Deposit Formation on Diesel Oxidation Catalysts
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RESE ARCH E X haust Aftertreatment
Deposit Formation on Diesel Oxidation Catalysts
Deposits on components of the Exhaust Aftertreatment (EAT) are often only detected
after the failure of the respective component. Within the FVV project EAT Clogging
(FVV project no. 1271), the Technical University of Braunschweig investigated which
parameters have an impact on deposit formation and in which stages this takes place.
The investigations include dynamic endurance runs and stationary tests.
AUTHORS
Dipl.-Ing. Kevin Friese
was Research
Associate at the Institute
of Internal Combustion
Engines (IVB) of the
Technical University
Braunschweig (Germany).
Prof. Dr.-Ing. Peter Eilts
is Head of the Institute of
Internal Combustion
Engines (IVB) at the
Technical University
Braunschweig (Germany).
Dr. rer. nat.
Bernhard Lüers
is Technical Specialist Die-
sel Systems at FEV Europe
GmbH Aachen (Germany).
68 © IVB
1 MOTIVATION 2 TEST BENCH SETUP
2 TEST BENCH SE TUP
3 TEST PREPAR ATION The test bench is equipped with a 2-l diesel engine. In compari-
4 ENDUR ANCE RUNS son to the series configuration, the exhaust system was modified
5 ACTIVE REGENER ATION to achieve the most uniform flow possible. The DOC has been
6 C ONCLUSION AND OUTLO OK positioned further downstream of the turbine of the exhaust gas
turbocharger. Due to the modified location of the DOC, the heat
losses are increased compared to the series application. To pre-
vent the gas flow in the inlet funnel of the DOC from breaking off,
the opening angle here is 8°. Furthermore, secondary fuels can be
injected into the EAT system using a reciprocating pump and an
evaporator. Since the influence of water condensation on the for-
mation of deposits is not initially investigated and is therefore to
be prevented, a low mass flow of dry shop air is introduced into
the exhaust system in phases when the engine is not running.
3 TEST PREPARATION
Within the project, deposits are created using two different
approaches. On the one hand, endurance runs are carried out
wherein the engine is operated in dynamic driving cycles, and on
the other hand, conditions of active Diesel Particulate Filter (DPF)
regeneration are simulated. For this purpose, the engine is oper-
1 MOTIVATION ated at a stationary point and fuel is injected into the exhaust sys-
tem using a reciprocating pump and an evaporator.
Deposit formation on components of the EAT system [1–4] has to A reference cycle is first developed for the dynamic driving
be avoided, as the catalytic activity of the affected components cycles; the corresponding curves of the engine speed, the torque
can be reduced. Against the background of the Real Driving Emis- and the resulting temperature are shown in FIGURE 1. The reference
sion (RDE) legislation, this is to be regarded as extremely critical. driving cycle consists of five combinations of set values of engine
Diesel Oxidation Catalysts (DOCs) are, due to their positioning, the speed and acceleration pedal position, wherein an idling operation
first components of the EAT system of diesel engines most point is included. The settings are transmitted to the Engine Con-
affected by carbonaceous deposits, hence the research project trol Unit (ECU) and the brake controller. In further driving cycles
focuses on them. these parameters are varied.
FIGURE 1 Representation
of the reference driving
cycle (© IVB)
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RESE ARCH E X haust Aftertreatment
Label Test Parameters
Run 06 refers to the reference driving cycle mentioned above
and the speed, torque and temperature curves shown in FIGURE 1.
Run 06 Reference
In run 07 the temperature in the exhaust system is increased com-
Run 07 Highest temperature pared to the reference, while in run 08 the engine speed is increased.
Run 08 Higher speed
Run 29 also shows higher temperatures in the exhaust system com-
pared to run 06, but lower than in run 07. In run 30 the engine is
Run 29 Higher temperature
operated without Exhaust Gas Recirculation (EGR), while in run 31
Run 30 Higher temperature without EGR a low load operating point is set instead of an idling point as in the
Run 31 Higher temperature without light-out reference cycle. This means that the DOC is always above the light-
off temperature in order to oxidize typical hydrocarbons. In addition,
Run 38 Higher temperature (20 % (v/v) RME and 80 % (v/v) EN 590)
in run 38 a blend with 20 % (v/v) biodiesel (Rape seed Methyl Ester,
Run 39 Higher temperature (DMA, sulfur content ~1000 ppm (m/m)) RME) is used and in run 39 marine gas oil (classified as DMA), with
the same load and speed characteristics as in run 29.
TABLE 1 Overview of the performed endurance runs (© IVB)
All driving cycles are repeated until a running time of about
60 h is reached. Once every hour, the engine is operated at a ref-
erence point (2500 rpm, 46 Nm) to determine the increase in dif-
In trials for active regeneration, the engine is operated with fuel ferential pressure over the respective DOC. Every 10 h the endur-
according to EN 590. For this purpose, the engine is first warmed ance runs are interrupted to take photos of the DOC’s front surface
up and then a stationary operating point is set. The test duration and to substitute catalyst cores having deposits with fresh drilling
is 6 h, with the mass flow of the fuel injected into the EAT system cores. In order to exchange samples, catalyst cores were predrilled
being 0.1 kg/h in all runs. The temperature upstream of the DOC and stabilized using mounting mats.
is set by the engine load; due to the closed ECU different tem- In all endurance runs deposit formations occur. Due to the
peratures come with different boost pressures. In the results resulting reduction of the effective flow cross-sectional area of the
shown, fuel according to EN 590 is used as secondary fuel. DOCs, the mean gas flow velocity increases along with an extended
running time. As a result, the differential pressure over the DOCs
increases as well, FIGURE 2. In almost all cases, the differential
4 ENDURANCE RUNS
pressure decreases after 11, 21, 31, 41 and 51 h of running time.
Within the project, eight endurance runs, each with one DOC, are This is due to the drill core changes and the resulting increase in
carried out. An overview of the performed endurance runs is given effective flow cross-sectional area.
in TABLE 1. The variation parameters mentioned here represent the From the increase of the differential pressure over the DOC at
change parameters defined for the tests; other parameters change the reference operation point over time, it can be concluded that
accordingly. deposit formation is strongly affected by engine-out emissions
FIGURE 2 Increase of the
differential pressure (© IVB)
70
of soot and hydrocarbons. Run 29, 38 and 39 show the same
speed and torque courses, resulting in very similar temperatures
and exhaust gas mass flow courses. This means that the bound-
ary conditions at the DOC are more or less the same. The differ-
ences in differential pressure are due to the fuel and the asso-
ciated emissions.
Further dependencies cannot be clearly determined, since a
change in driving cycle means that multiple impact factors influ-
encing deposit formation will inevitably also change at the same
time. For example, the increase of engine load in run 07 in com- FIGURE 3 Stages of deposit formations (© IVB)
parison to run 06 leads to increased temperatures in the EAT
system and at the same time to increased gas velocities due to
increased boost pressures while hydrocarbon and soot emissions
change as well. It should be noted that run 06 has the lowest In order to assess the thermal stability of the deposits, thermo-
temperatures, followed by run 29 and run 07, which has the high- gravimetric analyses were performed. This is important because
est temperatures. Nevertheless, further analyses within the the removal of existing deposits in the EAT system requires differ-
project suggest that the temperature over the DOC influences ent temperatures in the exhaust gas system. The normalized mass
deposit formation. loss of the deposits is shown in FIGURE 4. This illustrates clearly
The photographic documentation after every 10 hours of endur- that the engine operation mode, when comparing run 29 and 30,
ance run shows different stages of deposit formation, which are highly affects the thermal stability of the deposits. The same
shown in FIGURE 3 exemplarily. First, the effective flow cross- applies to the fuel used and the resulting raw emissions, which
sectional area of the individual channels decreases homogeneously can be seen by comparing run 29, 38 and 39. Temperature is
over the entire cross-section of the monolith, FIGURE 3 (1). Subse- another influencing factor, where higher temperatures in the cycle
quently, output or migration of deposits occurs partially, FIGURE 3 lead to thermally less stable deposits.
(2). In some channels the effective free cross-sectional area
increases. At the same time, the effective flow cross-sectional area
5 ACTIVE REGENERATION
decreases in other channels, whereby individual channels can also
become completely clogged. In the endurance runs 06, 29, 31 and In the trials on active regeneration, the influence of flow velocity
39 there is also an output or migration, respectively, in which the and temperature on the formation of deposits by unburned hydro-
monolith is visible again, FIGURE 3 (3). In the mentioned endurance carbons is investigated under stationary conditions. FIGURE 5 shows
runs, the measured differential pressure increase is highest at the images of drill cores after the tests at different speeds and tem-
reference point. peratures taken with the Scanning Electron Microscope (SEM).
FIGURE 4 Thermal stability
of deposits (© IVB)
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RESE ARCH E X haust Aftertreatment
1500 rpm and temperatures of about 260 and 310 °C respec-
tively, the specimens are more strongly blocked than the speci-
mens with comparable temperature and higher engine speed.
6 CONCLUSION AND OUTLOOK
Within the research project, deposits on DOCs were generated
using two different procedures. On the one hand, the engine was
operated in dynamic driving cycles, on the other hand, fuel was
injected into the EAT system in stationary tests. With both proce-
dures, the formation of deposits took place within short periods
of time. The extent to which deposits were formed on the mono-
liths and the characteristics of the deposits depended strongly on
the boundary conditions during formation.
Deposit formation is a complex process, which is influenced by
many factors. In order to weigh the individual influencing param-
eters, it is expedient to change the properties of the exhaust gas
after leaving the combustion chamber. The multi-ashing system
used in open pore particle filters in the current FVV project Open
Pore Filters could be applied to investigate the influence of the
flow velocity in dynamic driving cycles. The evaluation of the tem-
perature influence is also possible.
Besides deposit formation, it should be investigated how to
remove existing deposits, for example by oxidation using oxygen
or nitrogen dioxide or by shear forces at high gas velocities.
REFERENCES
[1] Kumar, A.; et. al.: Impact of Carbonaceous Compounds Present in
Real-World Diesel Exhaust on NO x Conversion over Vanadia-SCR-Catalyst.
In: SAE International Journal of Engines 03/2016, pp. 1598-1603
[2] Watanabe, T.; et. al.: New DOC for Light Duty Diesel DPF System.
In: SAE Technical Paper Series (2007), no. 2007 01 1920. International
Fuels and Lubricants Meeting, Kyoto, 2007
[3] Nakane, T.; et. al.: Investigation of the Aging Behavior of Oxidation Catalysts
Developed for Active DPF Regeneration Systems. In: SAE Technical Paper Series
(2005), no. 2005-01-1759. World Congress & Exhibition, Detroit, 2005
[4] Nakano, K.; Okano, H.: Study on the Prevention of Face-Plugging of Diesel
Oxidation Catalyst (DOC). In: SAE Technical Paper (2018), no. 2018-32-00.
Small Engine Technology Conference, Düsseldorf, 2018
FIGURE 5 SEM Images of samples from active regeneration trials (© IVB)
At constant engine speeds the deposit formation’s dependency
THANKS
on temperature becomes clear. Deposit formation shows a maxi- The research project (FVV project no. 1271) was performed by the Institute of
mum in the temperature range of around 220 to 300 °C. This is Internal Combustion Engines (IVB) at the Technical University of Braunschweig
probably due to the fact that at lower temperatures the chemical under the direction of Prof. Dr.-Ing. Peter Eilts. Based on a decision taken by
reactions leading to deposit formation are too slow, whereas at the German Bundestag, it was supported by the Federal Ministry for Economic
higher temperatures the deposits or their precursors can be oxi- Affairs and Energy (BMWi) and the AIF (German Federation of Industrial
dized. The comparison of the specimens resulting from similar Research Associations e. V.) within the framework of the industrial collective
temperatures at different engine speeds shows that higher speeds research (IGF) program (IGF No. 19460 N/1). The project was conducted by an
and thus higher mean gas flow velocities decrease the deposit expert group led by Dr. Bernhard Lüers, (FEV Europe GmbH). The authors
formation. This is obviously due to higher shear forces. It is gratefully acknowledge the support received from the funding organizations,
known from the endurance tests that deposit output only occurs from the FVV (Research Association for Combustion Engines e. V.) and from all
when the monolith is severely clogged. Therefore, at a speed of those involved in the project.
72
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