lecture set 02 road vehicle performance
DESCRIPTION
Road Vehicle PerformanceTRANSCRIPT
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102321 TRANSPORTATION ENGINEERING
LECTURE SET # 2
Road Vehicle Performance
1
Main Reference: Principles of Highway Engineering and Traffic Analysis, Fred L.
Mannering – Scott S. Washburn, 5th edition, 2013, John Wiley & Sons, Inc.
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Outline
1. Resistance a. Aerodynamic
b. Rolling
c. Grade
2. Tractive Effort
3. Acceleration
4. Braking Force
5. Stopping Sight Distance (SSD)
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Main Concepts
• Resistance
• Tractive effort
• Vehicle acceleration
• Braking
• Stopping distance
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Main Concepts
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Main Concepts
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Resistance
Resistance is defined as the force impeding
vehicle motion
1. What is this force?
2. Aerodynamic resistance
3. Rolling resistance
4. Grade resistance
grla RRRmaF
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Aerodynamic Resistance Ra
Composed of:
1. Turbulent air flow around vehicle body (85%)
2. Friction of air over vehicle body (12%)
3. Vehicle component resistance, from radiators
and air vents (3%)
2
2VACR fDa
from National Research Council Canada 7
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Aerodynamic Resistance Ra
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Aerodynamic Resistance Ra
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Aerodynamic Resistance Ra
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Aerodynamic Resistance Ra
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Rolling Resistance Rrl
Composed primarily of
1. Resistance from tire deformation (90%)
2. Tire penetration and surface compression ( 4%)
3. Tire slippage and air circulation around wheel ( 6%)
4. Wide range of factors affect total rolling resistance.
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Rolling Resistance Rrl
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Rolling Resistance Rrl
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Example 2.1
Solution
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Solution (Cont’d)
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Grade Resistance Rg
Composed of
– Gravitational force acting on the vehicle
gg WR sin
gg tansin
gg WR tan
Gg tan
WGRg
For small angles,
θg W
θg
Rg
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Grade Resistance Rg
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Example 2.2
Solution
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Solution (Cont’d)
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Available Tractive Effort
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Available Tractive Effort
The minimum of:
1. Force generated by the engine, Fe
2. Maximum value that is a function of the
vehicle’s weight distribution and road-tire
interaction, Fmax
max,mineffort tractiveAvailable FFe
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Available Tractive Effort
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Maximum Tractive Effort
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Maximum Tractive Effort
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Engine-Generated Tractive Effort
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Engine-Generated Tractive Effort
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Engine-Generated Tractive Effort
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Solution (Cont’d)
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Engine-Generated Tractive Effort
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Engine-Generated Tractive Effort
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Vehicle Acceleration
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Solution (Cont’d)
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Solution (Cont’d)
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Solution (Cont’d)
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Braking Forces
• Front axle
• Rear axle
L
fhlWF rlr
bf
max
L
fhlWF
rlf
br
max
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See page 28 in textbook for complete derivation.
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Braking Forces
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Solution (Cont’d)
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Solution (Cont’d)
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Theoretical Braking Distance
(2.43) sin2
2
2
2
1
grlb
b
fg
VVS
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Solution (Cont’d)
Theoretical Braking Distance
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Practical Braking Distance
50
It is important to note that Eq. 2.47 is consistent with Eq. 2.43 (the
theoretical stopping distance ignoring aerodynamic resistance).
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Stopping Sight Distance (SSD)
• Worst-case conditions
– Poor driver skills
– Low braking efficiency
– Wet pavement
• Perception-reaction time = 2.5 seconds
• Equation
rtV
Gg
ag
VSSD 1
2
1
2
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Solution (Cont’d)
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Solution (Cont’d)