furtther development of a smoke sensor for diesel engines

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Proceedings of ICEF2005 ASME Internal Combustion Engine Division 2005 Fall Technical Conference

September 11-14, 2005, Ottawa, ON Canada

ICEF2005-1302

FURTHER DEVELOPMENT OF A SMOKE SENSOR FOR DIESEL ENGINES

D. Gould, D.P. Gardiner, M. LaViolette and W.D. Allan Dept. of Mechanical Engineering, Royal Military College of Canada

Kingston, Ontario K7K 7B4 email: [email protected]

Proceedings of ICEF2005ASME Internal Combustion Engine Division 2005 Fall Technical Conference

September 11-14, 2005, Ottawa, Canada

ICEF2005-1302

ABSTRACT

This paper describes experimental research aimed at developing an on-board smoke sensor for diesel engines. The sensor element was similar to a conventional spark plug. Electrical heating of the insulator was used to prevent carbon fouling from the diesel soot. The sensing element created sparks within the exhaust pipe and changes in smoke levels were detected through analysis of the voltage levels of the sparks. The system was tested in a heavy duty diesel engine equipped with Exhaust Gas Recirculation (EGR)) and compared with reference measurements of the Filter Smoke Number (FSN).

The experiments showed good sensitivity to step changes in smoke levels (accomplished by varying EGR levels) at smoke levels below 0.5 FSN. However, the sensor suffered from temperature induced signal drift and was unstable under some circumstances. The use of a spark plug with a smaller electrode tip diameter improved the signal stability. It is proposed that measurement and control of the electrode temperature will be necessary to control the signal drift.

INTRODUCTION

It has been shown that an on-board diesel smoke sensor could be useful for control and diagnostic applications [1,2]. These include closed loop EGR control, transient fuel control and on-board diagnostics for exhaust aftertreatment systems. Laboratory instruments capable of real time smoke measurements exist [3,4] but a rugged inexpensive sensor suitable for direct installation in a diesel exhaust pipe (like the Exhaust Gas Oxygen sensor of a gasoline engine) is not currently available. Approaches that would require a sampling system or optical access to the exhaust have been considered to be unsuitable for vehicle applications [2].

A number of researchers have studied approaches to smoke sensing based upon the electrical properties of the soot particles. These approaches involve sensors which insert electrodes into the exhaust flow. In one type of sensor, the electrodes are used to detect the naturally occurring electrical charge of the soot particles and no voltage is applied to the

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electrodes by the sensor circuit [5,6,7]. A more recent development is a type of sensor where a high voltage bias is imposed between a pair of electrodes [8]. The electrical conductivity of the carbonaceous particles causes current flow across the electrode gap.

The approach described in this paper also uses a sensor with electrodes. However, in this case, the bias between the electrodes is so high that a spark is created. During the period following breakdown when spark current flows across the gap through the spark, the spark sustaining voltage is monitored. Changes in smoke levels are detected based upon changes to the spark voltage caused by the soot particles.

This concept was introduced in an earlier publication [9]. The present paper describes the results of ongoing work to improve the sensitivity to low smoke levels and to understand and overcome problems that affected the consistency of the measurement.

BACKGROUND

Figure 1 shows the voltage waveform of an actual spark with the phases of the spark identified based upon the definitions provided by Maly [10]. Following breakdown, the spark is sustained at voltage levels that are relatively low compared with the peak voltage needed to create the spark. The difference in voltage levels between the arc phase and the glow phase is due primarily to the lower cathode fall voltage of the arc phase. This lower cathode fall voltage exists because an arc discharge achieves electron emission from tiny molten hot spots on the cathode surface.

Glow discharges have a cold cathode electron liberation mechanism which requires a higher voltage to sustain the spark. Low current sparks (<50 mA) are predominantly glow discharges because the current density needed to create and maintain arc hot spots is unavailable. However, brief periods of arc activity can be observed during the first few microseconds of the spark when the highest current levels are experienced.

1 Copyright © 2005 by ASME

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Figure 1: Spark Voltage Waveform It was shown in earlier work [9] that the presence of

carbon at the cathode could increase the duration of the arc phase and lower the minimum current threshold for an arc phase to occur. This behaviour offered a means of detecting the carbonaceous soot particles in diesel exhaust by determining whether a spark was in arc phase or glow phase at a given elapsed time after breakdown.

An initial version of the sensor was developed and tested [9] and results from this work are shown in Figure 2. These tests showed a reasonable correlation between the sensor signal and the FSN in the range of 1-3. However, the results were inconsistent from test to test and there was poor sensitivity at low smoke levels. The remainder of the paper describes new results using an improved apparatus and focusses on measurements of smoke levels below 1 FSN.

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Figure 2: Correlation Between First Generation

Smoke Sensor Signal and Filter Smoke Number (1000 rpm, 3.25 bar BMEP, variable EGR).

Adapted from Allan et al. [9]

EXPERIMENTAL DETAILS Test Engine The test engine was a Steyr model WD615.98 9.7 litre 6-cylinder heavy duty direct injection diesel, mounted on a water brake engine dynamometer. The engine was originally turbocharged and intercooled in its vehicle application (a 10 ton heavy logistic military vehicle) and had a compression ratio of 15.5:1. For the tests described in this paper, the turbocharger was removed (converting it to a naturally aspirated engine) so that a simple EGR system (a valve controlled duct between the intake and exhaust pipes) could be used to vary exhaust smoke

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levels over a wide enough range at a fixed speed and load. Turbocharged diesel engines normally require special hardware to provide EGR when the intake manifold pressure exceeds the exhaust pressure. The EGR was not cooled as is normally done with production engines to maximize NOx reductions. The EGR was used only to increase the exhaust smoke levels and the actual EGR rates were not measured. Test Conditions The engine was tested at 1000 rpm and 150 Nm torque (1.9 bar BMEP). This test condition was selected because with the simple EGR system, it was possible to vary the exhaust smoke levels from a Filter Smoke Number (FSN) below 0.15 to FSN values above 1. The authors recognize that the engine setup was not representative of current diesel technology and that tests over a wide range of operating conditions will be required to properly validate the smoke sensor concept. The approach used for the present work simply provided a means of exposing the developmental sensor to the desired range of smoke levels for early proof-of-concept evaluations. Reference Measurements Reference smoke measurements were obtained using an AVL 415S smoke meter. This device performs automatic smoke measurements based upon the classical filter paper method. Unlike older manual and automated piston pump approaches, it uses continuous sampling through a diaphragm pump. The sampled volume can be varied in relation to the soot concentration and the volume used to calculate the smoke values is corrected to standard pressure and temperature conditions of 1 bar and 25°C, respectively.

The output of this smoke meter is expressed as a FSN from 0 to 10. It is essentially an accurately defined Bosch Smoke Number that accounts for the pressure and temperature of the sampled volume. During the remainder of this paper, the reference smoke measurements will be presented as FSN values.

Figure 3: Smoke Sensor System


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Smoke Sensor The smoke sensor system is depicted in Figure 3. The

latest version of the smoke sensor is based upon a prototype electrically heated spark plug provided by a spark plug manufacturer. The heating element also serves as a temperature sensor so that the insulator temperature may be held constant at different exhaust temperatures. This arrangement has proven to eliminate carbon-fouling problems while requiring only modest heater power levels (typically <25W).

The spark plug was fired by a programmable current ignition system which allowed the spark current to be adjusted so that few arc discharges were produced when no soot was present. The spark voltage was attenuated by a 1000:1 probe. The signal conditioning system compared the voltage of each spark to a known threshold and determined whether the discharge was in arc mode or glow mode at a selected time after breakdown. The smoke sensor signal (0-5 volts) corresponded to the percentage of sparks that were in arc mode.

In comparison with the work described in [9], sensitivity to low smoke levels was improved through higher fidelity measurements of the spark voltage and by focusing on measurements very early after the breakdown phase of the spark. Note that the timing of the breakdown phase is depicted in Figure 1.

RESULTS AND DISCUSSION

Signal Response to Variations in Smoke Levels Figure 4 shows an example of the response of the sensor to variations in smoke levels caused by changes in EGR flows. For each EGR cycle, the EGR valve was opened from the fully closed position to a target EGR level and then returned to the fully closed position. The closed EGR valve position resulted in smoke levels between 0.12 – 0.15 FSN while the different open positions provided smoke levels between 0.22 – 0.97 FNS.

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It can be seen that the sensor exhibited a substantial change in signal level to each up or down step in EGR including those which generated differences in smoke levels as low as 0.1 FSN. However, the relationship between the sensor signal and the actual FSN value was inconsistent. For example, the signal level for the closed EGR value setting varied between 1.5 and 2 volts and was unrelated to the small variations in the actual FSN values for this condition. We have used the term “signal drift” to refer to this problem.

This behaviour is to a degree analogous to the response of a piezoelectric pressure transducer. When used for engine cylinder pressure measurements piezoelectric transducers measure changes in pressure rather than absolute pressure and must be referenced to a know pressure at some point in the engine cycle [11]. Similarly the smoke sensor responded to changes in smoke levels but had an inconsistent baseline or zero level. As shown in Figure 5, the resulting correlation between the smoke sensor signal and actual FSN values exhibited a substantial amount of scatter.

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Figure 5: Correlation Between Heated Spark Plug Smoke Sensor Signal and Filter Smoke Number

(1000 rpm, 1.9 bar BMEP, Variable EGR)

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Figure 4: Smoke Sensor Signal at Varying EGR Levels (Heated Spark Plug)

[Numerical values are FSN values for steady EGR valve positions.]


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Considering the piezoelectric pressure transducer analogy, it was decided to examine the response of the smoke sensor in terms of the change in signal (∆ Sensor Voltage) occurring during a change in EGR valve position vs the accompanying change in Filter Smoke Number (∆ FSN). The “delta plot” depicts what the correlation might be if the baseline drift of the smoke sensor could be controlled. As shown in Figure 6, plotting the data in this manner removed much but not all of the scatter. Evidently drift in the baseline signal was not solely responsible for imperfections in the correlation. Thus, variations or drift in the effective gain or span of the signal and/or underlying inconsistencies in the sensor’s response at a given smoke level also existed.

y = 1.1656x + 0.5485R2 = 0.6468

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Figure 6: Correlation Between Change in Heated Spark Plug Sensor Signal and Change in Filter Smoke

Number (1000 rpm, 1.9 bar BMEP, Variable EGR)

Signal Drift and Instability The origins of the signal drift problem may be explained

by examining the temperature sensitivity of the sensor. Figure 7 shows the response of the sensor to changes in operating temperature induced by varying the power supplied to the spark plug’s electric heater. The resistance of the heating element (which has a positive temperature coefficient of resistance) was measured to provide a relative indication of the sensor temperature. It can be seen that increasing the temperature reduced the sensor signal and vice versa. Thus the sensor was sensitive to temperature as well as smoke. This was problematic because increasing the EGR levels to increase the smoke also raised the temperature of the exhaust gas and exhaust pipe; consequently the sensor temperature rose as the EGR was increased.

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Figure 7: Effect of Electric Heating on Sensor Signal

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Figure 8 shows the behaviour of the sensor signal during two consecutive EGR cycles. It can be seen that the signal increased abruptly when the EGR valve was opened and the FSN increased from 0.12 to 0.52. However, the signal drifted downwards during the period the EGR valve was open and the sensor was warming up. When the EGR valve was closed (and the exhaust temperature fell), the initial signal drop was followed by upward drift in the sensor signal as the sensor cooled off.

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Figure 8: Signal Drift During EGR Cycles (Numerical values are FSN values for steady EGR valve

positions.)

Note that the sensor signal levels at the beginning and end of the plot were almost identical despite the fact that the smoke levels were 0.12 and 0.61 FSN, respectively. This shows that the temperature sensitivity could offset the smoke sensitivity completely under some circumstances. Maintaining a constant heater temperature (which determines the insulator temperature) did not eliminate the temperature sensitivity problem. It is likely that the temperature of the center electrode tip must be controlled and temperature sensing within the electrode will be required. Another problem that was encountered can be described as “short term instability”. A particularly severe example of this is shown in Figure 9. It can be seen that abrupt changes in the signal level could occur during operation at a constant EGR level despite a constant FSN.

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Figure 9: Short Term Instability Behaviour (Numerical values are FSN values for steady EGR valve

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Examination of the spark plug electrodes (Figure 10) indicated that the spark location could change during operation, as the outer edge of the 1.85 mm diameter center electrode had a polished appearance due to arcing. (Note the clean appearance of the insulator nose due to the electric heating.) It was hypothesized that the spark might encounter variations in soot particle build-up as it moved across the relatively large surface area of the electrode and this could cause abrupt changes in the signal.

In order to investigate this possibility, a spark plug with a much smaller “active” electrode area was tested. This spark plug had a 1.0 mm diameter platinum pins on the center and ground electrode so the spark remained focused over a smaller area. The platinum spark plug was not electrically heated but proved to be suitable for brief tests at low smoke levels where serious insulator fouling could be temporarily avoided. As shown in Figure 11, the spark remained confined to the smaller area of the platinum pins. Note also, the fouled appearance of the insulator in contrast with the electrically heated spark plug in Figure 10.

Figure 10: Arcing Pattern on Heated Spark Plug

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Results with the platinum spark plug are shown in Figure 12. Short-term instability problems were markedly reduced. As shown in Figure 13, good signal response was achieved for variations in smoke levels as low as 0.1 FSN.

The correlation between the sensor signal (using the unheated platinum spark plug) and the Filter Smoke Number is shown in Figure 14. In Figure 15 the data is plotted in the same manner as in Figure 6 showing the relationship between the change in smoke sensor signal and the change in Filter Smoke Number during changes in EGR valve position. From these figures, it is apparent that further work is needed to achieve a more consistent relationship between the sensor signal and actual smoke levels.

Figure 11: Arcing Pattern on Platinum Pin Spark Plug

Newer spark plug designs with even smaller precious metal electrode pins (platinum and iridium) are now available. Future work on the smoke sensor concept will employ these designs to eliminate short-term instability along with electrode temperature sensing and active heater control to overcome temperature-related signal drift problems.



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Figure 12: Smoke Sensor Signal at Varying EGR Levels (Unheated Platinum Spark Plug) (Numerical values are FSN values for steady EGR valve positions.)

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(Numerical values are FSN values for steady EGR valve positions.)

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Figure 14: Correlation Between Unheated Platinum Spark Plug Smoke Sensor Signal and Filter Smoke

Number (1000 rpm, 1.9 bar BMEP Variable EGR)

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y = 0.6777x + 0.3818R2 = 0.8708

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Figure 15: Correlation Between Change in Unheated

Platinum Spark Plug Sensor Signal and Change in Filter Smoke Number

SUMMARY

Further development of a diesel smoke sensor has been carried out using an electrically heated spark plug as the exhaust probe and analysis of the spark voltage waveforms to detect changes in smoke levels. It has been demonstrated that this concept offers good sensitivity to step changes in smoke levels (accomplished by varying EGR levels) at smoke levels below 0.5 FSN. However, the signal drifts as the exhaust gas and exhaust pipe (where the sensor is mounted) warm up or cool down. Maintaining a constant heater temperature (which determines the insulator temperature) cannot eliminate this temperature sensitivity. It is likely the temperature of the center electrode tip must be controlled and temperature sensing within the electrode will be required.

The sensor has also exhibited short-term instability problems. This is believed to be related to movement of the spark between different locations on the electrode surface. Preliminary tests using an unheated spark plug (which is very vulnerable to fouling) with a smaller electrode tip diameter



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have shown a marked improvement in short-term signal stability.

It is recognized that there are numerous other variables which may have the potential to impact the sensitivity of the smoke measurements such as differences in particulate characteristics (due to the factors such as fuel composition, engine design and operating conditions), flow conditions at the spark gap and the composition of the exhaust gas. Further work will be required to address these and other issues.

ACKNOWLEDGMENTS The financial support of Canada’s Program of Energy

Research and Development (PERD) is gratefully acknowledged. The authors also wish to thank Dr. Ron Patrick of ECM: Engine Control and Monitoring for providing the heated spark plug. The assistance of Stephen Guy and Dan Faux with the experimental setup is gratefully acknowledged REFERENCES 1. Zhao, F. and Asmus, T., “Diesel Closed Loop Control via

Smoke Sensor”, presented at the Department of Energy Sensor Workshop, Jan. 25-26, 2000, Berkely, CA.

2. Anon, “Particulate Sensors – Requirements, Recommen-dations/Issues”, Minutes of the Department of Energy Sensor Workshop, Jan. 25-26, 2000, Berkeley, CA.

3. Schrami, S., Heimgartner, C., Will, S. and Leipertz, A. “Application of a New Soot Sensor for Exhaust Emission Control Based on Time Resolved Laser Induced Incandescence (TIRE-LII)”, SAE Paper #2000-01-2864, Society of Automotive Engineers, Warrendale, PA, 2000.

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4. Halsch, C., Beck, H.A. and Niessner, R., “A Photoacoustic Sensor System for Time Resolved Quantification of Diesel Soot Emissions”, SAE Paper #2004-01-0968, Society of Automotive Engineers, Warrendale, PA, 2004.

5. Schweimer, G.W., “Ion Probe in the Exhaust Manifold of Diesel Engines”, SAE Paper #860012, Society of Automotive Engineers, Warrendale, PA, 1986.

6. Collings, N., Baker, N. and Wolber, W.G., “Real-Time Smoke Sensor for Diesel Engines”, SAE Paper #860157, Society of Automotive Engineers, Warrendale, PA, 1986.

7. Hong, G., Collings, N. and Baker, N.J., “Diesel Smoke Transient Control Using a Real-Time Smoke Sensor”, SAE Paper #871629, Society of Automotive Engineers, Warrendale, PA, 1987.

8. Warey, A., Hendrix, B. and Hall, M., “A New Sensor for On-Board Detection of Particulate Carbon Mass Emissions from Engines”, SAE Paper #2004-01-2906, Society of Automotive Engineers, Warrendale, PA, 2004.

9. W.D.E. Allan, R.D. Freeman, G.R. Pucher, D. Faux, M.F. Bardon, D.P. Gardiner, “Development Of A Smoke Sensor For Diesel Engines”, SAE Paper #03-01-3084, SAE Powertrain Congress, Pittsburgh PA, October, 2003.

10. R. Maly, 1984, Spark Ignition: Its Physics and Effects on the Internal Combustion Process, Fuel Economy in Road Vehicles Powered by Spark Ignition Engines, Hilliard, C. John ed., Plenum Press.

11. Amann, C.A., “Cylinder Pressure Measurement and Its Use in Engine Research”, SAE Paper #852067, Society of Automotive Engineers, Warrendale, PA, 1985.



furtther development of a smoke sensor for diesel engines

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