Soot filtration and regeneration behaviour of particulate filters for Diesel passenger cars

The economic diesel engine with it's low fuel consumption as vehicle powertrain is an important contribution to the intensively discussed CO₂ challenge. Unfortunately, diesel engines typically have high NO_x and particulate emissions. Apart of improvements in engine technology, those emissions can only be reduced applying complex exhaust after-treatment devices. In the last years, DeNO_x systems and particulate filter systems have been intensively developed and partially been applied in series. Tenneco Automotive in Edenkoben (Germany) is currently running an investigation programm for optimisation of particulate filter systems for passenger cars.

1. Introduction

The advantages of modern direct injection turbocharged diesel engines are lower fuel consumption and higher torque at low speed ranges. These merits which are highly valued by the end consumer have led to a considerable increase in their market share within Europe. In some countries the 50 % mark has already been broken. An average market share of 50 % is expected in Europe in 2005.

Although some of the diesel passenger cars produced by leading manufacturers have already reached limits which fall below those set for Euro 4 for the year 2005, the image of diesel engines with respect to harmful emissions is not particularly positive. As the Title Figure clearly demonstrates, during emission tests Euro 3 certified vehicles still emit clearly visible amounts of particulates which are deposited on the test filter plate and blacken it. As opposed to this, in tests with the same vehicle but equipped with a DPF system the filter plate is still white after the emission tests - the emitted particulates have been filtered out almost 100 %.

There are concerns that these ultra fine particulates that are absorbable by the lungs can cause lung cancer. Although this suspicion has never been conclusively proven in laboratory tested animals or in humans, certain institutions such the Federal Environmental Agency (UBA) and Greenpeace believe that by using particulate filters in vehicles with diesel engines the risk of cancer can be reduced.

According to statements made by the UBA they plan to advocate a further reduction of the permissible limits for particulate emissions which will effect the use of par- ticulate filters. If necessary, tax concessions should promote the introduction of such technology. Contrary to this, the German automobile industry, in particular, is of the opinion that particulate emissions should be preferably reduced by measures carried out on the engine. Their claim is that particulate filters would have a negative impact on fuel consumption and engine performance and would cause a considerable increase in manufacturing costs. Furthermore, reliable, periodical regeneration of the filter charged with particulates would not be without problems. The filter would have to be serviced and this would increase running costs.

Despite of all the possible optimisation steps on engine side such as turbo charging, combustion control and, in particular, optimisation of the dynamic operating mode, it is expected that in view of particulate emission the diesel engine will never reach the level of gasoline engines. Contrary to the situation in California and Japan, currently defined European particulate limits do not require the use of particulate filters. However, in view of the ecopolitical situation and in order to improve the image of diesel engines their use would seem to be unavoidable in the future. Having recognized these ecopolitical aspects, PSA introduced particulate filters in the year 2000 in several Peugeot and Citroen brand vehicles. Our company was involved in the development of these particulate filters. In order to have long-term, reliable operation the choice of the filter material and careful adaptation of the regeneration strategy is of crucial importance.

2. The Diesel Particulate Filter System (DPF)

Diesel particulate consists of a core of elementary carbon (C) surrounded by chemical exhaust gas components such as polyaromatic hydrocarbons (PAH), sulphates, water and metal oxides from engine wear and oil additives. Conventional oxidation catalysts convert only the organic fractions and thus contribute in only a small share to the reduction of the particulate emissions (< 30 %). However, these catalysts are not in the position to completely remove the particulates. At present, the only reliable method is by filtering the particulates using a suitable filter element and subsequent regeneration. During the regeneration process the soot particulates are burnt off after the appropriate thermal conditions are met.

2.1 Filter Elements

In the past decades various, mostly very effective, filter technologies have been developed for filtering out particulates. Principle distinction is made between the following typical types:

• Honeycomb filters: these are made of cordierite or silicon carbide (SiC) using an extrusion procedure. Other materials are in the course of development. The exhaust gas flows through the wall of the filter (wall flow filter), whereby the soot particulates are filtered out and a layer of soot is formed (surface filter)
   
• Cartridge filters: these are made mainly from ceramic fibres that have been rolled up to form so-called filter candles. The fibres consisting of silicon- or alumina- oxides are specially treated to improve filtration efficiency. During flow through the cartridge filter the particulates are deposited between the fibres (deep-bed filter). A soot layer is not formed
   
• Sintered metal filters: in this case metallic fibres are sintered together to make filter plates. Various types of filter shapes can be formed from these plates. During flow through the filter plate the particulates are deposited on its surface (surface filter)
   
• Ceramic foam filters: these foam parts are made of aluminium oxide or SiC. When the exhaust gas flows through, the soot particulates are deposited in the structure of the filter (deep-bed filter). The filtration efficiency can be controlled by the pore size

Figure 1 shows the ratings that are made for the various types of filters with respect to specific performance parameters. Apart from costs, the filtration efficiency, the maximum permissible soot accumulation before regeneration (temperature stability, regeneration frequency), the ash storage capacity (back pressure) and the ash cleaning capability are also decisive when choosing a filter. Further benchmark criteria are weight, availability, customer acceptance and compatibility with the various regeneration methods.

The rating matrix shows some "knockout" criteria for different filters. For example, low acceptance is found for the conventional cordierite filter for passenger cars due to its bad thermo-mechanical properties, in spite of its considerable cost advantage. Also fibre filters find low acceptance because of their high costs and their low filtration efficiency. The basis used for the evaluation is the Ibiden SiC filter that is already in use in serial production in passenger cars. However, there is need for improvement, above all, with respect to cost, weight and the ash storage capacity, which in general determines the size of the filter. Furthermore, if extensive application of the DPF-system would start in all European diesel passenger cars, availability could cause problems. Therefore, it makes sense to examine alternatives to the Ibiden SiC filter with similar characteristics.

2.2 Regeneration Strategies

Figure 2 gives an overview of the main possibilities that exist for oxidation (burning off) of the soot particulate.

In order to effectively burn a C-particle with oxygen (O₂), high temperatures (> 600 °C) are needed. In diesel exhaust systems, these temperatures can hardly be met even under full load operation.

Through the use of additives based on cerium (Ce) or iron (Fe) these ignition limits can be reduced. In principle, the additive is mixed with the fuel, but it is also possible to coat the additive directly onto the filter. Using additives which are ready available today soot burns at approx. 500 °C. During the course of development of the additives a further reduction of the ignition temperature can be expected.

The advantage of additive usage is that the ignition temperature is reached during regular driving more frequently or that, during low-load operation, the expenditure of energy needed to reach regeneration temperature is lower. Apart from this, regeneration is also considerably accelerated by the additive resulting in a shorter regeneration phase. On the other hand, additives produce residual ashes that in addition to the unavoidable oil ashes may clog the filter sooner. Furthermore, during the highly exothermal regeneration with an additive, high temperature peaks occur which must considered when choosing the filter material and its support system. The aim of future additive developments, therefore, is especially to use reduced dosages (less ash) and to have smoother regeneration temperatures profiles.

In principle, it is also possible to burn off the soot with nitrogen dioxide (NO₂). Here much lower temperatures (approx. 250 °C) are needed than when using O2. This oxidation is very slow and nearly not exothermal. The NO₂ is produced by oxidation of NO, either in an upstream Oxicat (CRT^TM from Johnson Matthey), or by the catalytic coating of the filter itself. Unfortunately, the possibility of NO₂-regeneration is limited to 450 °C because NO₂ is not stable at higher temperature levels. Furthermore, it should be noted, that an appropriate NO to soot ratio is crucial for acceptable efficiency, which might become an issue for future NO_x-optimised vehicles. Whether O₂ or NO₂, both procedures have in common that they do not work under certain normal driving conditions (e.g. low-load operation in town traffic). Additional energy, therefore, is always needed in order to achieve regeneration conditions. This results in a discontinuous accumulation and regeneration process. There are various possibilities for providing the energy required:

• Post-injection of fuel and exothermal reactions in the Oxicat
• Electric heater or ignition device (available electric power in the 12V system is max.1.0-1.5kW!)
• Fuel Burner
• Non Thermal Plasma (NTP), whereby the soot is regenerated by oxidation with O-radicals, which can be created by the plasma even at the lowest temperatures. The electrical power required for today's systems is under 1 kW

Looking again at Figure 2, different strategies for the regeneration are indicated:
1. Increase of the temperature into the NO₂ regeneration range
2. Increase of the temperature into the O₂ regeneration range
3. Increase of the temperature into the additive supported O₂ regeneration range.

Generally, the supply of energy to initiate the regeneration process increases the fuel consumption. On the other hand, soot accumulation in the particulate filter increases the back pressure and thus also increases fuel consumption. Figure 3 shows the regeneration characteristics for two regeneration strategies (1 and 3)

In the first strategy when critical backpressure level has been reached only little energy needs to be provided in order to reach an exhaust gas temperature of approx. 300 °C. Regeneration takes place very slowly and is not complete. Although less energy needs to be provided it has to be provided over a longer period. As the filter is only partly regenerated it still has a high backpressure level (fuel consumption) and needs to be regenerated again after only a short driving period.

As opposed to this, in the third strategy more energy has to be provided to achieve the ignition temperature of approx. 500 °C but this only for a short time because as a result of the additive supported reactions the filter is already completely regenerated after 3 - 5 minutes. As the filter is completely regenerated, the backpressure level is low again and the time till the next regeneration is comparatively long.

It can be clearly seen that the additive supported strategy 3 is energy-wise the more favourable.

The design of a DPF must always be made in direct relation to the chosen regeneration strategy. Figure 4 shows the energy balance for two filter sizes and the respective amount of regeneration energy required. On the assumption that there is a maximum permissible backpressure level, a small filter has to be more frequently regenerated than a large filter. The bars on the chart showing the sums demonstrate that it is more advantageous from an energy point of view to have a larger filter and therefore less frequent regeneration. Furthermore, due to its bigger ash storage capacity, a larger filter increases much slower in back pressure with ash accumulation which again has a beneficial effect on fuel consumption. Opposed to this are the higher costs that arise for larger filters, the heavier weight and, possibly, space limitations.

3. The PSA DPF-System

In the year 2000 PSA became the first car manufacturer to introduce a filter system for diesel engines in passenger vehicles in Europe. Information on this system has already been widely published. It is in serial production at Gillet. As it represents the gauge against which other filter concepts are measured, it is described briefly once more in the following:

The filter element is an extruded and segmented honeycomb made of SiC.

Pressure sensors monitor the amount of soot accumulation in the filter in the exhaust system as well as by an accumulation model in the engine control unit developed by PSA. Finally, if both controls do not indicate regeneration, it will be initiated automatically all 620 km.

_ In order to support the regeneration process, an additive based on Cerium is added to the fuel. For each refuelling it will be automatically mixed to the fuel from a tank which is integrated in the fuel tank. The additive tank has sufficient additive for approx. 80,000km and is refilled during the service inspection.

Regeneration conditions are set up in two steps: through retarded injection the oxidation catalyst near the engine is activated, in which then post-injected fuel is highly exothermally "burnt". As a result the required regeneration temperature of 500 °C is reached. Thus, PSA regenerates in accordance with strategy 3 described above.

PSA regeneriert somit nach der oben beschriebenen Strategie 3.

At present a cleaning process removes the ashes in the filter. Field results have shown that the service interval could also be significantly lengthened without any technical changes having to be made.

4. The Gillet Advanced Engineering DPF-Programme

As an exhaust system supplier, Gillet's main interest in a particulate filter system is the filter itself. In view of the various performance criteria as cost, thermo-mechanical durability, regeneration characteristics, backpressure and ash management appropriate filters must be selected and applied. As already described, these filters must be carefully adapted to the regeneration strategy, selected by the car manufacturer, in view of layout (material, volume, position etc.) and design (support system, geometry, flow impingement etc.). In the Gillet advanced engineering programme the performance of the various filter technologies is continuously being examined and compared.

4.1 The Test Benches

An engine test bench with a supercharged 2.0 litre diesel engine with exhaust recirculation is available for the soot loading and regeneration tests. The engine is equipped with common-rail injection. Regeneration of the particulate filter can be initiated via access to the engine management. Soot and ash deposits on the filter are determined using a high accuracy-weighing machine under clearly defined conditions.

The soot and ash distribution for various soot accumulation conditions is determined on the cold flow rig by measuring the flow distribution over the outlet section of the filter. The pressure loss over the filter can also be measured very precisely and reproducibly.

A vehicle is available for tests on the emission roller dyno for measuring the limited emissions in the standard emission cycles.

The efficiency of the filter canning is tested on the hot vibration test bench.

4.2 Tested Products

During filter benchmarking the Ibiden SiC filter was examined in direct comparison with the Denka SiC filter. The specifications of both filters are summarized as follows:

Ibiden filter:
D = 5.66" x 6" (2.47l) and 5.66" x 10" (4.12l),180cpsi/14mil, typical pore diameter = 10µm

Denka filter:
D = 5.66" x 140mm (2.27l), 169cpsi/14mil, typical pore diameter = 20µm.

Further tests were carried out with an HJS sintered metal filter. As ist development status does not meet with the current status, the results are not published in this article. Tests using the new generation of the HJS sintered metal filter (the "Jetfilter") are ongoing.

Compared to the known cerium additive Eolys^TM from Rhodia that is used today in series production, and of which a dosage of 25ppm is normally mixed in the tank with the diesel fuel, three further additives were tested. The additives in question are new cerium and iron additives that in combination with highly effective filters have been approved by the German Federal Environmental Agency (UBA). Furthermore, an additive containing platinum was tested. The dosing rates were made according to the suppliers' recommendation.

4.3 Test Cycles

In order to examine regeneration behaviour, the filters were loaded with differing amounts of soot in a soot accumulation cycle, Figure 5, simulating as closely as possible the ECE-test cycle. In this cycle the exhaust gas temperatures are typically between 150 °C and 250 °C, which prevent stochastical regenerations. After reaching a defined soot loading level, the filter is removed and soot distribution and backpressure measurements are carried out on the flow test bench.

In the subsequent regeneration cycle, Figure 6, speed and load are increased, which at first also increases the backpressure. As soon as this begins to sink again, indicating the start of regeneration, the engine is switched to idle. Now, the high amounts of oxygen in the exhaust gas cause the highest possible exhaust gas temperatures during regeneration. In order to examine the ash behaviour the individual additive dosages were increased tenfold to shorten the test time. In the ash accumulation cycle the engine was operated at 3500 1/min and full load interrupted all 4 hours by 20min idling. Since the filter is operated at about 550 °C the soot regeneration is ensured. The increase in back pressure results therefore only from the accumulated ashes. However, it should be considered here, that because of the additive over-dosage the relation between oil ashes and additive ashes is different from that which occurs under real driving conditions.

5. Results

All the additive test series were evaluated considering the following performance criteria:

• Ash production and the resulting increase in back pressure
• Ignition temperature (start of regeneration)
• Peak temperature during regeneration
• Time delay of the exothermal reaction
• Filter damage

5.1 Maximum Permissible Soot Accumulation

When determining the maximum permissible soot accumulation of a filter, two aspects have to be taken into consideration:

• How does the filter back pressure increases with the soot accumulation?
• Which is the critical soot loading where filter damage occurs? (with additive!)

Figure 7 shows the backpressure behaviour of the Ibiden filter in comparison to the Denka filter. As the filters were of different sizes, the deposited amounts of soot were given in g/m² instead of g/l. At low soot loading the Ibiden filter showed better backpressure behaviour. However the Denka filter proved better with higher loading. The break-even point is at approximately 5.6 g/m². This behaviour can only be explained by differing soot storage behaviour that will be confirmed by microscopic structure analysis.

Filter damage (fissures) were determined in the Denka filter at an accumulation of 23 g/l. Similar damage did not occur in the Ibiden filter until 28 g/l.

5.2 Partial Regeneration

What happens when regeneration is interrupted, for example if the engine is turned off prematurely? The consequences of such partial regeneration were examined using an Ibiden filter. The filter was first loaded homogenously with 14 g/l soot, Figure 8 b.

The subsequent regeneration was interrupted at 8 g/l. Figure 8 c clearly shows that the hot central areas were regenerated first. A relatively high accumulation remained in the surrounding area. Next the filter was burdened again with 14 g/l soot, whereby the soot again was more or less evenly deposited over the filter cross-section. As a consequence, the surrounding edge area was overloaded, Figure 8 d resulting in extreme local temperature peaks above 1050 °C during the subsequent regeneration. As an interruption of the regeneration cannot always be avoided such temperature peaks must be taken into consideration when selecting and applying the filter.

5.3 Trapped Ashes and Pressure Loss

Ash deposits in the DPF during accelerated ash loading on the engine test bench differ in general from the deposits that have been found after normal road driving conditions. Figure 9 a shows an example of the wall deposits, which is typical for tests carried out on the engine bench. The x-ray picture in Figure 9 b shows, however, that under vehicle operating conditions on road the ashes fill the filter from the back to the front. Here, using a new additive, it was found, that after approx. 70,000 km the 5.66" x 9" filter was only filled up to about 15 % of ist ash storage capacity. Optimistically it could be concluded that with a slightly bigger filter, even when using an additive, the useful life of the engine (approx. 240,000 km) could be covered without cleaning the filter.

Figure 10 indicates, how ash deposition affects the backpressure of honeycomb filters. The wall deposits reduce the hydraulic channel diameter that increases the pressure loss exponentially with a shift of four. When the filter is filled up from the back to the front, the filtration surface will be reduced, which has up to a certain level a much lower impact on pressure loss. These effects are of course taken into account when predicting the pressure loss for different loading scenarios with our prediction tool.

5.4 Comparison of the Additives

Figure 11 shows the comparison of the various additives with respect to ash production, ignition temperature and peak regeneration temperature at filter outlet. All the additives examined performed significantly better in comparison to Eolys^TM, which clearly documents the progress which has been made in the course of the development of additives. For all tests, regeneration was initiated at 20 g/l of soot loading.

As opposed to Eolys^TM and additive A which both need an exhaust temperature of approx. 500 °C to ignite, regeneration with additive C occurred already at 420 °C. This temperature level is reached considerably more often under regular driving conditions which significantly reduces the frequency of forced regeneration and thus the expenditure of energy. A slight disadvantage of additive C is the peak temperature behind the filter of 600 °C. It is in fact 100 °C lower than that of Eolys^TM, but considerably higher than that of additive A. Here, only temperatures of max. 400 °C occur due to the somewhat slower regeneration process, which could lead to the use of possibly more cost favourable alternatives to the SiC filter. It should, however, be clarified once more that these peak temperatures were measured behind the filter. Inside the filter significantly higher temperatures are reached.

Compared to Eolys^TM it was also possible to reduce the ash production by more than 50 % due to the significantly lower dosing rates that are required for the new additives. This also has a positive effect on the back pressure which for additives B and C is about 40 % lower than for Eolys^TM after about 50 hours accelerated ash loading on engine bench. This result is in good correlation to the result of the 70,000 km vehicle test described above, Figure 9 b.

6. Filter Back Pressure Prediction

Based on the numerous test results obtained in the filter benchmark study, a calculation model was developed that predicts the pressure loss in honeycomb filters. Besides soot accumulation, the deposit of ashes from oil and the additive are also considered in the calculation.

The following flow losses occur during flow through the filter, Figure 12:

• Channel in- and out-flow (turbulent)
• channel flow (laminar with turbulent inlet effects)
• flow through the porous filter wall (laminar and turbulent)

In 1999 (SAE1999-01-0468) A.G. Konstandopopoulos and E. Skaperdas presented a non-linear differential equation of the 2nd order for the calculation of local channel outlet velocity, considering, in particular, the turbulent losses in the porous walls at high mass flow rates.

Our contribution to the further development of this model consists in numerically solving the above-mentioned equation. The parameter, describing the turbulent effects of the flow through a porous medium could be precisely derived from our measurement data. Finally, we added to the model the above-mentioned losses of the channel inflow and outflow, so that we are able to calculate a complete filter, now. Figure 13 demonstrates the excellent correlation of our model with very precise measurements taken for filters at different soot loadings even at high flow rates.

Details of the calculation program will be given in a separate presentation in the autumn of 2002.

7. Outlook

During the last two years, our company has produced and supplied more than 100,000 particulate filters to PSA. So far, no field failure were observed or reported that could raise doubt about the functionality of the filter system. The current fuel additive used, Eolys^TM, which is based on cerium, has proven to be highly effective and reliable. The ash deposits resulting from the additive were considerably less than expected. As was shown in our study, the amounts of ash can be further reduced by the development of new additives. This would enable the possibility of a more compact filter or could enable an extension of the cleaning intervals to above 200,000 km.

The choice of silicon carbide (SiC) as a material for the filter and the monolith with a channel structure in combination with the regeneration strategy has also proven to be efficient.

Even if particulate filters are not required in order to meet current legal gravimetric particulate emission limits, the serial use of filters seems to be an appropriate measure to take in order to improve the image of diesel passenger cars. Finally, with its significantly lower fuel consumption when compared to gasoline engines, the Diesel will have a positive contribution in meeting the global CO₂ emission targets for climate protection.