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2023-01-0342
Published 11 Apr 2023
Real Time Observations of Water Entering and
Leaving Internal Combustion Engine Oil, Over Both
Standard Engine, ICE and Plug-in Hybrid, PHEV
Dynamic Drive-Cycles
Richard Butcher, Nathan Bradley, and Timothy Powell bp Castrol
Citation: Butcher, R., Bradley, N., and Powell, T., “Real Time Observations of Water Entering and Leaving Internal Combustion Engine Oil,
Over Both Standard Engine, ICE and Plug-in Hybrid, PHEV Dynamic Drive-Cycles,” SAE Int. J. Advances & Curr. Prac. in Mobility 6(1):133-144,
2024, doi:10.4271/2023-01-0342.
This article was presented at the WCX SAE World Congress Experience, April 18-20, 2023.
Received: 15 Nov 2022
Revised: 30 Jan 2023
Accepted: 31 Jan 2023
Abstract
D
ue to the global drive for carbon neutrality, passenger
vehicle gasoline engines are transitioning to higher
levels of electrification, such as hybrid electric vehicles
and plug-in hybrid electric vehicles, HEVs and PHEVs.
Compared with conventional internal combustion, ICE only
operation, the combustion engine in a HEV or PHEV typically
operates for shorter periods. In turn the engine coolant and
lubricant temperatures are often lower. Such cooler engine
running is particularly noticeable for a variety of conditions
including short journeys in charge-sustaining mode, urban
motoring, a journey length towards the end of the electric range,
at cold ambient temperatures, or a combination of these conditions. All C-type piston rings allow limited combustion gases
to escape through the ring end-gap. Though the crankcase
ventilation system will remove the blowby gases into the engine
Introduction
D
ue to the global drive for carbon neutrality, passenger
vehicle gasoline engines are transitioning to higher
levels of electrification, such as hybrid electric
vehicles, HEVs and plug-in hybrid electric vehicles, PHEVs.
PHEVs are attractive in their ability to enable electric vehicle,
EV, driving, where an electric range of typically 30 to 50 miles
(48 to 81 km) is currently available. This is coupled with the
re-assurance of an internal combustion engine, ICE, when
required; thus, side-stepping EV range anxiety, charger
anxiety and frustration, whilst the world builds the necessary
EV infrastructure.
The combination of electric and ICE drive within a PHEV
powertrain leads to a wider range of engine operating conditions. These are influenced by vehicle parameters such as drive
battery state of charge, SOC; driving conditions, including
altitude changes. Also influenced by driver demands, such as
drive mode selection, ranging from EV to Sport. Therefore,
opportunities for engine operation with low engine oil
temperatures are more common, in contrast to a traditional
air inlet system, the crankcase blowby gases are able to mix
with the lubricating oil as it returns to the sump. However, the
ability of fuel and water in the blowby to evaporate from the
engine oil is influenced by temperature. This paper presents
sensor data showing, in real time, how this water exists in the
blowby gases within the airspace of the engine crankcase. With
direct reading sensors it shows how the water level in the oil
increases and decreases in key areas within a running engine
over the WLTC drive-cycle. Data for a standard internal
combustion engine is compared with a plug-in hybrid cycle, in
charge-sustaining mode. A range of lubricant formulations are
tested. A comparison is established, between the observed water
content sensor data over a single cycle with the widely known
Karl Fischer titration method over a longer run test. Aspects of
Karl Fischer titration sample practical preparation are discussed,
these are required for a used oil sample with high water content.
ICE only vehicle. These reduced temperatures for an earlier
generation of PHEVs are outlined in [1].
For an ICE only vehicle, short engine operation times
where engine oil temperatures remain low are exclusively
associated with very short trip distances from cold start, which
represents an uncommon situation [2].
For a PHEV however, the possibility of short engine
operation coupled with low engine oil temperatures is more
likely. Either the total trip distance is less than the E-range of
the vehicle and so operating conditions can force short engine
operation, where short periods of higher output are demanded,
or the total trip distance is marginally above the E-range
forcing short engine operation to complete the trip. The
increased range of possible scenarios is illustrated in Figure 1.
Previous published work has identified three driver
groups [2]; which complete urban, commute or escape trips,
with average daily distances of 26, 43 and 78 miles, (42, 69,
126 km) respectively. For a typical current PHEV, the urban
and commuter distances are such that there is a high probability of short engine operation, assuming that the vehicle
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FIGURE 1 The opportunity window for short engine ontime for a PHEV in normal operation is shown.
drive battery is partly or fully charged. Furthermore, with the
recent introduction of larger drive battery PHEVs, for example
[3], with an electric range of 62 miles (100 km), the possibility
of short engine operation during ‘escape’ trips becomes
more likely.
[4] predicts that more of the global population will
be living in megacities year on year. Consequently, the share
of urban motoring will increase unless other large infrastructure factors successfully mitigate this. Combined with the
increasing electric range of current PHEVs, the changes in
driving behaviour within large urban areas leads to shorter
engine operation scenarios. A recent Transport for London,
TFL, report describes this. TFL typically use ANPR, Automatic
Numberplate Recognition. The report shows that average
traffic speeds on central London roads during December 2010
were ~9 mph (~15 km/h) and have reduced to ~7 mph (~11
km/h) during the same period in 2019 (an increase in speed
was noted during 2020 due to the impacts of the coronavirus
pandemic on traffic volumes) [5]. This re-enforces the idea
that for a short urban or commute style trip across large urban
populations the mode of engine operation for a PHEV vehicle
is likely to be short bursts, where water accumulation
is possible.
Used engine oil analysis from conventional ICE only
operation typically shows little water content, due to the
typical trip lengths and engine oil temperatures of up to ~100
°C, as shown later in this paper. For a hybrid vehicle however,
the lower engine operating temperatures due to short operation mean the engine oil is potentially more susceptible to fuel
and water accumulation. Others have also noted this increased
risk due to reduced water volatilisation [6].
A recent energy outlook to 2050, [7], identifies three
scenarios, “Net-Zero”, “Accelerated Transition” and “New
Momentum”. However, there are many uncertainties in any
future scenario for the energy transition. It also identifies signs
that the “New Momentum” scenario aligns with the broad,
current, global trajectory, with the least change from today.
New Momentum anticipates only 42 % of car and truck miles
will be electrified in 2050. Whereas the “Net-Zero”
requires 80 %.
Also, regarding the rate of progress, [8] notes that the EU
forecasts that 60 % of the European fleet in 2030 will consist
of cars with internal combustion engines. So, all vehicle
propulsion types need a reduced carbon footprint with a
speedy transition to control global warming.
This paper presents sensor data showing in real time, how
this water exists in the blowby gases within the airspace of
the engine crankcase. With direct reading sensors it shows
how the water level in the oil increases in key areas within a
running engine over the WLTC drive-cycle, also the water
leaving the engine oil. Data for a standard internal combustion
engine is compared with a plug-in hybrid cycle, in chargesustaining mode.
The data shown here is for engines installed and run
in-line with the high precision transient fuel efficiency type
engine test cell installations and test methodology approaches,
as outlined in paper SAE 2016-01-890, [9].
Measurement Protocol
Engine Information
For all testing, the engine used was a typical European engine,
which is installed in both production ICE and PHEV vehicles.
See Table 1 for further engine details.
Brief properties of the 95 RON E10 gasoline fuel used for
the test cell installation are also provided in Table 2.
A range of indicative SAE viscosity 0W-16 to 0W-20
passenger car engine oil formulations, with additive technology relevant to European and American markets, were
tested throughout this study. The target of this study was to
develop a robust test method and to show effects and areas of
difference. Therefore, any oil comparison results presented
are chosen to focus on interesting areas of different behaviours
between oils.
Chassis Dyno Installation
Before measuring the water content of the oil in the test cell,
the crankcase exhaust gas was measured for a fully operational
vehicle. This is to establish the reference water content in the
crankcase. For the full vehicle testing, crankcase gas
TABLE 1 Specification of engine used throughout this testing.
Feature
Description
Type
4 Stroke internal combustion
engine
Arrangement
4 cylinders inline
Total swept volume
In the range of 1 to 1.6 litres
Aspiration
Single Turbocharger + intercooler
Fuel system
Direct injection GDI
Output power [kW/litre]
50 to 90
Emissions level
Euro 6
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TABLE 2 Properties of E10 fuel used in the test
cell installation.
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FIGURE 2 Schematic showing location of sensors installed
on the engine, in the test cell.
Parameter
Method
Result
RON
DIN EN ISO 5164
95.7
MON
DIN EN ISO 5163
85.1
EtOH [vol %]
DIN EN 1601
10
Density at 15°C [kg/m3]
DIN EN ISO 12185
750
LHV [kJ/kg]
DIN 51900
41462
Initial Boiling Point [°C]
DIN EN ISO 3405 auto.
33.6
measurements were taken at the engine oil filler cap, into the
cylinder head. This was over a WLTC cycle, using a Fourier
Transform Infra-red, FTIR, instrument. This removed crankcase gas and contaminants to a remote instrument, in the
chassis dynamometer room. The instrument was at the end
of a 6 m heated line, with an estimated 2 second lag time.
Test Cell Installation
In the engine test cell, direct measurement sensors were used
to sense the water, but did not require removal of any oil and
gases from the engine.
Sensor Specification and Engine Locations For
online measurement of engine oil water content, four capacitance sensors were installed in the engine within the test cell
installation as described in Table 3 and shown in Figure 2.
The sensors were mounted in position using bespoke adaptors.
Once the sensors were installed in the engine, the engine tests
were run over a short time, so very little oil oxidation
took place.
The engine contains the crankcase, where oil, blowby
gases and air mix. The crankcase has two main airspaces. The
lower crankcase airspace is the region around the crankshaft.
The upper crankcase airspace is within the cylinder head.
The capacitance sensors give water-in-oil content
expressed as percentage sensor saturation. This is not so
helpful for comparing oils, where the saturation level is a
function of the oil formulation. The relationship between %
saturation and absolute water content in ppm is specific for
each oil. Hence, for each oil tested in the engine here, a careful
individual calibration was made from zero to saturation levels
of water. This enables data in ppm water to be generated. Some
examples of the calibrations made on several oils are shown
in Figure 3 to show the range of oil behaviours.
TABLE 3 Brief description of capacitance sensor locations in
the engine together with corresponding notation used
throughout the manuscript.
Sensor Position in Engine Oil
Circuit
Notation used in Figures
In pump pressure circuit, main oil
feed from oil pump to engine
Gallery
In bulk sump oil
Sump
In airspace above oil
Crankcase
In oil filler cap, at top of filler pipe
Filler Cap
FIGURE 3 The maximum measurable water content with
these sensors for 6 typical engine oils from 0°C to 80°C
highlighting differences in behaviour.
In general, as temperature increases, the % saturation
level of the oil reduces for a fixed water content. The saturation
line varies across the different oils emphasising the importance of comparing absolute water content. Potentially a laboratory oil test which uses a fixed water content may
be misleading, compared with real engine operation.
The test cell installation using the sensors together with
the calibrations enabled the absolute concentration of water
to be measured in ppm. The output was a linear signal from
the sensor, direct into the test cell control system, for simultaneous logging with all the other test cell parameters.
It is important to note that the sensors do not read above
these sensor saturation lines. Fortunately, for this engine
running over a single WLTC, the sensors typically showed
data below these lines, a key enabler of this rapid test method.
As a result, data was immediately available on water distribution inside the running engine after the 30 minutes of WLTC.
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FIGURE 4 The different engine speed traces for the two
vehicle types across the WLTC. In both cases the vehicle speed
trace is the same.
WLTC data is shown for upper crankcase water concentration, measured by suctioning out gases to the remote FTIR.
The pipe length was 6 metres, giving a 2 second delay. This
method is to an extent intrusive, as it may affect the balance
of blowby gas and ventilation air movement within the engine.
It is presented here for comparison purposes, ICE versus PHEV.
The driver for the blowby gas is the cylinder gas, the
engine cylinder exhaust water level is also shown for reference.
ICE Operation FTIR
Measurement
Drivecycle Whilst the WLTP is familiar to many as the
protocol for emissions and fuel consumption standards, it is
worthwhile reflecting on the provenance of the chassis dynamometer cycle, the WLTC, [10, 11 and 12]. It is a 4-part cycle
developed internationally from 476,000 miles (766,000 km)
of data, using 441 vehicles, by a variety of methods. These
included instructed drivers running a prescribed route, chase
cars mimicking the behaviour of traffic, and customer car
logged data. Data gathering was run over urban, rural and
motorway routes.
Of particular importance here, the urban road style of
driving, parts 1 and 2 was identified as the most similar style,
over the range of the countries who developed the WLTC:
Europe, Japan, and Korea. This is the hybrid cycle choice here,
which focusses on the cold start urban aspect. So WLTC part
1 and 2 have a particularly wide relevance for real-world
urban operation.
The WLTC is defined by a vehicle speed trace so in order
to run tests on an engine test cell the engine speed and torque
setpoints must be established. To achieve this, a vehicle with
the same engine and transmission was tested on a chassis
dynamometer. In the case of hybrid vehicles, the cycle
performed by the combustion engine will vary significantly
depending on powertrain architecture, battery capacity, user
selected drive modes and differences in OEM calibration. For
this reason, a representative cycle was used to replicate the
engine behaviour of a contemporary PHEV with approximately 30 miles (~48 km) of E-range; operating in chargesustaining, CS-mode. An engine speed comparison between
the recorded ICE WLTC and the representative PHEV WLTC
can be observed in Figure 4.
Chassis Dyno FTIR
Measurements
This section briefly discusses measurements of the crankcase
gases. These are formed from the PCV, positive crankcase
ventilation, air and the blowby gases. These blowby gases are
the main route for water to enter the engine oil, whilst running.
From the ambient air water level of ~1 % by mass, the crankcase water level rises up to 3 % as the engine load increases,
see Figure 5.
Gas is extracted continuously for these measurements,
so water content data is the water content of the blowby gas.
The oil is removed by the instrument filter. This is different
from the capacitance sensors in the next Test Bed section,
where there is limited airflow and mainly oil splash onto the
sensors. So, the sensors measure water content in the
incident oil.
PHEV Operation FTIR
Measurement
Similarly for the PHEV, from the ambient air water level of
~1 %, the crankcase water level rises to 3 % as the engine load
increases, see Figure 6. Also, there are clear periods of higher
water content, where the PHEV engine is operating at a
high load.
For both cases there is an apparent exhaust nominal
dilution factor of 6:1 in the crankcase. This is for the upper
engine where blowby gases pass through from the piston ring,
back into the inlet air system.
Oil enters this area through oil galleries, as it is pumped
to key parts of the cylinder head, valvetrain and fuel pump.
However, it then may mix with the blowby gas, as it returns
under gravity to the sump.
The air space above the oil is partially separated from the
upper crankcase air space. Most of the moving parts are
FIGURE 5 FTIR Crankcase water measurements for ICE
vehicle over WLTC.
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FIGURE 6 FTIR Crankcase water measurements across the
WLTC for a PHEV.
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FIGURE 7 The measured water content from the four
sensors for an ICE standard WLTC cycle. Engine oil
temperatures are also shown.
in-between. The crankcase ventilation system guides the
blowby gases straight up to the breather system.
Significant amounts of water are seen in the upper crankcase blowby air, however they are not seen in the lower crankcase sensor, as they are partially separated. The only connection is the oil return down and blowby gas up route.
Test Bed Direct Water
Content Measurement,
in Lower Engine
ICE WLTC Operation
When the water content is monitored throughout a standard
WLTC, the leading sensor is the air space in the crankcase as
it is closer to the blowby gases, see Figure 7.
This sensor is believed to be covered in oil, the circulation
is the continuous spraying and splashing within the crankcase. Consequently, it will sense the concentration of water in
the oil in that space. It is shown in the chassis dyno experiments that the crankcase gases contain ~1 to 3 % water and
this is the driver for water ingress into the oil, as shown by
this sensor. This response is closely followed by an increase in
filler cap air water content, although the magnitude is
significantly lower.
As the water in the blowby gases is circulated and is
absorbed by the oil, the gallery oil sensor starts to register an
increase in water. The gallery sensor leads the sensor in the
middle of the sump oil, indicating that water containing oil
flows directly to the oil pick up, then distributed throughout
the engine.
As the test progresses the oil temperature increases, and
the net water absorption slows. At approximately 250 seconds
the crankcase air and gallery oil water sensors reach their peak
levels, then the oil begins to reject water quicker than absorb
it. At approximately 400 seconds the filler cap and the sump
also reach their peak – and all sensors converge towards the
baseline water content. The filler cap sensor continues to read
water for longer than the other sensors; it is at the end of the
closed filler pipe, in a relatively remote location, with low flow.
Overall, when compared with the 10,000 to 30,000 ppm
water in gases of the upper engine, the oil appears to reject
the majority of the water, however at lower temperatures some
water is absorbed into the oil.
PHEV WLTC Operation
Due to reduced engine-on time, the PHEV oil temperature
warm up profile is significantly slower than that of the ICE,
see Figure 8.
The oil temperature also varies more, as the extended
engine-off time gives the oil time to cool. This appears especially during engine-off conditions, due to the lack of mixing.
The bulk oil temperature is believed to be more stable.
As for the ICE, the crankcase water content starts with a
sharp increase at the first engine start, followed by the other
sensors. Again, the water content continues to rise steadily
until the oil temperatures reach ~40 °C, after which the water
contents reduce and converge.
The key difference is the amount of time taken to reach
40 °C. In the case of the ICE this was approximately 250
seconds, whereas in the case of the PHEV it was closer to 1000
seconds. The additional time the oil has spent held in a humid
environment allows the peak water content of the oil to
be significantly higher than that of the ICE – with gallery
sensor readings of ~550 ppm compared to ~300 ppm.
In the PHEV cycle the delay of sump oil water data was
significantly increased, seeing almost no increase in registered
water content for the first 700 seconds. This again
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FIGURE 8 The measured water content from the four
sensors for the PHEV running a WLTC cycle. Engine oil
temperatures are also shown.
demonstrating that the water is heterogeneously distributed
throughout the engine.
FIGURE 9 Water absorption behaviour of ICE operation
over a hot start WLTC.
FIGURE 10 Water absorption behaviour of PHEV operation
over a hot start WLTC.
WLTC Hot Starts
For completeness, data was also gathered for a hot start WLTC
for both ICE and PHEV, see Figure 9 and Figure 10. Here it
can be seen that the water content is evenly distributed and
low, stable within the range of 100 to 150 ppm for most positions in the engine.
Direct Comparison Between
PHEV and ICE for One Sensor
The differences between the PHEV and the ICE response to
water absorption can be seen more clearly by plotting a
comparison for a single sensor, as Figure 11 shows, for a cold
start WLTC immediately followed by a hot start WLTC. Here,
for clarity, just the gallery water content is plotted.
Once the oil temperature rises above ~40 °C, water is
rejected and the ppm level returned apparently close to
baseline, by the end of the cold start WLTC. In this case, it
remains stable over the subsequent hot start WLTC.
For the ICE cycle data shown here, the water peaks within
part 1 of the WLTC, say after ~300 seconds. So the scenario
for high water content is a short engine on time, which is
straightforward when the trip length is short. It is possible
that a driver could run a 2 miles (3.2 km) trip from a cold start
every time, though this is unusual [2]. Leading to little water
in the oil over ICE WLTC.
For the PHEV WLTC, the water content was higher, for
a longer time. The water ppm levels reduced after part 1 and
part 2 of the WLTC, Figure 11. So to achieve water accumulation, the engine must stop after part 1 and part 2, then
cooldown again, before restarting.
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FIGURE 11 A direct comparison of measured water content
in the gallery for ICE and PHEV operation over two backto-back WLTCs.
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2. A longer journey of say 35 miles (56 km), with 30
miles (48 km) E range included.
3. Battery at minimum SOC, or driver selects CS-mode,
for a short trip.
4. Battery full: trip distance just > range.
5. A hill at the end of the journey, so more power is
required. Here after a small movement of the pedal,
the engine may generate 50 kW for a minute from a
cold start. WLTC does not factor in altitude change,
whereas Euro 7 will do so, for
emission measurements.
6. Cold start and short trip, for cabin heating on a
cold day.
7. Driver behaviour: for example switching to economy
mode to override engine run, save fuel and/or
E-range. Or changing the cabin temperature set point
can cause the engine to switch on or off.
Test Cell Controlled Cooldown
Urban Hybrid Cycle,
Shortening the WLTC
With the benefit of the sensors, it was possible to rapidly
identify a shortened WLTC PHEV cycle, which gave an
increasing water content level, part 1 + part 2 then cooldown,
hereafter referred to as the Urban Hybrid Cycle. This represents multiple short engine duration on-times seen with
PHEVs. Importantly, this is possible for a range of
journey lengths.
In this paper sensor data is presented from a run of a few
Urban Hybrid Cycles, say 1 or 2. With a cooldown after
each cycle.
Also, tests were run for 85 cycles with ~2 hour controlled
cooldowns in-between each cycle. So over ~8 days. This representing multiple short engine running events. For each Urban
Hybrid cycle, the peak oil temperature reaches 40 °C.
It is reasonable to expect that a similar peak temperature
would be achieved for example: by a short cold start journey
of 5 miles (8 km) length, or a longer journey of say 35 miles
(56 km), with 30 miles (48 km) E range included.
Such short engine operation, for a variety of reasons, is
known to PHEV drivers over the last ~10 years of customer
use. A few cases are listed here, as examples.
From the author’s PHEV experiences, typical scenarios
where engine on-time is short include:
1. A short cold start journey of up to 5 miles (8 km)
length, with enough pedal attack to trigger an
engine start.
Between repeat cycles the engine was carefully and repeatably
cooled back to the starting temperature, 18 °C.
An example of such a cooldown is shown in Figure 12.
While the engine was stopped, the oil and coolant were circulated to ensure an even cooling. This process shows water loss
from the oil. Some of which is apparent, though actually
temporary. A full drying cycle is necessary to achieve a stable
start condition.
For the figures throughout this paper, unless stated otherwise the cooldown data has been omitted, as the focus is on
the actual running cycle data.
FIGURE 12
A typical cooldown.
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Engine Drying Procedure
Just as for the high precision fuel economy tests described in
[9], a reliable method of pre-test engine flushing and drying
is essential, to give a fair water content test.
Once a low temperature Urban Hybrid Cycle has been
run several times, there is considerable measured water in
the engine.
If a single standard cold start, ICE WLTC is run, the water
level is apparently reduced acceptably, for example to below
200 ppm. However, when the engine is cooled and restarted
a higher level of water is immediately observed, say 500 ppm.
This level of water carryover is not compatible with a high
precision test. It was apparent that some water was not
appearing on the sensors after a single WLTC, but was elsewhere in the engine, then re-joined the oil on the cooldown.
As a countermeasure, two WLTC cycles were run backto-back as Figure 13 shows. This provided an additional 30
minutes of hot flushing and drying, to completely remove the
more elusive water from the engine. After this procedure, the
water level reliably returned to less than 200 ppm, our quality
measure at this stage of the development.
Individual Sensor Results
Repeatability Over the Urban
Hybrid Cycle
As a result of all the efforts with the sensors, installation, cycle
controls, test detail development, cooldown and drying developments, useful levels of sensor repeatability over the urban
hybrid cycle was demonstrated.
FIGURE 13 Engine drying procedure consisting of two
back-to-back WLTCs.
FIGURE 14 View of typical sensor repeatability across the
Urban Hybrid Cycle.
TABLE 4 Typical range of sensor data.
Sensor
Position
Maximum Range Over Urban Hybrid Cycle +/ppm (test 1 minus test 2)
Gallery
-282 ppm
Sump
+50 ppm
Crankcase
+46 ppm
Filler Cap
-45 ppm
Figure 14 shows a summary of the level of repeatability,
of both sensor and test procedure, for all four positions.
Typical range of data for these repeats is shown in Table 4.
Procedure Repeatability
Figure 15 shows the results of an ABBA test matrix, this
compares two different oils, oil A and oil B. Here two cycles
are shown.
This figure is to show that the test procedure, including
flushing and drying, was capable of high-quality repeatability
of +/-30 ppm water and clear lubricant differentiation.
Between each cycle shown here, there was a two-hour
gentle cooldown. This has been removed for clarity of the
running phase data.
Some water loss was observed, during cooldown. This is
thought to be due to natural evaporation, accelerated a little
by the oil circulation of the cooling circulation system.
Of prime importance is that this controlled cooldown
was identical for each test, so they are comparable.
Oil Type General Observations
To demonstrate key aspects of oil behaviour, three engine oils
are compared in this section. Below are some observations,
which outline different types of oil behaviours. An overarching observation is that there are differences between the
oils tested. Also it should be noted that these observations
refer to a short running time, so may not correspond to more
prolonged engine running.
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FIGURE 15 ABBA repeatability matrix comparing two
formulations in PHEV operation over two Urban Hybrid Cycles.
There was a full cooldown between each cycle.
FIGURE 16 Oil gallery water content over two Urban
Hybrid Cycles. There was a full cooldown between each cycle.
Figure 16 is an example test data matrix to show that the
test shows different oil formulations give different, repeatable
effects. Hence this method can differentiate between oils.
Oil B absorbed water at a low rate, then did not reject
water so much during the cooldown. Oil C absorbed water at
a high rate, then rejected water at a higher rate. Consequently,
the net results for oils B and C were similar.
Whereas Oil D absorbed water at a high rate, then showed
very low water loss during the cooldown phase. Consequently,
Oil D ended up with the highest overall water content. The
net water accumulation result appears to be the result of these
two clear effects, both water gain rate and water loss rate.
Brief Comparison with
Karl Fischer (KF) Titration
After Longer Engine
Running Time
The water sensor data gave immediate insights for up to an
hour of engine running, with data showing typically less than
1,000 ppm water in the oil. No sample taking and/or post-test
analysis was necessary for these measurements. Though it
must be recalled that the sensors required calibrating for each
oil tested.
141
To compare the sensor data with longer engine running
water build-up, the Urban Hybrid Cycle was repeated 85 times
with several oil samples taking throughout. This extended test
duration took ~192 hours compared with the ~0.25 hours
required to complete one Urban Hybrid Cycle, with the
sensors recording.
The water content of the oil samples taken from the
engine were measured using Karl Fischer titration [13]
method. This is a common automotive lubricant industry
water content measurement for fluids that relies on analysis
of a small aliquot of test sample taken from the total sample.
The data shown here are to give some indication of water
build-up over a longer time. The short time sensor results are
encouraging, as they show interesting relationships with the
longer run-time results. Also, to an extent the longer tests
validated the sensor data.
Further discussion on the importance of sample preparation before conducting such water measurements on used
engine oil samples is included later in the manuscript.
Figure 17 shows the used engine oil water content results,
for two oils, measured with both methods. The single water
sensor cycle data has been extrapolated to 85 cycles, for
comparison purposes.
For the upper graph oil D, the water content of the oil is
seen to increase following an approximately linear trend with
the end of test sample containing >5 % water. This is at a higher
rate than predicted by the sensors. Results up to 1.5 % water
show a smaller error.
For the lower graph, a different oil E, the sensors align
more closely with the sample data, up to ~3 % water. Above
this the water level increases at a faster rate.
At the end of the tests, both oils show over 5 % water
content. This high-water content demonstrates the impact of
repeatedly carrying out short engine operation at low engine
oil temperatures.
Currently it is believed that in 15 minutes the sensors give
a useful indication of water content build-up over the first ~30
cycles which could take ~75 hours. Thirty Urban Hybrid
Cycles, one after another is quite a severe case: 10 cycles a week
for 3 weeks, with no weekend higher temperature, for example.
Also of note, the oil appears to show a change in water
pick up after these initial 30 cycles. However further work is
underway in this region.
Used Engine Oil Analysis,
Sample Preparation
The small sample required for KF titration water testing, 0.25
g to 3 g, is generally an advantage. When trying to describe
the properties of a large volume of fluid though, such as the
total engine oil fill in a vehicle, it is believed that for best results
some extra consideration is required.
For a homogeneous sample, such as a correctly blended
fresh engine oil, the preparation required before measurement
is limited, as typically any water present is small and evenly
distributed throughout the fluid. However, for a potentially
heterogeneous sample, such as a used engine oil with high
water levels, for example one which has been exposed to hybrid
operating conditions, careful preparation is critical to
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FIGURE 17 Two different oils, Oil D and Oil E. Actual water content measurements made according to ASTM D6034 compared
with predicted water content for all four sensors based on the sensor values measured across one Urban Hybrid Cycle. Two repeats
of the Urban Hybrid Cycle have been used to create a prediction band. Error bars have been included for the KF water data using
the reproducibility value from the ASTM D6304 method [13].
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143
accurately report the total water content of the sample. Others
have also reported the importance of such sample preparation
in relation to producing homogenous emulsion samples [14].
For example, a used engine oil sample after running 85
Urban Hybrid Cycles is shown in Figure 18. Stratification can
be seen visually after storing the sample; there is a bulk phase
on top and a denser white phase at the bottom.
Water content analysis of 0.5 g aliquots taken from
different regions of the sample shown in Figure 18, by KF
titration, shows a large variation which correlates with the
visual separation seen. In isolation these results can lead to
very different conclusions. While seeing the sample visually
in a beaker highlights the heterogenous nature of the sample,
if an aliquot for water content measurement is taken directly
from an opaque container or oil can, as used to collect the
drain oil from the engine in the first place, this may not
be so clear.
For a heterogenous engine oil sample, preparation is
therefore critical before taking an aliquot for measurement,
in order to ensure the result accurately describes the total
fluid. In Figure 19 different preparation techniques are
explored and their influence on the subsequent KF titration
water measurements is shown.
After testing different methods of sample preparation, it
was shown that shaking the sample container vigorously can
mix the fluid sufficiently, so that there is a more uniform
distribution of water throughout the total sample. This may
not be practical for a large volume of oil, so a mixing method
was also evaluated, which also produces a considerably more
homogenous sample.
FIGURE 19 The impact of different sample preparation
methods on the measured water content of two drain oil
samples, collected from two separate 85 Urban Hybrid
Cycle tests.
FIGURE 18 An example of the variability of KF titration
water measurements seen for a used engine oil sample
depending on where in the sample the aliquot for analysis
is taken.
Conclusions
Throughout this manuscript KF titration water measurements reported were achieved by using the mixing procedure
described, targeting a fair, representative result.
1. Use of these responsive sensors and a suitable test
protocol delivers repeatable water content data within
engine oil.
2. Engine oil water content in a running internal
combustion engine varies dynamically with the
engine drive-cycle and engine oil temperature.
3. For the ICE over WLTC, the water accumulates
quickly, with gallery oil water content peaking within
Part 1, ~3 km (1.8 miles) distance. By comparison, for
the PHEV over WLTC, the gallery oil water content
peaks at the end of Part 2, ~8 km (5 miles). As the
PHEV can travel with EV miles, then the later
represents a range of journey lengths.
4. During cold operation and engine warm up, there is a
wide range of measured water content, for the range
of locations within the engine measured here.
5. Oil formulation affects the measured water content in
the engine oil, in a running engine.
6. Running multiple test cycles allows the measured
water content from the sensors over a single WLTC,
to be compared with Karl Fischer titration water
measurements over a much longer operation time.
The relationship appears to be oil dependant. At high
water levels an increased accumulation rate is shown.
More work is required, this will be the subject of a
future paper.
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7. These responsive sensors give useful data after a few
minutes of engine running. So are a suitable tool for
understanding how oils behave in the engine and
breather system.
8. For best results, Karl Fischer titration water
measurements require a sample from a homogeneous
mix in order to representatively measure water
content from a larger volume of fluid.
Acknowledgments
The authors would like to respectfully acknowledge all
contributing members of the bp Castrol Bochum, Germany
and Pangbourne, UK team; for agile teamwork together, which
produced the data and insights presented here. In particular:
Robert Spragg, Christian Bomholt, Ludwig Stenner, Luigi
Aleo, Melanie Harlos, Antje Hoyer-Bracke, Kai Müller and
Jörg Ludemann.
The authors would also like to thank the leadership of bp
for granting permission to publish this work.
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Contact Information
richard.butcher@castrol.com
Definitions/Abbreviations
CO2 - Carbon dioxide
CS mode - Charge sustaining mode, where drive battery
charge at end of cycle = charge at start of cycle.
EU - European Union
EURO 7 - Next future European emission standard
EV - Electric vehicle, a mode of PHEV operation
FTIR - Fourier Transform Infrared spectroscopy
HEV - Hybrid Electric Vehicle
ICE - Internal Combustion Engine
KF - Karl Fischer method - ASTM D6304 method [13]
OEM - Original Equipment Manufacturer
PCV - Positive Crankcase Ventilation
PHEV - Plug-in Hybrid Electric Vehicle
SOC - State of Charge of a drive battery
WLTC - World Light duty Transient Cycle
WLTP - Worldwide Harmonized Light-vehicles Test Procedure
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