CONSEIL INTERNATIONAL
DES MACHINES A COMBUSTION
20th INTERNATIONAL CONGRESS ON COMBUSTION ENGINES
THE MEASUREMENT OF PISTON RING
OIL FILM THICKNESS IN A TWO-STROKE
MARINE DIESEL ENGINE
by
Stewart Moore
BP Group Research & Engineering, Sunbury on Thames
LONDON 1993
INTERNATIONAL COUNCIL
ON COMBUSTION ENGINES
THE MEASUREMENT OF PISTON RING
OIL FILM THICKNESS IN A TWO-STROKE
MARINE DIESEL ENGINE.
Stewart Moore Ph.D C.Eng., M.I.Mech.E
BP Group Research & Engineering Sunbury on Thames
Middlesex UK
ABSTRACT
An understanding of the mechanism of piston ring lubrication is essential for the correct formulation of cylinder lubricants and design of piston rings. In this paper a technique is described which uses capacitance transducers to measure the oil film directly. Sensors fitted in the cylinder liner allow both the instantaneous minimum filth thickness and the operating profile of the running surface of the individual piston rings to be determined. By fitting several sensors around and along the liner, the film thickness can be monitored at any position in the stroke, and the effect of changes hi
operating variables and lubricant parameters determined.
The design of the sensors is described, along with the associated measuring system. Results are presented from operating slow-speed engines showing the range and type
of measurements that can be made.
RESUME
Afin d'arriver a la composition correcte des lubrifiants des cylindreset de bien dessiner des segments de piston, c'est essentiel 'de comprendre la methode de letir
lubrification. Ce document decrit une technique qui emploie des detecteurs de capacite pour directement mesurer le film du lubrifiant.
La fixation des detecteurs dans la chemise du cylindre le rend possible de determiner le minimum instantane de l'epaiseur du film et le profit des segments de piston. Si l'on installe plusieurs detecteurs autour et au long du cylindre, on peut surveiller l'epaiseur du film a n'importe quel point de la course du piston, et on pew constater l'effet des changements des variables de fonctionnement et des parametres de lubrification.
Le dessin des detecteurs et le systeme de mesure sont decrites. Ce memoire presente des resultats obtenus pendant l'operation des moteurs a longuecourse et montre
INTRODUCTION.
Capacitance transducers have been used for a number of years by the author to
examine the piston ring lubrication conditions in automotive diesel engines (1) (2) (3). Although the lubrication has been shown to be hydrodynamic over most of the stroke, oil starvation in the inlet region of the ring severely depleted the film thickness below
the simple hydrodynamic value. A large part of this starvation was due to the
interaction of the individual rings, combined with the fact that in most automotive engines, oil is supplied to the liner via splash from the sump.
In a two-stroke marine engine the lubrication conditions are quite different and lubricant is supplied directly to the liner walls. To examine the mechanism of piston ring lubrication in such engines, and to determine the influence of mechanical and
lubricant parameters on film thickness, a series of measurements has been carried out on full size marine engines. This paper describes the instrumentation and operating technique and presents some results from engines showing the range of measurements that can be made.
INSTRUMENTATION. 2.1 Measurement Technique.
The capacitance transducer consists of a small coaxial probe fitted flush with the front
face of the cylinder liner. As the ring passes the probe, the electrical capacitance tothe ring is measured. If there is a continuous oil film between the ring and the probe, and the dielectric properties of the oil are known (dielectric constant only changes very slightly for used oil), then the capacitance can be interpreted as a film thickness. To minimise electrical breakdown and give adequate resolving power it is necessary to
make the transducer small. This leads to typical capacitance values of theorder 0.5 - 5 pF. Three terminal measurement techniques must be used to ensure that these
capacitances can be measured accurately in the presence of large stray capacitances
from the connecting leads. A schematic view of the active end of a transducer is
shown in Fig. 1.
22 Sensor Design.
The design of the sensor has evolved from the original simpler sensors used in
automotive applications. The design is essentially the same, but has been strengthened to withstand the high cylinder pressures near top-dead-centreand to give a longer operating life in service. For use with piston rings up to 10 mm wide, the central conductor (see Fig. 1) would be typically 1 mm diameter, while for ring widths greater than this, 2 mm diameter would be more suitable. The diameter of the central
electrode does affect the resolving power of the transducer slightly. The greaterthe diameter, the larger the edge rounding effect on the ring as the sensor passes over it.
The individual components of the sensor are assembled into a tapered cast-iron plug
and the plug pressed into a reamed tapered hole in the liner at the point where the measurement is to be made.
If sensors are to be fitted into a new liner, then it is possible for the final machining to
be carried out after they have been fitted. If not, then the sensors should be fitted
slightly proud of the surface and then carefully machined backby hand. Running-in, in
the engine, completes the bedding-in process. Care is needed when fitting sensors into wave-cut cylinders to ensure that the active part of the transducer is not positioned below the initial running surface of the liner.
PISTON RING
FIG. SCHEMATIC VIEW OF ACTIVE END OF SENSOR.
2.3 Calibration.
In principle, the transducer should obey the parallel plate capacitance formula. That is, the capacitance is proportional to the area of the centre of the sensor and the dielectric constant of the intervening medium, and inversely proportional to the distance between the plates (the ring and liner in this instance). One problem with using a co-axial sensor of this type in an engine, is that to avoid smearing the active front surface of the transducer when the oil film breaks down, it is necessary for the clearance between the central conductor and the screen to be larger than might be necessary in a static
measurement situation. This clearance means that, as the distance to be measured increases, the capacitance gauge progressively underestimates the real value of the film thickness due to a fringing field effect. It is therefore necessary for each sensor design
to be calibrated against a known distance scale. However, for the large diameter
sensors used in the marine engine test work, it is found from calibration checks that up to a distance of 30 jam, the difference between the calculated and measured
capacitance value is so small that it can be neglected. Thus, as the oil film thicknesses are usually considerably less than this, the parallel plate formula gives a very good approximation to the real value.
CYLINDER LINER
SCREEN
2.4 Data acquisition.
All measurements have been made using a Fylde FE420CD capacitive displacement
amplifier. This gives an output of 0 -10 volt, proportional to the capacitance being detected. A variable gain output stage allows a scalar to be used so that a measuring range from 0.1 - 100 pin can be covered on the same instrument. The exact film
thickness range covered obviously depends on the diameter of the transducer used. For test bed measurements, the normal signal collection method was to watch the signals being displayed on line, and then to capture the required data on a portable data
logger for later interpretation. For the measurements made on the test ship however, a dedicated data logger was built. This was software driven so that the type of logging
(sensor number, logging frequency, engine stroke, number of rings examined, etc.)
could be varied to suit the type of measurements being made. Clearly during a three
month voyage, with the film thickness being logged automatically, it is possible to
collect an enormous amount of data. It is necessary therefore to be very selective in
the type of data logged and the logging period. Flexibility built into the logger also allowed the system to be used in an on-line mode, with rapid logging at high frequency, so that particular experiments could be carried out.
TEST ENGINES.
3.1 Engine Type.Although a variety of engines have been used to date in this work, the measurements reported here will be from two particular slow speed marine diesel engines designated engine A and engine B.
Engine A was a MAN B&W 6L35MC engine operated on a test bed and engine B a New Sulzer Diesel 6RTA62 fitted into a container vessel trading regularly between
Europe and the Far-East. Both engines were comprehensively instrumented with temperature sensors in addition to the film thickness transducers. The MAN B&W
piston was fitted with 4 rings and the Sulzer piston with 5 rings.
3.2 Fuel.
The test bed engine (engine A) was operated on gas oil with a typical S content of 0.2%. Engine B was operated on residual fuel with atypical S content of 3.2%.
33 Cylinder Lubricants.
The MAN B&W engine (engine A) was operated on a range of BP cylinder lubricants,
while the Sulzer engine (engine B) was operated continuously on BP CL050M. This is an SAE 50, 70 TBN premium grade cylinder oil. Feed rates were varied on both
engines depending upon the measurement programme being carried out.
RESULTS.
4.1 Description of signal.
Before presenting the results in detail, it will be helpful to describe a typical trace from
one of the sensors. Fig. 2 shows a capacitance trace from the top ring of engine A, measured on a sensor fitted just below second ring tdc. The measurement was made
on the compression stroke, so the top face of the ring appears on the left hand side of the figure and the bottom of the ring on the right hand side. The trace is a plot from an oscilloscope screen and the shape of the trace is a direct representation of the
operating profile of the piston ring.
FIG. 2 TYPICAL OUTPUT FROM TRANSDUCER SHOWING REPEATABILITY OF SIGNAL OVER 100 CYCLES.
When interpreting the shape, it must however be realised that the vertical scale represents typically 40 - 501.im, while the width of the trace equates to the actual width of the ring which is typically 8 -15 mm. Thus the resulting trace gives an extremely exaggerated ring profile. The minimum film thickness is shown by the distance between the zero voltage reference line and the actual profile. This trace is a composite of 30 seconds continuous output from the sensor (100 engine revolutions) and therefore gives a good impression of signal repeatability. It is seen that the minimum film thickness is almost constant throughout the exposure. The slight increase in width at the sides of the trace indicates small changes in the vertical
position of the ring. Thiscan be caused by either movement of the ring in its groove,
or slight stroke to stroke variations in engine speed.
42 Ring Behaviour and Film Thickness Measurements.
The attitude of the top ring between the compression and expansion strokes is shown in Fig. 3 for engine A. These traces were measured on a sensor fitted just below 2nd ring tdc with the engine operating at 75% load. As the oscilloscope alwaystriggers from left to right, the leading edge of the ring is always on the left hand side of the
trace. Thus in Fig. 3a the top of the ring is on the left hand side, while in Fig. 3b the
bottom face of the ring ison the left hand side. The fact that
one trace is almost a
direct reversal of the otherindicates that the ring is
not tilting between the strokes. It
will also be seen that the minimum film thickness is thinner on the expansion stroke. This is expected and will be discussed in more detail later.
COMPRESSION STROKE
EXPANSION STROKE
iFIG.3 RING PROFILE COMPARISONS BETWEEN COMPRESSION & EXPANSION STROKES. MEASUREMENTS MADE ON ENGINE A.
Fig. 4 shows a similar set of results, but this time for both top and second rings and
with the measurements made on the engine B. The measurements were obtained
shortly after fitting new rings to the piston and some of the original machining marks on the face of the rings can be seen in the form of signal spikes in the generally smooth
traces. There is a similar pattern of behaviour between these two strokes as that
already shown in Fig. 3, although the second ring presents a noticeably flatter profile
'against the liner than the top ring does. One point of particular interest is the similarity
between the top ring profiles on the two different engine types, which, although having different ring widths, at this stage have run for a similar number of hours.
When the top ring is fully run-in, the shape presented to the cylinder liner near tdc
changes from a tapered to a flat surface. This is due to the top edge of the ring,which
is pressed against the liner during early running (see Fig. 4), being worn away during
It
I
It
running-in. This gives the ring a flat appearance at tdc but a tapered appearance at
mid-stroke, where the attitude of the ring changes.
TOP RING TOP RING
COMP STROKE EXP STROKE
2ND RING COMP STROKE
2ND RING EXP STROKE
FIG.4 TOP & SECOND RING PROFILES MEASURED
CLOSE TO SECOND RING TDC ON ENGINE
The change in attitude of the ring between tdc and mid-stroke positions is clearly seen in Figs. 5a and 5b. These were obtained from engine B after more than 1000 total ring
running hours. These signals are a direct output from the data logger and were
obtained on the same compression stroke. The data points on each trace are individual
voltage values. The lower trace (Fig. 5b) shows a tapered ring profile recorded on a
sensor fitted near mid-stroke, while the upper trace (Fig. 5a) shows the profile of the same ring on the same stroke but this time measured on a sensor close to top ring tdc. Although the ring looks highly tapered in the mid-stroke result, it must be remembered that because of the scale of the figure the ratio of ring width to profile height is almost 1000:1. The unusual dip in profile, close to the centre of the ring on the inlet side, will be discussed later.
As expected, the film thickness is considerably thicker at mid-stroke than at tdc. This is due to several reasons. Not only is the velocity of the ring highest near mid-stroke, but the temperature and pressure are lower and this increases oil viscosity and reduces
ring loading. However, it is significant that even at the highly stressed conditions near the top of the liner, a continuous oil film is still seen.
8° r
it) ;I:1 I!I I I 4 I I 1-1 I I I IHHII I I 1,1
I UM II
I1 I I I 1 II1I I 1 1, 1.:11-6:1,10I (b) PROFILE NEAR MID-STROKEFIG. 5
L
.
f-F4-1-1-H-1--1-H-1-H I I 1 I 1-1 1-1I i7 II
1 ii IHul
i(a): PROFILE NEAR TDC
70T
TOP RING PROFILE AND FILM THICKNESS RECORDED FROM ENGINE 13 AT 100% LOAD.,
It is of interest to compare the output from sensors fitted diametrically opposite each other in the liner. Not only does this give an indication of ring stability, but it also allows film thickness around the liner to be compared. In Fig. 6 such a comparison is
shown for two sensors mounted near mid-stroke in engine B. The measurements were made on a compression stroke. Fig. 6a is from the port side of the liner and Fig. 6b from the starboard side. In both cases the ring profiles and film thickness ratios are almost identical, suggesting that the ring is stable in the bore and the lubrication
conditions similar on both sides of the liner. The use of several such sensors fitted
around the liner has great potential for the study of lubricant spreadability.
II 60 10 70 -40 30 20 10 0
-40 30 20 8-J WI WI 10 c.) 0 11/111MMEMIMMIIHRINIMIMUNICHE IIMMIMIHRIHMIMHINIIIIIMHONTINliffelfilliff11101111 U. WI, 70
i(a); PROFILE ON PORT SIDE
(b) PROFILE ON STARBOARD SIDE
FIG. 6 VARIATIONS IN RING PROFILE AROUND LINER MEASURED CLOSE TO MID-STROKE ON ENGINE 'B
It was shown in Fig. 5 that, as expected, the film thickness near to tdc was
considerably thinner than at mid-stroke. This is mainly due to a reduction in forward velocity of the ring at the top of the stroke, which causes the oil film thickness to
decrease rapidly as the ring approaches the dead centres. The effect is similarat both
top and bottom dead centres but, because of the higher pressures and temperatures at
tdc, the effect is more severe there.
This reduction is shown in Fig. 7 for engine A, where the minimum film thickness ratio, for sensors fitted at a range of distances from top-dead-centreare given for 100% load
conditions. Sensor 5 was at 2nd ring tdc, sensor 6 at 3rd ring tdc and sensor 7 at 4th ring tdc. Because of the construction of the liner and cylinder itwas not possible to fit. a sensor close to top ring tcic in this engine. Results from compression and expansion strokes are given and tdc is shown in the centre of the figure. It is seen that the film thickness ratio falls rapidly as the ring approaches tdc and then increases again as the,
ring moves down on the expansion stroke. Film thicknesseson the expansion stroke
are thinner than on the compression stroke, as expected from the increased ring
0 41111 HNIMIIHUMMINHAIMMERIUMMINUMMININIE 80 -70 60 50 40 30 20
loading on this stroke. Some later measurements on this enginehave shown that
occasionally a thicker film of oil can be built up near top-dead-centre than just below it. This seems to be associated with fully run-in piston rings and is an effect that will be examined further in future work.
6 2 1 -0.9 0.8 LI zc 0.7 0 -4- Oh 0.5 -LT_ 0.4 0.3 0.2 0.1 SEN COMPRESSION STROKE SEN 6 SEN 5
FIG. 7 THE EFFECT OF SENSOR POSITION ON MINIMUM OIL FILM THICKNESS FOR COMPRESSION & EXPANSION STROKES ON ENGINE A.
The sensitivity of minimum film thickness to load isdemonstrated in Fig. 8. This is a typical result from a series of tests carried out on engine A. Results are shown for the
top ring on both strokes, at loads of 100% and50%, with the measurements made at
the 2nd ring tdc position. Films are thicker on the compression stroke than on the
expansion stroke, as has already been demonstrated in other results,and there is a clear load effect with an almost doubling of minimum film thickness on the expansion stroke when the load is reduced from 100 to 50%.
COMPRESSION STROKE TDC EXPANSION SMOKE 10 SEN 5 EXPANSION STROKE SEN 6 SEN 7 1% 50% 100% 50%
FIG. 8 THE EFFECT OF LOAD ON MINIMUM PISTON RING FILM THICKNESS ON ENGINE A. MEASUREMENTS MADE ON TOP RING AT 2ND RING TDC POSITION.
0
7
One advantage of a marine two-stroke engine for research of this nature is that the cylinder oil feed rate, and even the formulation type, can be varied while the engine is running. This allows lubricant parameters to be studied in great detail with almost
immediate results. Conventional wear testing methods, with the possible exception of
radioactive techniques such as TLA, all require long periods of running to obtain measurable data, and in some instances, dismantling of cylinder components may be
necessary to obtain wear values. In addition, long term testing often introduces
variables, such as fuel quality, which can be difficult to control and will almost certainly affect cylinder lubrication conditions.
5. DISCUSSION.
The results just presented clearly show that capacitance transducers can be used to measure piston ring oil film thickness in a two-stroke marine diesel engine, and that changes in film thickness ratio due to engine load and sensor position along the liner
can be easily determined. The sensors have proved to be robust in operation and
remain flush with the operating surface of the cylinder liner during service.
Repeatability of the signal has been shown to be good on both engine types, suggesting that a stable pattern of oil forms on the liner wall.
It was pointed out when presenting Fig. 3 and Fig. 7 that the film thickness on the expansion stroke was thinner than on the compression stroke. An obvious reason for
this is that the loading on the ring is higher at this position. However, in addition to this, there are hydrodynamic reasons for a reduction in film thickness after the ring has
passed through a dead centre, which are due to the squeeze film effect. As the ring
approaches either tdc or bdc the film thickness tends to reduce because the forward velocity of the ring is falling. When the ring is exactly at the dead centre the forward velocity of the ring is zero and it might be expected that the film thickness would also
be zero. This would happen if it were not for the fact that oil cannot escape instantly from beneath the rings running surface. This restricts the rate at which the film
thickness falls, and means that the film is still decreasing while the ring is stationary at each dead centre.
It is this so called squeeze film that protects the ring at the critical tdc position. By the
time the ring velocity increases again on the following stroke, the film thickness has fallen to below its initial tdc value and so film thicknesses after a dead centre are thinner than those before a dead centre. There are some complications to this simple explanation that depend on the symmetry of the ring profile, but generally minimum
film thickness can occur up to 10 degrees after the dead centres. The flatter the ring
profile the greater the contribution to oil film thickness from the squeeze film effect. The fact that in both Figs. 3 and 7 the measured film thickness is thinner after tdc than before, indicates that rings are behaving in an expected hydrodynamic manner and giving results similar to those observed in earlier work on small trunk piston engines (4).
It was shown in Fig. 5 that once the top ring had run-in, it presented a flat profile to the liner at tdc, whilst appearing tilted at the mid-stroke position. This behaviour has also been observed in many previous tests on trunk piston engines. Measurements of
the profile of worn rings after removal from the engine, and comparisons with the profile recorded in the engine by the capacitance transducer, have shown that generally the ring is sitting square in the bore at the mid-stroke position and that the flat profile at tdc is caused by the ring possibly twisting or tilting in its groove, or by elastic
deformation.
The slight dip in the profile on the inlet side of the ring (left hand side) in Fig. 5 is almost certainly due to air trapped in the ring inlet. The inlet of a piston ring may not always be completely filled with oil during operation and this means that a change in
dielectric constant occurs at the air/oil interface. As the typical dielectric constant for
oil is 2.3, compared to 1.0 for air, this change shows up as a slight variation in ring profile. This effect has been confirmed in previous measurements on small trunk piston engines using pressure transducers alongside the film thickness sensors (2).
6. CONCLUSIONS.
It has been demonstrated that capacitance transducers can be used in slow-speed marine engines to make useful measurements of piston ring oil film thickness and to monitor ring attitude.
Film thickness has been shown to vary along the liner wall in the expected hydrodynamic manner.
Film thickness is sensitive to engine load.
Oil film thickness is almost identical on opposite sides of the cylinder liner. Larger diameter sensors could be used to monitor piston movement.
It is concluded that the technique is an ideal tool for developing and assessing the performance of cylinder lubricants and piston rings.
ACKNOWLEDGEMENTS.
The author would like to thank BP Marine for permission to publish this paper, and
also to record his thanks to Mr. S. Greenland at BP Group Research and Engineering
for designing the data logger and writing the computer software.
The author would also like to record his thanks to numerous personnel from both MAN B&W and New Sulzer Diesel who have contributed to the success of this work.
REFERENCES.
(1) Hamilton, G. M. & Moore, S. L.
Measurement of the oil film thickness between the piston rings and liner of a small diesel engine.
Proc. I. Mech. E., 1974, Vol. 188, p253-261
Moore, S. L. & Hamilton, S. L.
The starved lubrication of piston rings in a diesel engine.
J. Mech. Eng. Sci., 1978, Vol. 20, No.6, p345-352 Moore, S. L.
Piston ring oil film thickness - the effect of viscosity. SAE Paper 850439, 1985
Moore, S. L. & Hamilton, G. M. The piston ring at top dead centre.
Proc. I. Mech. E., 1980, Vol. 194, No.36, p373-381
13 1(2)
(3)
