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)