Valvetrain Friction - Modeling, Analysis and Measurement of a High Performance Engine Valvetrain System Michele Calabretta and Diego Cacciatore, Automobili Lamborghini Phil Carden, Ricardo UK
www.ricardo.com RD.10/######.# © Ricardo plc 2010 Contents
Introduction
Test rig and measurement system
Mathematical model
Measured results
Analysis results
Comparison between measured and calculated data
Conclusions
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Valvetrain friction makes a significant contribution to the whole engine friction loss, esp. at low engine speed 2.4 Auxiliaries group The total amount depends on the valvetrain type, engine 2.2 Crankshaft group architecture, engine speed and lubricant temperature 2 etc. Valvetrain group 1.8 Reciprocating group Test data in general based on motored strip measurements 1.6
Valvetrain group contributing 1.4 – ~35% of total friction @ 1000 rpm 1.2
– Reduces with engine speed 1 – ~10% of total friction @ >6000 rpm 0.8
Valvetrain friction of high performance engine is WholeWholeengineengine nono loadload- - (bar) (bar) 0.6 relatively high, reasons being: – Use direct acting valvetrain – sliding contact between 0.4
cam and tappet 0.2 – Large valve diameter for high performance which 0 leads to high valve mass. Combined with high speed and large valve lift, leads to requirement of high 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 spring force Engine speed (rpm) Project Number Client Confidential – Client Name ## Month 2010 RD.10/######.# © Ricardo plc 2010 3 Contents
Introduction
Test rig and measurement system
Mathematical model
Measured results
Analysis results
Comparison between measured and calculated data
Conclusions
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Cylinder head from a V12 engine was used
Intake camshaft was driven by an electric motor connected to the end of the camshaft
Camshaft operated 12 intake valves via hydraulic tappets
Drive torque was measured by electric motor
Valve motion was also measured using a laser system to verify the physical dynamic behaviour
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Introduction
Test rig and measurement system
Mathematical model
Measured results
Analysis results
Comparison between measured and calculated data
Conclusions
Project Number Client Confidential – Client Name ## Month 2010 RD.10/######.# © Ricardo plc 2010 6 Mathematical model
Dynamic model of a single valvetrain modelled using VALDYN
Valvetrain represented as a series of lumped mass/inertia nodes connected by stiffness and damping
Cam node connected to a stiffness element representing camshaft bending and support stiffness
Tappet top stiffness modelled as a function of eccentricity of cam-tappet contact
Hydraulic tappet modelled as two mass nodes connected by a HLA element to account for the action of – High pressure chamber – Expansion spring – Check valve
Tappet connected to the valve using lash stiffness element representing the stiffness of the valve stem between the tip and the centre of mass
Valve and spring retainer modelled as a single mass node
Valve node connected to ground by another lash stiffness representing valve head and seat stiffness
Valvetrain have double spring pack
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The cam profile designed to meet the many conflicting requirements for – Engine breathing – Acceptable durability – High speed dynamics etc.
The spring pack designed to maintain contact between cam and follower at high engine speed
VALDYN calculated dimensions of cam tappet contact ellipse based on Hertzian theory
MOFT at cam-tappet contact calculated using isothermal EHL theory
Greenwood & Tripp model utilized for asperity contact friction force prediction
Lubricant Non-Newtonian behaviour taken into account
Total friction torque calculated due to oil shear and asperity contact effects
Camshaft bearing friction calculated using ENGDYN
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Introduction
Test rig and measurement system
Mathematical model
Measured results
Analysis results
Comparison between measured and calculated data
Conclusions
Project Number Client Confidential – Client Name ## Month 2010 RD.10/######.# © Ricardo plc 2010 9 Measured results – camshaft drive torque
Camshaft drive torque measured with standard steel tappets
Friction torque reduced with increasing camshaft speed
Increasing the oil supply temperature gave increased friction at low speed and reduced friction at high speed but the effect was quite small
No significant change in behavior at very high engine speed was observed - friction torque continued to decrease
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Steel tappets replaced with DLC coated ones
Comparison between whole valvetrain measurements at 90°C – Use of DLC gave a strong reduction in valvetrain friction • 24% at 350 rpm • 33% at 1000 rpm • 24% at 4000 rpm camshaft speed
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The final build involved removal of the valves, springs and tappets and operation with only the camshaft present
This build measured to quantify the losses at the camshaft bearings with low load to assist with the understanding of the relative contributions to the total system friction
Drive torque increased with camshaft speed
The highest loss occured at lowest oil supply temperature
Bearings are operating mainly in hydrodynamic lubrication regime – Losses are due to oil shear effect
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Introduction
Test rig and measurement system
Mathematical model
Measured results
Analysis results
Comparison between measured and calculated data
Conclusions
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Calculated camshaft bearing friction torque with and without valvetrain loads
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The cam/tappet friction power loss is a function of – Cam-tappet sliding velocity – Cam-tappet contact force – MOFT between cam and tappet
At low speed – Sliding velocity low – Cam-tappet contact force high on the nose low on the flanks – MOFT low
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Total power loss increased roughly linearly with speed and the largest loss occurred at the cam/tappet contact
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Introduction
Test rig and measurement system
Mathematical model
Measured results
Analysis results
Comparison between measured and calculated data
Conclusions
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First predicted camshaft bearing friction compared with the measured data
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Comparison of measured and calculated Comparison of measured and friction with standard steel tappets and calculated friction with standard friction coefficient of 0.045 steel tappets and variable friction coefficient
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Coefficient of friction required for good correlation at low engine speed
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Comparison between predicted and measured data for whole valvetrain friction as a function of oil supply temperature – standard steel tappets (on the left) and – DLC tappets (on the right)
Model shows the same trends as the measured data; friction increased with oil temperature at low speed and friction decreased with temperature at high speed
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Introduction
Test rig and measurement system
Mathematical model
Measured results
Analysis results
Comparison between measured and calculated data
Conclusions
Project Number Client Confidential – Client Name ## Month 2010 RD.10/######.# © Ricardo plc 2010 23 Conclusion
The model indicated that friction losses are dominated by losses at the cam/tappet contact particularly at low engine speed
Tappet/bore interactions were less important but still worth considering
Camshaft bearing losses were shown to make a very small contribution to total losses even at high engine speed
Model shown in this paper can be used to evaluate the effect on valvetrain friction of many variations in valvetrain design
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