LUBE OIL CONSUMPTION MEASUREMENT ON INTERNAL COMBUSTION ENGINESTRACK OR CATEGORY
Engine and Drivetrain
AUTHORS AND INSTITUTIONS
Bernhard Rossegger
Claudia Schubert-Zallinger
Andreas Wimmer
Institute of Internal Combustion Engines and Thermodynamics, Graz University of Technology, Austria
KEYWORDS
Applied Tribology: Automotive, Engines: Diesel Engines, Engines: Natural Gas Engines.
INTRODUCTION
Lube oil consumption (LOC) exerts a major influence on life cycle cost and emissions of internal combustion engines. As the LOC of
combustion engines must be reduced to meet future emission standards and advance economical operation, accurate LOC
measurement is necessary. Since state-of-the-art methods for measuring LOC are not fully satisfactory for different reasons, themotivation to develop new measurement systems is high. The deficiencies of currently available measurement techniques areillustrated by comprehensive literature research and tests on large engines using the SO2 method. Because tracer methods are
considered to be the most promising due to their ability to detect minimal levels of oil consumption, research is focused on
developing new tracer substances and/or detection methods. The main challenges in developing marker compounds are exemplified
by test bench experiments that use barium as tracer.
STATE-OF-THE-ART IN LOC MEASUREMENT
There are two general types of LOC measurement methods: direct measurement of lube oil in the oil reservoir and indirect
measurement of oil-borne substances in the exhaust gas and mathematical conversion to oil consumption as shown in Figure 1.

LOC measurement
Direct measurement
GravimetricIntermittent

On-lineVolumetricE.g., capacitive

Indirect measurement
Via tracer
Radioactive

Nonradioactive

Oil emission measurement
Mass
spectrometry

Flameionization

ParticulatematterFigure 1: State-of-the-art lube oil consumption measurement – overview, cf. [1]The former is conducted via gravimetric or volumetric and intermittent or continuous (on-line) measurement. The latter is mainly
based on detecting hydrocarbons or tracer substances that originated in the oil. As state-of-the-art combustion engines consume
quite low amounts of lubrication oil compared to their oil reservoir and the tendency is for this consumption to decrease,gravimetric and volumetric methods are – for different reasons – not satisfactory in terms of time and accuracy. Two main
challenges arise when methods of hydrocarbon detection are used, e.g., mass spectrometry as described in [2], flame ionization or
particulate measurement. First, lube oil components that take part at the combustion process and thus are oxidized to H2O, CO, CO2
etc. are ignored. Second, separation of fuel and oil-borne hydrocarbons is highly questionable especially when it comes tomeasurement on diesel engines. In the face of these challenges, the hydrocarbon method should be most strictly separated from
LOC measurement and thus called oil emission measurement. At present, tracer methods describe the most sensitive and accuratetechnique in LOC measurement. So as not to change the physical and chemical properties of the oil, the marker should ideally befound naturally in the base stock or additives. However, this does not apply to radioactive tracers, which also demand extra effort
regarding safety standards for humans, nature and infrastructure. As a result, research at the LEC generally focuses on nonradioactive tracers. Furthermore, tracer substances should be stable throughout the entire duration of the experiments – especiallyduring the combustion process. They should also be assignable to the lube oil and detectable in the exhaust gas. As all theserequirements apply to sulfur and its combustion product SO2, the SO2 method as described for instance in [3] is considered to be themost efficient and universally applicable in terms of engine category, size and fuel; however, trends to low SAPS oils (ACEA C112: total sulfur ≤ 0.2 wt%) might negatively affect its accuracy or sensitivity.
LOC MEASUREMENT ON LARGE ENGINES VIA THE SO2 METHOD
In the SO2 method, oil consumption is measured by determining sulfur balance at the inlet and at the exhaust (after the combustion
chamber). As shown in Figure 2 with a homogeneous natural gas operated engine, sulfur is introduced into the system via theoperating fluids (sulfur concentration in the air µ𝑆 is negligible), thus the sulfur mass concentrations in the oil 𝜇𝑆𝑜𝑜𝑜 and the natural𝑎𝑎𝑎

gas 𝜇𝑆𝑁𝑁 respectively have to be determined with high accuracy. Provided that the engine is operated homogeneously, a partial flowof the air/fuel mixture can be separated and directed through the sulfur detector as well; the benefits of this action will be explainedlater. As the air/fuel mixture may contain manifold sulfur compounds, these are only partially oxidized to SO2 during combustion.
Therefore, the sample is directed through an oxidizer in order to make sure all sulfur components are converted to SO2 by the hightemperature and ozone injection. To detect SO2, manifold methods have been developed, for example TOF-MS as described in [3]and IMR-MS [4].

Figure 2: Schematic depiction of SO2 measurement setup, cf. [5]The volumetric concentration of SO2 in the exhaust gas 𝜈𝑆𝑂2 is determined via IMR-MS and converted to a mass concentration via
molar masses of sulfur 𝑀𝑆 and the exhaust gas 𝑀𝑒𝑒.𝑔𝑔𝑔 . Finally, oil consumption 𝑚̇𝑜𝑜𝑜 can be calculated with equation [1] – cf. [4].
𝑚̇𝑜𝑜𝑜 =

𝜈𝑆𝑂2 ∙

𝑀𝑆

𝑀𝑒𝑒.𝑔𝑔𝑔

≈0

− µ�
𝑆𝑎𝑎𝑎

𝜇𝑆𝑜𝑜𝑜 − 𝜈𝑆𝑂2 ∙𝑀𝑆

𝑀𝑒𝑒.𝑔𝑔𝑔∙ 𝑚̇𝑎𝑎𝑎 +𝜈𝑆𝑂2 ∙

𝑀𝑆

𝑀𝑒𝑒.𝑔𝑔𝑔− 𝜇𝑆𝑁𝑁𝜇𝑆𝑜𝑜𝑜 − 𝜈𝑆𝑂2 ∙𝑀𝑆

𝑀𝑒𝑒.𝑔𝑔𝑔

Equation[1]∙ 𝑚̇𝑁𝑁

When propagation of uncertainty is calculated, it becomes apparent that the sulfur concentration in fuel exerts a major influence on
the accuracy of oil consumption measurement as the mass flow of natural gas is about three orders of magnitude higher than the“mass flow” of engine oil. The gas supplier’s odorization processes should be taken into account with natural gas operation inparticular. To provide an idea of the influence of the sulfur concentration in the fuel on oil consumption, Table 1 shows themeasurement uncertainties with a large natural gas engine assuming oil consumptions of 𝑚̇𝑜𝑜𝑙𝑒𝑒𝑒.1 = 0.5 𝑔/𝑘𝑊ℎ and 𝑚̇𝑜𝑜𝑙𝑒𝑒𝑒.2 =0.10 𝑔/𝑘𝑘ℎ for high uncertainty (±1 ppmw) and for low uncertainty (±0.1 ppmw) in measurement of sulfur concentration in thefuel (natural gas).
Table 1: Influence of the uncertainty of sulfur measurement in natural gas on LOC measurement uncertainty

Total sulfur in natural gas
12±1 ppmw

Total sulfur in natural gas
12±0.1 ppmw

Name

Variable

Units

Assumed specific LOCṁ oilexp.spec

g/kWh

0.50

0.10

0.50

0.10

g/kWh

0.05

0.05

0.03

0.02

urel

%

10.8

48.6

6.0

18.2

Specific uncertainty
Relative uncertainty

uspec

Specific lube oil consumption in g/kWh

0.5

Specific LOC
Average LOC
Total sulfur in natural gas
Average total sulfur

30

0.4

20

0.3

10

0.2

0

Total sulfur natural gas in ppm

If the sulfur in the natural gas is determined more accurately, LOC measurement accuracy with high oil consumption (0.5 g/kWh) canbe improved by 40 % and with low oil consumption (0.1 g/kWh) by even 60 %. Figure 3 shows endurance run LOC measurements ona large single cylinder gas engine where the sulfur concentration in the natural gas is measured regularly.

0.1

0.0

0

25

50

75
Time in h

100

125

150

Figure 3: LOC measurement on a single cylinder engine

With diesel engines, however, which cannot be operated homogeneously, online measurement of the fuel or fuel air mixture prior
to combustion is not a viable option. Moreover, sulfur concentration measurement of diesel fuel must be performed using UVfluorescence or a similar technology (RSD <± 1 %) because the standard specification sets a minimum reproducibility limit of±3.1 ppm at 10 ppm (as per EN ISO 20884), which would result in inacceptable measurement uncertainty with the SO2 method.
Nevertheless, the measurement uncertainty of the more or less fluctuating tracer amount in the fuel results in uncertainty in LOC
measurement (the lower the LOC, the higher the influence of fuel, the higher the measurement uncertainty). Thus research into
alternative tracer substances is being conducted at the LEC.
DEVELOPMENT OF NEW TRACER SUBSTANCES: BARIUM
The deficiencies of the SO2 method and the challenges it will face in terms of detection limits, accuracy and selectivity justify the
exploration of new tracer substances. As mentioned above, the ideal tracer should already be present naturally in the base oil oradditives so it does not have an impact on the lube oil’s physical and chemical properties. Consequently, barium compounds such as
the detergent additive barium sulfonate are being investigated. Barium sulfonate is a mixture of non-polar compounds such as
mineral oil and barium salts such as barium sulphate and barium carbonate (strongly polar). Artificial oil aging experiments haveshown that barium sulfonate can be blended into engine oil in a rather high concentration (up to 5 %) without significantly changing
the oil properties that affect oil consumption.
Because of the composition of the additive, it seemed likely that the barium salts would neither vaporize nor participate incombustion and thus would remain and accumulate in the oil sump. In this case, the increasing concentration might indicate a way
to measure oil consumption. However, long run experiments on a large gen-set gas engine showed that the concentration of barium
remained level, thereby indicating that barium must be consumed at the same rate as the engine oil. During further preliminary
experiments, oil consumption was calculated by analyzing the dilution of barium in the oil when barium-free oil was periodically
added. The result was oil consumption between 0.15 and 0.18 g/kWh with a small standard deviation. However, the barium
concentrations in the exhaust gas as well as deposits in the engine or infrastructure have not been measured yet for verification. To
measure barium via the exhaust gas, calculations similar to those in the SO2 method can be applied with the exception that theengine oil is considered to be the only barium source within the system boundaries of the engine. Since it is uncertain whether the
barium compound would transfer into a gaseous phase and reach the minimum detection limit of the IMR-MS when blended into
the engine oil, barium sulfonate was added to the diesel fuel at different concentrations (100, 500, 1000 and 2400 ppm) instead of
directly to the oil during feasibility testing. Because the lightest barium isotope has a molecular weight of 130 amu and barium has
seven stable isotopes, a significant peak pattern at a mass-to-charge ratio higher than 130 Da was expected (with ion molecule
ionization, multiple ionization and fragmentation can be excluded). However, for the sake of completeness mass-to-charge ratios
between 17 Da and 299 Da have been observed. Mass spectrum comparisons between the reference measurements (0 ppm of
barium in diesel fuel) and measurements at 2400 ppmw in diesel fuel are shown in Figure 4, Figure 5, Figure 6 and Figure 7.

Abundance in ppb

O2

100000H2S

H3O+ H218O

1000

0 ppm - 110 %CO22400 ppm - 110 %SO2 Xe+2

NO2
SO

10
17 20 22 24 26 28 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70
Mass-to-charge ratio in Da

Abundance in ppb

Figure 4: Mass spectrum between 17 and 70 Dalton
Xe

100000
C6H6

1000

C7H8

Kr

C8H10

10

71 74 77 80 83 86 89 92 95 98 101104107110113116119122125128131134137140
Mass-to-charge ratio in Da

100

10
142
144
146
148
150
152
154
156
158
160
162
164
166
168
170
172
174
176
178
180
182
184
186
188
190
192
209
211
213
215
217
219

Abundance in ppb

Figure 5: Mass spectrum between 71 and 141 Dalton

Mass-to-charge ratio in Da

100

299

296

293

290

287

284

281

278

275

272

269

266

263

260

257

254

251

248

245

242

239

236

233

230

227

224

10
221

Abundance in ppb

Figure 6: Mass spectrum between 142 and 220 Dalton

Mass-to-charge ratio in Da
Figure 7: Mass spectrum between 221 and 300 Dalton

Since masses from 130 Da to 299 Da are observed, it is clear that neither barium itself nor barium compounds can be detected by
the mass spectrometer. Signals occurring between 122 Da and 137 Da represent the ionization gas xenon.
However, a look at Figure 8 reveals that the results of the mass spectrometry are justified. On the left, significant changes in particle
filter samples are visible due to emission of barium containing particles. In addition to engine tests, thermogravimetric experiments
are conducted that include analyses of vaporization residues and comparison with particle filter samples via x-ray diffraction. The
right side of Figure 8 shows the existence of a significant similarity between the composition of the vaporization residues and theparticle matter emitted by the engine, thereby leading to the conclusion that barium sulfonate is not oxidized during the combustion
process but releases its organic moiety by vaporization and is emitted in its original form as barium salts.

Figure 8: Particle filter samples containing barium (left); X-ray diffraction of vaporization residues and engine particle matter, respectively(right)

Unfortunately, the amount of barium particle matter emitted is not high enough to enable accurate quantitative oil consumption (orin this case diesel consumption) measurement via measurement of particle mass.
CONCLUSION
Conventional lube oil consumption measurement methods (gravimetric, volumetric) are no longer feasible especially with large
engines as the oil consumption compared to the oil reservoir is currently too low and the amount of time required for measurement
is too high and thus cost intensive. For today’s lower detection limit and accuracy requirements, tracer methods have to be the toolof choice for measuring LOC. Even though the SO2 method has certain deficiencies, it has been found to be the most satisfactory
procedure in terms of accuracy, detection limit, cost and handling. However, future trends such as low sulfur fuels and LOW-SAPS(Sulphated Ash, Phosphorus, Sulfur) oil or sulfur adsorption systems in the field of biofuels are leading to low trace amounts of sulfur
in the exhaust gas. This will be a challenge for the IMR-MS detection limit or selectivity between oil and fuel consumption.
Experiments using barium as a tracer reveal the manifold challenges and interferences in the development of new tracer substances.
Barium as a tracer fulfills many of the requirements such as high oil solubility without significantly changing oil properties,
proportionality between tracer consumption and oil consumption, high selectivity (the oil is the only source of barium in thesystem), low cost and no safety restrictions when operating with the tracer. Unfortunately, online detection is possible with neither
IMR-MS nor comparable systems because high-boiling particles must be able to be detected. If proper detectors for measuring
barium salts at low concentrations in gas mixtures won’t be developed in the near future, different tracers will have to be found.
Another potential new tracer method is currently under investigation at the LEC and is hoped to be ready for presentation by 2018.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the financial support of the “COMET - Competence Centres for Excellent Technologies
Programme” of the Austrian Federal Ministry for Transport, Innovation and Technology (BMVIT), the Austrian Federal Ministry of
Science, Research and Economy (BMWFW) and the Provinces of Styria, Tyrol and Vienna for the K1-Centre LEC EvoLET. The COMET
Programme is managed by the Austrian Research Promotion Agency (FFG).
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