Transcript Thermal Issues in Braking

Thermal Issues in Braking
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Understanding the Problem
Designing brake systems to cope
Pad-Disc Interface
Friction Materials
Brake Fluids
Brake cooling
Retarders
Braking Duty -Main Concerns
DUTY
CONCERN
• City running (250-350°C)
Lining and rotor wear
• Alpine Descent (600-700°C)
Fade, wear, vaporisation, pedal travel
• Autobahn Stops
Disc cracking, hot judder
• Fade test (e.g. AMS )
Fade, pedal travel
City Driving
600
500
400
Wear Rate
• Critical effect of
temperature on brake
wear
• Front Spoiler can
severely reduce pad life
• Significant effect on
service intervals and cost
of ownership
300
200
100
0
Temperature (°C)
Alpine Descent Temperatures
Alpine Temperatures (°C)
800
• Grossglockner
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Industry Standard
10 miles
10 % gradient
600-700°C Discs
150-200°C Fluid
Driving
Soak
700
Front
600
500
400
Rear
300
200
Front Fluid
100
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5 10 15 20 25 30 35 40 45
Time (mins)
Autobahn Stops
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Up to 300 kw initial heat input
600-700°C Differential
Disc cracking
Hot judder (Blue spotting)
Cuore GLX
NEXIA 1.5 GL
ESPERO 1.8
ZX Break 1.9 TD
VOYAGER 3.3SE
VOYAGER 2.4SE
M3
Alpina B8 4.6
728i
535i (E39)
40
523i (E39)
328i (veh 1)
320i
318ti COMPACT
A8 4.2
A6 2.6
A4 2.8 (veh 2)
A4 1.8
Spider 2.0 TS
75 Spider 2.0 V6
Distance (m)
Fade – e.g. AMS Stopping distances
80
COLD
70
60
HOT
50
40 m
30
20
10
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Minimising the thermal input
- Designing brake systems to cope
• Avoid excessive duty on any brake
• Optimising brake distribution
Braking Distribution
• Commercial Vehicles
– Predominance and load sensing valves
– Coupling force control to spread duty levels
with advent of brake by wire
• Cars and light Commercials
– Front brakes have high duty
– Need to maximise rear braking duty
High Rear Thresholds
10
800
8
700
7
6
600
5
Net Effective Pressure
Thresholds
4
3
2
1
Large differential between
Front and rear axles
Front
500
Temp (°C)
Line Pressure (Bars)
9
400
300
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Front
Rear
Rear
200
High rear thresholds (e.g. arising from
integral PCRV in RWC)
- resulting in excessive front duty
100
Front Fluid
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00:00 05:46 11:31 17:17 23:02 28:48 34:34 40:19
EBD and ZCrit
(ZCrit = Decel at point of rear lock)
0.25
0.25
Typical PCRV Non ABS characteristic
Same system with EBD
0.2
0.2
PCRV
EBD
0.15
0.15
0.1
0.1
0.05
0.05
0
0
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
x axis = deceleration, y axis = proportion of rear braking
0.25
0.25
EBD failure gives rear lock @ 0.4 g
System revised to give rear lock @ 0.7 g EBD Fail
0.2
0.2
EBD Failure Zcrit 0.4 g
EBD Failure Zcrit 0.7 g
0.15
0.15
0.1
0.1
Resulting 30% Reduction in rear braking at 0.2g
0.05
0.05
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0.2
0.4
0.6
0.8
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0.2
0.4
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0.8
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Minimising Thermal Effects
• Minimising brake fluid temperature – pad under-layer
• Less heat into caliper gives increased lining degradation
• Heat needs to go somewhere
Bucket and Hole Analogy
Devised by Eric Thoms of Scania
Bucket Analogy
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Flow from tap = Brake energy in
Height of Water = Rotor temp
Size of hole = cooling capability
Plan area = rotor heat capacity
Alternative Strategies
• WIDE BUCKET
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Large Heat Capacity
Good Cooling
Low Temperatures
E.g. Aluminum MMC Discs
Alternative Strategies
• WIDE BUCKET
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Large Heat Capacity
Good Cooling
Low Temperatures
E.g. Aluminum MMC Discs
• TALL BUCKET
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Small Heat Capacity
Moderate Cooling
High Temperatures
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E.g. Thin solid steel or cast iron
disc with plasma ceramic
coating running at up to 1100°C
Methods of Determining the Rotor Size
• Benchmarking against current vehicles
• Single stop temperature rise
calculations
• Fade and Alpine descent prediction
• Sizing by packaging constraints
Single Stop Temperature Rise
> tt <
id
od
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Thickness t
Vent factor
Diameter OD
Diameter ID
Calc area
Calc vol.
Disc mass m
2.16
0.7
26.2
16.1
336
507
3994
cm
cm
cm
cm2
cc
g
Density
7.873
Spec. Heat S 550
Veh mass M 1910
Brake balance 80%
Max speed
230
= 63.9
g/cc
J/kg/°C
kg
front
km/h
m/sec
Calculating Temperature Rise
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K.E. = ½mV2 Total energy kinetic energy = 3898110J
Energy per front brake = 1559244J
(80% shared)
Calculate temperature rise T: Energy = mSdT
Therefore temperature rise, T = 710°C
Rotor Type
Drum
Solid Disc
Vented Disc
SSTR (°C)
350-400
550
600-650
If the above SSTR values are exceeded then in-service problems could occur
Single Stop Temperature Rise
• Indication for single stop on the
autobahn
• Reasonable quick indicator
• Empirical evidence for SSTR figure
• Shortcomings
– No allowance for the brake cooling
– Not for an AMS test or alpine descent
– Should not be relied upon for new designs
Fade and Alpine Descent Predictions
• Uses increased computing power
availability from PC’s and workstations
• Predictions can be much more involved
• Possible to calculate predicted
temperatures for a fade test and Alpine
descent
• Better judgements at the concept stage.
Fade and Alpine Descent Predictions
• Uses assumed cooling coefficients
(often based on a current vehicle)
• Significant changes in the brake cooling
rate will not be reflected in the
predictions.
• Currently most effective technique
Sizing by packaging constraints
• Often need to design the biggest rotor possible in the
packaging space available
• Method most often used in practice
• Engineer’s preference
• Brake rotors not often oversized
• Decisions made at the concept stage for brake sizing
made on optimistic assumptions
Why Maximise Rotor Size?
• Weight always increases
– (First prototype is always the lightest)
• Subsequent versions often include estate or
high performance derivatives
• Customer expectations continually change
– Increasing service intervals
• Changing technology
– e.g. reduced overrun braking on auto boxes
• If the brake size is marginal, then subsequent
lining choices are compromised (limited to
high mu linings)
Maximising Disc Size
• Wheel size tends to define rotor sizing
• Wheel Types
– Steel wheels more restrictive
– Alloy wheels not normally fitted to base-line
– Cold formed fabricated alloy wheels worst
Caliper clearances
• Clearances
– Wheel to Caliper
– Caliper to disc
• Caliper stiffness defined by
“b” affecting:
– Pedal travel
– Pressure distribution
– Aluminium calipers
• Require greater b dimension
a
b
Pressure Distribution
• Effect of uneven
pressure distribution
• Localised Heating
• 6 stops from 80 km/h
(IBT < 100°C)
Detailed disc design
• Large thermal gradients cause differential
expansion of the disc brake material
– Thermally unstable disc is likely to induce cracking
and both hot and cold judder
– Energy needs to be fed into friction ring evenly
– Stable disc design with minimal coning
– Minimal runout and bolt-up distortion
– Suitable disc material
Stable Disc Design
Thermal Effects
• Thermal stresses greater than mechanical
stresses
• Coning gives rise to
– Increased pedal travel
– Taper wear
– Uneven heat input (risk of forming hot bands)
Disc Coning and Undercuts
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• Minimise stiffness of top hat and undercut
Disc Material
• Thermal Stresses >> Mechanical
• Maximising Thermal Conductivity
– Graphite Flake Structure
– High Carbon Cast Iron (Free Graphite)
– Silicon,Titanium,Vanadium, Copper,
Molybdenum
– Ferrite and Pearlite different characteristics
Casting Perfection
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The perfect material doesn’t exist
Important to understand parameters
Casting processes vary
Partnership with the right foundry
essential
Brake Cooling
ACTUAL AIR FLOW ACTUAL AIR FLOW FLOW
• Critical effect on brake temperature and
performance
CL
ACTUAL AIR FLOW
FRONT SPOILER
INTENDED AIR FLOW
Fig 1 Air flow along spoiler
Brake Cooling Issues
• Complex air flow through wheels
– Steel and alloy wheels cars and 4x4
– Wheel trims
• Wind tunnel tests
– Various techniques
• Timing of cooling tests
– Need to establish cooling before B.I.W. tooling
(Testing first prototype can be too late)
Computational Fluid Dynamics
• C.F.D. can be used to predict air flow
– Flow around body
– Flow through ventilated disc
– Wheel design
• CFD can be performed early in program
– At stage in programmed where body in
white design can still be influenced
Component Temperatures
• Service Problems Include:
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Brake fluid reactions with components at elevated temperatures.
– Certain combinations of fluid and brake pipe can suffer adverse reactions at
sustained elevated temperatures. Cu/Zn is extracted from the brake pipe
and reacts with the fluid.
– Reservoir platiciser reacting with brake fluid at elevated temperatures
– Certain combinations of hose and fluid producing deposits in fluid
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Collapsing Vacuum pipes
Nylon pipes may survive elevated temperatures if carrying positive air
pressure, but can collapse due to combination of internal vacuum and
temperature.
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Seizing Vacuum non-return valves
Non-return valves can seize shut if subjected to elevated temperatures,
especially in the presence of fuel
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ABS sensors melting
Metropolitan Police usage can generate temperatures of up to 800°C and melt
ABS sensors, especially if dirt shields are not used.
Component Temperatures
• Need to perform thorough hot room tests
• Take great care with heat shield deletion
Summary
• Need to spread heat energy between
axles
• Need to optimise component design
and materials
Example of Improved Cooling
Forced cooling of ventilated discs
• Need for BMW Autosport to improve the
brake cooling on M3 motor sport vehicles
• Increasing engine power brought about a
marked increase in braking duty
• A solution was to introduce forced cooling into
the vents in the front discs.
Cooling improvement
• Inner race rotates at half rate of outer race
• Supplementary race to force cooling air
• Reduction of 80°C at Nurburgring
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Vented Disc
Fan
AIR FLOW
Supplemetary
bearing race