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Measuring Oil Films in Automotive Engines
Prof Rob Dwyer-JoyceThe Leonardo Centre for Tribology,University of Sheffield
Tribology DaysTrollhatten Nov 2011
Where does my Petrol Go?
Oil Films are fast, thin, and small
• The best way to minimise friction is to separate the surfaces
• High viscosity leads to churning losses
• Need just the right film thickness
• Very thin films 10nm to 100 μm• Fast moving (m/s) over small areas (mm2)
• Conventional metrology won’t work…• Callipers, rulers, dial gauges
• Strain gauges
• Inductance displacement sensors
Talk Today
• Ways to Measure Oil Films• light
• electricity
• sound
• Ultrasonic Reflection to Measure Oil Films• basic principles
• Some experiments on engines• Tiny transducers
• Ring and skirt imaging
Using Light
• Optical Interference• Just like the oil films on puddles of water
Using Light
Cameron & Gohar, Nature 1963Johnston, Wayte, Spikes, STLE, 1991Dwyer-Joyce & Heymer, Leeds-Lyon, 1996
Courtesy of Connor Myant and Philippa Cann, Imperial College
Ultra Thin-Films…
Johnston, Wayte, Spikes, STLE, 1991Cann, Hutchinson, Spikes, Tribology Trans 199Guangteng G, Spikes HA. Trib Trans 1996
In an Engine Cylinder• Transparent engine block• Glass, Sapphire set into the liner• Use flourescence to estimate film thickness (LIF)
Greene, Wear 1969Tian et al, MIT, SAE 2005Sei et al, JSAE 2000
Using Electricity
In an Engine…
Hamilton, Tribology Int 1980Sherrington & Smith, Trib Int 1985Sochting & Sherrington, IMechE J 2009
Using Sound• Ultrasound - Tiny displacement waves above frequency of human hearing
(>20kHz)
• Usually generated by piezo electric transducers
• Reflect from interfaces
Acoustic Impedance Mismatch• The acoustic impedance is important• Reflection Coefficient, R• R=Ampl r / Ampl i
21
21zzzzR
+−= cz ρ=
Incident wave, i Transmitted wave, t
Reflected wave, r
1 2
Response of an Oil Layer
incident wave
reflected wave
transmitted wave
-90-70-50-30-101030507090
0 0.5 1 1.5 2
Time, microseconds
Am
plitu
de, m
V
-90-70-50-30-101030507090
0 0.5 1 1.5 2
Time, microseconds
Am
plitu
de, m
V
Thick filmtime of flight method
Thin film<100μm
012345678
0 5 10 15
Frequency (MHz)
Am
plitu
de
Pulsed Wave Packet & FFT
0 0.5 1 1.5
Time (us)
Am
plitu
de
FFT
R = reflected signal / incident signal = A/BR = f(frequency, film thickness)
A B
System Layout
Captured pulses
Digital oscilloscope (or PC card)
Control signal
Signal processingUltrasonic pulser receiver
Transducer
Interface
Reflection at a Layer
• Wave Propagation in Each of the Layers
1111 )( c
xic
xi
eRexuωω
+=−
00000 )( c
xic
xi
eReTxuωω
+=−
222 )( c
xi
eTxuω−
=
T1=1R1
T0R0
T2
)()()()(
20
20
hhhuhu
σσ ==
x)0()0(
)0()0(
01
01
σσ == uu
A Simple Spring Model
hBK =
2)/2(11
zKR
ω+=
B=bulk modulush=film thickness
What is Stiffness…
•In Lubricated Contacts dudpK nom=
P
‘du’ big
P
‘du’ small
What is Stiffness…
•In Dry Contacts dudpK nom=
PP
When Area=0 K=0 As Area→100% then K → infini
‘du’ big ‘du’ small
Modified TE77• Phoenix Tribology TE77
• Piston ring – liner attachment
• Sections cut from ring and liner• Cast-iron liner (27.40x17.5 mm)
• Ring bending fixture
• Friction force transducer
• Ultrasonic Pulser FMS100• Pulsing, Digitiser, PCV
• Labview interface
2.5 mm (length), 1.35 mm (width) and 0.2 mm (thickness)
The piezoelectric sensor (8 mm diameter)
Piezo-Electric Sensors
0 10 20 30 40 50-2000
-1500
-1000
-500
0
500
1000
1500
2000
Data points
Am
plitu
de (m
V)
ReferenceRing
0 100 200 300 400 500-3000
-2000
-1000
0
1000
2000
3000
Time (ns)
Am
plitu
de (m
V) III
II “II” pulse is reflected from the inner side of liner.
Ultrasonic Reflections
7 8 9 10 11 12x 10
4
-2000
-1500
-1000
-500
0
500
1000
1500
Stacked Pulses
Digitised Point Number
Am
plitu
de
AAFFT was applied to each of these stacked pulses
ReferenceofAmp.FFT Pulses ReflectedofAmp.FFT=R
1400 1600 1800 2000 2200 24000
0.2
0.4
0.6
0.8
1
Pulse Number
Ref
lect
ion
Coe
ffic
ient
1400 1600 1800 2000 2200 24000
0.5
1
1.5
2
2.5
3
3.5
4
Pulse Number
Oil
Film
Thi
ckne
ss (µ
m)
B BF
a)
‘Stacked’ Reflections
Test PlanTwo tests procedures were followed varying both load and speed.
Load Tests Speed RampTestsLoad From 40 N to 180 N 60 N and 140 N
Speed 10 Hz From 2.5 Hz to17.5 Hz
Temperature 20ºC 20ºC
Stroke 15 mm 15 mm
Lubricant S20 and 15W40 S20 and 15W40
LubricantSpeed of Sound(m/s) at 20 ºC
Kinematic Viscosity (cSt.) at 20 ºC
S20 1502 43
15W40 1448 300
Properties of the lubricants used in the tests.
20 40 60 80 100 120 140 160 180 2000.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
Load (N)
Fric
tion
Coe
ffic
ient
S2015W40
Load Ramp
20 40 60 80 100 120 140 160 180 2000
0.4
0.8
1.2
1.6
2
Load (N)
Oil
Film
Thi
ckne
ss, h
(µ)
S2015W40
0 2.5 5 7.5 10 12.5 15 17.5 200.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
0.11
Speed (Hz)
Fric
tion
Coe
ffic
ient
S20 *15W40 *15W40 S20
* Load: 60 N
Speed Ramp
0 2.5 5 7.5 10 12.5 15 17.5 200
0.5
1
1.5
2
Speed (Hz)
Oil
Film
Thi
ckne
ss, h
(µ)
S 20 *15W40 *15W40S20
* Load: 60 N
λ is ranging from 0.5 to 3 (boundary to mixed regime).
0 0.5 1 1.5 20
0.02
0.04
0.06
0.08
0.1
0.12
Minimum Oil Film Thickness (µm)
CO
F
S2015W40
The dimensionless film thickness parameter
λ=hmin/ σc
(where σc =0.5)
Data Assembled onto a ‘Stribeck’ Cu
A Lawnmower Engine
• Single cylinder – air cooled
• Flywheel and brake disk
• Speed encoder
• Load cell on the brake disk
• Same ultrasonic system as before
SensorLocation
Observe 3 rings (including two oil ring rails)
Observe the skirt film - during power stroke
Skirt shows signs of 'piston slap' with a transient film with very repeatable frequency content
Loaded - 2500 rpm
Idling 1200rpm
Compression Ring Films
Piston Ring Oil Film Measurement (Encyclopaedic)
ConclusionsOil films are difficult to measure‘direct’ measurement not possible – need a physical reflection phenomenonMy group specialises in using reflected soundEasiest on large films (hydrodynamics) ut recent work on enginesAdvantage of no need to penetrate cylinder barrel –non-invasivePossible to implement on a fired engine (no cross talk from spark)Resolution is the biggest challenge
The End.
www.leonardocentre.net
