Environ. Sci. Technol. 1992,26,930-934
Application of Tunable Diode Laser Spectroscopy to the Real-Time Analysis of Engine Oil Economy Keith R. Carduner,**t Alex D. Colvln,t Richard Y. Leong,t and Dennis Schuetzlet Research Staff, SRL S-3061, Engine Testing Laboratories, Ford Motor Company, Dearborn, Michigan 4812 1-2053
Gervase I . Mackay, Davld R. Karecki, and Harold I . Schlff Unisearch Associates Inc., 222 Snidercroft Road, Concord, Ontario, L4K 185 Canada
Tunable diode laser spectroscopy (TDLAS) of oil-derived SO2in automotive exhaust demonstrated acceptable repeatability in determination of oil consumption at steady-state engine operating conditions. The response time of the instrument was approximately 30 s, the time related to the flow rate of the sampling system. Instrument sensitivity is sufficient to measure SO2levels of 0.1-1 ppm required for the oil consumption determination. Typical exhaust gas species were investigated for their interference effects and were observed to have less than a 10% interference on the SO2signal for mixing ratios with SOz.typical of automotive exhaust. Water, on the other hand, did show a significant, but compensatible interference. Carbon deposition under rich engine conditions was observed, is expected to be a problem for any analytical device, and is best solved by using a heated sampling line. 1. Introduction
Internal combustion or diesel engine oil economy is a key factor in the customer’s perception of engine quality. Evaluation of customer concerns has shown that many auto owners consider the need to add additional oil between oil changes to reflect problems with engine design or performance. In addition, high oil consumption has been correlated with premature aging of catalytic converters. The level of oil consumption is also diagnostic of engine function or malfunction, as the case may be. For this reason, it is of considerable importance to have instrumentation available both to the engine developers and to the engine plant engineers to accurately and quickly measure engine oil consumption. One would like this capability both for steady-state operation and in transient mode of engine operation as the engine speedlload point is varied during a standard engine operation test cycle. A logical approach to the determination of oil consumption is embodied in the drain and weigh or continuous weigh methods. These techniques use the change in weight of the oil charge before and after a time period of steady engine operation. These tests normally require 3-6 h of engine operation and, as one can imagine, can become time consuming should multiple measurements be desired to ensure accuracy. The length of time required for the test also opens the possibility for occurrence of a number of interferences that can affect the result. These include dissolution of fuel in the oil charge that can effectively lead to a “make oil” result when the weight of the dissolved fuel surpasses the weight of the consumed oil. Moreover, the test is not applicable to the measurement of oil consumption during a transient-mode test. An alternative approach is to add a tracer to the oil that is then monitored in the exhaust stream during engine operation. By knowing the concentration of the tracer both in the exhaust stream and in the oil charge, it is possible to calculate the amount of oil that has been consumed +Research Staff. Engine Testing Laboratories.
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during the test. A radiometric method has appeared (1) that involves the addition of a radioactive tracer to the oil, which is then observed in the exhaust. A further tracer technique monitors the exhaust concentration of a chemical element that is naturally occurring in engine oil. In this technique, sulfur, a natural constituent of oil, is observed in the exhaust in the form of SO2that results when oil is burnt during the combustion process (2). A number of techniques to monitor the concentration of SOzin the exhaust stream have appeared. These include a batch method to collect the total amount of SOz during a given time interval (2), an optical absorbance method that is applicable in real time employing flame photometry (3),and the use of NDIR (nondispersive infrared), which compares the exhaust both with and without catalytic SOz removal to determine the level of SOz ( 4 , 5 ) . All have been demonstrated to be good S-trace methods. Nevertheless, batch methods are not applicable to real-time measurement and therefore may not be used for transient-mode analysis. The absorbance techniques using flame photometry and NDIR both suffer from potential interferences in the exhaust that either can limit their low-level sensitivity or can produce erroneously high SO2 readings. A still further technique for S-trace makes use of a coulometric determination of SO2 by passing a fraction of the exhaust stream through a electrochemical cell through which a current will flow in direct proportion to the amount of SOz in the gas stream (6). The electrochemical reaction that occurs in the cell is highly selective for SOz. An easily measurable cell current is generated that is therefore exactly equivalent to the mass of SOz that is passing through the cell in unit time. The major interferences that have to be considered are only incomplete conversion of organic sulfur to SO2 and any SO2 from sulfur that is in the engine fuel. An excellent description of the technique, examples of its use, the chemical reactions occurring in the electrochemical cell, and a description of the mathematical calculations giving oil consumption from SOz concentration are provided elsewhere (7,8). As part of an investigation into the viable methods of S-trace for oil economy, we also studied the use of tunable diode laser spectroscopy. This technique has seen application to both ambient air monitoring (9) and monitoring of diesel exhaust (10). The advantages of tunable diode laser spectroscopy (TDLAS) including applicability to a variety of gases, inherent selectivity to the species of interest, rapid response, and high sensitivity exceeding the ppm range make the technique a viable candidate for application to the oil consumption problem. In this report, we show that the tunable diode laser (TDL) is indeed useful for the current analytical problem by comparison against the sulfur coulometer technique that had previously been calibrated against the drain and weigh technique. 2. Experimental Section 2.1. TDL Absorption Spectrometer System. The
TDL spectrometer was a TAMS-150 produced by Uni0013-936X/92/0926-0930$03.00/0
0 1992 American Chemical Society
t
HsNe
1
-T Vehicle Exhaust
Ill
2 slm
Valve
Catalyst
lm Controller
Valve
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0 1 ppm
Source 01
Calibration SO2
Figure 1. (a) Functional block diagram of the TAMS150 tunable diode laser spectrometer. (b) Block diagram of the exhaust sampllng and calibration gas introduction system.
search Associates, a block diagram being given as Figure la. Extensive description of this instrument has been provided elsewhere (11). With reference to Figure l a , a few features need to be noted here because of their relevance to the current application. First, the White cell uses multipass optics to give a total path length of over 150 m. This gives the TDL sensitivity toward SOz in the 0.01-2 ppm range required for oil consumption measurements. As a rule of thumb (qualitative), at an oil-consumption level of 1 qt per 2500 miles using oil that is 0.5 w t % S, the SOz will be in the exhaust at the 0.5-1.0 ppm level (not including any S from the fuel, to be discussed below) depending on the engine speed. Cell alignment is provided by the visible HeNe laser. The White cell is operated at low pressure, and thus no heating of the sample line is required to prevent condensation of water in the exhaust sample. At low pressure, the absorption features of the gaseous exhaust components are sharp, and so an auxiliary cell RC1 containing a high concentration of SOz is included with the system and is used to internally lock the laser frequency to the center of the sharp vibration-rotation mid-IR absorbance feature used to monitor the analyte. This feature is also responsible for the selectivity typical of TDL. 2.2. Sampling and Calibration Procedure. Sample air enters the White cell through a small Teflon tube (6 mm 0.d.) located as shown in Figure l a near the optical entrance to the cell. The sample is directed to the White
cell through the sampling system given in Figure lb. The Teflon tube was fitted with a 2-pm filter and then to a three-way valve. A platinum catalyst was inserted in the sample line between the three-way valve and the Teflon tube to ensure complete conversion of S-containing species to SOz. This is not a restriction related to the operation of the TDL, but rather reflects the fact that burnt oil can be in a partially oxidized condition in the exhaust depending on the engine operating condition. One side of the valve was connected to a flow-limiting stainless steel capillary inserted into the exhaust manifold that restricted flow into the cell to 1-2 standard L/min. With this restriction on flow, the residence time of the exhaust in the cell was about 30 s. This is acceptable for the determination of engine oil consumption under steady-state conditions, but would be too slow for transient cycles. The cell could have easily been operated at a higher flow rate, thus achieving faster response, and this is anticipated for future testing of the TDL. A motorized valve and pump arrangement, MV in Figure 1, controlled the pressure in the White cell to 10 Torr. The other arm of the three-way valve was used to introduce calibration gas into the cell. The calibration gas was certified to be 0.1 ppm SOz. Short lines were used for the calibration gas system to minimize loss of the trace constituent and to minimize the response time to the added calibration gas. In operation, the calibration gas was introduced into the cell and a spectrum was acquired. It was found that the calibration spectrum varied less than 10% during the course of operation of the device so that the calibration spectrum only needed to be acquired at the start of the day's testing. After calibration, the measurement is made by admitting the exhaust sample, acquiring its spectrum, and then comparing the spectral intensity through a least-squares fitting procedure to the calibration spectrum. In effect, the relative areas of the sample versus calibration spectrum gives the concentration of SOz in the exhaust sample flow. 2.3. Choice of Analyte Absorbance Feature. The rotation-vibration spectrum of SOz gives rise to infrared absorption in the 1350-cm-l region. Choice of a specific feature in this region was made with respect to the lack of significant interference by absorptions of HzO, CO, COz, CHI, NO, and NOz. After choice of a specific absorption, potential interferences due to CO, COz, and HzO were checked by monitoring the change in signal for a standard mixture of 100 ppb SOz in either zero air or zero air containing 10% CO, 10% COz,or 2% HzO.Interferences due to NO, NOz, and CH4 were investigated by interposing in the optical path short cells, containing the pure gas at 25 Torr, with optical densities equivalent to ca. loo0 ppm NO, NOz, and CHI. The levels chosen to test the interference by these various gases are representative (except for water) of typical exhaust gas concentrations. Except for water, changes in SOz signal were observed to be negligible, as illustrated in Figure 2. The ca. 5% decrease in signal observed for NO and NOz (and CHI not shown) are due to absorption of ca 5% of the laser power by the windows of the optical cells containing these gases. The slight decrease in the cases of CO and COz are due to a slight increase (ca. 3%) in the gas flow, which acts to decrease the SO2 mixing ratio by a corresponding amount. Water showed an interference that translates into approximately 10 ppb equivalent SOz absorbance per percent of water. With water at approximately 10% of the volume of typical auto exhaust for gasoline operation (this could be as high as 20% for vehicles run on alcohol fuels), this would represent about a 100 ppb spurious SOz signal. For Environ. Sci. Technol., Vol. 26, No. 5, 1992 931
140 Zero Air
120 m f 0
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Da D)
.E
.-
a
+ 10% CO2 + 10% CO
+Zero Alr
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lation for oil consumption in grams per hour for both the TDL instrument and the coulometer are made using the following equation:
+ NO2 Cell
GM = KSG(S2 - SI)/SO(S3 - S2)
60
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0 0
2
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Tlme (Minutes]
Flgure 2. TDLAS response to ca. 100 ppb SO2 In the presence of
CO,, CO, NO, and NO,.
the present time, this interference was compensated by mathematical correction of the raw data on SOz concentration, assuming water to be 10% of the sample. This level of interference by water needs to be more seriously considered when oil consumption measurements in excess of 7500 miles per quart are needed. As shown above, the SO2content of the exhaust could be as low as 0.1 ppm, or 100 ppb. It should be noted that the water interference arises from broad-band absorption of the laser radiation by the water in the exhaust stream. If the water concentration remains approximately constant over the conditions of a test, then calibration of the instrument by introduction of known mixing ratios of SOp at the air intake of the engine will compensate for this interference. Alternatively, since the water content of the exhaust can be accurately calculated, a mathematical correction for the interference by water can always be made and one can continue to use the calibration scheme given in Figure lb. Removal of water from the sample flow prior to entering the White cell is possible with a permeation dryer. This was not used since it was feared that, given the high concentration of water in auto exhaust, the dryer would have to be replaced often. The SOz spectral line shape of the calibration gas was found to be essentially identical to the line shape of SO2 in a typical exhaust sample (correlation coefficients derived from fitting the sample spectra to the calibration gas spectrum were 0.98 or better), indicating that the interferences resulting from any other exhaust components that were not specifically checked are negligible. 2.4. Sulfur Coulometer. During testing, the oil consumption determination by the TDL method was compared against a similar measurement by the sulfur coulometer. The coulometer had previously been evaluated against the traditional drain and weigh method and was used instead of the latter since the time available to test the TDL was limited. The coulometer is described in detail elsewhere (6). Unlike the TDLAS, the coulometer draws a sample near atmospheric pressure and, therefore, requires a heated line to bring exhaust to the electrochemical cell where the SO2 concentration is determined. 2.5. Test Engine. All testing of the TDL was performed in an engine dynamometer cell using a research model 3.8-L V6. No special modifications to the engine were required other than the sampling systems for the TDLAS and coulometer, and a line that allowed injection of SO2into the intake manifold. The dynamometer was equipped with instruments to measure engine air intake flow, fuel flow, rpm, and exhaust gas oxygen content. 2.6. Test Fuel and Oil. For determination of oil consumption by sulfur trace, the exact sulfur content of the oil and fuel in weight percent is required. The calcu932
Envlron. Scl. Technol., Vol. 26, No. 5, 1992
(1)
where K is a constant comprising a number of numerical factors, SG is the weight per hour of sulfur in a stream of SOz gas that is introduced periodically into the engine intake manifold, S3 - S2 is the signal size resulting from the doping stream (determined either from absorbance using TDL or coulometrically using the coulometer), S2 - S1 is the SOz signal resulting from burnt oil, and SO is the fractional content of sulfur in the oil on a weight basis. The determination of oil consumption at a steady-state engine condition proceeds according to three steps. First, a background signal is acquired from the ambient air that will later enter through the engine air intake. This is signal level S1. Then, with the engine at a steady operating condition, 52 is measured, which represents the contribution of S from consumption of oil and fuel and from ambient air. Finally, with the engine condition unchanged, the doping SO2stream is added to the intake, and signal level S3 is determined; this represents the sum of consumed oil and fuel and the doping stream. Signal level S2 needs to be corrected by subtracting the contribution from SO2arising from sulfur in the fuel. This correction is made using the following equation:
S2corrected = s2 - FFSgmCC
(2)
where FF is the fuel consumption rate, S,, is the fractional percentage of S in the fuel, and CC is the coulometer constant in MA/ppm or in the case of the TDL, an absorbance cross section derived from the calibration of the TDL. For the test of the TDL, the oil was 0.47% S and the fuel was 2 ppm S by weight. 3. Results 3.1. Linearity of Instrument Response. Linearity of instrument response by the TDL to changes in the amount of SOz in the exhaust was determined and compared to that of the coulometer by adding known concentrations of SO2to the engine intake. This test was run at a variety of engine speedlload points to determine whether any engine conditions would result in anomalous TDL output. As a point of information, the speed/load point of an engine is a quantitation of the steady-state engine operation characterized by a specific engine speed in rpm and manifold absolute pressure as controlled by the position of the throttle. At so-called wide-open throttle, the manifold pressure approaches atmospheric pressure and is said to be at high load. At idle, absolute pressure is ca. 1/3 atmospheric and the engine is said to be at low load. Data representative of this part of the analysis of the TDL system is given as Figure 3. With the engine running at 1500 rpm/17.3 mmHg (ca. 1/2 atmospheric pressure), a fast idle condition, the SO2 concentration measured by the coulometer was observed to be directly proportional to the calculated (added) SO2 concentration. The added SOzconcentration is determined from a knowledge of engine airflow and the flow from the SO2tank. A straight line with a slope of 45" with respect to the axes indicates that the response of the coulometer is linear. For the TDLAS, the slope is reduced to 31'. This reduction is a result of differences in the calibration procedures for the TDLAS and the coulometer. The TDLAS was calibrated as described in section 2.2 using an auxiliary SO2 source. The coulometer, on the other hand, is calibrated using the SO2that is added to the engine air intake
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Flgure 5. Oil consumption at various engine conditions as determined by the coulometer and the TDLAS.
Added SO2 Concentration (PPMl
c
2
Engine Condition [Speed/Loadj
7
Flgure 3. Total system sensltlvtty to SO2 added to the engine air intake. Englne condition, 1500 rpm/17.3 mmHg.
20
1
10111213
Time Sequence Flgure 4. Oil consumption versus time at a typical engine condition as determined by the coulometer and the TDLAS. Oil consumption corrected for contributlon from fuelderived sulfur. Englne condition, 1500 rpm117.3 mmHg.
manifold. In other words, the coulometer should automatically have a calibration curve that is characterized by a 45O line in Figure 3. The experiment checks whether the line is straight and that indeed the response is linear. The fact that the line for the TDL is at 31" indicates that some of the SO2 added to the intake manifold is being lost as it travels through the engine. The difference in slope between the coulometer and the TDL indicates, in fact, that approximately one-third of the SO2injected into the intake manifold is lost as it travels through the engine. In practice, it is better to calibrate using the procedure for the coulometer since the measurement is corrected for loss of SO2 by the system before the gas reaches the point of measurement. Linear least-squares regression on these data gave standard deviations for the residuals of 0.056 ppm for the TDLAS and 0.040 ppm for the coulometer. 3.2. Measurement System Stability over Time. Figure 4 illustrates the periodically determined oil consumption in grams per hour (calculated from the measured SOzlevel) by the 3.8-L V6 using the SO2 standard addition technique for the calibration described in section 2.6 while the engine ran at 1500 rpm/17.3 mmHg intake depression.
The graph gives the oil consumption as corrected for the contribution to the SO2 signal from the fuel. The calculated oil consumption as determined by the TDLAS varied from 9.02 to 18.12 g/h, while the coulometer indicated consumption to be between 14.72 and 19.97 g/h. The average obtained by the TDLAS and the coulometer were 15.67 and 18.54 g/h, respectively. The lower figure given by the TDLAS appears to be related to the difference in calibration between the TDL and the coulometer, as discussed in section 3.1. On the basis of the comparison of efficiency (response to SO2 originating in the engine) discussed in the previous section, it would be expected that if the coulometer measures SO2 corresponding to 18 g/h, then the TDL would see approximately two-thirds the amount of SO2 and give an oil consumption of about 12 g/h. For the test of Figure 4,the concentration of SO2 in the exhaust was about 0.7 ppm as determined by the coulometer. This means that every 0.1 ppm of SO2 corresponds to about 2.5 g/h of oil consumption. When you remember that the water interference at a level of 10% exhaust water is about 0.1 ppm (for gasoline), then it is plausible to explain the difference between the oil consumption level determined by the TDL of 15.67 g/h and the expected value of around 12 as a result of water interference on the SO2 absorption peak. Despite this difference in absolute value of oil consumption, both instruments show the same trend of oil consumption by the engine. The variation in oil consumption by the engine over time is felt to be typical for long-term engine operation. It is quite clear that the TDL technique shows excellent long-term stability. Similar experiments run at different engine conditions gave substantially the same results. In addition, the results of a simultaneous drain and weight analysis involving weighing the oil before and after the test gave an oil consumption of 17 g/h. 3.3. Oil Consumption at Different Speed/Load Points. With the engine running at different speed/load points (Figures 5 and 6), the oil consumption measured by the coulometer and the TDLAS was observed to be comparable except when the engine was running at the speed/load point of 2000 rpm/0.2 mmHg (Figure 5). For all of these data, the difference in calibration as discussed above and the effect of water in the exhaust were used to correct the raw determination of oil consumption by the TDL. The 2000 rpm/0.2 mmHg operating point is a rich engine operating condition characterized by an air to fuel Environ. Sci. Technol., Vol. 26, No. 5, 1992
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120
-
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3000 RPM fl.4' -. . Ho .-"
. I I _
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1500 RPM 15' Hg
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2500 RPM 7.7" Hg
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0 1
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4 5 Engine Condition [Speed/Load)
6
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Flgure 6 . Oil consumption at various engine conditions as determined by the coulometer and the TDLAS.
ratio (A/F) of 11.3, which is substantially below the stoichiometric A/F of 14.6. Also, the discrepancy did not occur during the first 15 min of engine operation. This suggests that this discrepancy is probably due to the excess of partially burnt fuel and carbon in the exhaust gas, the carbon buildup eventually affecting the analyzers. In addition, high levels of hydrocarbons may result in a positive interference for the TDLAS. Further investigation will be performed, and it should be pointed out that it is not clear from the data the extent to which error may be ascribed to either the TDL or the coulometer. 4. Conclusions
The TDLAS showed high repeatability in measured oil consumption for a given engine-operating condition. Although not shown, the response time of the instrument was approximately 30 s, the time related to the flow rate of the sampling system. It is expected that significantly faster response time could be achieved with an optimization of the sampling system and that the TDL would not be limited in application to transient engine cycles not investigated during the experiments described above. TDL sensitivity was observed to be somewhat less than the coulometer, although this is believed to be related to the calibration technique. A number of typical exhaust gas species were investigated and were observed to have less than a 10% interference on the SOz signal for typical mixing ratios of the species and SOs. Water, on the other hand, does have a significant interference. Much of this interference can be corrected for by calibration under the operating conditions of the test. Furthermore, since the fraction of water in the exhaust is easily calculated, this interference can be compensated for by correction of the
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raw data. The problem with carbon deposition under rich conditions is expected to be a problem for any analytical device and is best solved by using a heated sampling line. The TDLAS has been repackaged to simplify its use in a vehicle or engine dynamometer environment. Further studies are planned to determine the application of this instrument to the real-time analysis of other emission components. While no disadvantages of TDL were observed, it was felt at the time of the test (January 1989) that the complexity of the then form of the TDL precluded routine use in the engine-testingenvironment. Recent advances in the development of portable, robust versions of this device necessitate reevaluation of this conclusion, a process that will be undertaken during the upcoming year. Further applications of the TDLAS to measure other exhaust gas constituents can be envisioned. These include, for example, the measurement of exhaust gas aldehydes from alcohol fuel vehicles and the measurement of methane from natural gas vehicles. The 1990s are expected to be a time of great change in transportation fuel technologies. Each fuel will need to be evaluated with regard to its impact on the total auto emissions picture, and for this reason increases in instrumentation reliability, ease of use, ruggedness, and accuracy will be required on a continuous basis. The TDL technique can be expected, we believe, to play an important role in the retooling of the modern auto emissions laboratory. Registry No. SOz, 7446-09-5.
Literature Cited Kawamoto, J.; Yamamoto, M.; Ito, Y. SAE Prepr. 1974, No. 740543. Hanaoke, M.; Ise, A.; Nagasaka, N.; Osawa, H.; Arakawa, Y.; Obata, T. SAE Prepr. 1979, No. 790936. Iizumi, S.; Koyama, T. SAE Prepr. 1986, No. 860545. Usami, I.; Miyake, H.; Saitoh, 0.;Ishida, K.; Miyatake, K. SAE Prepr. 1982, No. 820055. Maeda, Y.; Inoue, T.; Nakada, M.; Hamada, Y. SAE Prepr. 1986, No. 860544. Carduner, K. R.; Colvin, A. D.; Leong, R.; Bissell, H. SAE Prepr. 1992, No. 920655. Butler, J. W.; Korniski, T. J.; Colvin, A. D.; Jary, E. H. SAE Prepr. 1987, No. 871913. Butler, J. W.; Korniski, T. J.; Colvin, A. D.; Jary, E. H. Automot. Eng. 1988, 96, 57-60. Schiff, H. I.; Hastie, D. R.; Mackay, G. I.; Iguchi, T.; Ridley, B. A. Environ. Sci. Technol. 1983,17, 352A-356A. Harris, G. W.; Mackay, G. I.; Iguchi, T.; Schiff, H. I.; Schuetzle, D. Environ. Sci. Technol. 1987, 21, 299-304. Mackay, G. I.; Schiff, H. I.; Wiebe, A,; Anlauf, K. Atmos. Environ. 1988,22, 1555-1564. Received for review June 27,1991. Revised manuscript received November 5, 1991. Accepted January 21, 1992.
