a comparison of simulation and engine test

A COMPARISON OF SIMULATION AND ENGINE
TEST DATA FOR VARIOUS SKIRT PROFILE
AND CAM SHAPES
By
Raymond R. Istenes, Jr.
Zollner Pistons, LLC.
Fort Wayne, Indiana
ABSTRACT:
A study was conducted to compare piston
wear and noise levels observed in engine
tests to those determined using PISDYN
from Ricardo. Three skirt profile and four
cam shapes were investigated in addition to
skirt coating. PISDYN simulations showed
increased wear levels that were consistent
with engine test results. There was some
disagreement between the location of wear
from the simulation and test results.
Potential sources for this discrepancy are
discussed and future simulations will be
performed. However, it was determined that
the PISDYN code can be used as an
effective tool to optimize the skirt profile
and cam for reduced piston wear and noise.
INTRODUCTION:
The reduction of noise and wear is of utmost
importance when designing a piston for
most engine applications. Critical engine
operating conditions for noise and wear
include cold start, cold idle, hot idle, and hot
under load (1). Even a reliable engine that is
labeled by consumers as noisy is
unacceptable. As a result, the optimization
of the piston to reduce noise from piston
slap and land contact is extremely desirable.
Engine parameters such as peak cylinder
pressure, peak engine speed, stroke, and
connecting rod length can greatly effect
piston motion within the cylinder bore.
Critical piston design parameters include
compression height, pin hole offset, total
weight, center of gravity location, and the
skirt design. Factors such as skirt thickness
profile, clearance, profile, cam, and total
length can all be optimized to reduce piston
noise and wear.
Of the mentioned parameters pin hole offset,
profile, and cam are those which the piston
designer has the most freedom to
manipulate. Other piston parameters can be
modified but they usually have more of an
impact on other areas of engine performance
or function and in many cases can not be
altered.
However, the time from initial design to
production is constantly being reduced
which necessitates the need to accurately
predict and reduce piston noise using
analytical methods. At Zollner Pistons this
is accomplished by using PISDYN and other
analytical tools to optimize the skirt design
for structural integrity and minimal noise.
Three parametric studies are presented in
this paper which show the effects of skirt
profile, cam, and skirt coating on skirt
performance during hot scuff testing. Other
piston parameters such as skirt thickness
profile and pin hole offset were not varied
during this study. Values of hydrodynamic
pressure, asperity contact pressure, wear
load, and frictional forces are shown for
each case. Photographs from the actual
engine tests are also included for
comparison with the simulated results.
PISTON AND ENGINE DATA:
The piston and engine used in this study are
current production models. As a result,
critical information such as pressure traces
and piston temperatures were available. The
peak cylinder pressure was 65.5 bar and the
pressure trace is shown in Figure 1.
Additional engine and piston parameters are
shown below:
Engine Type
V8
Bore Size
90.19 mm
Stroke
90.00 mm
Piston Weight
356.0 grams
Pin Offset
0.127 mm
Head Clearance
1.000 mm
Skirt Clearance
0.0203 mm
Pin Diameter
22.00 mm
Peak Cylinder Pressure 65.5 bar @ 4500
RPM
In order to perform an elastohydrodynamic
simulation information must be input to
define the skirt cutout profile, skirt thickness
profile, skirt profile, and cam. The skirt
cutout profile was determined at 11 points
and the skirt thickness profile was
determined for nine locations in both the
axial and circumferential directions. This
information was obtained from a parametric
solid model of the piston. Skirt profile
information was input at 21 axial locations
on the skirt. This was done to accurately
capture the skirt shape for all profiles
considered. In addition, the cam was input
for the top and bottom of the skirt and was
elliptical for all cases considered. It should
also be noted that the skirt cutout profile,
skirt thickness profile, skirt profile, and cam
were symmetrical for all cases.
A finite element model was needed to
determine piston temperatures, and the skirt
compliance matrix. The finite element
model was created using PISFLEX with a
9x9 mesh to define the lubricated zone of
the skirt. The model included the crown
offset option. Heat transfer coefficients
were applied to simulate measured
temperatures. Displacements due to thermal
loading were calculated using PISFLEX and
included in the PISDYN simulations. In
addition, displacements arising from
pressure and inertia loads were also
included.
It should be noted that bore distortion and
land contact parameters were not input into
the PISDYN simulations and liner
temperatures were taken as default values.
The effects of these omissions will be
discussed.
SKIRT PROFILE VARIATION:
PISDYN simulations were conducted for the
same piston configuration with varied skirt
profile shapes. Figure 2 illustrates the three
skirt profiles that were considered. The
three profiles have the same basic shape
below the maximum diameter which occurs
approximately 10.0 mm below the pin hole
centerline. The three profiles have very
distinct shapes above the maximum
diameter location. Profiles 2 and 3 were
derived in an attempt to fill out the bore near
the top of the skirt and reduce the rocking
motion of the piston in the bore, which
contributes, to noise. In addition, each case
had 0.508 mm of straight elliptical cam
applied to the skirt.
A team from Zollner Pistons and the
customer performed an audible noise
evaluation. The noise team evaluated piston
noise at a steady state condition of 1800
RPM under no load. The findings of the
team, which could be rated from 1 being
unacceptable to 10 being completely
acceptable, were as follows for Profiles 1-3.
Profile 1
Profile 2
Profile 3
6.8
6.9
7.2
The results of this test showed that Profiles 2
and 3 had a marginal effect on noise.
However, high wear was observed on
Profile 2, which is undesirable but will be
discussed later. Accelerometer and other
noise evaluations were conducted which
supported the above findings but that data
will not be included in this paper.
Tables 1a and 1b shows the effect of skirt
profile on the peak hydrodynamic pressure,
peak asperity contact pressure, maximum
wear load, noise rank, and wear rank.
Profile 2 had the highest values for all
variables and Profile 1 is a minor
improvement over Profile 3. It is observed
that the wear rank corresponds directly with
asperity contact pressure and can be used as
an indication of wear. Figures 3 and 4 (2)
show the skirt contact patterns from a hot
scuff engine test of Profiles 2 and 3. Only
cylinders #5-8 are shown but cylinders #1-4
showed similar results. A photo was not
available for Profile 1 but the appearance
was similar to that observed for Profile 3.
The area of wear on piston #8 of Profile 2,
indicates that the profile was close to a scuff
condition.
The area of wear predicted by PISDYN is
lower and outboard on the skirt as compared
to the high wear areas in Figure 3. This may
have been caused in part by the use of the
PISFLEX-generated model instead of a
more accurate solid model of the piston for
the finite element calculations. The
omission of bore distortion information will
also effect the obtained results. Figures 5
through 7 show the sum of the rotational and
lateral kinetic energy, the asperity friction
force on the skirt, and the piston rocking
angle, respectively. Figure 5 shows that the
piston with the highest kinetic energy had
the least favorable noise performance and
that the noise rank increases with increasing
kinetic energy. Figure 6 shows that Profile
2 which exhibited the highest wear and the
highest asperity pressure also had the
highest asperity friction forces on the skirt,
which corresponds to the wear rank. Figure
7 shows that Profiles 2 and 3 exhibited
reduced piston rocking in the cylinder bore
which was their design intent. However, the
fuller profile did result in increased wear in
Profile 2.
SKIRT COATING:
Skirt coating was applied to Profile 1 and
the scuff test and audible noise evaluation
were again performed. Wear patterns were
nearly identical for Profile 1 both with and
without coating. In addition, the audible
nose rating was 8.3, which was a significant
improvement over the previous results.
Since the coating had little effect on piston
secondary motions a nose evaluation could
not be based on kinetic energy for the coated
piston.
CAM VARIATION:
PISDYN simulations were also conducted
for the same piston configuration with
varied cam. The skirt profile for each case
was Profile 2 which showed the highest
wear in the preceding study. The various
cams that were tested are shown below:
Cam 1
Cam 2
Cam 3
Cam 4
0.508 mm (Top)
0.457 mm (Bot)
0.457 mm
0.406 mm
0.508 mm
It should be noted that all of the cams re
elliptical and that Cam 4 is the same as
Profile 2 in the previous study. Also, the
cam for Cam 1 varies linearly from the top
to the bottom of the skirt. Tables 2a and 2b
shows the effect of cam on the peak
hydrodynamic pressure, peak asperity
contact pressure, maximum wear load, and
wear rank. Hydrodynamic pressure, asperity
contact pressure and wear load all increased
with a reduction in cam. Figures 8 through
10 illustrate the skirt contact patterns from a
hot scuff engine test of Cams 1, 2, and 3.
Cams 1 and 4 passed the scuff test and both
had 0.508 mm of cam at the top of the skirt.
Similarly, Cam 2 scuffed two cylinders and
shown elevated wear levels in one other
cylinder. Cam 3 scuffed 4 cylinders and
showed signs of high wear levels in the
remaining four cylinders.
Figures 11 and 12 show the sum of the
rotational and lateral kinetic energy and the
asperity friction force on the skirt. Figure
11 shows that Cam 4 had the highest peak
kinetic energy. This would indicate that
Cam 1 should produce more favorable noise
results than Cam 4. However, variations in
kinetic energy are smaller than those in the
profile study and an audible noise evaluation
was not conducted. Figure 12 shows that
Cam 3, which exhibited the least desirable
hot scuff performance, had the highest
asperity skirt friction forces. Also the
magnitude of the friction force is nearly 6
times that of Cams 1 and 4 which
successfully completed the engine test.
Again, it is observed that the wear rank
increased with the asperity pressure and the
asperity friction force.
CONCLUSIONS:
1. Direct correlation was present between
engine test results and PISDYN output
parameters such as hydrodynamic
pressure, asperity contact pressure, and
wear load.
2. Areas of high wear were predicted by
using PISDYN; however, the location of
the wear was lower and outboard of the
wear observed during engine test. Not
including bore distortion or land contact
and using a solid model generated by
PISFLEX instead of a more accurate
solid model of the piston are possible
reasons for this discrepancy. The
temperature distribution was also known
in the circumferential direction at the top
of the skirt but the variation in the axial
direction was estimated. This is another
potential source for error.
3. Skirt coating was shown to reduce piston
noise but a reduction in kinetic energy
was not observed, in the simulations.
However, this occurred since secondary
motions of the skirt are not greatly
altered with the application of skirt
coating.
4. Rotational and lateral kinetic energy
plots showed correlation with audible
noise levels. In addition, the reduced
rocking angles produced by Profiles 2
and 3 resulted in reduced noise;
however, a skirt profile that is less
resistant to scuffing will result.
5. Skirt frictional and boundary lubrication
forces followed the observed trends for
skirt wear and scuffing. The amount of
asperity contact friction and wear load
could be directly related to engine test
data.
ACKNOWLEDGEMENTS:
I would like to thank John Whitacre and Kay
Colbert from Zollner Pistons for their
technical input for this paper as well as
Mathias Perchanok, Beejal Parmar, and
Denise Rowe from Ricardo for their input
and assistance.
REFERENCES:
1. Whitacre, John., “Automotive Gasoline
Engine Piston Noise, Sources and
Solutions,” SAE Paper 901491, 1990.
2. Colbert, Kay., Zollner Pistons Technical
Report # 1781, June 8, 1993.
3. PISDYN Version 2.5 Documentation
and Users Manual, Ricardo Software,
1998.
Table 1a, Predicted Skirt variables Versus Profile
Profile 1
Profile 2 (Cam 4)
Profile 3
Peak
Hydrodynamic
Pressure [Pa]
3.569x107
4.775x107
3.83x107
Peak Asperity
Contact
Pressure [Pa]
1.32x106
6.024x106
2.14x106
Wear load
[MW/m2]
6.26
28.32
7.33
Peak Kinetic
Energy
[kg.m2/s2]
1.2x10-4
1.5x10-4
2.4x10-4
Table 1b, Experimental results Versus Profile
Profile 1
Profile 2 (Cam 4)
Profile 3
Wear Rank
1= lowest
1
3
1
Noise Rank
1= best
3
2
1
Table 2a, Predicted Skirt variables Versus Cam
Cam Drop
[mm]
Cam 1
Cam 2
Cam 3
Cam 4 (Profile 2)
.508 (top)
.457 (bottom)
.457
.406
.508
Peak Asperity
Contact
Pressure [Pa]
1.088x107
64.16
6.239x107
7.025x107
4.77x107
2.34x107
5.13 x107
6.024x106
177
484
28.3
Table 2b, Experimental results Versus Cam
Cam 1
Cam 2
Cam 3
Cam 4 (Profile 2)
Wear Rank
1= lowest
1
3
4
2
Wear load
[MW/m2]
Peak
Hydrodynamic
Pressure [Pa]
5.0x107
Figure 3: Wear Patterns from Profile 2.
Figure 4: Wear Patterns from Profile 3.
Figure 8: Wear Patterns from Cam 1.
Figure 9: Wear Patterns from Cam 2.
Figure 10: Wear Patterns from Cam 3.