Performance of an Automobile

7/29/2019 Performance of an Automobile

1/46

DESCRIPTION OF AN

AUTOMOBILE


2/46

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.


3/46

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.


4/46

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.


5/46

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


6/46

PERFORMANCE OF AN

AUTOMOBILE


7/46

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.


8/46

The torque produced by the engine is

transmitted through the drive line to the road

wheels to propel the vehicle.


9/46

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


10/46

-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.


11/46


12/46

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.


13/46

Let Tw=Torque at driving wheel

Tw= G x t x TE

Where G = Gear Box Ratiot = Overall Transmission Efficiency

TE =Engine Torque


14/46

Engine Torque TE =(B.P. x 60 x 1000/2N ) in

Nm

Where B.P is in kW


15/46

Tractive effort,

F = Tw

/Rw

Where Rw Radius of the driving wheel


16/46

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


17/46

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


18/46

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


19/46

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.


20/46

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.


21/46

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.


22/46

The main forces which oppose the motion of a

vehicle are

1. Rolling resistance

2. Wind or air resistance

3. Gradient resistance


23/46

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


24/46

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


25/46

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


26/46

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.


27/46

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 )


28/46

When the vehicle is moving along a level road,

total resistance

Rtotal = Rr + Ra and

While moving up a gradient

Rtotal = Rr + Ra + Rg


29/46


30/46


31/46


32/46


33/46


34/46


35/46


36/46


37/46


38/46


39/46


40/46


41/46


42/46


43/46


44/46


45/46


46/46

Date post:	14-Apr-2018
Category:	Documents
Upload:	rajnish-kumar
View:	218 times
Download:	0 times

Performance of an Automobile

Documents