Automobile Engg Unit 01

7/27/2019 Automobile Engg Unit 01

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Automobile Engineering ACET

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Mr.S.Sanjithkumar/AP/Department Of Mechanical Engineering

UNIT I

1.1 GENERAL LAYOUT OF AUTOMOBILE

Fig.1.1: General Layout of Automobile

It contains the source of power, i.e. engine, the frame, which supports the engine, wheels,body, transmission, the braking system and the steering. It also gives support tosuspension system and springs. Besides these parts

1.2 COMPONENTS OF THE AUTOMOBILEThe automobile can be considered to consist of five basic components:

1. The Engine or Power Plant: It is source of power.2. The Frame and Chasis: It supports the engine, wheels, body, braking system,

steering, etc.3. The transmission which transmits power from the engine to the car wheels. It

consists of clutch, transmission, shaft, axles and differential.

4. The body.5. Accessories including light, air conditioner/hearer, stereo, wiper, etc1.3 FUNCTIONS OF MAJOR COMPONENTS OF AN AUTOMOBILE

(a)WheelsThe wheels which are four in number are fitted below the car chassis to support

the load of the vehicle and passengers as well as run the car. They are fitted with hollowrubber tyres filled with air in rubber tubes under sufficient pressure necessary forcarrying the load. The shocks caused by road irregularities are absorbed by them. Byfitting springs between the wheels and the vehicle allowing the vertical movement of thewheels in relation to vehicle, greater part of unevenness of road surfaces is taken care

of.

(b)Front AxleIt is used for steering front wheels carried on stub axles swiveling upon kingpins

the axle extremities. Steering arms and track rod link the two stub axles together for


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swiveling them by a steering wheel about the king pins. The steering wheel linked to oneof the stub axle by a shaft, a gear box and suitable linkage is operated by the drivershand wheel. An axle in which one-piece beam was used to support the vehicle throughsprings (axle and spring arrangement) was previously used. Now, an arrangementknown as independent front suspension has replaced the axle and spring arrangement.

Under the control of springs, the wheels are free to rise and fall vertical independently ofeach other.

(c)Rear AxleRear axle or driving axle is a tube like shaft enclosing driving shafts with suitable

bearings for rotating the wheels. It is used for fixing the rear wheels. It is enlarged at thecentre for enclosing the final drive gears used for providing main speed reductionbetween the engine and the driving wheels. The change of direction from the line ofpropeller shaft to the transverse line of the axle shafts is also provided by the rear axle.

(d)Power UnitPower unit consist of an internal combustion engine. It is usually mounted at the

front end of the car. The clutch and the gear-box are placed immediately behind it. Thethree components (engine, clutch and gear-box) are assembled into a single unit.

(e)The Suspension SystemThe various parts are attached to the basic structure by means of springing

suspension system. This system is used to prevent the road shocks to the various vehiclecomponents and the occupants and to preserve stability under various road conditions.There are two distinct types of suspension system:

1. The conventional system; where the road springs are attached to a rigid beamaxle. It is mostly used in the front axle of commercial vehicles and for rear axle ofall types of vehicles.

2. The independent system; in this system there is no rigid axle. Each wheel is freeto move vertically without any reaction on its mating wheel. This is mostly usedis small rear axle suspensions.

(f) Transmission SystemThis system or mechanism carries the power from engine to the wheels of the

vehicle. It is bolted through the clutch to the engine at the front and the rear to thesprings which are connected to the chassis to prevent the engine vibrations fromtransmitted to vehicle. It generally consists of a friction clutch, a gear-box providingthree, or four different ratios of torque (output to input) a propeller shaft fortransmitting the torque output from gear-box to the rear axle, a final gear reduction in

the rear axle and a differential gear for distributing the final torque between the wheelsequally. In certain cases clutch is replaced by fluid fly wheels. The function of clutchand gear-box is completely served by the hydraulic torque converter. The variouscomponents of the power train or the transmission system are: clutch, gear-box,propeller shaft, universal joint and the differential gear.

(g)Gear BoxIt consists of various types of gears which are constantly in mesh. The Gear

changes are made by sliding the dogs. The main function of the gear-box is to providethe necessary variation to the torque applied by the engine to the road wheel accordingto operating conditions. The necessary variations are provided due to the presence ofdifferent gear ratios among various meshing gears.

(h)Propeller ShaftIt is universally jointed shaft. Its function is to transmit the power from the rear

end of the gear-box to the final reduction gear in the axle. The vertical movements ofthe rear axle relative to the frame are also accommodated. In construction, it is an


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ordinary Hookes joint, the small and limited angular displacement in the rubber jointsis advantageous in damping out torsion vibrations.

(i) Universal JointDue to the flexing of the road springs, the rear axle is constantly moving up and

down. The propeller shaft fitted to the rear axle must also be free to move up and down.To permit the turning of the propeller shaft when this movement is taking place,universal joints are fitted at each of its ends. Therefore, the relative movement betweenthe engine and the driving wheel is maintained by the universal joint.

(j) The Differential GearThe differential gear carries the power from the propeller shaft to the rear wheel

axles. It helps the two rear wheels to turn at different speeds when rounding a curve.The outer wheel must over-run the inner wheels when taking a turn. The differentialgear also ensures that the final output torque is equally distributed between the twowheels without any consideration of their relative speeds.

(k)ClutchIt is a friction type uncoupling device. It consists of a single steel disc faced withsuitable friction material. It is clamped between two surfaces directly driven by theengine. For disengaging the clutch, the two surfaces are positively separated bypressing the clutch pedal. The main function of the clutch is to take up the drivesmoothly from the engine and to release or disengage that drive whenever desired. Thedisengagement of clutch is required while changing the gear or bringing the vehicle torest.

(l) FrameFrame is the foundation for carrying the engine and body of the vehicle as well as

steering, power train etc. by means of springs, axles, rubber pads etc. The frames aremade of box, tubular channel or U-shaped section, welded or riveted together. In orderto make them rigid to withstand the shocks blows twists and the vibrations met duringthe operation, cross bracing or cross members are used. When the engine, wheels,power trains, brackets and steering systems are fitted on the frame, the assembly isknown as the chassis. Frame curves upwards in shape at the rear to provide space forthe rear springs and to provide space for the turning of the front wheels when steered,it is tapered at the front. There are two types of frame constructions in use.

1. Conventional pressed steel frameHere all the mechanical components are attached to it and the body is

superimposed on it. This is a standard practice for all commercial vehicles and in

some private and open cars.

2. The frameless constructionHere the body is designed I such a way as to combine the function of the body

and the frame. The components are generally attached to the frame than to the bodydirectly. This is preferably used in case of a closed car where the roof screen pillars;door pillars and rear panel are the main parts.

1.4 CLASSIFICATIONS OF VEHICLES

There are numerous types of automobiles found in different parts of the world. Withrespect to different purposes, the various types of automobiles are classified as under:

With Respect to the Usei.) Auto-cycles and Mopedsii.) Scooters and Motorcyclesiii.) Cars, Station wagons and Pick-upsiv.) Lorries (Buses) and Trucks


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v.) Tractors. With Respect to Capacity

i.) H.T.V. or Heavy Transport Vehicles or Heavy Motor Vehicles: Bus,Coaches, Truck,Tractor.

ii.) L.T.V. or Light Transport Vehicles, or Light Motor Vehicles: Cars, Jeeps,Motorcyclesiii.) Medium Vehicles: Minibus, Station wagon.

With Respect to Fuel Usedi.) Petrol Vehicles: Jeeps, cars, Motor Cyclesii.) Diesel Vehicles: Truck, Bus, Tractor, Bulldozer, Mercedesiii.) Gas Vehicles- Coal gas, Gas turbine or Producer gas Vehicles.iv.) Electric Vehicles- Using electric storage batteries or accumulators to drive

electric motors attached to the front or rear wheels, e.g. Heavy cranes,battery truck, cars and forklifts.

v.) Steam vehicles: Steam road rollers, it is now obsolete. With Respect to Wheels and Axles

i.) Two wheelers: Motor cyclesii.) Three wheelers: Tempos, Auto Rickshaws, Tricyclesiii.) Four wheelers: Cars, Jeeps, Buses, Trucks (some buses and trucks have

six tyres out of which four are carried on the rear wheels for additionaltraction.

iv.) Six axle wheelers (10 tyres) vehicles. With Respect to the Motion

i.) Reciprocatingpiston enginesii.) RotaryWankel engine, Gas turbine

With Respect to the Suspensioni.) Conventionalleaf springii.) Independentcoil, torsion bar, pneumatic

Fig 1.2 General Classifications of vehicles


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Fig 1.3 Classifications of vehicles

1.5 ENGINE

An engine is a device, which transforms one form of energy into another form. Normally,most of the engines convert thermal energy into mechanical work and therefore they arecalled heat engines.

Heat engines can be broadly classified into two categories:i. Internal Combustion Engines (IC Engines)ii. External Combustion Engines (EC Engines)

1.5.1 External Combustion and Internal Combustion EnginesExternal combustion engines are those in which combustion takes place outside the

engine whereas in internal combustion engines combustion takes place within the engine.For example, in a steam engine or a steam turbine, the heat generated due to thecombustion of fuel is employed to generate high pressure steam, which is used as theworking fluid in a reciprocating engine or a turbine. In case of gasoline or diesel engines,the products of combustion generated by the combustion of fuel and air within the cylinderform the working fluid.

1.5.2 Application of IC and EC EnginesIC Engines

1. GASOLINE ENGINEAutomotive, Marine Aircraft

2. DIESEL ENGINE - Automotive, Marine, Power, Locomotive

3. GAS ENGINESIndustrial Power

EC Engines1. STEAM ENGINESLocomotives, Marine

2. STEAM TURBINEPower, Large Marine


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1.5.3 Classification of Internal Combustion Engines

Internal Combustion engines are of two types,i. Rotary enginesii. Reciprocating engines

(a) Two stroke & four stroke engines(b) Petrol & diesel engines.

1.6: I.C ENGINE.Shows the cross-section of a single cylinder spark ignition internal combustion

engine. Description of different components of this engine is given below.

Fig 1.4 Cylinder -The cylinder is that part in which air-fuel mixture is sucked, compressed,

ignited and expanded. Cylinder Block -Cylinder block is made by casting and is used to support the cylinder

in position. Piston -Piston reciprocates inside the cylinder. Combustion Chamber -The space enclosed between cylinder and upper part of the

cylinder forms the combustion chamber where fuel-air mixture burns. Piston Rings -Piston rings are provided on the piston. These are used to seal the high

pressure side (cylinder) and low pressure side (crank case), i.e. to prevent leakage ofgases. There is one oil ring also which is used to scrap the lubricating oil at thecylinder surface so that it returns to crank case.

Spark Plug -A spark plug is put near the top of the cylinder or in the cylinder head. Itis used to ignite the fuel-air mixture by generating a spark in petrol engines.

Fuel Injector-Fuel injector is used in diesel engines in place of spark plug. Piston Rod -Piston rod or connecting rod connects the piston and crank. Gudgeon Pin -It is provided on the piston. It joins the piston and connecting rod. Crank Pin -Crank pin joins the crank and piston rod. Crank -Crank and the piston rod convert the reciprocating motion of piston into

rotary motion of the crank shaft. Crank Shaft -It is supported on bearings attached to the crank case. Crank Case -It is the main body of the engine to which cylinder is connected. Valve Mechanism -A mechanism to open and close the suction and exhaust valves is

also provided in four stroke engines. Top Dead Centre (TDC) -Top dead center is the upper most position upto which

piston moves. Bottom Dead Centre (BDC) -Bottom dead centre is the lower most position upto

which piston comes down.


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Bore (D) -Bore is the diameter of piston on cylinder. Stroke (L) -The nominal distance through which the piston moves from one extreme

position (say TDC) to other extreme position (say BDC). Suction Manifold -Suction or intake manifold is the pipe through which air and

petrol mixture enters the cylinder (through suction valve).

Exhaust Manifold -Exhaust manifold is the pipe through which burnt gases passfrom cylinder (through exhaust valve) to the silencer of the engine. Stroke Volume -The volume of the cylinder between TDC and BDC is known as

stroke volume. Clearance Volume-It is the volume of cylinder left above TDC, i.e. between TDC and

top of cylinder.

1.7 GENERAL SPECIFICATIONS OF AUTOMOBILE ENGINEEngine specifications may include following details:

(a) Model Designation: Model designation as specified by manufacturer.(b)Engine Configuration: Numberof cylinders and their arrangement.(c) Fuel System: Fuel system with carburetor or with multi-point fuel injection

(MPFI).(d)Displacement Volume: Strokevolume of all cylinders.(e) Ignition System(f) Maximum Horse Power(g) Maximum Torque

Example: Engine Specification of Santro Car.Model Designation:Hydraulic epsilon engine.Configuration:In-line-4 cylinder.Fuel System:Multi-point fuel injection (MPFI).Displacement:1086 cc.Ignition System :Distributorless.

Maximum Horse Power (BHP/rpm) : 63 at 5500 rpm.Maximum Torque (kgm/rpm) : 9.8 kg at 3000 rpm.

1.8 WORKING OF FOUR STROKE PETROL ENGINEIn four stroke engines, one cycle is completed with completion of four strokes. Mainfeatures of all the strokes are discussed below and their sketch is given in Figure

Fig 1.5 Four stroke petrol engine1.Suction or Intake Stroke

Initially the piston remains n top dead centre (TDC) position, suction valve is open andexhaust valve remains closed. The piston now moves downward and the petrol and airmixture (charge) enters into the cylinder. When piston reaches bottom dead centre(BDC). The cylinder fills with the petrol air mixture. At this moment, suction valve


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closes. This completes one stroke. Crank turns by 180o, i.e. it completes halfrevolution.

2.Compression StrokeBoth the valves (suction and exhaust) are closed. The piston moves upwards fromBDC to TDC position. The charge is compressed inside the cylinder, i.e. its pressureincreases and volume decreases. Along with pressure temperature also increases. Thecrank completes next half of revolution.

3.Working or Expansion or Power StrokeWhen the piston reaches the TDC position spark plug generates spark and the chargeis ignited and combustion of mixture takes place. Because of burning of fueltemperature and pressure of gases increases tremendously, both the valves remainclosed. The gases expand in the cylinder and push the piston downward andtherefore, work is done by the gases on the piston. The crank revolves and completes

next half revolution. The reciprocating motion of the piston is converted into rotarymotion of crank-shaft by piston rod and crank. During expansion, volume of gasesincreases. All the power for running the engine is obtained during this stroke.

4.Exhaust StrokeThe suction valve remains closed but exhaust valve opens. The piston moves fromBDC to TDC. The burnt gases are pushed out of the cylinder due to movement ofpiston. The cylinder pressure falls down to little above atmospheric pressure. Thiscompletes the next half revolution of the crank. By this time, crank shaft completestwo revolutions and one engine cycle is completed with the completion of four strokes.After this the same process is repeated again and again.

1.9 WORKING OF FOUR STROKE DIESEL ENGINEThe main features of all the four strokes in diesel engines are given below:

Fig 1.6 Four stroke diesel engine

1.Suction or Intake StrokeInitially piston is at top dead centre (TDC), exhaust valve is closed but suction valveopens. Piston moves downwards towards bottom dead centre (BDC). As suction valveis open, air enters into the cylinder. It is important to note that only air enters thecylinder during suction in case of diesel engines. Cylinder is full of air when piston


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reaches BDC and suction stroke in completed. Crank shaft or crank rotates by 180o,i.e. it completes half revolution.

2. Compression StrokeBoth the valves (suction and exhaust) are closed, piston moves from BDC to TDC.Volume of air decreases and pressure and temperature increases. When the pistonreaches TDC, this stroke is completed and the crank completes next half revolution.By this time crank has rotated by 360o.

3. Expansion or Power StrokeAt the end of compression stroke, both the valve remains closed. The injector fitted inthe cylinder head injects diesel fuel in the high temperature air. The temperature is sohigh that the fuel, i.e. diesel starts burning at constant pressure. The pressure andtemperature increases further due to combustion of fuel. The gases in the cylinder

push the piston downwards from TDC to BDC and expansion process takes place. Thevolume of gases increases and work is obtained in this process. The reciprocatingmotion of piston is converted into rotary motion of crank shaft through piston rod andcrank.Expansion process is completed when piston reaches BDC. The crank rotates by nexthalf revolution. This stroke is called power stroke because power of work is obtainedin this stroke.

4. Exhaust StrokeAfter completion of expansion stroke, the piston starts moving upwards from

BDC to TDC. Suction valve is close, exhaust valve is open. As the piston moves, itpushes the burnt gases through the exhaust vale. Thus, exhaust takes place. The

cylinder becomes empty as the piston reaches TDC. The exhaust stroke is completed.Crank has now completed two revolutions and all the four strokes are now completed.This completes one engine cycle. These cycles are repeated as engine continues to run.

1.10 WORKING OF TWO STROKE PETROL ENGINETwo stroke and four stroke engines are different in the method of filling the

cylinder with fresh charge and also in the removal of burnt gases from the cylinder.In a four stroke engine these processes are performed by the movement of pistonduring suction and exhaust stroke. In four stroke engines these are suction andexhaust valves where as suction (inlet) and exhaust (outlet) ports are cut in the walls

of cylinder.

Fig 1.7 Two stroke petrol engine


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1. Exhaust and TransferenceFigure shows the exhaust and transfer process. When the piston moves from TDC toBDC, i.e. downwards after expansion of gases, the piston uncovers the exhaust port.The burnt gases start going out of the cylinder. Simultaneously the slightly

compressed charge in the crank case is forced into the cylinder through transferport. The deflector on the piston crown deflects this charge and the fresh chargemoves in the upward direction. This fresh charge pushes the burnt gasesout ofcylinder. During this process, some fresh charge may also leave the cylinder throughexhaust port.

The process of cleaning of cylinder, by pushing burnt gases by fresh charge, isknown as scavenging

2. CompressionWhen the piston moves upwards from BDC to TDC, transfer port and exhaust

ports are closed. Compression of charge, present in the cylinder takes place. Duringthis motion the inlet valve open and fresh charge enters the crank case. When thepiston reaches TDC, compression process is completed.

3. Ignition and ExpansionAfter compression, spark plug generates spark and ignition of fuel takes place.

Rapid rise in pressure and temperature takes place at constant volume. At this stageboth transfer port and exhaust port are closed. Expansion of burnt gases takes placeat the piston moves downward from TDC to BDC. The gases push the piston withgreat force and power is obtained during this process. Simultaneously, slightcompression of fresh charge, present in crank case takes place.

Exhaust and transfer of charge takes place and cycle is repeated again. Thus,the cycle is completed in two strokes of piston and one revolution of crank shaft. Incase of petrol engines, fresh charge consists of air petrol mixture which comes fromcarburetor after mixing.

1.11 WORKING OF TWO STROKE DIESEL ENGINES

Fig 1.8 Two stroke diesel engine


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Working of two stroke diesel engines is similar to that of petrol engines except the followingdifferences:

(a) Fuel injector is provided in the cylinder head in place of spark plug.(b)Only air enters the crank case. After slight compression, it is passed to cylinder

and compressed in the cylinder.

(c) At the end of compression strokes fuel injector injects diesel into compressed air.Due to high temperature of air, diesel starts burning.

1.12 COMPARISON BETWEEN TWO STROKE AND FOUR STROKE ENGINES

Four stroke engine Two stroke engine

1One power stroke for every tworevolutions of the crankshaft.

1One power stroke for each revolution ofthe crankshaft

2 There are inlet and exhaust valves in theengine. 2 There are inlet and exhaust portsinstead of valves.

3Crankcase is not fully closed and airtight.

3 Crankcase is fully closed and air tight.

4 Top of the piston compresses the charge. 4Both sides of the piston compress thecharge

5Size of the flywheel is comparativelylarger.

5Size of the flywheel is comparativelysmaller

6 Fuel is fully consumed. 6 Fuel is not fully consumed.

7 Weight of engine per hp is high. 7Weight of engine per hp is comparativelylow.

8 Thermal efficiency is high. 8 Thermal efficiency is comparatively low.

9 Removal or exhaust gases easy. 9Removal of exhaust gases comparativelydifficult.

10 Torque produced is even. 10 Torque produced is less even

11For a given weight, engine would giveonly half the power of two stroke engine. 11

For same weight, two stroke enginegives twice the power that of four strokeengine.

12All types of speed are possible (high andlow).

12 Mostly high speed engines are there.

13 It can be operated in one direction only. 13It can be operated in both direction(clockwise and counter clockwise).

1.13 COMPARISON OF DIESEL ENGINE WITH PETROL ENGINE

Diesel engine Petrol engine

1It has got no carburetor, ignition coiland spark plug.

1It has got carburetor, ignition coil &spark plug.

2Its compression ratio varies from 14:1 to22:1

2Its compression ratio varies from 5:1 to8:1.

3 It uses diesel oil as fuel. 3It uses petrol (gasoline) or powerkerosine as fuel.

4Only air is sucked in cylinder in suctionstroke.

4Mixture of fuel and air is sucked in thecylinder in suction stroke.


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5it has got fuel injection pump andinjector

5It has got no fuel injection pump andinjector, instead it has got carburetorand ignition coil.

6Fuel is injected in combustion chamberwhere burning of fuel takes places due

to heat of compression.

6Air fuel mixture is compressed in thecombustion chamber when it is ignited

by an electric spark.7

Thermal efficiency varies from 32 to38%

7Thermal efficiency varies from 25 to32%

8Engine weight per horse-power is high.E

8Engine weight per horsepower iscomparatively low.

9 Operating cost is low. 9 Operating cost is high.

10Compression pressure inside thecylinder varies from 35 to 45 kg/cm2and temperature is about 500C

10Compression pressure varies from 6 to10 kg/cm2 and temperature is above260C.

1.14 THE VARIOUS FORCES ACTING ON THE VEHICLE(a) Drag force (Fx)(b)Lift force (Fz)(c) Cross wind force (F y)

a. Drag force:Force of air drag is acting in the direction of vehicle motion with

the wind acting along the longitudinal direction axis (opposite to the direction ofmotion of vehicle). This force is also called air resistance.

The total aerodynamic drag can be calculated by using the equation.

Fx=CxV2A/2Where

Cx-drag coefficient -density of airV- velocity of airA-projected area of the vehicle viewed from front.

b. Lift force:Aerodynamic lift force is the vertical component of the resultant force caused by thepressure distribution on the body.

Lift force can be calculated by using the equation.

Fz=CzV2A/2Where

Cx-drag coefficient -density of air.

The aerodynamic lift will tend to reduce the pressure between the tyres and theground which causes loss of steering on the front axle and loss of traction on therear axle.

c. Cross wind forceCross wind force is acting in the lateral direction, on the side of the vehicle. This isformed by the asymmetric flow of air around the vehicle body.

These forces are acting at the centre of pressure instead of centre of gravity andhence cause moments as follows:


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Fig 1.9 moments

i.) Pitching moment:Pitching moment which cause by the drag force or lift force. This moment makesthe rear wheels lift off the ground and further reduces the available traction.

ii.)Yawing moment:Yawing moment which is caused by the cross wind force Fyabout Z axis.iii.) Rolling moment:

Rolling moment which is caused by the cross wind force F yabout Z axis.

1.15 THE VARIOUS RESISTANCES ACTING ON THE VEHICLE

The thrust known as tractive effort provided by the engine at the driving wheelsvaries at different engines speeds and gear positions. A moving vehicle is opposed bythe various forces known as resistances.

1. Rolling resistance The rolling resistance is mainly due to the friction between thewheel tyre and the road surface. It mainly depends upon the load on each road

wheel, type of tyre tread, wheel inflation pressure and type of road surface.Mathematically, rolling resistance

Rr = KWK = constant of the rolling resistance, andW= Weight of the vehicle in newtons.

2. Wind or Air resistance- The wind or air resistance depends upon the shape and sizeof vehicle body, air vehicle and speed of the vehicle. It increases as the square ofvehicle speed owing to which much importance is given to streamlining and frontalarea of modern automobiles. When calculating air resistance, air velocity is usuallyneglected.

Fa=CaV2A/2

Where

Ca-drag coefficient

-density of air

V- Velocity of air

A-projected area of the vehicle viewed from front.

3. Gradient resistance The gradient resistance is due to the steepness of roadgradient. It depends upon the vehicle weight and the road gradient. Mathematically,

Rg=W sin

Where


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W = Weight of the vehicle in newtons, and = Inclination or gradient of road.

It may be note that when the vehicle is moving along a level road, it has toface rolling and air resistance. When the vehicle moves up the gradient, it has toencounter the gradient resistance in addition to the rolling and air resistances.

The Power required to propel a vehicle is proportional to the totalresistance to its motion and speed.

Pv = R x V --------------(neglecting transmissionlosses)

= ( R x V) / t ---------------(considering transmissionlosses)Where

R = Total resistance in newtons,V = vehicle speed in m/s, andt = Transmission efficiency.

1.16 THE FACTORS AFFECTING TRUCK FUEL ECONOMY

1.16.1 Performance Factors

Fuel consumption is a function of power required at the wheels and overall engine-accessories-driveline efficiency. Factors that affect fuel consumption at steady speeds overlevel terrain are:

1.16.2 Power Output-Engine-Accessory-Driveline System1. Basic engine characteristics; fuel consumption vs. RPM and BHP.2. Overall transmission and drive axle gear ratios.

3. Power train loss; frictional losses in overall gear reduction system.4. Power losses due to fan, alternator, air-conditioning, power steering, and any otherengine-driven accessories.

Fig 1.10 Drage force Vs Vehicle speed

1.16.3 Power Required - Vehicle and Tires

The horsepower required for a vehicle to sustain a given speed is a function of the vehiclestotal drag. The greater the drag, the more horsepower is required. The total vehicle drag canbe broken into two main components; aerodynamic drag and tire drag. Factors affecting

these components are:

1.16.4 Factors Influencing DragAerodynamicVehicle speedVehicle Frontal area


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Automobile Engineering ACETVehicle ShapeTireVehicle Gross WeightTire Rolling Resistance

Both aerodynamic drag and tire drag are influenced by vehicle speed. It is important,though; to note that speed has a much greater affect on aerodynamic drag than on tire

drag, Gains in fuel economy can be made by either optimizing or reducing some of thefactors affecting drag.

1.16.5 Type of Vehicles affects aerodynamic drag

The type of vehicle affects aerodynamic drag through its size (frontal area) andshape. The following illustration shows two tractor-trailer combinations which, as a resultof their shorter height (h2 and h3), have smaller frontal areas than the standard van-typetrailer. Trailer shape has a large impact on the aerodynamic drag of the tractor-trailercombination. Some examples of trailers that have lower aerodynamic drag shapes are:

Fig 1.11 The type of vehicles affects aerodynamic drag

Automobile Engg Unit 01

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