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Overview of Diesel particulate filter systems sizing approaches
Applied Thermal Engineering 121 (2017) 537–546
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Overview of Diesel particulate filter systems sizing approaches Antiopi-Malvina Stamatellou, Anastassios Stamatelos ⇑ University of Thessaly, Mechanical Engineering Department, Volos, Greece
h i g h l i g h t s Application of particulate filters to large Diesel engines is moving at slow pace. Rational sizing of the filters becomes essential in these applications. An overview of Diesel particulate filter systems sizing approaches is presented. An improved filter sizing methodology is necessary for large engines. Filter system design improvements may significantly reduce filter size.
a r t i c l e
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Article history: Received 24 January 2017 Revised 4 April 2017 Accepted 9 April 2017 Available online 26 April 2017
a b s t r a c t Although application of Diesel particulate filters in modern automotive Diesel engines is commonplace, their introduction to large Diesels as locomotive or marine engines is moving at a slower pace. One important reason for this delay is the large volume of filter required which is not easy to accommodate in this type of equipment. Thus, rational sizing of the filters becomes essential in these applications. It is observed that DPF systems for large Diesel engines are usually oversized. Possible reasons are discussed in this paper. With the present status of technology and the concern for compact and cost-optimized exhaust treatment systems a new design methodology is needed. This paper summarizes progress in the specific fields of application and attempts to formulate a filter sizing methodology that would lead to feasible solutions with regard to space requirements and backpressure penalty. Ó 2017 Elsevier Ltd. All rights reserved.
Contents 1. 2. 3. 4. 5.
6. 7.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Filter backpressure characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PM emissions standards for various engine categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Marine engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of engine size and power on DPF size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors affecting average pressure drop in diesel filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Engine-out particulate emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Filtration efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Vehicle driving mode or engine operation mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of filter size on regeneration behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Today, application of Diesel particulate filters (DPF) [1] is commonplace in automotive Diesel engines. As particulate emissions ⇑ Corresponding author. E-mail address: [email protected] (A. Stamatelos). http://dx.doi.org/10.1016/j.applthermaleng.2017.04.096 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
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standards become increasingly stringent for heavy duty engines and off-road machinery, Diesel particulate filters are also introduced to these engine categories. On the other hand, the evolution of emissions standards and aftertreatment technology resulted in the priority of NOx aftertreatment by means of SCR (selective catalytic reduction) over particulate in heavy duty engines [2–6]. DPF may be applied in the near future to big inland waterway vessels,
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Nomenclature
Dp L D cpsi ww
a
R
l
Mg k0soot kw d
qs
pressure drop [Pa] filter length [m] filter diameter [m] cells density [cells/in.2] filter wall thickness [m] channel Dp coefficient ideal gas constant [J/molK] dynamic viscosity of exhaust gas [kg/ms] molecular mass of exhaust gas [kg/kmol] layer permeability [m2] ceramic wall permeability [m2] channel hydraulic diameter [m] soot layer density [kg/m3]
or even to ships in specific congested areas with strict emissions regulation. Filter material is Silicon Carbide (SiC), Fig. 1, and cordierite [7] (2MgO-2Al2O3-5SiO2, Fig. 2). Filtration efficiency varies in the range 70–95% of total particulate matter. Higher efficiency, exceeding 95% is reported in the literature for the solid particulate fraction, which involves elemental carbon and metal ash. However, much lower efficiencies are reported for the soluble organic fraction (SOF) of the particulate [8–10]. A large variety of filter system design concepts and regeneration techniques (filter cleaning by incineration of the accumulated particulate) exist to date, at various production stages [11–19]. Table 1 presents a classification according to the main regeneration techniques employed. Different regeneration techniques may be combined in a single system (e.g. engine management combined with catalytic fuel additives) [20,21]. The need for a compact size of DPF to be fitted underhood, close to the engine to keep exhaust temperatures high, along with the mass produced of millions of units since 2000, resulted in compact sizes of DPF for passenger car application [17,20]. On the other hand, DPF for large engines are not yet mass produced and in several cases tend to be rather oversized. The reasons could be related to the low degree of commercialization, but also to some unrealistic requirements of conformance tests that are discussed here. As a matter of fact, most of the systems in production stage are oversized, leading to higher installation costs, unnecessary reduction in useful space and other side effects. These problems could be partially eliminated by proper sizing of the filter. The reduction of filter size, in addition to the reduction of the equipment cost, leads to increased filter temperature levels, due to reduced thermal inertia and heat losses of the filter.
N Af Vd w
number of open channels total filtration area [m2] engine displacement [m3] soot layer thickness [m]
Abbreviations DPF Diesel Particulate Filter SiC Silicon Carbide SOF soluble organic fraction VOF volatile organic fraction
Fig. 2. Cordierite filter for an off-highway machine.
Fig. 1 shows a photo of SiC Particulate filter for a 2-L automotive Diesel engine, with a higher magnification photo showing the soot layer deposited inside the channel and an electron microscopy photo showing the texture of the different porous layers. Fig. 2 shows a cordierite filter for an off-road machine (loader). Cordierite is selected in most cases for off-highway machinery. The design
Fig. 1. SiC Particulate filter for a 2-L automotive Diesel engine, with higher magnification photo showing the soot layer deposited inside the channel and an electron microscopy photo showing the texture of the different porous layers [22].
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Table 1 Classification of existing DPF systems according to regeneration techniques employed [17].
Continuous regeneration
Discontinuous regeneration
Active regeneration Generation of reactive agents: Plasma regeneration (Ozon, OH-radicals, NO2) Micro wave regeneration Exhaust gas Burner Electric Heater Electric oven (dismantling filter) Engine management increases exhaust temperature (e.g. post – injection, EGR, compressor bypassing)
and mass production of filters with more channels per unit inlet area has reduced filter size, since a smaller filter size may accommodate the same soot mass with similar backpressure levels. Moreover, as explained above the filter stays warmer and regenerates more frequently. Due to the interaction of a significant number of parameters with filter size, before formulating a sizing methodology, it is useful to discuss the effect of each parameter. Current commercially available DPF systems for heavy duty engines demonstrate significant variations in substrate size, as normalized with respect to engine volumetric efficiency and nominal power. It seems that, after three decades of DPF application, there is no universally acceptable philosophy of filter sizing for the specific engine category. On the other hand, substrate sizing turns out to play an essential role in overall performance, reliability, cost and feasibility of a DPF system. A role which becomes increasingly critical as the engine size increases. The purpose of this paper is to present an overview of the situation in the different Diesel engine categories and suggest rational guidelines for filter sizing, taking into account the boundary conditions for each engine category and application. 2. Filter backpressure characteristics A fundamental requirement for a specific DPF system is to produce an acceptable backpressure which does not significantly affect fuel consumption [23]. This sets a lower limit to filter size. Pressure drop across the filter is normally produced by the following factors [23–26] (see Fig. 3): Dp wall: affected by porosity distribution, mean pore size and wall thickness.
Passive regeneration Generation of reactive agents: CRT System (NO2 production upstream DPF) Catalytic fuel additive Catalytically coated Filter
Dp channel: pressure drop from flow friction within the channels. Dp soot: the accumulated soot acts as an additional filter and imposes additional pressure drop. As seen in this Figure, exhaust backpressure of an engine with a specific filter installed increases almost linearly with exhaust flowrate assuming a constant exhaust temperature (the effect of exhaust temperature is seen separately in Fig. 4). The data of this Figure are produced by simplified modeling of filter pressure drop: When one takes into account the effect of exhaust temperature on exhaust density, the effect of exhaust gas flow rate on the three above-mentioned components of filter Dp may be approximated by the following expressions [24,27]:
Dpchannel ¼
a l R T L m_ Mg N p d
4
ð1Þ
Dpwall ¼
_ R T l ww m M g p Af k w
ð2Þ
Dpsoot ¼
_ RT lm W M g p Af k p
ð3Þ
where the dynamic viscosity of the exhaust gas is approximated by the following expression:
l ¼ 3:55 107 T 0:679
ð4Þ
Different ceramic filter composition yields a different increase in pressure drop over a given time, because of the variation in mean pore size and porosity distribution. However, the requirements for high filtration efficiency led to optimized ceramic filtration properties producing specific, acceptable levels of pressure drop
Fig. 3. Variation of the three components of filter pressure drop versus exhaust gas flowrate (Sic DPF, 5.66 6 in.).
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Fig. 4. DPF backpressure as function of engine exhaust gas temperature and flowrate (5.66 6 in. filter, 2-L engine).
(<200 mbar). The calculations of Fig. 3 are assuming a 144 mm diameter 152 mm length SiC DPF with 200 cpsi, 0.4 mm wall thickness, 9 lm mean pore size and an exhaust gas temperature of 200 °C. The variations of soot permeability (kp in Eq. (3)) with the type of soot deposition are even more pronounced. Moreover, soot particles demonstrate a significant ability for water adsorption [28] and hydrocarbon adsorption, which may significantly decrease permeability as water and hydrocarbons impregnate the porous structure of the soot. As regards exhaust temperature, as shown in Fig. 4, filter backpressure increases with exhaust gas temperature, due to the increase in exhaust gas velocity. However, the calculations of this diagram are considering only dry soot, without the effects of adsorbed water and hydrocarbons that complicate matters [8]. Backpressure levels should be kept low, because backpressure reduces engine torque and increases brake specific fuel consumption (bsfc) of the engine.
An increase in exhaust backpressure leads to an equivalent decrease in engine mean effective pressure, as approximated by the following expression [29]:
I
W cycle ¼
pdV þ ðpin pex Þ V d
ð5Þ
The effect of DPF backpressure on the engine fuel consumption may be assessed based on the additional fuel consumption required to restore engine output. For example, an increase of 200 mbar in average filter backpressure leads to a 2% deterioration in brake specific fuel consumption [29]. Naturally, fuel consumption increase is also affected by the vehicle driving mode. Exhaust backpressure of a loaded filter increases rather sharply with the particulate mass accumulated in the filter channels (see Fig. 5, based on calculations with Eqs. (1)–(3)). Keeping a low level of DPF loading requires frequent regeneration.
Fig. 5. Effect of soot loading on filter backpressure (5.66 6 in. filter, 2-L engine).
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Fig. 6. Effect of varying filter size (varying filter length with constant diameter of 5.66 in.) on the pressure drop characteristics of a 2-L car engine with constant exhaust gas temperature of 200 °C.
Fig. 6 shows exhaust backpressure levels produced by different filter sizes on a passenger car engine, as function of exhaust flow (200 °C exhaust gas temperature). 3. PM emissions standards for various engine categories The PM emissions’ standards for passenger Diesel Cars were 0.14 g/km in the European Regulation Euro 1 in 1992 (European Driving Cycle, Maximum Speed 120 km/h, Total Distance 11.3 km). They were reduced to 0.1 g/km (1996) and further to 0.05 g/km (Euro 3/2000). From this point on the DPF became necessary. In 2014 the limit became stricter to 0.005 g/km. The same standards are valid for light trucks. The evolution of European Legislation was different for Heavy Duty Vehicles. Until 1999 only steady-state test was performed on the vehicle engine (ECE R-49, 13 points). Emission standards, expressed in g/kW h weighted over the 13 points, were 0.36 g/ kW h in 1992. They dropped to 0.02 g/kW h in 2000 (Euro III) and further to 0.01 g/kW h in 2013 (Euro VI). Since 1999 the engine is subjected to a transient test as well. The limit was set to 0.01 g/ kW h in 2013. After 2000 the DPF is gradually introduced in Heavy Duty Vehicles in order to conform to the standards [16]. Similarly, EU Stage IV emission standards for non-road diesel engines were introduced in 2014, setting the limit in PM emissions to 0.025 g/kW h. Fig. 7 shows a DPF combined with an SCR device to meet these requirements. To represent emissions during real conditions, a new transient test procedure—the Non-Road Transient Cycle (NRTC)—was developed in cooperation with EPA. The NRTC is run twice—with a cold and a hot start. The results are weighted 10% for cold start and 90% for hot start. The test is used in parallel with the steady-state test, ISO 8178 C1, referred to as the Non-Road Steady Cycle (NRSC) [31]. Application to larger Diesels as locomotive or marine engines is moving at a slower pace [32]. Nevertheless, there exist hundreds of retrofits to locomotive engines, in countries with strict emissions standards like Switzerland [11]. Although most retrofits were rather oversized, clean engines may also be produced by incorporating a compact DPF close to the engine (Fig. 8). In this way, a more rational filter volume is attained and the overall system per-
Fig. 7. Combined DPF and SCR after-treatment device for off-highway Diesel engine (EPA Tier 4, <560 kW, EU Stage IIIB) [30].
formance improved. According to EU Stage III/B (2012) emissions standards for rail traction engines PM emissions for locomotive engines are limited to 0.025 g/kW h which would need a DPF. Fig. 8 shows a more compact, optimized DPF installed by the engine manufacturer for a 2000 kW (76.3 L displacement) locomotive engine. A total of 10 £12 8 in. monoliths are included in the package (about twice the engine displacement – see Fig. 9). To activate filter regeneration, internal engine temperature management strategies as A/F reduction, compressor bypassing and postinjection are employed by the engine ECU to substantially raise the exhaust temperatures to the required levels. EU Stage III A legislation also includes Inland Waterway Vessels which were allowed to emit up to 0.5 g/kW h PM in 2009. The limit will be gradually adjusted to 0.015 g/kW h in 2020 for engines with Net power greater than 300 kW. The legislation is less demanding for Vessels with Net power less than 300 kW and may be accommodated without DPF.
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Fig. 9. Internal design of the MTU DPF of Fig. 8. External dimensions L W H = 2.72 2.1 0.9 m [33].
Fig. 8. SiC particulate filter for a locomotive engine (MTU rail engine 16 V 4000 RX4, 1: DPF, 2: engine) [33].
3.1. Marine engines For the purpose of emission regulations, marine engines are divided into three categories based on displacement per cylinder. Category 1 concerns small engines with less than 5 L displacement per cylinder. Category 2 concerns medium sized engines with 5–30 L per cylinder, with technology similar to locomotive engines (up to 8000 kW), used to provide propulsion for tugboats, supply vessels, fishing vessels, etc. also used for auxiliary power generators in ships. Category 3 marine diesel engines typically range from 2500 to 70,000 kW, powering container ships, oil tankers, bulk carriers, cruise ships. Emission control technologies for the last category engines are difficult to implement, mainly due to the residual fuel they are burning (a by-product of distilling crude oil to produce lighter products). This fuel has high viscosity and density, which affects ignition quality, as well as high ash, sulfur and nitrogen content in comparison to marine distillate fuels. Residual fuel parameters are variable because its content is not regulated. As an example, PM emissions of a large container vessel 2-stroke engine burning residual fuel with 2.05% sulfur lie in the range of 1.1–1.75 g/kW h (as PM2.5), for engine load ranging between 10–70% [34]. A crude oil tanker’s two-stroke engine, fuelled with residual fuel, was measured to a weighted PM mass emissions factor of 1.60 g/kW h for the main engine and 0.141 g/kW h for the auxiliary engine [35]. Emissions from marine diesel engines have been regulated since 1999 through a number of rules, applicable to different engine categories, overlapping with the regulations for mobile, landbased off-road machinery. The most recent are the 2008 Category 1/2 Engine Rule, introducing Tier 3 and Tier 4 emission standards for marine diesel engines. The Tier 4 emission standards are modeled after the 2007/2010 highway engine program and the Tier 4 non-road rule, with an emphasis on the use of emission aftertreatment technology. To enable catalytic aftertreatment methods, the EPA established a sulfur limit of 500 ppm in marine fuels (not residual fuels). The 2009 Category 3 Engine Rule, introduced Tier
Fig. 10. Soot filter for a yacht Diesel engine. Filter regeneration effected by the two burners at the left of the picture [36].
2/Tier 3 standards in harmonization with 2008 Amendments to IMO MARPOL Annex VI. Fig. 10 shows a DPF for a large yacht Diesel engine. The DPF system volume may be demanding in these applications and size optimization is critical in certain applications. Special measures could be taken to significantly reduce filter volume. As an example, compact high speed ferries could be equipped with low volume filters that will be engaged only at engine startup in the harbor [37]. Regeneration of this type of DPF could be effected by dismantling the monoliths and placing in an electric oven regeneration facility every one or two days.
4. Effect of engine size and power on DPF size The large oversizing of DPF for off-highway and larger Diesel engines can be seen in the example of Fig. 11. In this Figure, filter volume for Diesel particulate filters OEM installed in Diesel passenger cars is plotted versus nominal engine power [17,20,39]. In the same diagram, a series of Diesel filters offered by a specific DPF manufacturer is plotted as function of the engine nominal power suggested for retrofitting. Correlation of filter volume to
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engine power should be straightforward, since engine power is directly related to exhaust gas flowrate that is the major engine operating parameter affecting filter backpressure (Figs. 3–6). A linear regression to the data for the specific manufacturer shows that suggested DPF volume in liters is correlated to the nominal engine power by the following relation:
V filter ¼ 0:2Pengine 1:2
ð6Þ
where Vfilter is expressed in liters and Pengine is in kW. If one compares the respective data for the passenger car applications on the same Figure, it is apparent that filter size in passenger cars is significantly smaller for the same engine nominal power. The situation is better depicted in Fig. 12, where the ratio of filter volume to engine displacement is plotted. Volume of the DPF offered for retrofitting off-highway machinery is several times (up to 9 times) higher than engine displacement, while the volume of DPF applied by the car manufacturers as OEM equipment is only a little larger than engine displacement volume. This oversizing of DPF for off-highway engines can only in part be attributed to the higher engine-out emissions of this engine category. A typical PM emissions limit of 0.02 g/kW h is prescribed for off-road machinery compared to a limit 0.01 g/kW h placed for heavy duty vehicles. Of course, a retrofit filter size for the same displacement older Tier engine would require a larger filter size relative to a new engine of the same size/power output but with much lower engine-out particulate emissions. As already discussed in the example of Fig. 8 (locomotive engine), careful DPF system design and its placement close to the engine may lead to low filter volume also for these large Diesel engines. A hint to the reason for filter oversizing in retrofitting application may be found if one studies the California Air Resources Board (CARB) certification procedure for Diesel exhaust after-treatment systems [40]. Certification according to the level 3 requires the attainment of 85% or greater efficiency of the filter in the reduction of particulate matter. This certification takes into account the type of engine application (stationary, power generation, off-highway machinery, etc.) and the type of engine (Diesel, with or without turbocharger, without Exhaust-Gas Recirculation (EGR), mechanically or electronically controlled, certified off-road engines meeting 0.2 g/bhp-h
diesel PM or less based on certification or in-use emission testing). A minimum Exhaust Temperature level is set for Filter Regeneration (e.g. 400 °C by use of catalyst in fuel), which must be kept for a minimum of 30 min, to allow the completion of regeneration. During the test, the engine is allowed to operate for a maximum of 300–720 consecutive minutes below the above-mentioned passive regeneration temperature. The manufacturer may set a minimum number of cold start and 30 min idle sessions before filter regeneration is required (from 10 to 24 cold starts, see the example of Fig. 13). Also, a number of hours of operation is set (say 2000 h or more) before disassembly and cleaning of the filter is required. Finally, a specific fuel is employed for the test (California diesel fuel with less than or equal to 15 ppm sulfur or up to 20% biodiesel blend). The diluted and cooled diesel particulate that is collected during emissions testing consists of two types of particles: (a) fractal-like agglomerates of primary particles 15–30 nm in diameter, composed of carbon and traces of metallic ash, and coated with condensed heavier end organic compounds and sulfate; (b) nucleation particles composed of condensed hydrocarbons and sulfate [42]. However, the soot layer collected on the DPF wall has a modified texture and composition [34]. As an example, Fig. 14 presents the results of Thermogravimetric Analysis (TGA) and Heat Release Analysis by Differential Scanning Calorimetry (DSC) of two soot samples received from a central channel of a SiC filter installed on a 2-L displacement, common-rail, high pressure injection (up to 1500 bar) Diesel engine. The first sample was taken after prolonged engine operation (filter loading mode) at 1800 rpm–30 N m engine operation point (250 °C filter wall temperature) and the second sample after filter loading at 1800 rpm–70 N m operation point (350 °C filter wall temperature) [43]. A careful inspection of the results of Fig. 14, reveals desorption of water vapor in the interval 50–100 °C, followed by desorption of adsorbed hydrocarbons (Volatile Organic Fraction of the particulate) in the interval between 350 and 500 °C. At soot sample temperatures higher 500 °C one observes significant mass reduction associated with heat release due to the ignition of the dry soot. Obviously, the composition of particulate emitted by the Diesel engine varies with load. Operation at lower load produces higher
80 70 y = 0.0002x - 0.0012
filter volume [dm3]
60 50 40 30
off-highway engines
20
automove engines
10 0 0
50
100
150
200
250
300
350
400
engine power [kW] Fig. 11. Filter volumes in liters offered by a specific DPF manufacturer as function of engine power [38], compared to DPF volume for mass production passenger cars.
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Fig. 12. Ratio of DPF filter volume to engine displacement for selected off-highway engine retrofits and passenger car OEM applications.
Fig. 13. Recorded filter backpressure levels during a CARB certification test that lasted about 30 days [41].
VOF content in the particulate, whereas operation near full load produces dry soot, due to the higher exhaust has and filter temperatures [22]. It is known from the literature that VOF may be oxidized at temperatures as low as 200 °C (in the presence of catalysts) and also may desorb from the particulate at higher temperatures, or even re-adsorb at low temperatures [44]. If one takes into account that adsorption capacity of the soot layer depends on the accumulated soot mass and adsorbed hydrocarbons and water (at low temperatures, below 100 °C) may significantly decrease soot layer permeability lp (see Eq. (3)), it becomes clear that a filter of large size and heat capacity, placed at a distance from the engine, may produce very high pressure drop, thus requiring oversizing of the filter. For example, if the certification test requires numerous cold starts (Fig. 13) and the filter is far downstream from the engine, significant filter oversizing may be required. Adsorption and desorption of soot volatile organic fraction is frequently observed in real world operation of Diesel engines equipped with DPF. Numerous experiments and tests have been conducted that show this behaviour. Fig. 15 is an example of this type of tests, that demonstrates the desorption phenomena of volatile hydrocarbons from soot.
The experiment was carried out with a 2-L displacement, common rail automotive Diesel engine that was subjected to load steps on the engine bench. In the recorded HC emissions signal (green line), hydrocarbon desorption is apparent at various load steps in this experiment. At time = 4000 s, engine operation point is changed from 3000 rpm/40 N m to 2450 rpm/100 N m. The effects on the filter wall temperatures are measured by thermocouples T/ C9, (centerline) and T/C8 (periphery) [8]. The regeneration process is extremely complex and monitoring filter backpressure only vaguely hints to the evolution of soot mass in the filter. This is confirmed by recent research works applying dynamic neutron radiography to the measurement of 3-D distribution of soot in the filter channels [45–48].
5. Factors affecting average pressure drop in diesel filters As discussed in the previous sections, the selection of filter size for a specific engine application should be based on the average pressure drop of the filter in typical engine operation. Filter size must be adequate to keep backpressure levels lower than the
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tests or 0.1 g/kW h for heavy duty vehicles and 0.2 g/kW h for offroad machinery. 5.2. Filtration efficiency The effect of engine soot emissions on the filter loading rate is significantly affected by filtration efficiency, since the filter loading capacity will be determined by the integral of particulate emissions multiplied by filtration efficiency for a required period of time between regenerations. Total particulate matter emissions of an engine at a given operating point consist of a number of different substances. Filtration efficiency varies for each category of substances. For example, filtration efficiency of SiC or cordierite filters is higher than 95% for dry soot, (mainly produced at high engine load); however it drops significantly at part load operation, due to the low efficiency of the filter in collecting the volatile fraction of particulates which is higher at part load. Fig. 14. Results of Thermogravimetric (TGA) and Heat Release (DSC) Analysis of two soot samples received from a filter channel, the first loaded with the 2-L displacement, common-rail Diesel engine operating at 1800 rpm–30 N m (250 °C filter wall temperature) and the other at 1800 rpm–70 N m (350 °C filter wall temperature) [43].
TF in (oC) TF wall periphery (oC) Speed (rpm/100) Torque (N.m)
TF wall centre (oC) ΔP filter (mbar) HC (ppm C1)
160
300
120
200
80
100
40
0
0
1200
2400
3600
Speed (rpm/100), Torque (Nm), HC (ppm C1)
Filter backpressure (mbar), Temperature (oC)
400
5.3. Vehicle driving mode or engine operation mode Prevailing vehicle driving modes or engine operation modes affect DPF loading rate in several ways: there exist engine operation points associated with high particulate emissions and a high solid fraction in the particulate emitted. Fortunately, these operation points are associated with high exhaust gas temperatures, which favour partial or complete regeneration of a well-designed DPF system. A criterion affecting the sizing of mainly passively regenerated filters is the engine-out PM/NOx ratio. The CRT requires a minimum NOx/PM ratio for proper operation, and thus its operation depends on the vehicle’s duty cycle. A successful passive operation of the filter requires that the exhaust gas reaches a sufficient temperature and meets certain conditions. The regeneration of the system can be enhanced, within certain limits, by increasing the size of the catalyst and the filter and/or by increasing the noble metal loading in the catalyst. Nevertheless, if the application is unsuitable or the duty cycle is too cold, the filter may be plugged with soot or experience uncontrolled regenerations [49,50].
0
Time (sec)
6. Effect of filter size on regeneration behaviour
temperatures are measured by thermocouples T/C9, (centerline) and T/C8 (periphery) [8].
Fig. 15. Desorption of soot volatile organic fraction can be observed in the recorded HC emissions signal (green line) at various load steps in this experiment. At time = 4000 s, engine operation point is changed from 3000 rpm/40 N m to 2450 rpm/100 N m. The effects on the filter wall temperatures are measured by thermocouples T/C9, (centerline) and T/C8 (periphery) [8]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
200 mbar threshold set by engine manufacturers. Average pressure drop of the filter will be determined by the following factors: engine out particulate emissions (steady state and transient mode) filtration efficiency of the DPF prevailing vehicle driving modes or engine operation modes regeneration frequency
As mentioned above, regeneration frequency of a specific DPF system depends on the vehicle driving conditions or prevailing engine operation modes. Active regeneration systems are normally installed in passenger cars and heavy duty vehicles, as well as locomotives and yachts (where burner systems are a common choice). Regeneration is activated when DPF backpressure exceed certain levels. Passive regeneration by use of catalysts is also present in car and vehicle systems. Frequent activation of regeneration refrains the engine of excessive exhaust backpressure, but at the same time could dissipate energy for regeneration. For this reason, fuel consumption increase is limited by optimized DPF design and placing the filter closer to the engine, profiting from the use of fuel additives or catalytically coated filters. Reduction of DPF size, inside the backpressure range permitted by engine manufacturers, generally improves regeneration behaviour due to the faster heating of the filter. For this reason, DPF size should not normally exceed twice the engine displacement volume.
5.1. Engine-out particulate emissions 7. Conclusions As already discussed in Section 3, engine-out particulate emissions of modern vehicle Diesel engines have been significantly reduced by engine design and engine management measures. Typical average emissions are of the order of 0.2 g/km in the legislated
Although the need of ceramic particulate filter size optimization is well established in practical applications of Diesel Particulate Filter, the manufacturers’ approaches do not yet converge in this area.
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Filter oversizing is not uncommon, especially in off-road and large engine applications. Filter backpressure rise characteristics is the main factor affecting filter sizing in real world applications. This is determined by engine soot emissions, prevailing vehicle driving mode or engine operation modes, average filtration efficiency and regeneration frequency. The situation is complicated by adsorption – desorption phenomena of volatile organic fraction and water vapour, which modify significantly filter pressure drop characteristics during low exhaust temperatures’ operation. Careful study of filter sizing approaches in the various fields of applications reveals that at least in the case of off-road machinery and large engines, a certain degree of oversizing is applied by several DPF manufacturers. The most probable cause of this tendency is the demanding tests for type approval, which may impose severe boundary conditions to the size optimization problem, especially when the filter is distant from the engine and part load engine operation prevails. Filter system design improvements, raising filter temperature levels, may significantly reduce filter size and overall system performance. As a rule of thumb, filter volume should by no means exceed twice the engine displacement volume. Application to large marine engines which consume residual fuel is a special case with very high particulate emissions that cannot be filtered by a rational filter size. In this case, a reduced size DPF could be configured that would only collect smoke emitted at start-up of the main or the auxiliary engines. References [1] C. Benaqqa et al., Morphology, physical, thermal and mechanical properties of the constitutive materials of diesel particulate filters, Appl. Therm. Eng. 62 (2) (2014) 599–606. [2] I. Nova, E. Tronconi, Urea-SCR Technology for deNOx after Treatment of Diesel Exhausts, Springer, 2014. [3] B. Guan et al., Review of state of the art technologies of selective catalytic reduction of NOx from diesel engine exhaust, Appl. Therm. Eng. 66 (1–2) (2014) 395–414. [4] B.K. Yun, M.Y. Kim, Modeling the selective catalytic reduction of NOx by ammonia over a Vanadia-based catalyst from heavy duty diesel exhaust gases, Appl. Therm. Eng. 50 (1) (2013) 152–158. [5] K. Johansen et al., Passive NO2 Regeneration and NOx Conversion for DPF with an Integrated Vanadium SCR Catalyst, SAE International, 2016. [6] S. Nakagawa et al., A Study of after-Treatment System for Heavy Duty Trucks at Low Temperature Conditions, SAE International, 2016. [7] J.S. Howitt, M.R. Montierth, Cellular Ceramic Diesel Particulate Filter. SAE paper 810114. 1981. [8] G.A. Stratakis, G.S. Konstantas, A.M. Stamatelos, Experimental investigation of the role of soot volatile organic fraction in the regeneration of diesel filters, Proc. Inst. Mech. Eng., Part D: J. Automob. Eng. 217 (4) (2003) 307–317. [9] W.A. Majewski, Wall-Flow Monoliths,. 2016. [10] B.A.A.L. van Setten, M. Makkee, J.A. Moulijn, Science and technology of catalytic diesel particulate filters, Catal. Rev. 43 (4) (2001) 489–564. [11] A. Mayer, Best Available Technology of Diesel particulate Filter Systems, TTM, Editor. http://vert-certification.eu/, 2008. [12] T.V. Johnson, Diesel Emission Control in Review. SAE paper 2008–01-0069. 2008. [13] W.A. Majewski, Diesel Filter Regeneration. www.dieselnet.com. 2016. [14] V. Palma et al., Catalytic DPF microwave assisted active regeneration, Fuel 140 (2015) 50–61. [15] M. Zheng, S. Banerjee, Diesel oxidation catalyst and particulate filter modeling in active – flow configurations, Appl. Therm. Eng. 29 (14–15) (2009) 3021– 3035. [16] S. Bai et al., Soot loading estimation model and passive regeneration characteristics of DPF system for heavy-duty engine, Appl. Therm. Eng. 100 (2016) 1292–1298. [17] H.-O. Herrmann et al., Partikelfiltersysteme fuer Diesel-Pkw, Motortechnische Zeitschrift 62 (9) (2001) 652–660.
[18] R.D. Nixdorf et al., Microwave-Regenerated Diesel Exhaust Particulate Filter, SAE International, 2001. [19] T. Yamamoto et al., Nonthermal Plasma Regeneration of Diesel Particulate Filter, SAE International, 2003. [20] O. Salvat, P. Marez, G. Belot, Passenger Car Serial Application of a Particulate Filter System on a Common Rail Direct Injection Diesel Engine, SAE International, 2000. [21] M.K. Khair, A Review of Diesel Particulate Filter Technologies, SAE International, 2003. [22] G. Stratakis, Experimental Investigation of Catalytic Soot Oxidation and Pressure Drop Characteristics in Wall Flow Diesel Particulate Filters PhD thesis, in Mechanical Engineering Dept, University of Thessaly, Volos, 2004. [23] G. Pontikakis, Modeling, Reaction Schemes and Kinetic Parameter Estimation in Automotive Catalytic Converters and Diesel Particulate Filters PhD thesis, in Mechanical Engineering Dept, University of Thessaly, Volos, 2003. [24] O.A. Haralampous et al., Diesel particulate filter pressure drop Part 1: modelling and experimental validation, Int. J. Engine Res. 5 (2) (2003). [25] E. Jiaqiang et al., Effect analysis on pressure drop of the continuous regeneration-diesel particulate filter based on NO2 assisted regeneration, Appl. Therm. Eng. 100 (2016) 356–366. [26] E. Xuereb, M. Farrugia, Review and Validation of Models of Pressure Drop across Diesel Particulate Filter and Particulate Loading Quantity, SAE International, 2016. [27] G.A. Stratakis, D.L. Psarianos, A.M. Stamatelos, Experimental investigation of the pressure drop in porous ceramic diesel particulate filters, Proc. Inst. Mech. Eng., Part D: J. Automob. Eng. 216 (9) (2002) 773–784. [28] B.V. Kuznetsov et al., Water adsorption and energetic properties of spark discharge soot: specific features of hydrophilicity, J. Aerosol Sci. 34 (10) (2003) 1465–1479. [29] A.M. Stamatelos, A review of the effect of particulate traps on the efficiency of vehicle diesel engines, Energy Convers. Manage. 38 (1) (1997) 83–99. [30] N.N., Industrial Engine Ratings Guide. 2014, CAT. [31] Dieselnet. Emission Standards. www.dieselnet.com. 2016. [32] J. Kubsh, Diesel particulate filter experience on marine engines, in: Marine Black Carbon Workshop, Ottawa, Canada, 2014. [33] G. Schaeffner et al., Diesel Particulate Filter: Exhaust aftertreatment for the reduction of soot emissions, MTU Friedrichshafen GmbH, 2011. [34] A. Liati, P. Dimopoulos Eggenschwiler, Characterization of particulate matter deposited in diesel particulate filters: visual and analytical approach in macro, micro- and nano-scales, Combust. Flame 157 (9) (2010) 1658–1670. [35] H. Agrawal et al., Emission measurements from a crude oil tanker at Sea, Environ. Sci. Technol. 42 (19) (2008) 7098–7103. [36] Hug-Engineering. 2010; Available from: http://hug-engineering.com/de/ anwendungen/yachten. [37] N. Kyrtatos et al., Design and experimental verification of a variable path exhaust gas prototype aftertreatment system for marine engines, in: 12th International Marine Design Conference, IMDC 2015, Tokyo, Japan, 2015. [38] N.N., NETT DPF Fact Sheet. Nett Technologies Inc. 2009. [39] G. Blanchard et al., Passenger Car Series Application of a New Diesel Particulate Filter System Using a New Ceria-Based Fuel-Borne Catalyst: From the Engine Test Bench to European Vehicle Certification, SAE International, 2002. [40] CARB, Diesel Certifications. Verification Procedure: Stationary. https://www. arb.ca.gov/. 1996, California Air Resources Board. [41] Miratech, The Combikat DPF, in www.miratechcorp.com. 2009. [42] M. Maricq, Chemical characterization of particulate emissions from diesel engines: a review, J. Aerosol Sci. 38 (11) (2007) 1079–1118. [43] G.A. Stratakis, A.M. Stamatelos, Thermogravimetric analysis of soot emitted by a modern diesel engine run on catalyst-doped fuel, Combust. Flame 132 (1–2) (2003) 157–169. [44] G.A. Stratakis, G.N. Pontikakis, A.M. Stamatelos, Experimental validation of a fuel additive assisted regeneration model in silicon carbide diesel filters, Proc. Inst. Mech. Eng., Part D: J. Automob. Eng. 218 (7) (2004) 729–744. [45] G.D. Harvel et al., Three-Dimension Deposited Soot Distribution Measurement in Silicon Carbide Diesel Particulate Filters by Dynamic Neutron Radiography, SAE International, 2011. [46] A. Strzelec, et al., Partitioning of Volatile Organics in Diesel Particulate and Exhaust, in deer 08. 2008. [47] T.J. Toops et al., Progression of soot cake layer properties during the systematic regeneration of diesel particulate filters measured with neutron tomography, Emission Control Sci. Technol. 1 (1) (2015) 24–31. [48] A. Strzelec et al., Neutron Imaging of Diesel Particulate Filters, SAE International, 2009. [49] M. Maly et al., Influence of the Nitrogen Dioxide Based Regeneration on Soot Distribution, SAE International, 2004. [50] R. Verbeek, M. van Aken, M. Verkiel, DAF Euro-4 Heavy Duty Diesel Engine with TNO EGR System and CRT Particulates Filter, SAE International, 2001.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Overview of Diesel particulate filter systems sizing approaches Antiopi-Malvina Stamatellou, Anastassios Stamatelos ⇑ University of Thessaly, Mechanical Engineering Department, Volos, Greece
h i g h l i g h t s Application of particulate filters to large Diesel engines is moving at slow pace. Rational sizing of the filters becomes essential in these applications. An overview of Diesel particulate filter systems sizing approaches is presented. An improved filter sizing methodology is necessary for large engines. Filter system design improvements may significantly reduce filter size.
a r t i c l e
i n f o
Article history: Received 24 January 2017 Revised 4 April 2017 Accepted 9 April 2017 Available online 26 April 2017
a b s t r a c t Although application of Diesel particulate filters in modern automotive Diesel engines is commonplace, their introduction to large Diesels as locomotive or marine engines is moving at a slower pace. One important reason for this delay is the large volume of filter required which is not easy to accommodate in this type of equipment. Thus, rational sizing of the filters becomes essential in these applications. It is observed that DPF systems for large Diesel engines are usually oversized. Possible reasons are discussed in this paper. With the present status of technology and the concern for compact and cost-optimized exhaust treatment systems a new design methodology is needed. This paper summarizes progress in the specific fields of application and attempts to formulate a filter sizing methodology that would lead to feasible solutions with regard to space requirements and backpressure penalty. Ó 2017 Elsevier Ltd. All rights reserved.
Contents 1. 2. 3. 4. 5.
6. 7.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Filter backpressure characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PM emissions standards for various engine categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Marine engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of engine size and power on DPF size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors affecting average pressure drop in diesel filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Engine-out particulate emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Filtration efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Vehicle driving mode or engine operation mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of filter size on regeneration behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Today, application of Diesel particulate filters (DPF) [1] is commonplace in automotive Diesel engines. As particulate emissions ⇑ Corresponding author. E-mail address: [email protected] (A. Stamatelos). http://dx.doi.org/10.1016/j.applthermaleng.2017.04.096 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
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standards become increasingly stringent for heavy duty engines and off-road machinery, Diesel particulate filters are also introduced to these engine categories. On the other hand, the evolution of emissions standards and aftertreatment technology resulted in the priority of NOx aftertreatment by means of SCR (selective catalytic reduction) over particulate in heavy duty engines [2–6]. DPF may be applied in the near future to big inland waterway vessels,
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Nomenclature
Dp L D cpsi ww
a
R
l
Mg k0soot kw d
qs
pressure drop [Pa] filter length [m] filter diameter [m] cells density [cells/in.2] filter wall thickness [m] channel Dp coefficient ideal gas constant [J/molK] dynamic viscosity of exhaust gas [kg/ms] molecular mass of exhaust gas [kg/kmol] layer permeability [m2] ceramic wall permeability [m2] channel hydraulic diameter [m] soot layer density [kg/m3]
or even to ships in specific congested areas with strict emissions regulation. Filter material is Silicon Carbide (SiC), Fig. 1, and cordierite [7] (2MgO-2Al2O3-5SiO2, Fig. 2). Filtration efficiency varies in the range 70–95% of total particulate matter. Higher efficiency, exceeding 95% is reported in the literature for the solid particulate fraction, which involves elemental carbon and metal ash. However, much lower efficiencies are reported for the soluble organic fraction (SOF) of the particulate [8–10]. A large variety of filter system design concepts and regeneration techniques (filter cleaning by incineration of the accumulated particulate) exist to date, at various production stages [11–19]. Table 1 presents a classification according to the main regeneration techniques employed. Different regeneration techniques may be combined in a single system (e.g. engine management combined with catalytic fuel additives) [20,21]. The need for a compact size of DPF to be fitted underhood, close to the engine to keep exhaust temperatures high, along with the mass produced of millions of units since 2000, resulted in compact sizes of DPF for passenger car application [17,20]. On the other hand, DPF for large engines are not yet mass produced and in several cases tend to be rather oversized. The reasons could be related to the low degree of commercialization, but also to some unrealistic requirements of conformance tests that are discussed here. As a matter of fact, most of the systems in production stage are oversized, leading to higher installation costs, unnecessary reduction in useful space and other side effects. These problems could be partially eliminated by proper sizing of the filter. The reduction of filter size, in addition to the reduction of the equipment cost, leads to increased filter temperature levels, due to reduced thermal inertia and heat losses of the filter.
N Af Vd w
number of open channels total filtration area [m2] engine displacement [m3] soot layer thickness [m]
Abbreviations DPF Diesel Particulate Filter SiC Silicon Carbide SOF soluble organic fraction VOF volatile organic fraction
Fig. 2. Cordierite filter for an off-highway machine.
Fig. 1 shows a photo of SiC Particulate filter for a 2-L automotive Diesel engine, with a higher magnification photo showing the soot layer deposited inside the channel and an electron microscopy photo showing the texture of the different porous layers. Fig. 2 shows a cordierite filter for an off-road machine (loader). Cordierite is selected in most cases for off-highway machinery. The design
Fig. 1. SiC Particulate filter for a 2-L automotive Diesel engine, with higher magnification photo showing the soot layer deposited inside the channel and an electron microscopy photo showing the texture of the different porous layers [22].
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Table 1 Classification of existing DPF systems according to regeneration techniques employed [17].
Continuous regeneration
Discontinuous regeneration
Active regeneration Generation of reactive agents: Plasma regeneration (Ozon, OH-radicals, NO2) Micro wave regeneration Exhaust gas Burner Electric Heater Electric oven (dismantling filter) Engine management increases exhaust temperature (e.g. post – injection, EGR, compressor bypassing)
and mass production of filters with more channels per unit inlet area has reduced filter size, since a smaller filter size may accommodate the same soot mass with similar backpressure levels. Moreover, as explained above the filter stays warmer and regenerates more frequently. Due to the interaction of a significant number of parameters with filter size, before formulating a sizing methodology, it is useful to discuss the effect of each parameter. Current commercially available DPF systems for heavy duty engines demonstrate significant variations in substrate size, as normalized with respect to engine volumetric efficiency and nominal power. It seems that, after three decades of DPF application, there is no universally acceptable philosophy of filter sizing for the specific engine category. On the other hand, substrate sizing turns out to play an essential role in overall performance, reliability, cost and feasibility of a DPF system. A role which becomes increasingly critical as the engine size increases. The purpose of this paper is to present an overview of the situation in the different Diesel engine categories and suggest rational guidelines for filter sizing, taking into account the boundary conditions for each engine category and application. 2. Filter backpressure characteristics A fundamental requirement for a specific DPF system is to produce an acceptable backpressure which does not significantly affect fuel consumption [23]. This sets a lower limit to filter size. Pressure drop across the filter is normally produced by the following factors [23–26] (see Fig. 3): Dp wall: affected by porosity distribution, mean pore size and wall thickness.
Passive regeneration Generation of reactive agents: CRT System (NO2 production upstream DPF) Catalytic fuel additive Catalytically coated Filter
Dp channel: pressure drop from flow friction within the channels. Dp soot: the accumulated soot acts as an additional filter and imposes additional pressure drop. As seen in this Figure, exhaust backpressure of an engine with a specific filter installed increases almost linearly with exhaust flowrate assuming a constant exhaust temperature (the effect of exhaust temperature is seen separately in Fig. 4). The data of this Figure are produced by simplified modeling of filter pressure drop: When one takes into account the effect of exhaust temperature on exhaust density, the effect of exhaust gas flow rate on the three above-mentioned components of filter Dp may be approximated by the following expressions [24,27]:
Dpchannel ¼
a l R T L m_ Mg N p d
4
ð1Þ
Dpwall ¼
_ R T l ww m M g p Af k w
ð2Þ
Dpsoot ¼
_ RT lm W M g p Af k p
ð3Þ
where the dynamic viscosity of the exhaust gas is approximated by the following expression:
l ¼ 3:55 107 T 0:679
ð4Þ
Different ceramic filter composition yields a different increase in pressure drop over a given time, because of the variation in mean pore size and porosity distribution. However, the requirements for high filtration efficiency led to optimized ceramic filtration properties producing specific, acceptable levels of pressure drop
Fig. 3. Variation of the three components of filter pressure drop versus exhaust gas flowrate (Sic DPF, 5.66 6 in.).
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Fig. 4. DPF backpressure as function of engine exhaust gas temperature and flowrate (5.66 6 in. filter, 2-L engine).
(<200 mbar). The calculations of Fig. 3 are assuming a 144 mm diameter 152 mm length SiC DPF with 200 cpsi, 0.4 mm wall thickness, 9 lm mean pore size and an exhaust gas temperature of 200 °C. The variations of soot permeability (kp in Eq. (3)) with the type of soot deposition are even more pronounced. Moreover, soot particles demonstrate a significant ability for water adsorption [28] and hydrocarbon adsorption, which may significantly decrease permeability as water and hydrocarbons impregnate the porous structure of the soot. As regards exhaust temperature, as shown in Fig. 4, filter backpressure increases with exhaust gas temperature, due to the increase in exhaust gas velocity. However, the calculations of this diagram are considering only dry soot, without the effects of adsorbed water and hydrocarbons that complicate matters [8]. Backpressure levels should be kept low, because backpressure reduces engine torque and increases brake specific fuel consumption (bsfc) of the engine.
An increase in exhaust backpressure leads to an equivalent decrease in engine mean effective pressure, as approximated by the following expression [29]:
I
W cycle ¼
pdV þ ðpin pex Þ V d
ð5Þ
The effect of DPF backpressure on the engine fuel consumption may be assessed based on the additional fuel consumption required to restore engine output. For example, an increase of 200 mbar in average filter backpressure leads to a 2% deterioration in brake specific fuel consumption [29]. Naturally, fuel consumption increase is also affected by the vehicle driving mode. Exhaust backpressure of a loaded filter increases rather sharply with the particulate mass accumulated in the filter channels (see Fig. 5, based on calculations with Eqs. (1)–(3)). Keeping a low level of DPF loading requires frequent regeneration.
Fig. 5. Effect of soot loading on filter backpressure (5.66 6 in. filter, 2-L engine).
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Fig. 6. Effect of varying filter size (varying filter length with constant diameter of 5.66 in.) on the pressure drop characteristics of a 2-L car engine with constant exhaust gas temperature of 200 °C.
Fig. 6 shows exhaust backpressure levels produced by different filter sizes on a passenger car engine, as function of exhaust flow (200 °C exhaust gas temperature). 3. PM emissions standards for various engine categories The PM emissions’ standards for passenger Diesel Cars were 0.14 g/km in the European Regulation Euro 1 in 1992 (European Driving Cycle, Maximum Speed 120 km/h, Total Distance 11.3 km). They were reduced to 0.1 g/km (1996) and further to 0.05 g/km (Euro 3/2000). From this point on the DPF became necessary. In 2014 the limit became stricter to 0.005 g/km. The same standards are valid for light trucks. The evolution of European Legislation was different for Heavy Duty Vehicles. Until 1999 only steady-state test was performed on the vehicle engine (ECE R-49, 13 points). Emission standards, expressed in g/kW h weighted over the 13 points, were 0.36 g/ kW h in 1992. They dropped to 0.02 g/kW h in 2000 (Euro III) and further to 0.01 g/kW h in 2013 (Euro VI). Since 1999 the engine is subjected to a transient test as well. The limit was set to 0.01 g/ kW h in 2013. After 2000 the DPF is gradually introduced in Heavy Duty Vehicles in order to conform to the standards [16]. Similarly, EU Stage IV emission standards for non-road diesel engines were introduced in 2014, setting the limit in PM emissions to 0.025 g/kW h. Fig. 7 shows a DPF combined with an SCR device to meet these requirements. To represent emissions during real conditions, a new transient test procedure—the Non-Road Transient Cycle (NRTC)—was developed in cooperation with EPA. The NRTC is run twice—with a cold and a hot start. The results are weighted 10% for cold start and 90% for hot start. The test is used in parallel with the steady-state test, ISO 8178 C1, referred to as the Non-Road Steady Cycle (NRSC) [31]. Application to larger Diesels as locomotive or marine engines is moving at a slower pace [32]. Nevertheless, there exist hundreds of retrofits to locomotive engines, in countries with strict emissions standards like Switzerland [11]. Although most retrofits were rather oversized, clean engines may also be produced by incorporating a compact DPF close to the engine (Fig. 8). In this way, a more rational filter volume is attained and the overall system per-
Fig. 7. Combined DPF and SCR after-treatment device for off-highway Diesel engine (EPA Tier 4, <560 kW, EU Stage IIIB) [30].
formance improved. According to EU Stage III/B (2012) emissions standards for rail traction engines PM emissions for locomotive engines are limited to 0.025 g/kW h which would need a DPF. Fig. 8 shows a more compact, optimized DPF installed by the engine manufacturer for a 2000 kW (76.3 L displacement) locomotive engine. A total of 10 £12 8 in. monoliths are included in the package (about twice the engine displacement – see Fig. 9). To activate filter regeneration, internal engine temperature management strategies as A/F reduction, compressor bypassing and postinjection are employed by the engine ECU to substantially raise the exhaust temperatures to the required levels. EU Stage III A legislation also includes Inland Waterway Vessels which were allowed to emit up to 0.5 g/kW h PM in 2009. The limit will be gradually adjusted to 0.015 g/kW h in 2020 for engines with Net power greater than 300 kW. The legislation is less demanding for Vessels with Net power less than 300 kW and may be accommodated without DPF.
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Fig. 9. Internal design of the MTU DPF of Fig. 8. External dimensions L W H = 2.72 2.1 0.9 m [33].
Fig. 8. SiC particulate filter for a locomotive engine (MTU rail engine 16 V 4000 RX4, 1: DPF, 2: engine) [33].
3.1. Marine engines For the purpose of emission regulations, marine engines are divided into three categories based on displacement per cylinder. Category 1 concerns small engines with less than 5 L displacement per cylinder. Category 2 concerns medium sized engines with 5–30 L per cylinder, with technology similar to locomotive engines (up to 8000 kW), used to provide propulsion for tugboats, supply vessels, fishing vessels, etc. also used for auxiliary power generators in ships. Category 3 marine diesel engines typically range from 2500 to 70,000 kW, powering container ships, oil tankers, bulk carriers, cruise ships. Emission control technologies for the last category engines are difficult to implement, mainly due to the residual fuel they are burning (a by-product of distilling crude oil to produce lighter products). This fuel has high viscosity and density, which affects ignition quality, as well as high ash, sulfur and nitrogen content in comparison to marine distillate fuels. Residual fuel parameters are variable because its content is not regulated. As an example, PM emissions of a large container vessel 2-stroke engine burning residual fuel with 2.05% sulfur lie in the range of 1.1–1.75 g/kW h (as PM2.5), for engine load ranging between 10–70% [34]. A crude oil tanker’s two-stroke engine, fuelled with residual fuel, was measured to a weighted PM mass emissions factor of 1.60 g/kW h for the main engine and 0.141 g/kW h for the auxiliary engine [35]. Emissions from marine diesel engines have been regulated since 1999 through a number of rules, applicable to different engine categories, overlapping with the regulations for mobile, landbased off-road machinery. The most recent are the 2008 Category 1/2 Engine Rule, introducing Tier 3 and Tier 4 emission standards for marine diesel engines. The Tier 4 emission standards are modeled after the 2007/2010 highway engine program and the Tier 4 non-road rule, with an emphasis on the use of emission aftertreatment technology. To enable catalytic aftertreatment methods, the EPA established a sulfur limit of 500 ppm in marine fuels (not residual fuels). The 2009 Category 3 Engine Rule, introduced Tier
Fig. 10. Soot filter for a yacht Diesel engine. Filter regeneration effected by the two burners at the left of the picture [36].
2/Tier 3 standards in harmonization with 2008 Amendments to IMO MARPOL Annex VI. Fig. 10 shows a DPF for a large yacht Diesel engine. The DPF system volume may be demanding in these applications and size optimization is critical in certain applications. Special measures could be taken to significantly reduce filter volume. As an example, compact high speed ferries could be equipped with low volume filters that will be engaged only at engine startup in the harbor [37]. Regeneration of this type of DPF could be effected by dismantling the monoliths and placing in an electric oven regeneration facility every one or two days.
4. Effect of engine size and power on DPF size The large oversizing of DPF for off-highway and larger Diesel engines can be seen in the example of Fig. 11. In this Figure, filter volume for Diesel particulate filters OEM installed in Diesel passenger cars is plotted versus nominal engine power [17,20,39]. In the same diagram, a series of Diesel filters offered by a specific DPF manufacturer is plotted as function of the engine nominal power suggested for retrofitting. Correlation of filter volume to
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engine power should be straightforward, since engine power is directly related to exhaust gas flowrate that is the major engine operating parameter affecting filter backpressure (Figs. 3–6). A linear regression to the data for the specific manufacturer shows that suggested DPF volume in liters is correlated to the nominal engine power by the following relation:
V filter ¼ 0:2Pengine 1:2
ð6Þ
where Vfilter is expressed in liters and Pengine is in kW. If one compares the respective data for the passenger car applications on the same Figure, it is apparent that filter size in passenger cars is significantly smaller for the same engine nominal power. The situation is better depicted in Fig. 12, where the ratio of filter volume to engine displacement is plotted. Volume of the DPF offered for retrofitting off-highway machinery is several times (up to 9 times) higher than engine displacement, while the volume of DPF applied by the car manufacturers as OEM equipment is only a little larger than engine displacement volume. This oversizing of DPF for off-highway engines can only in part be attributed to the higher engine-out emissions of this engine category. A typical PM emissions limit of 0.02 g/kW h is prescribed for off-road machinery compared to a limit 0.01 g/kW h placed for heavy duty vehicles. Of course, a retrofit filter size for the same displacement older Tier engine would require a larger filter size relative to a new engine of the same size/power output but with much lower engine-out particulate emissions. As already discussed in the example of Fig. 8 (locomotive engine), careful DPF system design and its placement close to the engine may lead to low filter volume also for these large Diesel engines. A hint to the reason for filter oversizing in retrofitting application may be found if one studies the California Air Resources Board (CARB) certification procedure for Diesel exhaust after-treatment systems [40]. Certification according to the level 3 requires the attainment of 85% or greater efficiency of the filter in the reduction of particulate matter. This certification takes into account the type of engine application (stationary, power generation, off-highway machinery, etc.) and the type of engine (Diesel, with or without turbocharger, without Exhaust-Gas Recirculation (EGR), mechanically or electronically controlled, certified off-road engines meeting 0.2 g/bhp-h
diesel PM or less based on certification or in-use emission testing). A minimum Exhaust Temperature level is set for Filter Regeneration (e.g. 400 °C by use of catalyst in fuel), which must be kept for a minimum of 30 min, to allow the completion of regeneration. During the test, the engine is allowed to operate for a maximum of 300–720 consecutive minutes below the above-mentioned passive regeneration temperature. The manufacturer may set a minimum number of cold start and 30 min idle sessions before filter regeneration is required (from 10 to 24 cold starts, see the example of Fig. 13). Also, a number of hours of operation is set (say 2000 h or more) before disassembly and cleaning of the filter is required. Finally, a specific fuel is employed for the test (California diesel fuel with less than or equal to 15 ppm sulfur or up to 20% biodiesel blend). The diluted and cooled diesel particulate that is collected during emissions testing consists of two types of particles: (a) fractal-like agglomerates of primary particles 15–30 nm in diameter, composed of carbon and traces of metallic ash, and coated with condensed heavier end organic compounds and sulfate; (b) nucleation particles composed of condensed hydrocarbons and sulfate [42]. However, the soot layer collected on the DPF wall has a modified texture and composition [34]. As an example, Fig. 14 presents the results of Thermogravimetric Analysis (TGA) and Heat Release Analysis by Differential Scanning Calorimetry (DSC) of two soot samples received from a central channel of a SiC filter installed on a 2-L displacement, common-rail, high pressure injection (up to 1500 bar) Diesel engine. The first sample was taken after prolonged engine operation (filter loading mode) at 1800 rpm–30 N m engine operation point (250 °C filter wall temperature) and the second sample after filter loading at 1800 rpm–70 N m operation point (350 °C filter wall temperature) [43]. A careful inspection of the results of Fig. 14, reveals desorption of water vapor in the interval 50–100 °C, followed by desorption of adsorbed hydrocarbons (Volatile Organic Fraction of the particulate) in the interval between 350 and 500 °C. At soot sample temperatures higher 500 °C one observes significant mass reduction associated with heat release due to the ignition of the dry soot. Obviously, the composition of particulate emitted by the Diesel engine varies with load. Operation at lower load produces higher
80 70 y = 0.0002x - 0.0012
filter volume [dm3]
60 50 40 30
off-highway engines
20
automove engines
10 0 0
50
100
150
200
250
300
350
400
engine power [kW] Fig. 11. Filter volumes in liters offered by a specific DPF manufacturer as function of engine power [38], compared to DPF volume for mass production passenger cars.
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Fig. 12. Ratio of DPF filter volume to engine displacement for selected off-highway engine retrofits and passenger car OEM applications.
Fig. 13. Recorded filter backpressure levels during a CARB certification test that lasted about 30 days [41].
VOF content in the particulate, whereas operation near full load produces dry soot, due to the higher exhaust has and filter temperatures [22]. It is known from the literature that VOF may be oxidized at temperatures as low as 200 °C (in the presence of catalysts) and also may desorb from the particulate at higher temperatures, or even re-adsorb at low temperatures [44]. If one takes into account that adsorption capacity of the soot layer depends on the accumulated soot mass and adsorbed hydrocarbons and water (at low temperatures, below 100 °C) may significantly decrease soot layer permeability lp (see Eq. (3)), it becomes clear that a filter of large size and heat capacity, placed at a distance from the engine, may produce very high pressure drop, thus requiring oversizing of the filter. For example, if the certification test requires numerous cold starts (Fig. 13) and the filter is far downstream from the engine, significant filter oversizing may be required. Adsorption and desorption of soot volatile organic fraction is frequently observed in real world operation of Diesel engines equipped with DPF. Numerous experiments and tests have been conducted that show this behaviour. Fig. 15 is an example of this type of tests, that demonstrates the desorption phenomena of volatile hydrocarbons from soot.
The experiment was carried out with a 2-L displacement, common rail automotive Diesel engine that was subjected to load steps on the engine bench. In the recorded HC emissions signal (green line), hydrocarbon desorption is apparent at various load steps in this experiment. At time = 4000 s, engine operation point is changed from 3000 rpm/40 N m to 2450 rpm/100 N m. The effects on the filter wall temperatures are measured by thermocouples T/ C9, (centerline) and T/C8 (periphery) [8]. The regeneration process is extremely complex and monitoring filter backpressure only vaguely hints to the evolution of soot mass in the filter. This is confirmed by recent research works applying dynamic neutron radiography to the measurement of 3-D distribution of soot in the filter channels [45–48].
5. Factors affecting average pressure drop in diesel filters As discussed in the previous sections, the selection of filter size for a specific engine application should be based on the average pressure drop of the filter in typical engine operation. Filter size must be adequate to keep backpressure levels lower than the
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tests or 0.1 g/kW h for heavy duty vehicles and 0.2 g/kW h for offroad machinery. 5.2. Filtration efficiency The effect of engine soot emissions on the filter loading rate is significantly affected by filtration efficiency, since the filter loading capacity will be determined by the integral of particulate emissions multiplied by filtration efficiency for a required period of time between regenerations. Total particulate matter emissions of an engine at a given operating point consist of a number of different substances. Filtration efficiency varies for each category of substances. For example, filtration efficiency of SiC or cordierite filters is higher than 95% for dry soot, (mainly produced at high engine load); however it drops significantly at part load operation, due to the low efficiency of the filter in collecting the volatile fraction of particulates which is higher at part load. Fig. 14. Results of Thermogravimetric (TGA) and Heat Release (DSC) Analysis of two soot samples received from a filter channel, the first loaded with the 2-L displacement, common-rail Diesel engine operating at 1800 rpm–30 N m (250 °C filter wall temperature) and the other at 1800 rpm–70 N m (350 °C filter wall temperature) [43].
TF in (oC) TF wall periphery (oC) Speed (rpm/100) Torque (N.m)
TF wall centre (oC) ΔP filter (mbar) HC (ppm C1)
160
300
120
200
80
100
40
0
0
1200
2400
3600
Speed (rpm/100), Torque (Nm), HC (ppm C1)
Filter backpressure (mbar), Temperature (oC)
400
5.3. Vehicle driving mode or engine operation mode Prevailing vehicle driving modes or engine operation modes affect DPF loading rate in several ways: there exist engine operation points associated with high particulate emissions and a high solid fraction in the particulate emitted. Fortunately, these operation points are associated with high exhaust gas temperatures, which favour partial or complete regeneration of a well-designed DPF system. A criterion affecting the sizing of mainly passively regenerated filters is the engine-out PM/NOx ratio. The CRT requires a minimum NOx/PM ratio for proper operation, and thus its operation depends on the vehicle’s duty cycle. A successful passive operation of the filter requires that the exhaust gas reaches a sufficient temperature and meets certain conditions. The regeneration of the system can be enhanced, within certain limits, by increasing the size of the catalyst and the filter and/or by increasing the noble metal loading in the catalyst. Nevertheless, if the application is unsuitable or the duty cycle is too cold, the filter may be plugged with soot or experience uncontrolled regenerations [49,50].
0
Time (sec)
6. Effect of filter size on regeneration behaviour
temperatures are measured by thermocouples T/C9, (centerline) and T/C8 (periphery) [8].
Fig. 15. Desorption of soot volatile organic fraction can be observed in the recorded HC emissions signal (green line) at various load steps in this experiment. At time = 4000 s, engine operation point is changed from 3000 rpm/40 N m to 2450 rpm/100 N m. The effects on the filter wall temperatures are measured by thermocouples T/C9, (centerline) and T/C8 (periphery) [8]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
200 mbar threshold set by engine manufacturers. Average pressure drop of the filter will be determined by the following factors: engine out particulate emissions (steady state and transient mode) filtration efficiency of the DPF prevailing vehicle driving modes or engine operation modes regeneration frequency
As mentioned above, regeneration frequency of a specific DPF system depends on the vehicle driving conditions or prevailing engine operation modes. Active regeneration systems are normally installed in passenger cars and heavy duty vehicles, as well as locomotives and yachts (where burner systems are a common choice). Regeneration is activated when DPF backpressure exceed certain levels. Passive regeneration by use of catalysts is also present in car and vehicle systems. Frequent activation of regeneration refrains the engine of excessive exhaust backpressure, but at the same time could dissipate energy for regeneration. For this reason, fuel consumption increase is limited by optimized DPF design and placing the filter closer to the engine, profiting from the use of fuel additives or catalytically coated filters. Reduction of DPF size, inside the backpressure range permitted by engine manufacturers, generally improves regeneration behaviour due to the faster heating of the filter. For this reason, DPF size should not normally exceed twice the engine displacement volume.
5.1. Engine-out particulate emissions 7. Conclusions As already discussed in Section 3, engine-out particulate emissions of modern vehicle Diesel engines have been significantly reduced by engine design and engine management measures. Typical average emissions are of the order of 0.2 g/km in the legislated
Although the need of ceramic particulate filter size optimization is well established in practical applications of Diesel Particulate Filter, the manufacturers’ approaches do not yet converge in this area.
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Filter oversizing is not uncommon, especially in off-road and large engine applications. Filter backpressure rise characteristics is the main factor affecting filter sizing in real world applications. This is determined by engine soot emissions, prevailing vehicle driving mode or engine operation modes, average filtration efficiency and regeneration frequency. The situation is complicated by adsorption – desorption phenomena of volatile organic fraction and water vapour, which modify significantly filter pressure drop characteristics during low exhaust temperatures’ operation. Careful study of filter sizing approaches in the various fields of applications reveals that at least in the case of off-road machinery and large engines, a certain degree of oversizing is applied by several DPF manufacturers. The most probable cause of this tendency is the demanding tests for type approval, which may impose severe boundary conditions to the size optimization problem, especially when the filter is distant from the engine and part load engine operation prevails. Filter system design improvements, raising filter temperature levels, may significantly reduce filter size and overall system performance. As a rule of thumb, filter volume should by no means exceed twice the engine displacement volume. Application to large marine engines which consume residual fuel is a special case with very high particulate emissions that cannot be filtered by a rational filter size. In this case, a reduced size DPF could be configured that would only collect smoke emitted at start-up of the main or the auxiliary engines. References [1] C. Benaqqa et al., Morphology, physical, thermal and mechanical properties of the constitutive materials of diesel particulate filters, Appl. Therm. Eng. 62 (2) (2014) 599–606. [2] I. Nova, E. Tronconi, Urea-SCR Technology for deNOx after Treatment of Diesel Exhausts, Springer, 2014. [3] B. Guan et al., Review of state of the art technologies of selective catalytic reduction of NOx from diesel engine exhaust, Appl. Therm. Eng. 66 (1–2) (2014) 395–414. [4] B.K. Yun, M.Y. Kim, Modeling the selective catalytic reduction of NOx by ammonia over a Vanadia-based catalyst from heavy duty diesel exhaust gases, Appl. Therm. Eng. 50 (1) (2013) 152–158. [5] K. Johansen et al., Passive NO2 Regeneration and NOx Conversion for DPF with an Integrated Vanadium SCR Catalyst, SAE International, 2016. [6] S. Nakagawa et al., A Study of after-Treatment System for Heavy Duty Trucks at Low Temperature Conditions, SAE International, 2016. [7] J.S. Howitt, M.R. Montierth, Cellular Ceramic Diesel Particulate Filter. SAE paper 810114. 1981. [8] G.A. Stratakis, G.S. Konstantas, A.M. Stamatelos, Experimental investigation of the role of soot volatile organic fraction in the regeneration of diesel filters, Proc. Inst. Mech. Eng., Part D: J. Automob. Eng. 217 (4) (2003) 307–317. [9] W.A. Majewski, Wall-Flow Monoliths,
[18] R.D. Nixdorf et al., Microwave-Regenerated Diesel Exhaust Particulate Filter, SAE International, 2001. [19] T. Yamamoto et al., Nonthermal Plasma Regeneration of Diesel Particulate Filter, SAE International, 2003. [20] O. Salvat, P. Marez, G. Belot, Passenger Car Serial Application of a Particulate Filter System on a Common Rail Direct Injection Diesel Engine, SAE International, 2000. [21] M.K. Khair, A Review of Diesel Particulate Filter Technologies, SAE International, 2003. [22] G. Stratakis, Experimental Investigation of Catalytic Soot Oxidation and Pressure Drop Characteristics in Wall Flow Diesel Particulate Filters PhD thesis, in Mechanical Engineering Dept, University of Thessaly, Volos, 2004. [23] G. Pontikakis, Modeling, Reaction Schemes and Kinetic Parameter Estimation in Automotive Catalytic Converters and Diesel Particulate Filters PhD thesis, in Mechanical Engineering Dept, University of Thessaly, Volos, 2003. [24] O.A. Haralampous et al., Diesel particulate filter pressure drop Part 1: modelling and experimental validation, Int. J. Engine Res. 5 (2) (2003). [25] E. Jiaqiang et al., Effect analysis on pressure drop of the continuous regeneration-diesel particulate filter based on NO2 assisted regeneration, Appl. Therm. Eng. 100 (2016) 356–366. [26] E. Xuereb, M. Farrugia, Review and Validation of Models of Pressure Drop across Diesel Particulate Filter and Particulate Loading Quantity, SAE International, 2016. [27] G.A. Stratakis, D.L. Psarianos, A.M. Stamatelos, Experimental investigation of the pressure drop in porous ceramic diesel particulate filters, Proc. Inst. Mech. Eng., Part D: J. Automob. Eng. 216 (9) (2002) 773–784. [28] B.V. Kuznetsov et al., Water adsorption and energetic properties of spark discharge soot: specific features of hydrophilicity, J. Aerosol Sci. 34 (10) (2003) 1465–1479. [29] A.M. Stamatelos, A review of the effect of particulate traps on the efficiency of vehicle diesel engines, Energy Convers. Manage. 38 (1) (1997) 83–99. [30] N.N., Industrial Engine Ratings Guide. 2014, CAT. [31] Dieselnet. Emission Standards. www.dieselnet.com. 2016. [32] J. Kubsh, Diesel particulate filter experience on marine engines, in: Marine Black Carbon Workshop, Ottawa, Canada, 2014. [33] G. Schaeffner et al., Diesel Particulate Filter: Exhaust aftertreatment for the reduction of soot emissions, MTU Friedrichshafen GmbH, 2011. [34] A. Liati, P. Dimopoulos Eggenschwiler, Characterization of particulate matter deposited in diesel particulate filters: visual and analytical approach in macro, micro- and nano-scales, Combust. Flame 157 (9) (2010) 1658–1670. [35] H. Agrawal et al., Emission measurements from a crude oil tanker at Sea, Environ. Sci. Technol. 42 (19) (2008) 7098–7103. [36] Hug-Engineering. 2010; Available from: http://hug-engineering.com/de/ anwendungen/yachten. [37] N. Kyrtatos et al., Design and experimental verification of a variable path exhaust gas prototype aftertreatment system for marine engines, in: 12th International Marine Design Conference, IMDC 2015, Tokyo, Japan, 2015. [38] N.N., NETT DPF Fact Sheet. Nett Technologies Inc. 2009. [39] G. Blanchard et al., Passenger Car Series Application of a New Diesel Particulate Filter System Using a New Ceria-Based Fuel-Borne Catalyst: From the Engine Test Bench to European Vehicle Certification, SAE International, 2002. [40] CARB, Diesel Certifications. Verification Procedure: Stationary. https://www. arb.ca.gov/. 1996, California Air Resources Board. [41] Miratech, The Combikat DPF, in www.miratechcorp.com. 2009. [42] M. Maricq, Chemical characterization of particulate emissions from diesel engines: a review, J. Aerosol Sci. 38 (11) (2007) 1079–1118. [43] G.A. Stratakis, A.M. Stamatelos, Thermogravimetric analysis of soot emitted by a modern diesel engine run on catalyst-doped fuel, Combust. Flame 132 (1–2) (2003) 157–169. [44] G.A. Stratakis, G.N. Pontikakis, A.M. Stamatelos, Experimental validation of a fuel additive assisted regeneration model in silicon carbide diesel filters, Proc. Inst. Mech. Eng., Part D: J. Automob. Eng. 218 (7) (2004) 729–744. [45] G.D. Harvel et al., Three-Dimension Deposited Soot Distribution Measurement in Silicon Carbide Diesel Particulate Filters by Dynamic Neutron Radiography, SAE International, 2011. [46] A. Strzelec, et al., Partitioning of Volatile Organics in Diesel Particulate and Exhaust, in deer 08. 2008. [47] T.J. Toops et al., Progression of soot cake layer properties during the systematic regeneration of diesel particulate filters measured with neutron tomography, Emission Control Sci. Technol. 1 (1) (2015) 24–31. [48] A. Strzelec et al., Neutron Imaging of Diesel Particulate Filters, SAE International, 2009. [49] M. Maly et al., Influence of the Nitrogen Dioxide Based Regeneration on Soot Distribution, SAE International, 2004. [50] R. Verbeek, M. van Aken, M. Verkiel, DAF Euro-4 Heavy Duty Diesel Engine with TNO EGR System and CRT Particulates Filter, SAE International, 2001.