HEV
Fundamentals of Vehicle movement
Δ- Mass factor depends on rotating components of power train
2
Vehicle Resistance
• Rolling resistance
• Aerodynamic drag
• Grading resistance
Rolling Resistance
a) Rolling resistance with hard road b)RR with soft surface
Rolling Resistance
• Rolling resistance on Hard surface is due to hysteresis
in the tire material
• This is due to deflection of carcass while the tire is
rolling
• This results into uneven distribution of the ground
reaction forces and leads to forward shift of resultant
ground reaction
• P*a = Rolling resistance moment
Rolling Resistance
• To keep the wheel rolling, a force F acting at the center
of the wheel is required to balance this resistance.
fr= Roiling resistance coefficient
rd = Effective tire radius
Equivalent force called as rolling resistance
Rolling Resistance
• For slope road
Rolling resistance coefficient depends on
1. Tire material
2. Tire structure
3. Tire temperature
4. Tire inflation pressure
5. Road Roughness
6. Road Material
7. Presence of liquid on the road
Rolling Resistance
In Vehicle performance calculation it is sufficient to consider
the fr as a linear function of speed
Aerodynamic Drag
• Force encountered by air to oppose the motion of
vehicle is called as AD
• It results from
1. Shape drag
2. Skin Friction
Shape Drag
Aerodynamic Drag
• As the air cannot move away instantaneously there is
high pressure ( leads to pushing force at the front)
• As the space is not filled by air results into low pressure
( leads to pulling force)
Skin Friction:
There is difference between the air velocities at the surface of the
vehicle and free stream.
This results into friction called as skin friction
Aerodynamic Drag
• It depends on
• Vehicle speed
• Vehicle shape
• Vehicle frontal area
• Air density
Where the Vw is the wind speed
Aerodynamic Drag
Where the Vw is the wind speed
Grading Resistance
Dynamic Equation of vehicle motion
Net force = Tractive efforts- Resistance
Tractive efforts for Ftf is zero for rear wheel drive and Vise versa
Maximum tractive efforts that tire ground contact can
support
For front wheel driven vehicles
For rear wheel driven vehicles
If the tractive efforts at wheels is more than this the driven
wheels will spin on the road leading to vehicle instability
Power train tractive efforts and Vehicle speed
Automobile power train Block diagram
Power train tractive efforts and Vehicle speed
Power train tractive efforts and Vehicle speed
Ideal performance of power plant for vehicle traction
Power train tractive efforts and Vehicle speed
• At higher speed,
increase in losses
reduces engine
torque. But power
continues to increase
due to improved
combustion.
• After peak power it
decreases due to
rapid decrease in the
engine torque.
Performance of typical Gasoline Engine
• This performance is far away from the ideal performance
• Therefore a multi Gear box is required with IC engines to modify this output
Power train tractive efforts and Vehicle speed
• Speed torque characteristics much
closer to ideal one.
• Single or double gear transmission
is sufficient.
• EM starts from zero speed.
• Voltage increases to its rated value
and flux remains constant upto
base speed.
• Above base point voltage remains
constant, flux get weakened,
power remains constant and
torque decreases hyperbolically.
Performance of traction motor
Power train tractive efforts and Vehicle speed
Modification of the engine torque at wheel with multigear arrangement.
Transmission Characteristics
• The maximum speed requirement of vehicle determines the GR
of highest gear.
• GR of lowest gear is determined by max tractive effort and the
gradeability.
• In between the GR are spaced such that TE and speed
characteristics are as close as the ideal one
Transmission Characteristics
Engine speed and Vehicle speed in different gear
Transmission Characteristics
Each gear ratio is calculated as given below
Factor Kg is calculated as
Instead of 3 take n-1
Transmission Characteristics
• Tractive effort for a four gear transmission Gasoline engine and
an electric motor with single gear transmission
• Thus electric drive satisfies the required tractive effort with
simple transmission compared to engine
Transmission Characteristics
• Tractive effort and speed of a car with an automatic
transmission
• With torque converter and three speed gearbox
Vehicle performance
• Vehicle performance is described by
• Maximum cruising speed
• Gradeability
• Acceleration
Vehicle performance
• Vehicle performance is described by
• Maximum cruising speed
• Gradeability
• Acceleration
Maximum Vehicle speed
• If the tractive efforts is more than resistance a vehicle
accelerates and Vice a versa.
• If they are equal vehicle reaches the maximum speed in the
highest gear as shown in figure
• In case these curve don’t intersects (usually due to large
power plants and large gear ratio) maximum speed of the
vehicles is calculated as
Npmax- maximum speed of the PP
Igmin- minimum gear ratio of transmission
Io- final drive gear ratio
Gradeability
For Road with small grade
For Road with large grade
Acceleration
For Road with small grade
The mass factor is given as
This considers the equivalent mass increase due to angular moments of
rotating components
Acceleration
Acceleration with Gasoline Engine Acceleration with Electric motor
and 4 gear transmission with single gear transmission
Acceleration
Acceleration time
Acceleration distance
Engine performance/fuel economy
Fuel economy performance of a typical gasoline engine
Engine performance/fuel economy
Engine power = resistance power
Engine speed
Fuel consumption in l/h SFC and fuel density
Kg/l for distance S at cruising
speed of V
Engine performance/fuel economy
Engine operating points with highest and second highest gears
Engine performance/fuel economy
• Actual driving of the vehicle is different than the constant speed driving as
explained in the previous case
• TO simulate the actual the real driving conditions various drive cycles are
been designed for urban driving and highway driving
EPA FTP75 cycle
Engine performance/fuel economy
Engine operating points in urban driving cycle
Engine performance/fuel economy
Engine operating points in highway driving cycle
Engine performance/fuel economy
Basic Techniques to Improve Vehicle Fuel Economy
1. Reducing vehicle resistance
2. Improving engine operation efficiency
3. Properly matched transmission
4. Advanced drive trains
Engine performance
Variation of torque, power and SFC w.r.t. speed of SI engine
Engine performance
Torque variation w.r.t. throttle opening of SI engine
Engine performance
Fuel consumption characteristics of a typical SI engine
Engine performance
Fuel consumption characteristics of a typical SI engine
Engine performance
Electric vehicles
• Electric vehicles use an electric motor for traction
and battery/fuel cell/ultra capacitors as a power
source
• Advantages over ICE vehicles
• Absence of emission
• Smooth operation
• Freedom from petroleum
• Higher efficiency
• Lower running cost etc.
Configurations of Electric vehicles
General configuration of EV indicating the various
essential components
Variety of possible Electric vehicles Configuration
EV configurations:
1 – Conventional driveline with multigear transmission and clutch :
M – Electric motor
C – Clutch
GB – Gear box
D – Differential
The clutch and gearbox may be replaced by an automatic transmission.
2 – Single-gear transmission without need of a clutch :
M – Electric motor
FG – Fixed gearing
D – Differential
With an electric motor that has a constant power in a long speed range, a fixed
gearing can replace the multispeed gearbox and reduce the need for a clutch as
gear shifting is not needed.
3 – Integrated fixed gearing and differential :
M – Electric motor
FG – Fixed gearing
D – Differential
The electric motor, the fixed gearing and the differential are integrated into a
single assembly while both axles point at both driving wheels.
4 – Two separate motors and fixed gearing with their driveshaft :
M – Electric motor
FG – Fixed gearing
The mechanical differential is replaced by using two traction motors. Each of
them drives one side wheel and operates at a different speed when the vehicle is
running along a curved path.
5 – Direct drive with two separate motors and fixed gearing (In-wheel drive) :
M – Electric motor
FG – Fifed gearing
The traction motors are placed inside the wheels. A thin planetary gear-set is
employed to reduce the motor speed and enhance the motor torque with inline
arrangement of the input and output shaft.
6 – Two separate in-line motor drives :
M – Electric motor
The electric motor is directly connected to the driving wheel. The speed control
of the electric motor is equivalent to the control of the wheel speed and hence
the vehicle speed. It requires the electric motor to have a higher torque to start
and accelerate the vehicle.
Design of Electric vehicles
• Performance parameters of vehicle are
• Maximum speed
• Acceleration
• Gradeability
And Mostly depends upon the traction motor
characteristics
Traction Motor Speed- Power and torque
characteristics
• Constant torque in low speed
• Constant power in high speed
• Speed ratio= x= Max. speed/base
speed
• Upto base speed the voltage
supply to the motor is increased
with speed by electronic converter
and flux kept constant
• At base speed the voltage
increases to supply voltage
• After base speed flux is weakened
hyperbolically with speed to keep
the power constant.
• The torque also decreases
Variable Speed Electric motor Characteristics hyperbolically with increase in
speed
Traction Motor Speed- Power and torque
characteristics
• With long power region torque is
increased. (x=6). This will improve
the acceleration and Gradeability
performance. Also simplified
transmission
• But motors has its limited speed
ratio
• PMM, x= 2
• Induction motor X= 4
• SRM, (Switched reluctance motor)
x=6
Speed and torque profile of 60 kW motor at 2. 4, and 6
speed ratio
Tractive effort and transmission requirement
Transmission selection single gear or multi gear depends on
motor characteristics . Discussed as below
