Oil Entry via Piston Top Land
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RESE ARCH Measuring Techniques
AUTHORS
Oil Entry via Piston Top Land
Investigations of the oil balance at the piston are important for optimizing future
engine concepts with regard to combustion processes and emissions. As part
Dr.-Ing. Thomas Ebert of the FVV project Fuel in Oil II (FVV No. 1225), the Institute for Combustion
is Project Manager
Engine Development Engines (VKA) at RWTH Aachen University developed and used a measurement
at Keyou GmbH
in Munich (Germany).
method for detailed investigations of the lubricating film thicknesses on the piston
top land. At the same time, the Institute for Analytical Measurement Technology
Hamburg e. V. (IAM-Hamburg) developed an exhaust gas analysis method to
investigate the oil emission mechanisms.
Ann-Christin Preuß,
M. Sc.
is Research Assistant at
the Institute for Analytical
Measurement Technology
Hamburg e. V. (IAM-
Hamburg) (Germany).
Prof. Dr.-Ing.
Stefan Pischinger
is Head of the Institute for
Combustion Engines (VKA)
at the RWTH Aachen
University (Germany).
Prof. Dr.-Ing.
Gerhard Matz
is Chairman of the
Institute for Analytical
Measurement Tech
nology Hamburg e. V.
(IAM-Hamburg)
(Germany).
© VKA
64 www.springerprofessional.com/automotive
1 MOTIVATION AND OB JECTIVES defined at three measuring positions on the cylinder liner: above
2 TEST ENGINES (measuring position 1) and below (measuring position 2) the first
3 ME THOD OLO GY OF THE OIL FILM THICKNESS ME ASUREMENT piston ring in the position of the piston at Top Dead Center (TDC)
4 ME ASUREMENT RESULTS and at a point of maximum fuel wall interaction (measuring posi-
5 E XHAUST G AS ANALYSIS AND RESULTS tion 3). The design and manufacturing process of the measuring
6 SIMUL ATION ME THOD OLO GY AND RESULTS probes are described in [5]. The key element of the measurement
7 SUMMARY method is the optical window, which seals the combustion cham-
ber and forms a complex multilayer system with air and oil as fur-
ther transparent media. Through a continuous refractive behavior
between oil and window material, it is possible to detect only the
oil layer on the piston top land. Fused silica as a window material,
which has a very similar light refraction behavior to engine oil, ful-
1 MOTIVATION AND OBJECTIVES
fills this requirement. By precisely focusing the layer to be mea-
Downsizing concepts with gasoline direct injection require new sured on the top land, layer separation can be further enhanced.
standards in combustion process development [1]. In earlier In addition, a window thickness of only 2.8 mm achieves minimal
research work, it was possible to prove clear interrelationships to signal degradation. Due to a maximum measuring rate of the mea-
the entry of lubricating oil into the combustion chamber both for suring device of only 4000 Hz, the investigations had to be limited
pre-ignitions [2] and particle emissions [3]. to low engine speeds.
Representing the system cylinder–piston–piston ring, to which
the highest contribution to the oil input can be attributed [4], the
4 MEASUREMENT RESULTS
research project focused on the mechanism of throw-off of oil
droplets from the piston top land as a result of inertial forces. One The results, FIGURE 1, FIGURE 2 and FIGURE 3, are displayed as an
objective was to carry out measurement-based investigations on image of the top land gap with the top land height on the y-axis
the oil balance at the piston top land as a source of a possible and the corresponding film thicknesses on the x-axis (light grey:
oil entry and numerical investigations on the oil entry behavior top land; dark grey: first piston ring). Successive measuring points
based on this. In addition, measurement-based analyses of aero- are superposed. For measuring position 1, FIGURE 1, all measuring
sol and oil emissions were used to reach conclusions about the time windows in the working cycle are preceded by a negative pis-
fundamental mechanisms involved. ton acceleration with inertial forces in the direction of TDC, as a
result of which the oil accumulation at the top land is shifted
toward the piston crown. Depending on the phase in the working
2 TEST ENGINES
cycle, the direction of the piston acceleration preceding the indi-
The measurements were carried out on two different engines. The vidual measurement time windows changes for measuring posi-
test engine for measuring the lubricant film thickness was a single- tion 3, FIGURE 2. Phase-dependently, the oil on the top land con-
cylinder gasoline engine (75 mm bore, 90.5 mm stroke) with direct sequently shifts toward the piston crown or piston ring. To inves-
gasoline injection. A single-cylinder gasoline engine with manifold tigate the influence of oil viscosity, the engine was conditioned
injection (90 mm bore, 96.3 mm stroke) from Helmut Schmidt to 30, 60 and 90 °C. The film thickness measurements were
University/University of the Federal Armed Forces Hamburg was carried out again at measuring position 1. FIGURE 3 shows the
used for the investigations using the exhaust gas analysis method. results for all three temperatures during expansion. The influence
of temperature and thus viscosity is clearly visible. At 30 °C in
particular, the oil appears to adhere strongly to the piston ring.
3 METHODOLOGY OF THE OIL
The dynamic behavior then becomes more pronounced toward
FILM THICKNESS MEASUREMENT
higher temperatures.
The oil film thickness measurements on the piston top land were At some points, the measurements give the impression that the
carried out using an interferometric film thickness measuring oil is strongly concentrated in the lower top land area. It is import-
device [5]. To perform the measurements, optical accesses were ant to note that missing measuring points in the upper area do not
FIGURE 1 Film thick-
ness measurement on
piston top land at
measuring position 1
(800 rpm, 5 bar pmi)
(© VKA)
MTZ worldwide 07-08|2021 65
RESE ARCH Measuring Techniques
FIGURE 2 Film thick-
ness measurement
on piston top land at
measuring position 3
(800 rpm, 5 bar pmi)
(© VKA)
FIGURE 3 Film thick-
ness measurement
on piston top land
at different engine
temperatures
(at measurement
position 1, expansion
phase, 800 rpm,
5 bar pmi) (© VKA)
necessarily mean that there is actually no oil present. Due to the spectrometric analysis method is based on the dependence of the
high oil film dynamics at the piston top land, it is possible that the spectrum of the oil emission on the underlying oil emission mech-
oil film is formed in such a way that it cannot be measured or only anism. While evaporation from the cylinder liner primarily leads to
with very low signal quality. a discharge of short-chain hydrocarbons, the proportion of long-
chain hydrocarbons is significantly increased in case of oil drop-
lets. FIGURE 4 (left) shows an evaporation spectrum that was mea-
5 EXHAUST GAS ANALYSIS AND RESULTS
sured immediately after the engine stopped. The spectrum in
In order to distinguish between evaporation from cylinder liner and FIGURE 4 (right) results from a complete evaporation of oil droplets
throw-off, scraping or reverse blow-by as oil emission sources, using the calibration unit [5]. The mass spectrometric analysis of
simultaneous measurements of the unburned hydrocarbons and hydrocarbon chain lengths is supported by correlation with the
the aerosol particles in the exhaust gas were carried out. For this aerosol particle emission.
a Quadrupole Time-Of-Flight (Q-TOF) mass spectrometer and a In order to provoke distinguishable oil emission mechanisms,
white light aerosol spectrometer system were used [5]. The mass the injection was interrupted. If the injection is switched off, the
FIGURE 4 Mass spectra to be expected
due to evaporation (left) and oil
droplet emission (right)
(© IAM-Hamburg)
66 www.springerprofessional.com/automotive
www.ut99.com
oil emission increases significantly, FIGURE 5 (1). The low propor-
tion of long-chain hydrocarbons and the non-increasing aerosol
particle concentration lead to the conclusion that wall film evap- HIGH PERFORMANCE
BLOW-BY-FILTER
oration is the primary oil emission source at this time.
15 s after the start of the injection interruption, FIGURE 5 (2), the
PLATFORM
proportion of long-chain hydrocarbons is significantly increased
(apparent in the comparison of the quotient of the summary high-
pass filters) and the aerosol particle concentration increases. This
leads to the conclusion that the decreasing wall film evaporation
due to the decreasing oil film temperatures is increasingly super- FOR COMBUSTION ENGINES
imposed by an oil droplet emission. It can be assumed that this is UP TO 4000 KW
due to an increasing oil film thickness at top land in combination
with the inertia forces. If the injection is reactivated, FIGURE 5 (3),
increased oil and aerosol particle emissions occur. The spectrum
shows a significantly increased proportion of long-chain hydrocar-
bons. This indicates an increased oil droplet emission. The evalu-
ation of the aerosol spectra shows that during the interruption of
injection, FIGURE 5 (2) oil droplets with a size of up to 2 μm are
emitted, while when combustion is restarted, FIGURE 5 (3), the aero-
sol particle diameters are clearly below 1 μm.
6 SIMULATION METHODOLOGY AND RESULTS
The investigations on the throw-off of oil droplets from the piston
top land due to inertial forces, which represent a complex multi-
phase problem, were carried out using a so-called Volume of Fluid
(VOF) approach. Due to the maximum two-dimensional resolution
of the oil film thickness measurements the actual three-dimen-
sional multiphase problem was reduced to a two-dimensional prob-
lem. The simulation domain includes the top land volume and a
small gas area above the piston. The influence of the cylinder pres-
sure is neglected compared to the effects of inertial forces. The
entire cell package, which is specified as a rigid dynamic grid, is Advantages
subjected to a translational behavior corresponding to the piston
movement. The phase initialization is based on the results of the • scalable platform
film thickness measurements.
The simulation results, FIGURE 6, illustrate the mechanism for
• mountable on the engine
detaching an oil droplet from the piston top land. The oil flows in • robust and service-friendly
the time range in the TDC area around the piston upper edge and
accumulates at the edge area of the piston crown. The interaction • configurable interfaces
of the stabilizing and destabilizing forces can be clearly seen.
A detaching process supposedly beginning in TDC is first stabi-
• integrated oil drain
lized and only begins about 20 °CA later, after a larger amount • compact unit RESIDUAL OIL CONTENT
AFTER FILTRATION
of oil with its destabilizing effect is present at the piston crown.
The droplets observed here have diameters in a range between • low pressure drop
RESE ARCH Measuring Techniques
FIGURE 5 Oil and aerosol particle
emission with 30 s injection
interruption at 3000 rpm,
15 Nm, 90 °C oil and cooling
water temperature); injection
switched off (1), injection inter-
rupted (2), injection switched
back on (3) (© IAM-Hamburg)
FIGURE 6 Detachment of an oil droplet at 6000 rpm and 260 °C oil temperature (© IAM-Hamburg)
nomena at the top land illustrate the mechanism for throw-off of
oil droplets from the piston as a result of inertial forces. Simulta-
neous measurements of oil and aerosol particle emissions show a
strong correlation.
THANKS
The research project (FVV project no. 1225) was performed by the Institute for
REFERENCES Combustion Engines (VKA) at the RWTH Aachen University under the direction
[1] Willand, J;. et al.: Limits on downsizing in spark ignition engines due to of Prof. Dr.-Ing. Stefan Pischinger and by the Institute for Analytical Measurement
pre-ignition. In: MTZworldwide 5/2009, pp. 56-61
Technology Hamburg e. V. (IAM-Hamburg) under the direction of Prof. Dr.-Ing.
[2] Dahnz, C.; Han, K.-M.; Magar, M.: Vorentflammung bei Ottomotoren: Unter-
suchung des Auftretens und der Ursache von Selbstzündungen vor Zündung- Gerhard Matz. Based on a decision taken by the German Bundestag, it was
seinleitung bei aufgeladenen Motoren mit hohem Verdichtungsverhältnis. Final supported by the Federal Ministry for Economic Affairs and Energy (BMWi) and
report FVV-Project No. 931, 2010
the German Federation of Industrial Research Associations (AiF) e. V. within the
[3] Dageförde, H.: Partikel bei Otto-DI: Untersuchung von Maßnahmen zur
Reduktion der Partikel-Anzahlemissionen bei Otto-DI-Motoren. Final report framework of the industrial collective research (IGF) program (IGF-Nr. 19089 N).
FVV-Project No. 1046, 2013 The project was conducted by an expert group led by Dr.-Ing. Marcus Gohl (APL
[4] Völtz, M.: Einfluß des Motorenöls auf den Ölverbrauch: Quellen und Mecha-
Automobil-Prüftechnik Landau GmbH). The authors gratefully acknowledge the
nismen des Ölverbrauchs im Motor. In: Krafthand 12/1997, n. p.
[5] Gohl, M.; et al.: Investigation of Oil Sources in the Combustion Chamber of support received from the funding organizations, from the FVV (Research Asso-
Direct Injection Gasoline Engines. SAE Technical Paper No. 2018-01-1811, 2018 ciation for Combustion Engines e. V.) and from all those involved in the project.
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