THERMAL PROPERTIES
Latest conclusion:
With the 90 cpsi technology the LiqTech WFF offer a soot load capacity at ~18 g/L in the expected life time of the trap. Such traps has been tested above 20 g/L without destroying the trap. With the 150 cpsi technology the WFF have a soot load capacity over 10 g/L.
Conclusions based on information below and various other SAE articles and practicle tests.
The filters used in the regeneration tests were uncoated, and no additive was used in the fuel or sprayed onto the filter to promote regeneration. These methods would reduce the regeneration temperature, and thereby normally give lower thermal loading than the tests with uncoated filters. With the latter, high temperatures are required to initiate a regeneration, resulting in high thermal loading. All tests were performed on low sulfur Shell City Diesel with less than 50 ppm Sulfur.
Pictures show the 5.8 litre segmented LiqTech WFF regenerating quite brutally as described below! And further mounted on the engine dynomometer.
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Material
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SiC
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Cordierite
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Radius - mm
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95
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95
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Length - mm
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205
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203
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Total volume - liter
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5.8
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5.8
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Active volume - liter
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5.8
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4.3
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Filtration area - m2
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2.9
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2.8
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Pore size - µm
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6 - 12
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34
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Wall thickness - mm
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0.8
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0.43
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Cell pitch - mm
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2.8
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2.53
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Specific area / m2/liter
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0.5
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0.48
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The engine regeneration tests were conducted on a Mercedes OM 616 4-stroke diesel engine mounted on a Super Flow SF-901 dynamometer. The dynamometer was equipped with external data acquisition for monitoring pressure drop and temperatures in exhaust and various places in the filter, as shown in Figure. Data was collected each 5 sec. and temperatures were measured with type K thermocouples (Ø 1.0 mm) introduced in the filter channels from the outlet side after soot accumulation. The filter compartment was provided with a simple diffuser to even out the soot and thermal load. The filter canning was open to the atmosphere at the outlet end. The monoliths were wrapped in an expandable insulation mat of the type Interam, XD, 4200 g/m2. Filter specifications are given in Table, the Stobbe SiC filter was composed of four quadrants. The Cordierite filter was a one-piece Celcor EX-66. Soot was accumulated under accelerated loading conditions with throttled intake, 37 % of rated speed and 50 % of max. power giving exhaust temperatures in the range of 400 to 450 °C. Before each regeneration, the filters were dismantled from the engine, released from both the canning and the insulation mat and weighed to an accuracy of 0.1 gram. After remounting the filter to the engine, the engine was run at accumulation conditions without throttled intake for another 5 minutes in order to reach reproducible temperature conditions from test to test. The additional soot accumulation during this 5 min. period was negligible.
THERMAL INERTIA - The term thermal inertia is defined as the inverse of the adiabatic temperature increase of the filter material possible for a certain soot load. Though disregarding the heat loss due to gas flow through walls and channels, the thermal inertia expresses the possibility of obtaining sharp exothermic peaks during uncontrolled regeneration: where: ms is the soot load in mass per area, Hu is the heat of combustion of the soot, cp is the specific heat of the filter material. Msp is the specific mass which is dependent on the design of the filter. One unit length of a channel has the mass of filter material given by:
where: t is the wall thickness and w is the channel width, as shown in Figure, P is the pore fraction and the material density.
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Step
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Continuous regeneration
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Severe regeneration
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1
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1300 rpm, 65 Nm, 5 min
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1300 rpm, 65 Nm, 5 min
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2
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3000 rpm, 110 Nm, 15 min
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3000 rpm, 105 Nm, 300-500 sec
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3
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1000 rpm, 8 Nm, continuing
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3500 rpm, 115 Nm, 100-200 sec
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4
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-
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1000 rpm, 8 Nm, continuing
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Regeneration modes for continuous and severe regeneration. Two types of regeneration were selected for study, a continuous regeneration and a severe regeneration. The former is an example of a moderate severity, because there is adequate flow to remove the energy released from the regeneration. The latter corresponds to the case where the regeneration is started and then the flow substantially reduced and oxygen content increased. In this situation particularly, the thermal inertia of the filter is expected to be a significant factor in determining temperatures.
SEVERE / UNCONTROLLED REGENERATION - The second step in the cycle raises the temperature in the whole filter starting a slow but continuous oxidation. When the temperature is stabilized, the rpm and load are increased in order to raise the filter temperature another ~100°C, then the engine is put into idle in order to initiate an uncontrolled regeneration. The severity of this cycle is controlled by the initial soot load on the filter and the length of the second and third steps during which the soot load is slightly diminished. In Ex-66 figure and Figure SiC, a SiC 5.8 liter filter is compared to a 5.8 liter Cordierite filter. Severe uncontrolled regeneration was performed according to the cycle given in Table 3. It is clearly seen that the temperature increase in the beginning of step 4 is lower in the case of SiC and that the temperature peaks during the uncontrolled oxidation are substantially higher for the Cordierite. The increase in filter temperature is consistent with that predicted by a comparison of the thermal inertias of the filters. A consequence of the lower temperatures is that the oxidation process in the SiC filter ceases and regeneration is limited to half the amount of the available soot compared to nearly full regeneration for the Cordierite filter.
These regeneration events have also been analyzed with the dynamic model described in the previous section. The model input is the material characteristics from Reference (1) and regeneration data chosen as close to experimental conditions, that is a reaction rate of 0.2 cm/s giving a total regeneration time of 150 seconds, exhaust gas temperature and initial filter temperature of 550°C and 700°C respectively. The remaining parameters in the model which are not defined are then restricted to the width of the reaction zone, the regeneration efficiency and an arbitrary convection factor controlling the rate of heat removal by the exhaust gas flow. The convection term was determined by comparison with temperature changes during periods where the filter changed in the absence of regeneration reactions. The higher thermal inertia of the SiC filter reduces the effects of the thermal loading during severe regeneration thus restricting the temperature peaks to fractions of the peaks seen in Cordierite filters. This feature will reduce the risk of unrestrained regeneration.
Check out SAE 950151 for more details
