Post on 04-Jan-2016
description
transcript
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.
Outline
1. Resistance a. Aerodynamic
b. Rolling
c. Grade
2. Tractive Effort
3. Acceleration
4. Braking Force
5. Stopping Sight Distance (SSD)
2
Main Concepts
• Resistance
• Tractive effort
• Vehicle acceleration
• Braking
• Stopping distance
3
Main Concepts
4
Main Concepts
5
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
6
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
Aerodynamic Resistance Ra
8
Aerodynamic Resistance Ra
9
Aerodynamic Resistance Ra
10
Aerodynamic Resistance Ra
11
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.
12
Rolling Resistance Rrl
13
Rolling Resistance Rrl
14
15
Example 2.1
Solution
16
Solution (Cont’d)
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
17
Grade Resistance Rg
18
Example 2.2
Solution
19
Solution (Cont’d)
20
Available Tractive Effort
21
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
22
Available Tractive Effort
23
Maximum Tractive Effort
24
Maximum Tractive Effort
25
26
Engine-Generated Tractive Effort
27
Engine-Generated Tractive Effort
28
29
Engine-Generated Tractive Effort
30
Solution (Cont’d)
31
32
Engine-Generated Tractive Effort
33
Engine-Generated Tractive Effort
Vehicle Acceleration
34
35
36
Solution (Cont’d)
37
Solution (Cont’d)
38
Solution (Cont’d)
Braking Forces
• Front axle
• Rear axle
L
fhlWF rlr
bf
max
L
fhlWF
rlf
br
max
39
See page 28 in textbook for complete derivation.
Braking Forces
42
43
44
45
Solution (Cont’d)
46
Solution (Cont’d)
47
Theoretical Braking Distance
(2.43) sin2
2
2
2
1
grlb
b
fg
VVS
48
49
Solution (Cont’d)
Theoretical Braking Distance
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).
51
52
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
53
54
55
Solution (Cont’d)
56
Solution (Cont’d)