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GT-VTRAIN Analysis of a Direct Acting and a Roller Finger Follower Follower Valvetrain Mike Dark 04th October 2004
Presentation Structure • Correlation Ex Exercise – Direct-Acting (DA) valvetrain
• Hydra Hydrauli ulic c Lash Lash Adjus Adjuster ter (HLA) (HLA) PumpPump-Up Up Inve Investi stigat gation ion – Roller Finger-Follower Finger-Follower (RFF) valvetrain
• Fric Fricti tion on com compa pari riso son n of DA DA and and RFF RFF valv valvet etra rain ins s
Direct Acting Valvetrain Correlation Exercise • Valve Valve veloc velocity ity,, and spri spring ng coil coil force force meas measure ured d for corr correla elatio tion n with GT-Vtrain model • Specia Specially lly instru instrumen mented ted ‘Head ‘Head Rig’ Rig’ (motor (motored) ed) cons constru tructe cted d allowing access to valve head • Valve Valve velo velocit city y measu measured red using using a Laser Laser Dopple Dopplerr Vibro Vibromet meter er • Valve Valve Spri Spring ng forc force e obtai obtained ned using using stra strain in gaug gauges es moun mounted ted on the spring
Direct Acting Valvetrain •
Valvetrain Schematic: -
•
GT-Vtrain Model: -
Direct Acting Valvetrain • Camshaft – Single branch (one valve) – Two bearings – Stiffness (Torsional and Bending) calculated within GT-Vtrain based upon shaft geometry
• Tappet and Valve Stiffness – Derived using FEA
• Valve Spring – Force vs. compression measured (1mm increments)
• Running Clearance calculated at 90°C
•
GT-Vtrain Model: -
Spring Model
FEA model of Tappet Contact Force
Tappet Crown Stiffness against Contact Position 400
350
300
Constraints
) 250 m m / N M ( 200 s s e n f f i t S
150
100
50
0 0
1
2
3
4
5
6
7
8
9
10 11 12 13
Contact Poistion From Tappet Centre (mm)
Model Correlation •
Majority of inputs easily obtained to a satisfactory level of accuracy; either measured directly or specified in drawings
•
Full parameter sweep performed to investigate model sensitivities
•
Most parameters had little influence on dynamic response when varied within expected tolerance band
•
Valve velocity correlation was found to be sensitive to tappet stiffness and damping ratio
Spring Force Correlation: 1000rpm
Spring Force Correlation: 3000rpm
Spring Force Correlation: 5000rpm
Spring Force Correlation: 7000rpm
Valve Velocity Correlation: 1000rpm
Low speed difference due to imperfect cam grinding
Valve Velocity Correlation: 3000rpm
Valve Velocity Correlation: 5000rpm
Valve Velocity Correlation: 7000rpm
DA Correlation Conclusion • Simple model resulting in good correlation to measured results • Spring dynamic behaviour captured to a high degree of accuracy at all speeds, particularly during the lift event • Valve velocity captured to a reasonable degree of accuracy – Critical features such as valve closing velocity well matched, although drifting at high speed (7000rpm+)
• Tappet stiffness and damping crucial to achieving good correlation • Further improvements in tappet modelling approach planned for future releases of GT-Vtrain will improve correlation in tappet dynamic behaviour
Roller Finger Follower (RFF) Model •
Single valve branch of RFF valvetrain modelled to investigate Hydraulic Lash Adjuster (HLA) pump-up
•
Observed on running engine between 7800rpm-7900rpm – Torque reduction – Increased exhaust port temperatures
•
GT-Vtrain model served two purposes: – Identify cause of pump-up – Investigate possible solutions to eliminate pump-up at engine speeds up to 8000rpm
Roller Finger Follower (RFF) Model •
Valvetrain Schematic: -
Finger
Roller
HLA
•
GT-Vtrain Model: -
RFF: HLA Pump-Up Identification •
Prime symptom of pump-up is valve not fully closing
•
Model shows loss of valve seat contact on BCR between 7700rpm to 8100rpm
•
Charts show contact on BCR at 7600rpm and at 8300rpm, but only intermittent contact at 7800rpm
Lift Event
BCR
Lift Event
BCR
Lift Event
BCR
RFF: Cause of HLA Pump-Up •
Low valvetrain contact forces as a result of inertia relief
•
Force at pivot reduced further, due to phase alignment of spring resonance and HLA resonance (due to air in oil) during the cam deceleration period
•
Lift Event
BCR
Resultant force at pivot reduced below pump-up threshold
Opening Flank
Closing Flank
RFF: Mass Reduction •
Reducing valvetrain mass reduces inertia relief during deceleration period
•
Possible methods of reducing mass: – Use of lightweight materials §
Titanium retainer / finger
– Hollow valve stem •
Reduction of about 10g required to delay onset of HLA pump-up to above 8000rpm
•
Requires extensive rework to achieve 10g reduction – 5g reduction achievable by using titanium retainer insufficient
Lift Event
BCR
RFF: Reducing Spring Fitted Length •
Simple addition of a shim beneath valve spring to reduce fitted length – Increases spring loads
•
Danger of increasing inter-coil clash
•
Maximum safe reduction of 0.5mm to avoid spring becoming coil-bound within tolerance range
•
Insufficient to delay onset of pump-up to 8000rpm
Lift Event
BCR
RFF: Reducing Spring Fitted Length •
Simple addition of a shim beneath valve spring to reduce fitted length – Increases spring loads
•
Danger of increasing inter-coil clash
•
Maximum safe reduction of 0.5mm to avoid spring becoming coil-bound within tolerance range
•
Insufficient to delay onset of pump-up to 8000rpm
•
1mm reduction required
Lift Event
BCR
RFF: Modified Acceleration Profile •
Modification of acceleration profile to reduce negative acceleration
•
Minimal change to lift profile
•
Delays onset of pump-up to over 8400rpm
•
Requires new cam to be ground
Lift Event
BCR
RFF: HLA Modifications • Three methods of HLA system modifications to avoid pumpup were looked at: – Increasing Leak-down (larger clearances) – Reducing oil feed pressure (lower pump-up force) – Reducing oil air content
• Increasing leak-down, or reducing oil feed pressure both have a negative effect on the HLA’s ability to take up lash • Oil air content reduces the bulk modulus (‘stiffness’) leading to low frequency oscillation • Any oil air content will cause HLA to resonate – only effective if air can be completely removed – Difficult (if not impossible) to eliminate air content – Air content should be minimised
RFF: Pump-up Conclusions • Pump up issue seen on test bed replicated in GT-Vtrain model • Caused by low force at finger pivot (due to inertia relief, amplified by system resonance) • Methods of resolving pump-up issue investigated; profile modification gives most scope to address issue
DA vs. RFF Friction Comparison Direct Acting
1.
Camshaft bearing friction
2.
Cam and tappet interface
3.
Tappet and valve interface
4.
Tappet and tappet bore friction
5.
Valve and valve guide interface
Roller Finger Follower
1.
Camshaft bearing friction
2.
Cam and roller interface
3.
Pivot (HLA) and finger interface
4.
Roller (needle) bearings
5.
Finger and valve interface
6.
Valve and valve guide interface
DA vs. RFF Friction Comparison • Valvetrains equalised as much as possible to separate effect of each system
Friction Comparison between RFF and DA Valvetrain 90
80
70
– Similar valve lift events – Same running clearances – Same friction coefficients – Same camshaft bearings – Same oil and temperature
60 ) s t t a 50 W ( n o i t c i r F l 40 a t o T 30
• Cam / Tappet / Roller surface parameters taken from drawings or measured
20
10
0 0
1000
2000
3000
4000
5000
Engine Speed (RPM) DA
RFF
6000
7000
8000
Friction Breakdown DA Friction Composition
RFF Friction Composition
80
80
70
70
60
60
50 ] W [ r e w o P 40 n o i t c i r F 30
50 ] W [ r e w o P 40 n o i t c i r F 30
20
20
10
10
0
0 1000
3000
5000
6000
6500
7000
7500
1000
3000
5000
6000
6500
7000
7500
Engine Speed [RPM]
Engine Speed [RPM] Right Bearing
Left Bearing
Right Bearing
Cam-follower Hydr.
Cam-follower Bdry.
Cam-follower Hydr.
Cam-follower Bdry.
Tappet Bore
Valve-guide
Finger Roller
Valve-guide
Left Bearing
Tappet Surface Roughness •
•
Majority of Friction in a new DA valvetrain is a result of asperity contact at the cam / tappet interface A reduction in surface roughness (which occurs in operation as surfaces bed in) can significantly reduce this friction
Friction Comparison between RFF and WL Valvetrain 90
80
70
60
) s t t a W50 ( n o i t c i r F40 l a t o T 30
20
10
0
0
1000
2000
3000
4000
5000
6000
Engine Speed (RPM) DA
New RFF
Worn RFF
7000
8000
Tappet Surface Roughness •
Majority of Friction in a new DA valvetrain is a result of asperity contact at the cam / tappet interface
•
A reduction in surface roughness (which occurs in operation as surfaces bed in) can significantly reduce this friction
•
This is due to a change in the contact distribution: – New tappet, asperity contact – Worn tappet, EHD contact
