Transcript Tire Rolling Resistance, Its Impact on Fuel Economy, and Measurement Standards

Tire Rolling Resistance, Its Impact on
Fuel Economy, and Measurement
Standards
Presented to the California Energy Commission
by
Tim J. LaClair, Ph.D.
CEC Workshop on Fuel Efficient Tires
Rolling Resistance Basics
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Definition: "Rolling resistance is the energy consumed per unit
distance and is equivalent to the scalar sum of all contact forces
tangent to the test surface and parallel to the wheel plane of the
tire" (SAE J2452)
Units are [J/m] or simply [N], but conceptually, rolling resistance
is better comprehended as a loss per distance than a force
Rolling resistance is primarily due to viscoelastic heat dissipation
in the rubber
Aerodynamic drag, friction in the contact patch, and friction with
the rim also contribute to the total rolling resistance, FR
CEC Workshop on Fuel Efficient Tires
Rolling Resistance Basics
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Loss contributions:
Aerodynamic
Drag
0 to 15%
Hysteretic losses
80 to 95 %
Tire/ground
and tire/rim
friction (<5%)
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RR Impact on Fuel Consumption
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Fuel energy is dissipated by many vehicle losses, including
rolling resistance:
(National Research Council, "Automotive Fuel Economy: How Far Should We Go?", 1992)
CEC Workshop on Fuel Efficient Tires
RR Impact on Fuel Consumption
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To better quantify and understand the contribution of rolling
resistance to fuel economy, Schuring (1988) proposed the
concept of the Return Factor (also referred to as energy ratio):
Return Factor 
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% Reduction in Fuel Consumptio n
% Reduction in Rolling Resistanc e
For passenger cars and light trucks, RF is typically between 1:10
and 2:10. This indicates that a 10% improvement in RR gives
only a 1-2% improvement in fuel economy.
For heavy trucks, RF can be higher, with a typical range between
1:10 and 3:10, hence the fuel savings potential for trucks may be
higher than for passenger vehicles
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RR Impact on Fuel Consumption
CEC Workshop on Fuel Efficient Tires
RR Impact on Fuel Consumption
In response to CAFE and a sense of environmental stewardship,
the tire and rubber industries have responded to dramatically
reduce rolling resistance since 1980:
175
150
Rolling Resistance Index
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125
100
75
50
1975
1980
1985
1990
1995
2000
Year
(Lowest Michelin FR high-volume construction at 80% of the T&RA 1.8 bar load, 2.6 bar, 80 kph)
CEC Workshop on Fuel Efficient Tires
Rolling Resistance Measurements
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In North America, the current rolling resistance standard is SAE
J2452, which superseded SAE J1269 in 1999
In 1995, EPA mandated a true load matching procedure which
would enable dynamometer simulation of road forces across a
range of speeds for emissions certification and fuel economy
testing
Therefore, for coastdown and performance modeling, auto
manufacturers require component-level data, including FR, as a
function of speed
The new method measures the dependence of FR on speed, in
addition to load and pressure, which is its primary benefit
CEC Workshop on Fuel Efficient Tires
Rolling Resistance Measurements
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Using the rolling resistance model and coefficients , , a, b, and
c deduced from the J2452 coastdown data:
tf
MERF 
 
2

dt
P
L
a
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bV
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cV

t0
tf
 dt
t0
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For North America, the natural choice of velocity cycles are the
EPA Urban and Highway cycles used for emissions certification
and fuel economy measurement
CEC Workshop on Fuel Efficient Tires
Rolling Resistance Measurements
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Using MERF the RR can be more accurately applied to fuel
economy predictions for a particular drive cycle
However, there is no direct correlation between RR values
measured with SAE J1269 and SAE J2452
Largely for this reason, SAE J2452 has been adopted slowly:
transition to the new standard is still not complete
For replacement and OE markets there is a bit of a mixture of
data between the two standards
Furthermore, for the replacement tire market RR is not a
significant performance target and in many cases data has not
been measured
CEC Workshop on Fuel Efficient Tires
Rolling Resistance and Other Performances
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Tire design requires balancing performances since changes in
design may change different performances in opposing directions
For the OE market, a tire is designed for a single vehicle and the
characteristics of the tire are optimized for that vehicle only
On the other hand, for the replacement market, tires need to be
designed to provide a balanced set of performances for a wide
variety of vehicle types with different handling characteristics,
etc.
For this reason, rolling resistance is frequently higher for
replacement market tires than a tire designed for use with a
particular vehicle
CEC Workshop on Fuel Efficient Tires
Rolling Resistance and Other Performances
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Rolling resistance is affected by many factors, both in tire design
and operating conditions:
– Tire Mass
– Rubber Formulations
– Inflation Pressure
– Speed
– Ambient/Tire Temperature
– Applied Drive Torque
– Surface Roughness
– Steer angle and camber/toe of vehicle
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Possible Approaches for Obtaining RR Data
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Obtaining RR data for which a direct comparison can be made
will require a significant effort and cost
Two basic approaches may be envisioned:
– RR testing could be performed, for several tires having a
range of RR values, at different sites in a “Round Robin”. A
correlation would be established between sites which would
then be applied to allow direct comparison of measured data
from the different sites
– Alternatively, an independent laboratory could be contracted
to perform all necessary RR testing
The first approach may be more cost effective since any existing
data could be used, but the second approach may provide more
reliable results