of 100 cycles and each cycle covered 600 km. The average vehicle speed observed during the city driving cycle trials was 15 - 20 km/h. In extra urban driving cycle the average speed observed was around 60 - 65 km/h. Details of test vehicles are given in table 1. BS III & BS IV certified fuel was used for vehicle A and B durability testing. These vehicles were running without any change in the calibration (same as European market calibration). Durability testing of these vehicles was conducted within and around Pune, India. The distance covered was around 30,000 km and 60,000 km in city and extra urban driving cycles respectively.

Based on A and B vehicle durability test results, re-calibration was done at Germany and new calibration data was used for C and D vehicles durability test. From the test results of A and B vehicles it was clear that DPF crack occurred due to the usage of BS III fuel. Hence, only BS III certified fuel was used for C and D vehicle's durability testing. The tests were mainly conducted in and around Bangalore, India.

Oil dilution is another critical parameter which needs to be monitored at periodic intervals to safeguard the engine against wear. In Indian driving conditions, the regeneration duration is expected to be longer as compared to Europe due to low average speed and longer idling duration. Hence, it is very important to monitor the oil dilution level periodically during durability testing. Oil samples were collected at periodic intervals of 2500 km and 5000 km in city and highway cycles respectively and samples were analyzed to ensure that oil dilution does not exceed the prescribed safe limits. BS III fuel samples were collected from various fuel stations across India to analyze the sulfur content and adulteration (if present) in fuel.

Mass emission test was conducted before starting the durability test as per Modified Indian driving cycle (MIDC) after instrumentation and filter installation on the test vehicles to assess the initial pollutants (CO, HC, NOx & PM) levels. At the end of durability test mass emission test was conducted again to check the impact of sulfur on DOC and DPF performance.

TEST RESULTS AND DISCUSSIONS: A AND B VEHICLES

The aim of durability test results presented here is to show the behavior of diesel particulate filter during regeneration under real driving conditions with European standard calibration. Figure 5 shows the exhaust gas temperature profile in DOC and DPF with BS IV fuel under city driving conditions. The vehicle speed, soot burning rate and pressure difference ([DELTA]P) across DPF are shown in figure 5. It can be clearly observed that, during the normal operation mode, the exhaust temperature varies from 200[degrees]C to 300[degrees]C. However, during the regeneration mode (from 3900 to 5800 seconds), the exhaust gas temperature has elevated to 700[degrees]C. This is achieved by controlling post injection quantity and timing. There are two basic principle techniques used to remove the particles from the exhaust gas namely; Oxidation by NO2 (Passive Regeneration) and Oxidation by O2 (Active Regeneration).

Passive Regeneration

During the normal operation mode, the loaded soot in the filter gets oxidized by the exhaust gas components of N[O.sub.2] and [O.sub.2] at lower temperatures around 250[degrees]C to 300[degrees]C. Engine out NO emission gets oxidized to N[O.sub.2] by DOC subsequently; it helps to oxidize soot in DPF. This reduction takes place at certain exhaust gas operating temperature range (Balance temperature). The balance temperature is defined as the temperature at which the same amount of soot in the DPF is oxidized as is emitted by the engine in the same unit of time.

N[O.sub.2] + C [right arrow] CO + NO (2)

N[O.sub.2] + C [right arrow] C[O.sub.2] + 2NO (3)

In figure 5, the passive regeneration behavior can be observed from 2100 to 3000 seconds during vehicle normal operation mode. Soot build-up is stabilized during this period on account of passive regeneration.

The N[O.sub.2] production in DOC is limited by the thermodynamic equilibrium. However, due to highly transient duty cycle (real driving conditions) the passive regeneration alone is not sufficient to burn the complete soot in the exhaust gas. However, it helps to increase the active regeneration interval.

Active Regeneration

When soot accumulation in DPF reaches specified limit, active regeneration must be triggered to prevent the excessive increase in back pressure which subsequently leads to higher pumping losses and fuel consumption. In order to burn the trapped soot in the DPF, the exhaust gas temperature at the DPF upstream (T5) must be increased enough to promote [O.sub.2] based soot oxidation (Thermal oxidation). Typically, Temperature in the DPF must be more than 600[degrees]C for initiating and sustaining the soot burning process until all soot particles are removed from DPF. This is achieved using in-cylinder late post injections which lead to high amount of unburned hydrocarbons in the exhaust gas. The unburned HC being oxidized (exothermic) in the DOC will contribute in reaching the targeted (>600[degrees]C) upstream DPF temperature. As this post injection does not burn in the combustion chamber, it does not elevate T3 (Exhaust manifold temperature) above the limit. Also, it is very important to manage injection timing, quantity and air-fuel ratio ([lambda]) to achieve desired T5 temperature without much impact on oil dilution. The active regeneration can be observed in figure 5 from 3950 to 5800 seconds. During this period the DPF temperature is controlled between 600[degrees]C to 700[degrees]C by adjusting post injection quantity and timing. Since the regeneration temperatures were well within the specified limit, it is called controlled regeneration.

DPF Pressure Drop Analysis

Figure 5 shows instantaneous pressure drop across DPF during engine normal operation mode as well as during the DPF regeneration mode. It can be observed that, the sudden increase in the pressure drop at the beginning of DPF regeneration (approx. at 3950 seconds) is due to an increase in the exhaust volumetric flow rate on account of increased inlet temperature. Hence, the pressure drop across the DPF rises significantly, but as soon as the soot oxidation becomes prominent after few minutes, the pressure drop starts decreasing gradually. This drop in pressure across DPF is a result of the soot oxidizing and reducing the soot loading in the DPF. At the end of regeneration, the exhaust gas temperatures drop drastically (due to post injection deactivation) causing a decrease in the pressure drop across the DPF. This is due to the fact that the volumetric flow rate decreases as the exhaust gas temperature drops. After the regeneration is complete, the soot starts accumulating again in the DPF causing a gradual increase in the pressure drop across the DPF.

Uncontrolled DPF Regeneration - Sudden Deceleration Scenario

During DPF regeneration the exhaust gas temperature is elevated to oxidize the accumulated soot in DPF. This process produces a considerable amount of heat. It is essential that the exhaust gas flow rate should be good enough to carry away this generated heat convectively. During moderate and high load operating conditions the exhaust gas flow rate is sufficient enough to maintain desired DPF regeneration temperature. During DPF regeneration, if exhaust gas flow rate suddenly drops then the heat produced by the soot oxidation may not be carried away completely as a result of which the DPF wall temperature will increase. This is one of the worst cases similar to drop to idle scenario that can be observed during DPF regeneration under real driving conditions.

Figure 6 shows the temperature profiles inside DOC and DPF with BS III fuel during regeneration under extra urban driving conditions. The regeneration temperature is well within the desired limit (<700[degrees]C) except at DPF rear middle position (DPF_Rear middle) where it has gone up to 1050[degrees]C during regeneration event. The root cause of this behavior is explained in figure 7.

As shown in figure 7, the vehicle speed drops from 115 to 42 km/h between 2553 to 2572 seconds. During this period the lambda increased on account of sudden vehicle deceleration. As a result, the O2 concentration in the exhaust gas increases and exhaust flow rate dropped considerably. Due to this condition there was sudden increase in regeneration temperature from 650[degrees]C to 1050[degrees]C. This temperature ramp is undesirable and called uncontrolled regeneration.

Uncontrolled Regeneration - Vehicle Overrun Scenario

In real vehicle operating conditions, vehicles are not likely to operate at a steady speed and load for extended periods of time. In many circumstances, the vehicle will encounter over run maneuvers when the driver takes his foot off from the accelerator pedal but still has the vehicle in gear. During this maneuver, only the ambient air is forced through the exhaust system without any combustion taking place in the cylinder which leads to higher [O.sub.2] concentration in exhaust gas.

Since, the durability test was conducted with BS III fuel, DOC and DPF poisoning (sulfur adsorption) and de-poisoning (sulfur desorption) were a periodic event. The de-poisoning process is viable if the exhaust gas temperature is elevated more than 600[degrees]C which means de-poisoning process is a part of regeneration event. However, the de-poisoning process does not completely remove the sulfur compounds from the substrate. Over the period of the vehicle run, part of the catalyst is permanently deactivated in DOC and DPF. Therefore, the DOC is unable to oxidize the unburned HC from the exhaust gas completely. Hence, a part of unburned HC is stored in DOC and also slips from DOC and stored in DPF. The rest escapes along with exhaust gas.

As shown in figure 8, the vehicle encountered overrun operation between 2128 to 2135 seconds. During this period engine load becomes zero and vehicle speed fluctuates between 90 to 55 km/h. As a result, the lambda value ([O.sub.2] concentration in exhaust gas) increased steeply and at the same time the stored unburned HC in DPF (HC slipped from DOC) reacts with [O.sub.2] instantaneously in the DPF and releases tremendous amount of heat in the DPF during the regeneration event. This leads to a temperature ramp in the DPF over 1030[degrees]C. This phenomenon is called as "uncontrolled regeneration at overrun conditions".

This increasing DPF wall temperature will cause the accumulated soot to oxidize even much faster and result in an uncontrolled regeneration (runaway regeneration). Because of the very high temperatures encountered during an uncontrolled regeneration, the DPF may be irreversibly damaged. Specifically, the DPF wall may crack due to thermal stresses and the catalyst may be deactivated.

DPF Investigation after Durability Test

After completion of A and B vehicles on-road durability test, the DOC and DPF filters were removed and analyzed in detail with respect to sulfur poisoning and potential mechanical damages. For this purpose the filters were de-canned and visual inspection was carried out before conducting destructive analysis.

Figure 9 shows B vehicle's DOC and DPF filters picture after decanning. Top view is upstream side and bottom is downstream side of the filters. These filters were removed after 60000 km vehicle run under extra urban driving cycle. These filters encountered more than 100 regenerations with BS III & IV fuels.

Figure 10 shows the diesel oxidation catalyst section cut view along the center axis. The filters were cut into two halves to get an insight into the filter interior. Soot deposits can be observed up to 2 inches from upstream of diesel oxidation catalyst. Diesel oxidation catalyst was not able to oxidize the soot due to the precious metals (Platinum and Palladium) deactivation (sulfur poisoning).

Figure 11 shows the transverse crack at rear side of diesel particulate filter. During vehicle B durability testing the diesel particulate filter regeneration temperature exceeded more than 1100[degrees]C in many occasions (uncontrolled regeneration). This was mainly due to sulfur poisoning along with highly transient drive cycle behavior. The root cause of uncontrolled regeneration has been discussed in detail in the earlier topics.

Fuel Analysis Summary

During the durability trials, BS III fuel samples were collected from various fuel stations across various locations in India to investigate the sulfur content and adulteration (if any) level in the fuel. The fuel sample analysis results are shown in table 4. All fuel samples were well within the BS III specified limits.

RE-CALIBRATION OVERVIEW

Based on the results obtained from A and B vehicles durability testing in city and extra urban driving cycles, re-calibration in terms of DPF regeneration strategy was necessary in order to protect the diesel oxidation catalyst and diesel particulate filter against excessive thermal load (Temperature ramp) during regeneration and also to prevent visible white smoke from exhaust. A strategy was evolved in order to get rid of excess sulfur present in BS III fuel which is responsible for elevating the temperatures in the DOC and DPF during regeneration; which can damage the catalyst (as observed in A and B test vehicles)

Sulfur poisoning is a critical issue which needs to be addressed for bad fuel applications. During normal combustion, sulfur in fuel reacts with oxygen and produces sulfur dioxide and sulfur trioxide which later converted as sulfates in diesel oxidation catalyst and deposited on the catalyst. This phenomenon known as sulfur poisoning reduces the catalytic conversion efficiency of the filter (DOC) thus preventing HC oxidation. This result in "HC Slip" across DOC and the unburned hydrocarbons gets accumulated partially in DOC and also in DPF over a period of time. Hydrocarbon slip is one of the major reasons for uncontrolled regeneration.

The objective of re-calibration was to prevent HC slip by periodically removing the stored HC in DOC and DPF; thus preventing sudden ramp in temperatures during regeneration. Also, this phenomenon helps in reducing visible white smoke. A Light off model (based on post injections) was introduced in order to periodically oxidize stored HC based on HC loading in DOC.

De-sulfurization strategy makes use of the inherent oxidation properties of sulfur namely oxidation temperature, rate of oxidation and its effect on soot oxidation. The strategy was to split the post injections during regeneration into two stages. The first stage was targeted for sulfur de-poisoning of DOC and DPF substrate and the second stage for soot oxidation. Sulfur compounds desorption begins when exhaust gas temperature reaches above 550[degrees]C. Once the soot loading limit in the filter is reached, it makes sense to regulate the desorption temperature by means of post injections for removing the sulfur from the filters. The second stage of regeneration is regulated by controlling post injection quantity and timing to maintain optimal soot burning temperatures between 600[degrees]C to 700[degrees]C.

TEST RESULTS AND DISCUSSIONS: C AND D VEHICLES

In this section, C and D vehicle's durability test results are presented to analyze the behavior of diesel particulate filter during regeneration period under real driving conditions after re-calibration with BS III fuel. Figure 12 shows the exhaust gas temperature profile in the DOC and DPF under city driving conditions. In addition the vehicle speed, soot burning rate and pressure difference ([DELTA]P) across diesel particulate filter are shown in figure 12. As seen in the figure, the temperatures inside DOC and DPF upstream are well below the desired temperature limit ([less than or equal to] 700[degrees]C). It can also be observed from the figure that, during the normal operation mode, the exhaust temperature varies from 200[degrees]C to 350[degrees]C. The soot build up was seen to be stabilized between 2200 to 3500 seconds on account of passive regeneration. However, during the regeneration mode (from 6000 to 8000 seconds), the exhaust gas temperature has elevated up to 790[degrees]C due to drop to idle driving behavior. During the entire durability testing this was the maximum temperature observed in the system. This temperature was observed in the thermocouple mounted at DOC middle position.

Figure 13 shows the DOC and DPF temperature profile during DPF regeneration in extra urban driving cycle. Temperature ramp inside DOC and DPF was well within the safe operating limits of the system even when the vehicle enters overrun operation during regeneration which is one of the worst case scenarios. Throughout the durability testing with C and D vehicles, no abnormal temperature shoot up in DOC or DPF was observed. As can be seen in the highlighted zone in figure 13, during the overrun phase a temperature ramp from 656[degrees]C to 674[degrees]C was observed in DOC (DOC_Middle). Temperature ramp during overrun condition was controlled by optimal use of post injections and by sulphur de-poisoning of substrate by means of split post-injections. Also, the lambda was well in control during overrun conditions by regulating post injections.

Regeneration in Extra Urban Cycle (Sudden Deceleration Case

In base calibration, the post injection was completely deactivated during DPF regeneration when the vehicle encounters sudden deceleration. As a result the lambda is increased drastically in the exhaust and leads to temperature ramp in DPF. In figure 7, from 2553 to 2572 seconds the vehicle speed drops from 115 to 42 km/h as a result of which lambda increases and leads to temperature ramp in DPF (DPF_Rear middle). After Recalibration, post injection was activated during the vehicle sudden deceleration conditions and also the EGR flow rate was increased considerably to control the lambda in the exhaust gas during regeneration. Figure 14 shows the temperature profile in DPF during the vehicle deceleration in extra urban driving cycle (Vehicle - D). It can be clearly seen from the figure that, from 9000 to 9011 seconds the vehicle speed drops from 106 to 45 km/h (possibly due to sudden braking). During this period the temperature inside DPF is well within the limit. The continuous post injection and higher EGR flow rate controls temperature ramps during this phase. This is a clear case of controlled regeneration even during sudden deceleration, as the temperature inside DPF is well within safety limits.

Regeneration Summary in City and Extra Urban Driving Cycles

Figure 15 shows the regeneration temperature peaks summary in diesel oxidation catalyst and diesel particulate filter in city cycle driving profile. The results show that majority of temperature peaks in DOC were well below 750[degrees]C and in DPF were well below 700[degrees]C during regeneration. Maximum temperature observed in city cycle during regeneration in DOC and DPF was 792[degrees]C and 715[degrees]C respectively. Few temperature peaks in DOC as can be seen in figure 15 are above 750[degrees]C on account of drop-to-Idling or sudden deceleration.

Figure 16 shows the regeneration temperature peaks summary in diesel oxidation catalyst and diesel particulate filter in extra urban driving cycle driving profile. It is clearly shown in the figure 16 that the temperature peaks in DOC and DPF are found to be clustered below or around 750[degrees]C. Few temperature peaks in DPF are found to be above 750[degrees]C; the maximum temperature peak observed during entire durability testing being 893[degrees]C. The reason for temperatures greater than 750[degrees]C was due to sudden drop in vehicle speed from full load conditions. After re-calibration, there was no abnormal DPF regeneration behavior observed throughout C and D vehicle's durability test in city and extra urban driving cycles.

Regeneration Interval & Duration Summary

Figure 17 shows the regeneration interval and regeneration duration summary in city driving conditions. The average regeneration interval observed in city cycle was 273 km. Passive regeneration was observed occasionally in city cycle due to exhaust gas temperatures more than 300[degrees]C which is evident as regeneration interval occasionally has gone above 400 km. The average regeneration duration observed in city cycle was 37 minutes. The regeneration duration was longer as compare to European driving conditions. This was due to low average driving speed and calibration strategy for desulfurization.

As compared to city cycle, the regeneration interval is longer in extra urban driving cycle on account of frequent passive regeneration. This is due to higher average speed (around 63 km/h) resulting in higher exhaust mass flow rates and temperatures which favor passive regeneration. Figure 18 shows the regeneration interval and duration summary in extra urban driving conditions. During both driving conditions, the oil dilution was well with in safe limits.

CONCLUSIONS

The effect of sulfur on diesel oxidation catalyst and diesel particulate filter has been investigated in this paper and results have been presented therein. In this regard, durability testing was conducted under real road driving conditions in India with BS IV and BS III fuel covering both city and extra urban cycles.

The investigation results of first phase of durability test shows that, there was no DPF performance degradation or abnormal temperature behavior during regeneration with respect to BS IV fuel. However, the low average driving speed and longer idling duration had an impact on oil dilution. Hence, the oil change interval was redefined for Indian driving conditions.

BS III fuel poses a challenge with respect to uncontrolled regenerations resulting in very high regeneration temperatures (>1000[degrees]C) which ultimately led to thermal crack of diesel particulate filter. The reason for uncontrolled regeneration was found to be sulfur poisoning of the catalyst which led to unburned hydrocarbon slip. This unburned hydrocarbon slip is responsible for elevating the temperatures during regeneration beyond the safety limits of the catalyst.

Based on first phase results, a robust regeneration strategy was developed for bad fuel applications (BS III) in order to control the temperature ramp during regeneration. The new calibration model was validated with respect to BS III fuel and Indian driving conditions as a second phase of durability test. The results show that the regeneration temperatures inside the filters were well within the safety limits even under worst case driving scenarios like drop-to-idle and overrun conditions. There was no abnormal regeneration behavior observed throughout the durability test. Oil dilution was also found to be well within the acceptable limits throughout the durability test.

REFERENCES

[1.] Rose, D., Pittner, O., Jaskula, C., Boger, T. et al., "On Road Durability and Field Experience Obtained with an Aluminum Titanate Diesel Particulate Filter," SAE Technical Paper 2007-01-1269, 2007, doi:10.4271/2007-01-1269.

[2.] Mooney, J., "Diesel Engine Emission Control Requires Low Sulfur Diesel Fuel," SAE Technical Paper 2000-01-1434, 2000, doi: 10.4271/2000-01-1434.

[3.] Rutland, C., England, S., Foster, D., and He, Y., "Integrated Engine, Emissions, and Exhaust Aftertreatment System Level Models to Simulate DPF Regeneration," SAE Technical Paper 2007-01-3970, 2007, doi:10.4271/2007-01-3970.

[4.] Zhan, R., Huang, Y. , and Khair, M., "Methodologies to Control DPF Uncontrolled Regenerations," SAE Technical Paper 2006-01-1090, 2006, doi:10.4271/2006-01-1090.

[5.] Singh, N., Rutland, C., Foster, D., Narayanaswamy, K. et al., "Investigation into Different DPF Regeneration Strategies Based on Fuel Economy Using Integrated System Simulation," SAE Technical Paper 2009-01-1275, 2009, doi:10.4271/2009-01-1275.

[6.] Kozak Miloslaw, Merkisz Jerzy, "The Mechanics of Fuel Sulphur Influence on Exhaust Emissions from Diesel Engines"

[7.] Zhan, R., Eakle, S., Spreen, K., Li, C. et al., "Validation Method for Diesel Particulate Filter Durability," SAE Technical Paper 2007-01-4086, 2007, doi:10.4271/2007-01-4086.

CONTACT INFORMATION

Mr. Dhinesh Kumar

Exhaust gas after treatment testing and calibration

Mercedes-Benz Research and Development India Private Ltd. (A Daimler Company)

Whitefield Palms, Plot No. 09 & 10, EPIP Zone, Phase 1, Whitefield Road, Bangalore 560066, India

Telephone: +91 80 6768 6304

dhinesh.kumar@daimler.com

Mr. Ashwhanth J Raju

Exhaust gas after treatment testing and calibration

Mercedes-Benz Research and Development India Private Ltd. (A Daimler Company)

Whitefield Palms, Plot No. 09 & 10, EPIP Zone, Phase 1, Whitefield Road, Bangalore 560066, India

Telephone: +91 80 6768 5343

ashwhanth.raju@daimler.com

Mr. Steffen Digeser

Daimler AG

Deputy General Manager (Exhaust gas after treatment team)

Bela-Barenyi-Str. 1

71063 Sindelfingen, Germany

steffen.digeser@daimler.com

DEFINITIONS/ABBREVIATIONS

DOC - Diesel Oxidation Catalyst

DPF - Diesel Particulate Filter

cDPF - Catalysed Diesel Particulate Filter

BS - Bharat Stage

PPM - Parts per Million

MIDC - Modified Indian Driving Cycle

PM - Particulate Matter

DPM - Diesel Particulate Matter

N[O.sub.x] - Nitrous Oxides

PN - Particle Number

SiC - Silicon Carbide

C - Carbon

CO - Carbon Monoxide

HC - Hydrocarbons

N[O.sub.2] - Nitrogen di-oxide

[O.sub.2] - Oxygen

S[O.sub.2] - Sulfur di-oxide

S[O.sub.3] - Sulfur tri-oxide

[H.sub.2]S[O.sub.4] - Sulfuric acid

Pt - Platinum

Pd - Palladium

T3 - Exhaust manifold temperature

T5 - DPF upstream temperature

ASTM - American Society for Testing and Materials (Technical standard)

CPSI - Cells per Square Inch

[DELTA]P/ DeltaP - Differential pressure across DPF

GPS - Global Positioning System

Dhinesh Kumar, Ashwhanth Raju, and Nitin Sheth

Mercedes-Benz R&D India Pvt. Ltd.

Steffen Digeser

Daimler AG

doi:10.4271/2016-01-0959
Table 1. Test vehicle details

Vehicle  Engine  Duty cycle   Distance, km     Fuel

   A     OM642      City         30000      BS III / IV
   B             Extra urban     60000
   C     OM642      City         30000        BS III
   D             Extra urban     60000

Table 2. Engine Specification

Engine Type                OM642

Number of Cylinders        V6
Bore x Stroke (mm)         83x92
Displacement ([cm.sup.3])  2987
Compression Ratio          15.5 : 1
Maximum Power              195 kW@ 3800 rpm
Maximum Torque             620 Nm @ 1600-2400 rpm
Lube Oil type              MB229.5(SAE0W-40)
Fuel Injection system      Common Rail Direct Injection
Charge System              Variable geometry turbocharger
Emission Control           Close-coupled Diesel Oxidation Catalyst
                           (DOC),
                           catalyzed Diesel Particulate Filter (cDPF)
Emission level             Euro 5

Table 3. DPF specification

                   Cell Density    Wall
Vehicle  Material     (CPSI)     thickness  Volume  Pt:Pd

   A                   270
   B       SiC                     10 mm    2.44 L  2 : 1
   C                   271
   B

Table 4. Fuel sample analysis summary

                                    BS III Fuel Analysis Summary- India
                                                Fuel Samples
S.No  Parameters                  S1   S2   S3   S4   S5   S6   S7   S8


 1    Sulphur Content,           281  292  286  261  334  330  320  326
      mg/kg
      Flash Point (Abel),
 2    deg C                       36   37   38   40   46   38   38   39

 3    Density at 15 [degrees]C,  822  860  840  839  827  827  826  826
      Kg/m3

 4    Water Content              106  104  102   64  110   42   45   49
      (Coulometric), ppm

 5    Cetane Index, -             52   44   55   53   57   51   52   51


                                 BS III Fuel Analysis Summary- India
                                      Fuel Samples  Test
S.No  Parameters                       S9  Limits   Method
                                                    ASTM D

 1    Sulphur Content,                285  Max      4294
      mg/kg                                350      2010
      Flash Point (Abel),                           IS 1448 (P
 2    deg C                            36  Min 35   20)2008
                                                    ASTM D
 3    Density at 15 [degrees]C,       835  820-     1298
      Kg/m3                                845      2012b
                                                    ASTM D
 4    Water Content                   104  Max      6304
      (Coulometric), ppm                   200      2007
                                                    ASTM D
 5    Cetane Index, -                  52  Min 46   4737
                                                    2010
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Article Details
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Author:Kumar, Dhinesh; Raju, Ashwhanth; Sheth, Nitin; Digeser, Steffen
Publication:SAE International Journal of Engines
Article Type:Technical report
Geographic Code:9INDI
Date:Sep 1, 2016
Words:6258
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