Measuring Oil Films in Automotive Engines

Transcription

1 Measuring Oil Films in Automotive Engines Prof Rob Dwyer-Joyce The Leonardo Centre for Tribology, University of Sheffield Tribology Days Trollhatten Nov 211 Where does my Petrol Go?

2 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 1nm to 1 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

3 Using Light Optical Interference Just like the oil films on puddles of water Using Light Cameron & Gohar, Nature 1963 Johnston, Wayte, Spikes, STLE, 1991 Dwyer-Joyce & Heymer, Leeds-Lyon, 1996

4 Courtesy of Connor Myant and Philippa Cann, Imperial College Ultra Thin-Films Johnston, Wayte, Spikes, STLE, 1991 Cann, Hutchinson, Spikes, Tribology Trans 199 Guangteng G, Spikes HA. Trib Trans 1996

5 In an Engine Cylinder Transparent engine block Glass, Sapphire set into the liner Use flourescence to estimate film thickness (LIF) Greene, Wear 1969 Tian et al, MIT, SAE 25 Sei et al, JSAE 2 Using Electricity

6 In an Engine Hamilton, Tribology Int 198 Sherrington & Smith, Trib Int 1985 Sochting & Sherrington, IMechE J 29 Using Sound Ultrasound - Tiny displacement waves above frequency of human hearing (>2kHz) Usually generated by piezo electric transducers Reflect from interfaces

7 Acoustic Impedance Mismatch The acoustic impedance is important Reflection Coefficient, R R=Ampl r / Ampl i Incident wave, i 1 2 Transmitted wave, t Reflected wave, r R z + z 1 2 = z = ρc z1 z2 Response of an Oil Layer incident wave reflected wave transmitted wave Amplitude, mv Time, microseconds Amplitude, mv Time, microseconds Thick film time of flight method Thin film <1μm

8 Pulsed Wave Packet & FFT Amplitude Time (us) FFT Amplitude A B Frequency (MHz) R = reflected signal / incident signal = A/B R = f(frequency, film thickness) System Layout Ultrasonic pulser receiver Signal processing Control signal Transducer Captured pulses Interface Digital oscilloscope (or PC card)

9 Reflection at a Layer Wave Propagation in Each of the Layers u u 1 ( x) ( x) u = e = T 2 ( x) iωx c1 iωx c e = T + R e iωx c1 1 + R iωx c2 2e iωx c e R 1 R T 1 =1 T x u () = u σ () = σ () u ( h) = u ( h) σ ( h) = σ 2( h) T () A Simple Spring Model R= 1 1+ (2K / ωz) 2 K = B h B=bulk modulus h=film thickness

10 What is Stiffness K = In Lubricated Contacts P dp du nom P du big du small What is Stiffness In Dry Contacts P P K = dp du nom du big du small When Area= K= As Area 1% then K infini

11 Phoenix Tribology TE77 Piston ring liner attachment Sections cut from ring and liner Cast-iron liner (27.4x17.5 mm) Ring bending fixture Friction force transducer Ultrasonic Pulser FMS1 Pulsing, Digitiser, PCV Labview interface Modified TE77 Piezo-Electric Sensors The piezoelectric sensor (8 mm diameter) 2.5 mm (length), 1.35 mm (width) and.2 mm (thickness)

12 Ultrasonic Reflections Amplitude (mv) I II II II pulse is reflected from the inner side of liner Time (ns) Amplitude (mv) Reference Ring Data points Amplitude A Stacked Pulses x 1 4 A Digitised Point Number Stacked Reflections FFT was applied to each of these stacked pulses FFT Amp.of ReflectedPulses R = FFT Amp.of Reference Reflection Coefficient Pulse Number Oil Film Thickness (µm) a) B F Pulse Number B

13 Test Plan Two tests procedures were followed varying both load and speed. Load Tests Speed RampTests Load From 4 N to 18 N 6 N and 14 N Speed 1 Hz From 2.5 Hz to17.5 Hz Temperature 2ºC 2ºC Stroke 15 mm 15 mm Lubricant S2 and 15W4 S2 and 15W4 Properties of the lubricants used in the tests. Lubricant Speed of Sound Kinematic Viscosity (m/s) at 2 ºC (cst.) at 2 ºC S W Load Ramp Friction Coefficient Load (N) Oil Film Thickness, h(µ) S2 15W Load (N) S2 15W4

14 Speed Ramp Friction Coefficient * Load: 6 N Speed (Hz) λ is ranging from.5 to 3 (boundary to mixed regime). S2 * 15W4 * 15W4 S2 Oil Film Thickness, h (µ) S 2 * * Load: 6 N 15W4 * 15W4 S Speed (Hz) Data Assembled onto a Stribeck Cu.12.1 S2 15W4.8 COF Minimum Oil Film Thickness (µm) The dimensionless film thickness parameter λ=hmin/ σc (where σc =.5)

15 A Lawnmower Engine Single cylinder air cooled Flywheel and brake disk Speed encoder Load cell on the brake disk Same ultrasonic system as before Sensor Location

16 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 Compression Ring Films Loaded - 25 rpm Idling 12rpm

17 Piston Ring Oil Film Measurement (Encyclopaedic) Conclusions Oil films are difficult to measure direct measurement not possible need a physical reflection phenomenon My group specialises in using reflected sound Easiest on large films (hydrodynamics) ut recent work on engines Advantage of no need to penetrate cylinder barrel non-invasive Possible to implement on a fired engine (no cross talk from spark) Resolution is the biggest challenge

18 The End.