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DESCRIPTION OF AN
AUTOMOBILE
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Following factors should be taken intoconsideration while writing down the descriptionof a vehicle.
1. Type.. Bus, Truck, Car, Motor Cycle etc.
2.Capacity. Carriage Capacity5 ton, 3 ton etc:4 or 6 or 35 or 45 seater etc.
3. Make.It is actual name allotted by themanufacturer. In most cases, the make alsoindicates capacity/H.P. of the engine fitted in thevehicle, such as Maruti 800.
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This means that Maruti make of car 800 c.c.engine, the total piston displacement is about800 c.c.
4.DriveThe description of an automobile withregard to drive may be given as follows:
i) Right hand or left hand drive
ii) 2 wheel drive or 4 wheel drive. This means asto how many wheels the engine power flows orhow many wheels are directly connected with theengine.
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Drive is usually indicated as under
Left Hand Drive : 4 x 4 (4 wheel drive)
Left Hand, Four wheel drive ; 4 x 4 means thevehicle contains 4 wheels and the engine
power could flow towards all the 4 wheels, 6 x
4 means that there are 6 wheels but the
engine power could flow towards 4 wheel
only.
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5. Model.Year of manufacture or special
Code Number allotted by the manufacturer.
Thus for the description of an automobile
following information will be required
Type, capacity, make, drive, model
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PERFORMANCE OF AN
AUTOMOBILE
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When fuel burns in the cylinder, pressures are
developed.
These pressure are transmitted to the
crankshaft by the piston and connecting rod
and torque is produced which sets the
crankshaft in motion.
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The torque produced by the engine is
transmitted through the drive line to the road
wheels to propel the vehicle.
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The torque is measured in Nm (SI units)
The actual power delivered by the engine is
known as Brake Power(B.P) and is measured
by dynamometer or prony brake.
B.P= 2NT/(60x1000) kW
Where T= Torque in Nm
N= Speed in r.p.m
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-the torque increases with the increase inengine speed upto a certain point after whichit starts to fall down even though the engine
speed continues to increase. The number ofrpm at which the torque begins to decreasedepends upon engine design. At higherspeeds, engine vacuum falls down and less
fuel enters the cylinders resulting in lesserforce available at the piston and hence the fallin toque as shown in the fig.
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The torque available at the contact between
driving wheels and road is referred to as
Tractive Effort.
Gear Box and final drive at differential act as
leverage to multiply torque which is inversely
proportional to speed.
If the gear speed is lowered, the torque shall
be increased in the same ratio and vice versa.
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Let Tw=Torque at driving wheel
Tw= G x t x TE
Where G = Gear Box Ratiot = Overall Transmission Efficiency
TE =Engine Torque
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Engine Torque TE =(B.P. x 60 x 1000/2N ) in
Nm
Where B.P is in kW
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Tractive effort,
F = Tw
/Rw
Where Rw Radius of the driving wheel
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The ratio between engine rpm and vehicle
speed depends upon overall gear ratio.
The vehicle having four speed gear box shall
have four different speeds and ratio between
engine rpm and vehicle speed shall be
different
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RPM of driving wheel = V/2Rw
Where V= vehicle speed in metres/min
Rw
= Radius of wheel in metres
Vehicle Speed
V= (Wheel Circumference x N)/G V= 2RwN/G m/min
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Engine RPM N= V x G/ 2Rw
Vehicle speed Vx1000/60 = 2RwN/G
Where V in Km/hr
The ratio between rpm(N) and vehiclespeed(V)
N/V= (1000xG)/(2Rw x 60)
V= 2RwN/G m/min
G=2RwN/V
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Power at Driving Wheels
The power available at the driving wheels to drivethe vehicle ranges from about 60 to 75%.
The various power losses which take place
between engine and the driving wheels are: i) Power loss due to friction of piston, bearing and
gears in the engine (the power available at engineflywheel is about 85%).
ii) Power loss from clutch to drive wheels due tofriction in clutch, gearbox, universal joints, finaldrive, differential and between tyres and ground.
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iii) Transmission line losses.
The power lost in transmission of power from
engine to road wheels reflects the transmission
efficiency () which is taken into account whilecalculating power available at road wheels.
The thrust known as tractive effort provided by
the engine at the driving road wheels varies atdifferent engine speeds and gear positions.
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A moving vehicle is opposed by various forces
known as resistances. For keeping the vehicle
moving a driving force or tractive effort (F)
equal to the sum of all the resistances has tobe applied to it. When F exceeds the sum of
the resistances, the excess value F will
accelerate the vehicle where as, when F is lessthan the sum of the resistances, the vehicle
will deaccelerate.
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The main forces which oppose the motion of a
vehicle are
1. Rolling resistance
2. Wind or air resistance
3. Gradient resistance
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Rolling Resistance (Rr):
It is mainly due to the friction between wheel
tyres and road surface. It depends upon
following factors
-load on each road wheel
-Type of tyre tread
-Wheel inflation pressure
-Nature of road surface
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It is measured in kg or N and is expressed askg/tonne or N/tonne of the vehicle weight oras a percentage of the vehicle weight. Rolling
resistance on an average type of road surfaceis between 1 to 2% of vehicle weight.
Rolling resistance
Rr
= kr
W kr = Constant of rolling resistance and
W = total weight of Vehicle
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Wind or Air Resistance (Ra)
This type of resistance depends upon the
following factors
-the shape and size of vehicle body
-Air Velocity
-Speed of the vehicle
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It increases as the square of the vehicle speed owing towhich much importance is given to streamlining and frontalarea of modern automobiles. In calculating air resistance,air velocity is usually neglected.
Air Resistance
Ra = kaAV
Where ka = Coefficient of air resistance
A= Projected frontal area, m
V= Vehicle speed , km/h
The values of ka for best streamlines cars, average cars andbuses and trucks are 0.00235, 0.0032 and 0.0046respectively.
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Gradient Resistance (Rg)
This resistance is due to steepness of road
gradient. It is subject to vehicle weight and road
gradient. It does not depend upon vehicle speed.
Gradient Resistance Rg= W/G or W sin
Where W= Total weight of vehicle
G= Gradient and
= inclination (for small values, tan = sin )
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When the vehicle is moving along a level road,
total resistance
Rtotal = Rr + Ra and
While moving up a gradient
Rtotal = Rr + Ra + Rg
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