Particulate matter sensor for on board diagnosis (OBD) of diesel particulate filters (DPF)
Enno Baars, Henrik Schittenhelm, Andreas Genssle, Bernhard Kamp, Thorsten Ochs Robert Bosch GmbH, Stuttgart, Germany
INTRODUCTION To meet current emission legislation standards in the EU and US modern diesel vehicles are commonly equipped with a trap for particulate matter, so called diesel particulate filter (DPF). While the required in-use monitoring (OBD) of the functional DPF efficiency can be provided by evaluating the pressure difference over the filter for current OBD-limits, it proves very difficult to meet the tightened regulations of upcoming CARB and EU legislation [1,2] with this oblique method. This calls for direct measurement of the particulate matter emissions with a novel sensor that provides sufficient accuracy and sensitivity as well as the ruggedness necessary for stable lifetime operation under exhaust-gas conditions.
PC OBD limit PC certification limit1) Preliminary proposal of EU commission
11
PM [mg/km]EU CARB
1)
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07-
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3+
3125
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> 1
760
kg180
50
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11
PM [mg/km]EU CARB
1)
MY
07-
09
MY
10-
12M
Y 1
3+
3125
Eu
ro 4
Eu
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ro 6
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til 1
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Figure 1: DPF-monitoring-limits according to OBD-legislation (CARB/EU).
EGS-PM
DPF
DOC
ECU
EGS-PM
DPF
DOC
ECU
Figure 2: Image of Bosch PM sensor with sensor control unit (bottom) and typical application in an exhaust aftertreatment system (top).
Figure 3: Exploded view of a resistive sensor element with inter-digital electrode (top), heater (centre) and temperature sensor (bottom).
PARTICULATE MATTER SENSOR Based on well-proven multi-layer technology Bosch has developed a ceramic exhaust-gas sensor for particulate matter (EGS-PM) which is shown in figures 2-4. On the sensing surface a DC voltage is applied to two inter-digital electrodes (IDE), on which soot particles are deposited from the exhaust gas during operation. The characteristic drop of resistance due to formation of electrically conductive soot paths between the electrodes can be used to determine the soot emission. An integrated heater and temperature sensor allow for controlled regeneration of the sensing element by oxidation of the deposited soot at elevated temperatures.
SENSOR PERFORMANCE ON ARTIFICIAL SOOT AND ENGINE TEST BENCH To evaluate the sensor performance and study different influences on the sensor signal the EGS-PM was operated both on a new soot particle test bench with an ethene burner and in the exhaust gas of a 4-cylinder, 2.2 litre engine with a common rail injection system and Euro4 application. The engine test bench was furthermore equipped with a variable bypass around the DPF to simulate variations of the filter efficiency as expected from DPF defects. On both test-benches the sensor shows excellent signal repeatability of 3-8% for 2 sigma (see figure 5).
By selecting engine operating points with similar exhaust gas parameters but different engine-out soot emissions and additionally varying the filter efficiency as described above it was possible to determine the sensor response characteristic over a wide range of particle concentrations. Figure 6 shows the typical inverse relationship between time-to-threshold and soot concentration which allows for improved discrimination of filter efficiencies at low emission levels. Furthermore the improvement in response-time by reduction of electrode-spacing is clearly visible.
Figure 4: Top-view of the inter-digital electrode including a magnified image of the soot deposit between electrodes.
Figure 5: Repeatability (right) of Bosch PM sensor as measured at a selection of speed / load combinations (left) within the engine map of a 4-cyl. / 2.2l common rail engine with DPF.
Figure 6: Hyperbolic relationship between soot concentration and time to threshold of the sensor signal as measured on a 4-cyl. / 2.2l common rail engine equipped with DPF and variable bypass.
SYSTEM INTEGRATION For reliable measurement of the small currents involved and ease of integration into vehicle projects the sensor comes equipped with a sensor control unit as shown in Figure 2. This SCU handles operation of all sensor components as well as data acquisition. Data is transferred to and from the engine control unit (ECU) via CAN-bus thereby allowing plug-and-play operation without the need for dedicated circuits within the ECU (Figure 7).
DPF OBD CONCEPT While emission limits set by legislation are defined with respect to specified driving cycles, engine-out emission can vary significantly for in-field driving due to driver behaviour and different engine load conditions. To overcome those influences we have developed an DPF OBD concept, shown in figure 8, that compares the measured EGS-PM response-time with a model based predicted response-time. The latter uses the simulated soot mass flow of the engine as well as other exhaust gas parameters and takes a limit DPF model into account to predict sensor behaviour downstream DPF.
Figure 7: DPF OBD by means of EGS-PM - system view.
Figure 8: DPF OBD monitoring concept based on EGS-PM sensor signal.
SUMMARY / CONCLUSION A concept of a resistive soot sensor was presented which consists of an inter-digital electrode for soot detection and an integrated heater and temperature-sensing element for controlled regeneration of the sensor. The natural hyperbolic characteristic of sensor response time vs. particle concentration was shown as well as excellent repeatability of 3-8 % (2 sigma) in measurements both on an ethene burner lab test bench and over a wide operating range of a 4-cylinder / 2.2l common rail engine. Ease of integration into existing vehicle projects due to the associated sensor control unit with CAN interface was outlined as well as an DPF OBD concept that allows for in-field diagnosis. REFERENCES [1] California Code of Regulations, Title 13, section 1968.2, Malfunction and Diagnostic System Requirements for 2004 and Subsequent Model-Year Passenger Cars, Light-Duty Trucks and Medium-Duty Vehicles and Engines (OBD II) [2] Official Journal of the European Union; Communication on the application and future development of Community legislation concerning vehicle emissions from light-duty vehicles and access to repair and maintenance information (Euro 5 and 6); (2008/C 182/08)
1
Exhaust Gas Sensor for Particulate Matter (EGS-PM)
GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©
Robert Bosch GmbH 2010. All rights reserved, also regarding any
disposal, exploitation, reproduction, editing, distribution, as well as in the event of applications for industrial property rights.
Diesel Systems
Particulate matter sensor for on-board diagnosis (OBD) of diesel particulate filters
14th
ETH Conference on Combustion Generated NanoparticlesZürich, August 3rd
2010
Enno Baars, Henrik
Schittenhelm, Andreas Genssle, Bernhard Kamp, Thorsten OchsRobert Bosch GmbH
2
Exhaust Gas Sensor for Particulate Matter (EGS-PM)
GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©
Robert Bosch GmbH 2010. All rights reserved, also regarding any
disposal, exploitation, reproduction, editing, distribution, as well as in the event of applications for industrial property rights.
Diesel Systems
Legislation
OBD limits
emission standards 1) preliminary Commission-proposal
18
PM [mg/mi]
EU
CARB
1)
MY
07-
09
MY
10-
12M
Y 1
3+
5040
Eu
ro 4
Eu
ro 5
Eu
ro 6
as o
f 1
.9.2
009
as o
f 1
.9.2
011
290
80
15
129
Legislation
CARB-legislation
for
DPF-
OBD (OBDII Nov. 9, 2007)
EOBD: preliminary
Commission proposal
(September 2007, PM and PN limit)
Requirements
Δp-based diagnostics not sufficient for OBDII requirements MY2013
EGS-PM necessary to fulfill OBDII legislation for PC/HD application (>MY2013)
EGS-PM
DPF
Oxid. Kat.
3
Exhaust Gas Sensor for Particulate Matter (EGS-PM)
GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©
Robert Bosch GmbH 2010. All rights reserved, also regarding any
disposal, exploitation, reproduction, editing, distribution, as well as in the event of applications for industrial property rights.
Diesel Systems
Resistive soot sensor: Overview
Soot particles are deposited from exhaust gas onto sensor surface
Formation of conductive pathways leads to decreased electrical resistance
Application of voltage to inter digital electrodes allows measurement of sensor current
Periodic thermal regeneration by means of integrated heater
electrode structure on ceramic substrate
Schematic drawing of the sensing element
electrode
heater
T-measurement
4
ther
mal
izat
ion
Exhaust Gas Sensor for Particulate Matter (EGS-PM)
GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©
Robert Bosch GmbH 2010. All rights reserved, also regarding any
disposal, exploitation, reproduction, editing, distribution, as well as in the event of applications for industrial property rights.
Diesel Systems
Schematic view of sensor signal over time
deposition of particles
sens
or r
egen
erat
ion
time
sensor signal
time to threshold / response time
current threshold
sensor signal
start of second measurement cycle
sens
or r
egen
erat
ion
ther
mal
izat
ion
5
Exhaust Gas Sensor for Particulate Matter (EGS-PM)
GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©
Robert Bosch GmbH 2010. All rights reserved, also regarding any
disposal, exploitation, reproduction, editing, distribution, as well as in the event of applications for industrial property rights.
Diesel Systems
heater
compressorsoot
generator
reference
measurement
technologies
position
of the
particle
sensors
mixer
Schematic setup of the artificial soot particle test bench
Ethene
burner produces highly reproducible soot concentration with adjustable particle diameter
Test bench will be calibrated against Micro Soot Sensor and Smokemeter (AVL), particle counter and SMPS
mass
flow controller
6
Exhaust Gas Sensor for Particulate Matter (EGS-PM)
GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©
Robert Bosch GmbH 2010. All rights reserved, also regarding any
disposal, exploitation, reproduction, editing, distribution, as well as in the event of applications for industrial property rights.
Diesel Systems
Sensor accuracy on artificial soot particle test bench
Accuracy will be defined based on reproducibility of the sensor signal
measured on test bench using burner-generated particulate matter
for a variation in soot concentration and gas volume flow
excellent signal repeatability: ±
3% (±
2 Sigma)
0
1
2
3
4
5
6
7
0 50 100 150 200 250 300 350 400Zeit in s
Sen
sors
tro
m in
µA
Messung 1Messung 2Messung 3Messung 4Messung 5Messung 6Messung 7Messung 8Messung 9Messung 10
Stationärmessung am Kennlinienprüfstandmittlere Auslösezeit tA = 343 +/- 10 s (+/- 3%)Reproduzierbarkeit bei +/-2 Sigma
PM-concentration:ca. 10 mg/m3
gas flow: 800 l/minSen
sor
curr
ent
in μ
A
time in sec
Stationary test bench measurementsAverage response time t = 343 +/-10 s (2 Sigma)
measurement 1measurement 2measurement 3measurement 4measurement 5measurement 6measurement 7measurement 8measurement 9measurement 10
7
Exhaust
Gas Sensor for
Particulate
Matter (EGS-PM)
GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©
Robert Bosch GmbH 2010. All rights reserved, also regarding any
disposal, exploitation, reproduction, editing, distribution, as well as in the event of applications for industrial property rights.
Diesel Systems
Repeatability
–
steady-state
engine
operation
Excellent
repeatability
throughout
wide
range
of engine
map
Engine
speed
(1/min)
Lo
ad(b
ar)
Speed
/ Load
combinationS
tan
dar
d d
evia
tio
no
fti
me
to t
hre
sho
ld(%
)
engine
test bench
4-cyl. / 2.2l / CRS
8
Exhaust Gas Sensor for Particulate Matter (EGS-PM)
GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©
Robert Bosch GmbH 2010. All rights reserved, also regarding any
disposal, exploitation, reproduction, editing, distribution, as well as in the event of applications for industrial property rights.
Diesel Systems
Regeneration interval vs. soot concentration
“Natural”
hyperbolic relationship between time-to-threshold und soot concentration
Steep slope for small concentrations
improved discrimination in the range of small soot concentrations
Increased sensitivity can be achieved by reduction of electrode spacing
engine
test bench
4-cyl. / 2.2l / CRS
adjustable
DPF bypass
9
Exhaust Gas Sensor for Particulate Matter (EGS-PM)
GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©
Robert Bosch GmbH 2010. All rights reserved, also regarding any
disposal, exploitation, reproduction, editing, distribution, as well as in the event of applications for industrial property rights.
Diesel Systems
Symmetrical drilling of DPF
at the inflow site and 8 drill-holes
Emission in FTP75 driving cycle:
PM ≈
18 -
19 mg/mi
Measurements done on a chassis dynamometer using a car with a common European calibration setup (6cylinders, engine displacement of 3l)
Sensor placed 655mm downstream DPF
Setup for the automotive exhaust line measurements
10
Exhaust Gas Sensor for Particulate Matter (EGS-PM)
GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©
Robert Bosch GmbH 2010. All rights reserved, also regarding any
disposal, exploitation, reproduction, editing, distribution, as well as in the event of applications for industrial property rights.
Diesel Systems
FTP75 Cycle, 3 l Common Rail (DPF 18 mg/mi)
Start of measurement after dew point has been reached
FTP75: DPF defect detection by means of EGS-PM possible
11
Task
DPF monitoring function is required to work reliably both in OBD
certification cycles and in the field
OBD emission limits are defined with respect to specified driving cycles
Challenge
Wide variation range of engine-out emissions in conjunction with variable driver behavior (e.g. soot emission in overrun ≃
0 mg/m³, soot emission during maximum acceleration ≥
100mg/m³)
Note: Only relevant for DPF monitoring if DPF partially deteriorated
RB solution
Decision “DPF OK”
/ “DPF not OK”
based on "filter efficiency"
Engine-out particulate mass emission-model used for comparison with sensor signal
Exhaust Gas Sensor for Particulate Matter (EGS-PM)
GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©
Robert Bosch GmbH 2010. All rights reserved, also regarding any
disposal, exploitation, reproduction, editing, distribution, as well as in the event of applications for industrial property rights.
Diesel Systems
DPF monitoring in the field
12
Exhaust Gas Sensor for Particulate Matter (EGS-PM)
GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©
Robert Bosch GmbH 2010. All rights reserved, also regarding any
disposal, exploitation, reproduction, editing, distribution, as well as in the event of applications for industrial property rights.
Diesel Systems
DPF monitoring conceptEngine soot simulation
Simulated soot mass upstream DPF
Simulated soot mass downstream DPF
Particulate Matter Sensor
EGS-PM
Sensor
signal
Comparison: measured response time
vs.expected response time
DPF OK
DPF not OK
model for particulate matter sensor signal
Measured response time
DPF monitoring
Expected response timefor a limit DPF
Measured response timefor built-in DPF
Limit DPF model
green Engine ECU
blue Particulate Matter Sensor EGS-PM
13
Exhaust Gas Sensor for Particulate Matter (EGS-PM)
GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©
Robert Bosch GmbH 2010. All rights reserved, also regarding any
disposal, exploitation, reproduction, editing, distribution, as well as in the event of applications for industrial property rights.
Diesel Systems
System view
CANSensor
Control Unit(SCU)
SensorDevice
Particulate Matter Sensor
ECU
ECU Component
Driver
DPF fault-detection
ECU:
Signal processing
DPF diagnostics
Dew point release
EGS-PM:
Sensor control
Data acquisition
Sensor diagnostics
Sensor signals
Dew point detection
Sensor diagnostics
Demand new measurement
14
Exhaust Gas Sensor for Particulate Matter (EGS-PM)
GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©
Robert Bosch GmbH 2010. All rights reserved, also regarding any
disposal, exploitation, reproduction, editing, distribution, as well as in the event of applications for industrial property rights.
Diesel Systems
Summary/Conclusions
Resistive soot sensor concept: IDE, heater and T-measurement
Excellent signal repeatability
artificial soot particle test bench: ±3% (e.g. @ 10mg/m3, 800l/min)
engine test bench: < ±5% (1s) (over wide range of engine map)
“Natural”
hyperbolic relationship between time-to-threshold and
soot concentration
improved discrimination at low particle concentrations
Sensor response time sufficient for DPF defect detection
FTP72 driving cycle with DPF (18 mg/mi)
Stand-alone sensor (CAN interface)
sensor management, regeneration control, signal processing, verification of sensor signal are controlled by SCU
Easy integration of the sensor into existing engine control system
Layout of concept for an in-field DPF OBD presented