CHAPTER-FIVE

Road Performance of
Motor Vehicle
 Force acting on vehicle

 Force acting on the vehicle can be classified as

1) The force which moves the vehicle

2) The force which offers resistance in its motion

 Force acting on vehicle
1) The traction force of the driving wheels arise as a result
of operation of the engine and this force depend on the
interaction b/n the driving wheels and the road.

2) The force of resistance includes

• The force of friction in the Drive line (Internal resistance)

• The force of resistance of the road

• The force of resistance of the air

• The gradient resistance

Tractive Force (Ft)
 The tractive force (Ft) is determine as

Moment on the axle shaft

Ft 
The radius of the driving wheel during uniform motion
Mt
Ft 
Rw
 The radius of the wheel can be
 Static radius (Rs)
 Radius of the wheel at stationary and vertical load condition

 Dynamic radius (Rd)

 The radius of the wheel during the rolling of the wheel
 Wheel Radius
 Dynamic radius (Rd)
 Rd increases with
 Less load Fz taken by the wheel and
 Higher air pressure (pt) in the tire
 Rolling radius of wheel (Rr)
 The radius of such an unmemorable ring which has the same angular &
linear velocity as the given elastic wheel
 Due to the driving torque
 The tire trends are compressed & the wheel makes a smaller path
 Due to the braking torque
 The tire trends are expanded & the wheel makes a large path
 Wheel Radius
 Effective radius (Rw)
 Due to the extreme complication in calculations and change in Rr due
to slip is small
 The numerical values of Rr, Rd & Rs are assumed equal

r  0.5Dr  Bt (1  t )
 Where Dr - the diameter of the rim
Bt - height of the tyre profile in a free state
λt - radial deformation coefficient of the tyre
0.1-0.16 for standard & broad-profile
0.2-0.3 for arched & pneumatic tyres
Traction Characteristics
 It is dependence of the tractive force on the motor
vehicles speed

 If the angular velocity ωw of the driving wheels and their

radii Rw are known then it is possible to find the speed at
which the vehicle would move on a road
V  W RW
e  W Gr
 e 
V  W RW    Rw
 Gr 
Traction Characteristics
 The traction characteristics of a motor vehicle
 is graphically represented,
 dependence on tractive force and speed
 Is plotted from the result of the chassis dynamometer or road
test data
e  r
 
e  witr  w ig iaux imain  v 
itr
Where ωe- angular velocity of the engine
ωw- Angular velocity of the wheel
itr- Over all gear ratio (G.R of transmission)
ig- Gear ratio of Gear box
iaux- Gear ratio of auxiliary transmission
imain - Gear ratio of final drive
Types of Resistance Force
 The different resistance force acting on the
vehicle
 Inertial resistance
 Road resistance
 Rolling resistance
 Gradient resistance
 Air resistance
 Inertial Resistance
 While the vehicle is in linear motion, an inertial resistance to
motion due to rotating parts also exists.
 Rotating masses can be grouped into
 Parts rotating at engine speed :
 crankshaft, flywheel, clutch etc.
 Parts rotating at transmission speed:
 transmission shaft, drive shaft etc.
 Parts rotating at wheel speed:
 Drive axles, wheels, tires, etc.
Transmission Efficiency
 Transmission losses are
 Friction force b/n the gear teeth in the gearbox and
driving axle in the bearings
 Overcoming friction of the gears against the oil & then
on its splashing
Pt  Pe  Pf
 Where Pt - Power deliver to the driving wheels
Pe - Effective engine power (free power)
Pf - the loss of friction power
Transmission Efficiency
 The values of Tf accounts two types of losses

 Hydraulic losses (Th)

 Friction losses b/n the teeth of gears & in U-Joints

(Tm)
Transmission Efficiency
 The hydraulic losses (Th) are caused by
 Churning, splashing of oil in the G. box & Axle casings

 The hydraulic losses depend on

 Angular velocity of gasses

 Viscosity

 Quantity of oil in the casing

 Road Resistance
 The interaction b/n a motor vehicle and road
involves an energy expenditure which can be
divided in to 3 groups

 Energy spent on traction

 Energy spent on deformation of tires and road

 Energy spent on vibration of Automotive parts

 Rolling Resistance
 The rolling resistance, Rr is due to deformation of road
and tire and to the dissipation of energy through impact.
 Rr = fr W
 20% weight reduction will give about 10-15% improvement of fuel
economy which about 4% is due to reduction in rolling resistance
 Its value mainly depends on
 vehicle speed,
 tire inflation pressure,
 vertical load on the tire,
 types of tire,
 the road surface.
GRADIENT RESISTANCE
 For a vehicle climbing up a gradient, the component of its
weight parallel to the road surface acts as a resistance to its
motion.
 Since θ is very small (40-50)
 Sin θ = tan θ

F g≈

tan θ = 1/x if is small enough (i.e. x > 5)

GR : Grade ratio (1/x)
G : Grade ratio in percentage
Power expenditure on ascent with gradient
FgV GRWV
Pg  
1000 1000
AIR RESISTANCE
 The air resistance is the force exerted by the air that
opposes the motion of a vehicle passing through it.
AIR RESISTANCE
 Air resistance composed of
 Form drag (55-60 %)
 Interference drag (12-18 %)
 System drag (10-15 %)
 Surface drag (8-10 %)
 Lift drag (5-8%)
 AIR RESISTANCE
 Form drag (55-60%)
 Mainly depend on the shape (profile) of the vehicle

The form drag could

reduce by
• Reducing the frontal area
AIR RESISTANCE
 Interference drag (12-18 %)
 All the components projecting away from the
basic shape like
 Door handle,
 Rear view mirror

 Causes flow separation or vortices which increase

the drag
• To reduce this drag no component should projected away
from the basic body
 AIR RESISTANCE
 System drag (internal flow) (10 - 15%)
 It is basically air flow inside the body & outside and
interaction
 Air flow through radiator affect this drag
 Side glass windows in open condition is also affect this drag
 Surface drag (8 -10 %)
 Fluid friction on over the metal panels
 All surface imperfection will increase
this drag
 fasting bolt and paint quality
AIR RESISTANCE
 Lift Drag (5-8%)
 Lift is a vertical force
 Resulting from the pressure difference above and
underneath a car
 Can be reduced by using stabilizers
 It decreases the weight of the car in effect
AIR RESISTANCE
 Drag (i.e. air resistance) and lift are usually mutually exclusive.
AIR RESISTANCE
 The typical modern automobile achieves a drag
coefficient of between 0.30 – 0.35.
AIR RESISTANCE
Acceleration Performance
 Maximum performance in longitudinal acceleration
of a motor vehicle is determined by one of two
limits:
 engine power or
 traction limits on the drive wheel.
 At low speeds tire traction may be the limiting
factor.
 At high speeds engine power may account for the
limits.
Power-Limited Acceleration
 Power-Limited Acceleration
 The flow of torque from the engine to
wheels can be derived as follows

 The rotational accelerations are related

by
Acceleration performance
for Manual Transmission
 The constant Engine power line is equal to the maximum power
of the engine, which is the upper limit of tractive effort available,
 The tractive force line for each gear is the image of the engine
torque curve multiplied by the ratios of the gear.
 For maximum acceleration performance the optimum Shift
point between gears is the point where the lines cross

The area between the lines for the

different gears and the constant power
curve is indicative of the deficiencies of
the transmission in providing maximum
acceleration performance
Plotting of Speed characteristics Curve
Example
 Calculate the external speed characteristics of a 4-s
carburetor engine developing a maximum effective power of
Pe max = 72 kw with crankshaft speed of ωN = 472 rad/s
 The effective power with an angular velocity of 136 rad/s

    2   
3

Pe  Pe max  e   e    e  
  N   N    N
For carburetor engine
 

 e  e   e  
2 3

Pe  Pe max 0.87  1.13      For diesel engine with open

 N  N    N   Combustion chamber
 solution
 The engine power @ 136 rad/s engine speed
 136  136  2  136 3 
Pe  72       25 kw
 472  472   472  

 The effective torque at the angular velocity

Pe 25,000
Te    184 Nm
e 136
parameters Angular velocity of crankshaft in rad/sec
136 220 304 388 472 556
Pe (kw) 25.0 41.9 57.0 67.6 72.0 67.5
Te (Nm) 184 190 188 174 152 121
Speed characteristics Curve
200
Power Curve
Torque curve
180

160

140
P(kw) and T(Nm)

120

100

80

60

40

20
0 100 200 300 400 500 600
engine speed
Plotting Traction characteristics curve
 Plot the traction curve characteristics for a car from the
following data
 i1=3.51 i2=2.26 i3=1.45 iiv=1.0
 Imain=4.1 r=0.33 ηt=0.9
 The value of the torque at different engine speeds are
tabulated in the pervious example

ωe (rad/s) 136 220 304 388 472 556

Te (Nm) 184 190 188 174 152 121
solution
 When the car run in 1st gear
 @ ωe=136 rad/s and Te=184 Nm
 The speed of the wheel (ωt)
e r 136 0.33
t    3.12 m / s
i g imain 3.51 4.1

 The Corresponding traction Force (Ft)

Ft  i g imain  tr   3.51 4.10.9  7230

Te 184
r 0.33
 Plotting Traction Curve
Gear parame Angular velocity of crankshaft in rad/sec
Ratio ters
136 220 304 388 472 556
1st V(m/s) 3.12 5.05 7.00 8.90 10.85 12.80
Gear
Ft (N) 7230 7450 7390 6820 5960 4750
2nd V(m/s) 4.85 7.85 10.80 13.80 16.80 19.80
Gear Ft (N) 4660 4810 4769 4400 3850 3030
3rd V(m/s) 7.55 12.20 16.90 21.50 26.20 30.90
Gear Ft (N) 2980 3080 3050 2820 2460 1960
4th V(m/s) 11.001 17.70 24.5 31.20 38.00 44.80
Gear Ft (N) 2026 2120 2100 1940 1700 1350
Traction Force Characteristics Traction Force characterstics

8000
I Gear
II Gear
7000
III Gear
IV Gear
6000

5000
Traction Force (N)

4000

3000

2000

1000

0
0 5 10 15 20 25 30 35 40 45 50
Speed of wheel (m/s)
Acceleration performance
for Automatic Transmission

 Automatic transmission provide somewhat different performance,

more closely matching the ideal because of the torque converter
on the input
 At low speed
 The output torque will be several times that of the input
 The torque input to the transmission is twice the torque of the engine
 At higher speed
 The transmission input approach
the engine speed
 The torque ratio drop to unity
 Acceleration performance
for Automatic Transmission

 Tractive effort performance for 4 speed AT

 Because of the slip possible with the fluid coupling, the
torque curves in each gear extended down to zero speed
without stalling

The intersection b/n

The road load curve and
any Of the tractive effort
curve is the max
Speed that can be
Sustained in that gear
Thanks!