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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)

MY

07-

09

MY

10-

12M

Y 1

3+

3125

Eu

ro 4

Eu

ro 5

Eu

ro 6

un

til 1

.9.2

011

w.

RW

> 1

760

kg180

50

9

80

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

.9.2

011

w.

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80

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

http://www.intranet.bosch.com/ds/topics/edc/204_sensors_actuators/Soot/

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GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©



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


GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©



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


GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©



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


GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©



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

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GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©



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 | ©



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

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GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©



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

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GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©



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

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GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©



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


GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©



Diesel Systems

DPF monitoring in the field

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GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©



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

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GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©



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

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GS-SI/ENX4-Bs | 08/03/2010 | F10_1080 | ©



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

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