In-situ measurement and numerical solution of main journal bearing lubrication in actual engine environment.
In recent years, Computer Aided Engineering (CAE) analyses have been used frequently, allowing a lower number of prototype tests and speedier automobile development. It is anticipated that such a trend will continue. When calculations are made for the CAE analyses, it is often the case that sliding surface data tend to be insufficient and experimental values are used instead. The authors paid attention to such sliding surfaces, and have been collecting sliding surface data of actual engines for the last few years.
The distribution of an actual load on a reciprocating engine's main journal (1,2,3,4) is an example of such data. Normally, the simple method will be used to calculate an engine's bearing load from the measured data of the engine's cylinder pressure. However, more accurate data of bearing loads are required when engine downsizing and weight reduction are studied.
Therefore, the authors selected thin film sensors which are proven to measure oil film pressure distribution (5), and attached them to an engine to measure the load distribution during the engine's combustion. Furthermore, the measured load distribution was compared with that obtained by using the simple method based on measured cylinder pressure values, which is a proven and frequently used method in bearing design activities.
This time, a horizontally-opposed engine for General Aviation (GA), which is characterized by its capability to nullify second and higher vibrations, was used to collect data.
CALCULATED BEARING LOAD
This study used a 4-cylinder horizontally-opposed engine with 230 hp output power, 3500 [cm.sup.3] displacement and water cooling.
Unleaded premium gasoline for automobiles was used as the engine's fuel. Honda pure Ultra engine oil Mild 10W-30 was used as the engine's lubricant.
A 6061B pressure sensor with water cooling manufactured by Kistler was attached to the engine's #1 piston to measure the engine's cylinder pressure at the Wide Open Throttle (WOT). Figure 1 shows the engine's waveform data during its combustion with respect to its crank angle.
Following equations by Gross et al. (6) were used to calculate the composition load of the engine's inertia force and explosion load on the engine's bearing based on the waveform data shown in Figure 1.
F = [P.sub.g][[pi][d.sup.2]/4] - [m.sub.e]r[[omega].sup.2] (cos [theta] + cos 2[theta]/[lambda])
[P.sub.g] = Cylinder pressure
D = Bore diameter
[m.sub.e] = Piston mass weight + Connecting rod mass weight
r = Crank radius
[[omega].sup.2] = Angular acceleration
[theta] = Crank angle
[lambda] = Connecting ratio
Figure 2 shows the composition load of the explosion load and the inertial force calculated from the measured cylinder pressure values per cylinder.
Such composition loads were calculated for each bearing, and the loads of the 4 cylinders were added. Figure 3 shows the changes of the added loads on individual bearings.
Characteristically the opposed-pistons in a horizontally-opposed engine cancel out each other's loads, and its load change is smaller than that of an in-line 4-cylinder one. However, it was found that the #2 bearing was subjected to a large load whose maximum value would be [+ or -] 56 kN. In order to verify how the bearings were subjected to actual loads, thin film sensors were attached to the bearings to measure the actual loads on the bearings during the engine's combustion.
MEASUREMENT OF OIL FILM PRESSURE DISTRIBUTION OF BEARING BY THIN FILM SENSOR
Several papers have reported results of pressure distribution measurement on sliding surfaces by thin film sensors (7,8,9,10). The authors also manufactured similar thin film sensors, and measured the distribution of the load to which the sliding surface of the main journal bearing was subjected.
Figure 4 shows the engine the authors used and locations where the thin film sensors were installed. The red marks on the bearings indicate the locations where the thin film sensors were installed.
Figure 5 shows the pressure-sensitive sections of the thin film sensor for the engine's main journal which was used for the measurement. The diameter of each pressure-sensitive section is 0.8 mm, and 5 pressure-sensitive sections are arranged symmetrically along the bearing's rotational direction. The thin film sensor's main characteristics are that its structure is such that its lead wire is led out from its side so that the widths of the section to which the thin film sensor is attached and sections with no thin film sensor attached are the same when subjected to pressure, and that a unique DLC insulating film is used to protect the thin film sensor.
TAIHO KOGYO SA250 units were used as the bearings, and the films were formed on the units after the surface overlays had been removed from the areas where the sensors were to be placed. The sensor was composed of 3 layers: a unique Diamond Like Carbon (DLC) insulating protective film whose thickness is 2 [micro]m, a sensor film whose composition is the same as that of a strain gauge and whose thickness is 0.4 [micro]m, and an insulating film whose thickness is 2 [micro]m.
After the films were formed, the thin film sensor was subjected to changing pressure and temperature in an autoclave, and the relation between the thin film sensor's output signal and the load to which the thin film sensor was subjected was investigated. Based on the data gained from the investigation, the thin film sensor's output signal during the engine's combustion experiment was converted to a corresponding load.
RESULTS OF OIL FILM PRESSURE MEASUREMENT USING AN ENGINE
After it was verified that the engine's warming-up (where the crank rotation speed = 1000 rpm, and [P.sub.b] = 40 kPa) made the oil temperature constant, the crank rotation speed was increased to 2000 rpm ([P.sub.b] = 40 kPa), and then to 3000 rpm ([P.sub.b] = 40 kPa) for the experiment. The sensors were used to collect data at the crank rotation speed of 3000 rpm (WOT).
Figure 6 shows bearing load changes at the #1 and #3 bearings during the engine's combustion with respect to the crank angle. Although each thin film sensor has 5 pressure-sensitive sections, the change of their maximum is plotted. The direction of this figure's Y axis is the opposite of Figure 3's.
It can be seen that the loads shown in this figure are in general smaller than those calculated from the measurement of the engine's cylinder pressure. Therefore, the calculated values and the measured ones are plotted on the same axis to compare them with each other. Figure 7 shows the changes of the bearing load calculated from the cylinder pressure and those measured by the thin film sensors.
What was found is as follows. For the #1 bearing, when the crank angle was about 380 degrees, the measured load was -12 kN while the calculated one was -20 kN. For the #2 bearing, when the crank angle was about 20 degrees, the measured load was 8 kN while the calculated one was 20 kN: the measured load is less than half of the calculated one. Furthermore, when the crank angle was about 560 degrees, the measured load was -9 kN while the calculated one was -20 kN: the measured load is also less than half of the calculated one. What caused these differences was investigated.
CAE ANALYSIS OF CONNECTING ROD
The authors suspected that the deformation of a connecting rod11) which transfers an explosion load to a bearing might be related to the finding that some measured loads were actually less than half of corresponding ones calculated from the cylinder pressure measurement. Therefore, Abaqus 6.12-4 was used to calculate how much the connecting rod deforms when a load of 60 kN, which is a typical cylinder pressure value, is applied to the connecting rod's upper end whose surface receives a piston pin. Figure 8 shows how much the connecting rod deforms and the result of a von Mises stress analysis.
The analysis's result revealed that when the connecting rod was subjected to an explosion load, the connecting rod's rod part deformed by 0.215 mm to become doglegged because of stress concentration there. This implied that the bearing was not directly subjected to the explosion load, and the load was attenuated by the connecting rod.
The deformation of the connecting rod's rod part is supposed to be one of the main reasons why some measured loads were less than half of corresponding calculated ones. However, friction, the elastic deformation of the crank shaft, and the deformation of the piston pin and the piston caused by the piston ring's sealing at the time of explosion should also be taken into account in addition to this deformation. As a follow-up on of this paper, the authors plan to conduct such measurements and CAE analysis.
The experiments conducted for this study have yielded the following findings:
* It was found that at certain crank angles the actual values measured by thin film sensors were less than half of the bearing loads calculated from the measured cylinder pressure values.
* The connecting rod's deformation was studied when it was subjected to an explosion load. As a result, it was found that the rod part deformed by 0.215 mm, and became doglegged.
These findings suggest that only calculations based on the measured cylinder pressure value cannot be used to determine the actual load to which the bearing is subjected during an engine's combustion, and that what should be taken into account includes the connecting rod's deformation as well as the piston ring's friction and the crank shaft's deformation which were not studied in this paper.
The authors envision that in-situ measurement with thin film sensors will be employed to more precisely predict sliding surfaces' behaviors, realizing more accurate CAE analysis, and that this in turn will further advance engine downsizing and weight reduction.
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Kenji Matsumoto, Hironori Harada, and Yuki Ono
Honda R&D Co., Ltd.
Tokyo City University
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|Author:||Matsumoto, Kenji; Harada, Hironori; Ono, Yuki; Mihara, Yuji|
|Publication:||SAE International Journal of Fuels and Lubricants|
|Article Type:||Technical report|
|Date:||Jun 1, 2016|
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