AUTOMOBILE ENGINEERING

            Automobile is a self propelled vehicle.  Steam engines are external combustion engines.

IC Engines	Steam Engines
Easy to start and Stop.	Takes time, not so easy.
Lighter	Heavier
A small tank is sufficient.	A boiler is needed to store water and produce steam.

           In a IC engine, because of heat the gas expands but volume remains constant and hence temperature raises.

Fuel feed system :

Petrol Engines:

           There are two types of fuel pumps.  They are mechanical and electrical fuel pumps.  It contains the following parts

• Fuel tank, Pump, filter

• Carburetor,

• Intake manifold and

• Gauge to indicate the driver the fuel level in fuel tank.

Diesel Engines:

        There are two methods of fuel injection.  They are air blast injection and Airless or solid injection.  It contains the following parts.

• Fuel tank, Filter

• Injection pump,

• Injector and

• Fuel gauge.

Super Charging:

            The process of supplying to the engine the A / F mixture above the atmospheric pressure is called as super charging.  The following are the objectives of super charging.

• To reduce the weight / Horse power.

• To reduce the space occupied by engine.

• To maintain power at high altitudes where less oxygen is available as in aircrafts.

Governors:

            In SI engines carburetor are responsible for delivering the proper mixture of air and petrol.  But in CI engines it is achieved by Governors.

Exhaust silencers:

            As the exhaust valve opens, high pressure exhaust gas is released which causes pressure wave in the air producing noise.  The noise frequencies are 50 - 500 Hz and 3000 to 10000 Hz.  To reduce noise engine exhaust is connected to a silencer or a muffler.  Various type of mufflers are Baffle type, wave cancellation type, resonance type, absorber type and combined resonance and absorber type.  

        Baffle types are less efficient.  Length of gas paths are so adjusted such that crest of one wave coincides with the trough of another wave canceling each other.  This type of muffler does not eliminate noise completely.  Gas flowing through Resonance muffler does not experience high resistance.  Series of resonators reduce the noise of the fundamental and higher harmonics.  Sound absorbing materials are kept surrounding the perorated tube through which the exhaust gases pass.  During high pressure fluctuations the gases pass through the perforations to the sound absorbing materials, when these fluctuations are reduced and thus the noise gets reduced in intensity.  The silencers may be straight flow type or reverse flow type.

Air Cleaner:

            The intake air should be cleaned if it contains dirt and dust, it will damage the engine.  It acts as inlet system silencer and arrests flame due to back firing if any.  It is mounted at the air entrance of carburetor.  The following are the different types

1. Oil bath type - The filter contains a filter element wetted with oil.  At the bottom there is a separate oil pan.  Air from the atmosphere enters through circumferential gap.  Air hits oil in the oil pan.  Large dust particles are removed.  then air passes through filler element.  Further cleaning takes place.  Then the air enters the engine.  Maximum efficiency of oil bath type air cleaners is about 98%.  Oil bath air cleaners can be designed with centrifugal pre-cleaners.

2. Dry type - In this the filter element is paper or felt.  Felt filters are not efficient as paper filters.  But are more efficient than oil bath air filters.  paper filters can be as efficient as 99.99%.

3. Oil wetted type.

4. Paper pleated type and

5. Centrifugal type.

Engine Cooling:

            In IC engine the temperature of the gases inside the cylinder vary from 35o C to 2750o C during the cycle.  Obviously at such high temperature the metals will loose their characteristic and piston will expand the seize the liner.  An efficient cooling system removes 30 - 35 % of heat generated.  Too much heat removal will reduce efficiency.  There are 4 types of cooling.  They are

1. Air cooling

2. Water cooling

3. Liquid cooling and

4. Stream cooling.

Lubrication:

           It is applied between the moving parts.  It is necessary to reduce wear and tear, reduce friction and also acts as a cooling medium.  A lubricant should possess the following important properties.

Flash point :  The lowest temperature at which the oil will flash when a small flame is passed across the surface.  It happens due to the volatilization of liquid particles in the oil.

Fire point : When the oil is further heated after flash point, the oil will burn continuously.

Cloud point :  When the oil is cooled it becomes solidified and becomes cloudy at this point.

Lubrication system:

            Following are the different types of lubrication system.

1. Petrol system

2. Splash system.

3. Pressure system

4. Semi pressure system and

5. Dry sump system.

           Following are the parts of lubricating system.

1. Oil tank

2. Pump ( Gear pump, Rotor pump, plunger pump and Vane pump )

3. Cooler

4. Oil pressure gauge and

5. Oil level indicator.           

Battery:

            It constitutes the electrical system.  It needed to start the engines.  There are three types of batteries.  They are lead alkaline batteries, Alkaline batteries and Zinc air batteries.

Ignition System:

           It supplies a high voltage of 30000 Volts across a small gap in spark plug.  The ignition system contains a battery, switch ignition distribution, ignition coil, spark plug and necessary wings.  There are two types.  They are battery and Magneto ignition system.

Battery Ignition	Magneto Ignition
Current is obtained from battery.	Current from magneto.
Less costly.	More costly.
Good sparking even at low speeds.	Poor sparking at high speeds.

Gear Boxes:

Functions of gear box: 

            It allows the engine to run at different speeds to maintain its power and regulate its torque.  Gear box is essential when the vehicle is to be reversed.

Types of gear boxes:

• Sliding mesh gear box,

• Constant mesh gear box

• Synchro mesh gear box and

• Planetary gear box.

Epicyclic gear box:  

            It is a speed gearbox also known as sun and planet gearbox.  In these gear boxes on sliding dogs or gears are provided to engage the gears.  Tightening the brake bands on the gear drums performs changing the gear. It consists of a ring gear and planer gears with a carrier.  Any one of them can be held from rotation by means of brake bands for obtaining different speeds.

Live axle and dead axle: 

            Dead axles only support the wheels but do not transmit any power.  While a live axle apart from supporting the wheels also transmits torque to the rear wheels.

Axle breather: 

            In order to maintain the pressure of oil in the rear axle, so that the oil is not forced past the oil seal, an axle breather is used.  This increase in pressure is caused by expansion of air due to heat from the gears.  The axle breather is placed in axle casing.    

Reverse shaft in gear box: 

            This shaft is used to reverse the direction of drive by bringing into the mesh the larger gear wheel on the main shaft with the lay shaft is known as reverse shaft.

Functions of differential gear:  

            Is to keep both the rear wheels at the same speed in straight travel and make the outer rear wheel to rotate faster than the inner one during turn.

Forces on rear axle: 

            The axle shaft transmits drive from the differential to the rear hub.  The various stresses to be resisted by these shafts are

• Bending stress due to the weight of the vehicle.

• Torsional stresses due to driving and bracing torque.

• Shear forces due to vehicle weight.

• Tensile and compressive stresses due to side thrusts or cornering forces.

Brakes:

            There are different types of brakes in automobiles.  They are Mechanical, Hydraulic, pneumatic, vacuum, electrical and combined vacuum and hydraulic.  Molded pulp, compressed fabric, woven and impregnated asbestos sheet are used as brake liners.

Requirements of braking fluid:  Following are the requirements of a braking fluid.

• It must have a high boiling point and low freezing point.

• It must be chemically stable.

• It must remain fluid at low temperatures but must retain good film strength at high temperature.

• It must have good lubricating properties.

• It must be non-corrosive and must not attack rubber or metallic parts.

Braking requirements of a vehicle:

• Application of brakes should bring he vehicle to a relatively quick stop on any type of road.

• The braking system components must require minimum maintenance.

• The pedal effort required to produce maximum deceleration should be negligible and should not vary with condition of the road.

• The braking action should not involve and noise, or drift the vehicle away from its desired path.

• Provisions for quick heat dissipation must be incorporated.

Leading and trailing shoe: 

            The leading shoe is the first shoe after the cam in the direction of rotation.  The friction between the shoe and the drum pushes the tip of the leading shoe harder in contact with the drum and pushes it off at its toe, where as the trailing shoe tip is throw away off the brake drum, as the drum rotates against.

Fading of brakes: Higher vehicle speeds give rise to excessive temperatures during braking..  Such a high temperature results in fast wear of the lining and brake fade i.e. heat temporarily changes the friction properties of the brake linings and brake pads.

Tyres:

           They are mounted on wheel rims to carry load and provide a cushioning effect.  There are two types of Tyres.  They are tubed tyres and tubeless tyres.