ic engine fuels

8/9/2019 Ic Engine FUELS

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THERMAL ENGINEERINGUNIT-1


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FUELS

Fuels can be in solid, liquid or gaseous

state.

Principle constituent of any fuel are

carbon and hydrogen.


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SOLID FUELS

WOOD : It is natural available and used as

commercial fuel in some industries.

PEAT : It is a mixture of decayed vegetablematter with water. It is used in gas producer

plant.

COAL : It includes all solid fuels from lignite

to anthracite . The quality of coal increases

from lignite to anthracite


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FORMATIONOF COAL

PEAT

LIGNITE

BITUMINOUS COAL

ANTHRACITE COAL


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SOLID FUELS

CHARCOAL : It is prepared by destructivedistillation of wood.

COKE : It is a solid residue left after thedestructive distillation of certain soft coals. Itis mainly used in blast furnace.

BRIQUETTED COAL : It is a block of

compressed coal dust. PULVERIZED COAL : Reducing coal to powder

or dust is pulverized coal.


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LIQUID FUELS (Typical oil pool

formation)


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LIQUID FUELS (Typical oil pool refining)


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PETROLEUM

Petra = rock + oleum = oil

It is a mixture of hydrocarbons like paraffins ,

olefins, naphthenes and aromatics , with some

sulphur and other impurities.Refining of petroleum is carried out through

fractional distillation.

Petroleum based liquid fuels are gasoline ,

kerosene , diesel oil , fuel oils and lubricating oils.

Non petroleum based liquid fuels are benzol,

alcohol, acetone and di - ethyl ether.


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GASEOUS FUELS

Gaseous fuel present no difficulty regarding mixingwith air , however , it creates the problem of storageand handling large volumes especially in automobiles .Different gaseous fuels are :

Natural gas : Its primary constituent is methane (85-99%) .Other constituents are hydrocarbons , inert gases, traces of hydrogen sulphide and water.

Natural gas is an excellent fuel for SI engines. It need

not be vaporized unlike liquid fuels , hence cold enginestarting is easier especially at low temperature andcold start enrichment , which is the major source of COemission , is prevented. It has high ignitiontemperature and excellent antiknock properties.


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GASEOUS FUELS

Liquified petroleum gas (LPG) : It is a product ofpetroleum gas , principally propane , propyleneand butane.

This gas can be liquefied at normal temperaturesubjecting to a moderate pressure. It has higherheating value compared to gasoline.

It is suitable for IC engines because of itsavailability and low carbon content.

It has self-ignition temperature and a high octanenumber, which makes it suitable for SI engines.


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GASEOUS FUELS

Producer gas : It is produced by burning

carbonaceous material with large deficiency of air

and treating with steam. It has low heat value.

Coal gas : Coal is heated to a temperature of1500C in the presence of very little air to

decompose.

Hydrogen : hydrogen, as an SI engine fuel

acquires special significance in view of its

unlimited supply potential and almost non-

polluting characteristics. But it is a costly

automotive fuel


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INTERNAL COMBUSTION ENGINE


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FLASH POINT

The safety of a diesel oil is measured by its flashpoint and fire point.

It is the lowest temperature at which a fuel willvaporize sufficiently to form a combustiblemixture of fuel vapour and air above the fuel.

It can be determined by heating a quantity of fuelin a special container, while passing a flame overthe liquid to ignite vapours.

A distinct flash of flame occurs when the flash

point temperature has been reached.A minimum flash point of 65C is specified for

safety.


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FIRE POINT

It is the temperature at which enough vapours

will rise to produce a continuous flame above

the liquid fuel.

The flame must sustain at least for 5 sec.

The fire hazard increases with increase in

volatility.

As the volatility of diesel oil is lesser than

gasoline ,it is safer under most circumstances.


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Heat of formation : The amount of heat

evolved or absorbed in the formation of onemole of a substance from its component

atoms is known as the heat of formation. It is

the heat change when substance formed.

Heat of combustion : The amount of heat

produced when one mole of a substance is

burned in oxygen. It is the heat produced by

combustion.


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NORMAL COMBUSTION

The combustion in a gaseous fuel-air mixture ignited bya spark is characterized by a rapid development offlame that starts from the point of ignition and spread

out in a continuous manner without any abrupt changein its speed and shape, is normal combustion.

Combustion spreads to the envelope of mixture at arate depending primarily on the temperature of flame

front and secondarily on the temperature and densityof surrounding envelope.

The amount of heat generated in the process ofcombustion out of mass fuel is known as calorific value.


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THEORETICAL PRESSURE Vs CRANK

ANGLE DIAGRAM


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ABNORMAL COMBUSTION AUTOIGNITION

ANDDETONATION

If the temperature and pressure of the endgas (the gas ahead of the flame front) are highenough, it will ignite spontaneously before the

flame front reaches it, then it is calledabnormal combustion.

It varies from the process of normalcombustion only at the end of combustion.

The sudden inflammation of remainingportion of the end-gas is called autoignition.


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DELAY PERIOD

In autoignition, the charge must remain above

a critical temperature for a certain length of

time known as the IGNITION DELAY OR DELAY

PERIOD.

During the delay period, certain chemical

reaction takes place which prepare the change

for autoignition.


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COMBUSTIONWITH DETONATION


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DETONATIONORKNOCKING

Knocking or detonation is characterized by highpitched metallic ringing sound.

In autoignition the burning is almost

instantaneous which results in an extremely rapidrelease of energy causing pressure of end-gas toincrease almost 3 to 4 times.

The excess pressure gives rise to severe pressure

waves which strike the cylinder wall and set tovibrate, causing sound.

The flame speed is of the order 300 to 1000 m/s.


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DETRIMENTAL EFFECTSOF DETONATION

Noise and vibration: The pitch of the sound depends upon thesize and shape of the combustion space and the velocity of wave at

speed of sound in cylinder gases . It also causes vibration of various

engine parts.

Increase in heat transfer: The increase in the rate of heat transfer isbecause of higher temperature of gas in detonating engine due to rapid

completion of combustion. The pressure waves scour away the protective

layer of stagnant gas on cylinder.

Pre ignition:-Ignition of the mixture by hot surface within the combustion space,before the normal spark ignition occurs is called pre ignition.

-Over heating of the spark plug electrodes leads to pre ignition.

-The burning time losses are generally increased and the power and efficiency are

reduced.


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Mechanical damage:

Power and efficiency: Increase in heat loss of cylinder cause reduced

power output and loss of thermal efficiency.

Carbon in the exhaust: Over long period of running of engine, largeamount of carbon is deposited. As a result of detonation, deposited

carbon is not only blown out as exhaust but also leaves the surface ofcombustion chamber and top of piston becomes rough and pitted.

-The impact pressure causes the fracture of aluminium alloy

piston

-The increase in temperature causes overheating and burning of the

gaskets between the cylinder and its head, electrodes and insulators

of spark plug, piston crown and the head of valves.

-In severe cases of detonation, increased cylinder and piston

temperature causes collapsing of piston crown and sometimes the

burning of cylinder head.

-Under detonating condition, puffs of gray smoke occurs and during night, puffs

are seen as light yellow flashes. Puffs of black exhaust smoke represents free

carbon in exhaust gas.


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OCTANE NUMBER The fuels octane number is the measure of a fuels resistance to knock in SI

engines.

The higher octane number (ON) indicates high resistance to knock and higher

compression ratio maybe used without knocking.

The octane number depends upon the engine design and the operating condition

during test.

The octane number scale is based on two hydrocarbons which define the ends ofthe scale.

The scale has been set up in which isooctane (2-2-4-trimethyl pentane) being a

very good anti-knock fuel is arbitrarily assigned a rating of 100 octane number.

On the other hand, normal heptane has very poor anti-knock quality and it is

given a rating of zero octane number. A gasoline is rated 90 octane number if its tendency to detonate in the test

engine is the same as that of a mixture of 90% isooctane and 10% normal

heptane by volume.

Octane number is determined by either research method (TESTING CODE: ASTM

D -2700) or motor method (TESTING CODE : ASTMD -2699)


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SUPERCHARGING

Supercharging is a method of increasing the power output of the engine without

increasing its weight and size. This is made possible by increasing the density of thecharge, which ensures greater amount of charge aspirated into the same stroke

volume.

Supercharging can be defined as the admittance of more charge into the engine

cylinder than what the engine can take during the normal suction stroke.

USES:

The three basic methods used for supercharging are MECHANICAL

SUPERCHARGING, TURBOCHARGING and PRESSURE WAVE SUPER CHARGING.

-It is used in high altitudes where the suction pressure of charge in cylinder

reduces, which in turn reduces the power output.

-In case of motor vehicle climbing hill, the power output reduces. Therefore,

with increase in height the degree of supercharge is increased.

-It is necessary in case of racing-car engines and aircraft engines where specificoutput is of prime importance.

-It is used where a limited space is only available for the engine.


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TURBOCHARGING In turbocharging, a turbocharger (a compressor and a turbine mounted on a

single shaft) is used to increase the inlet air density. The exhaust gases from the engine having sufficient energy to drive the

turbine which in turn drives the compressor mounted on the same shaft,

which increases the inlet air density before the air enters the cylinder.

The combination of an engine driven compressor and a turbocharger is

used in large marine engines.

The combination of 2 stage turbocharging provides very high boost pressure

to obtain higher power output.

In the arrangement of turbocompounding, the second turbine is directly

geared to the engine drive shaft, thus increasing engine power andefficiency.

The arrangement of charge cooling with a heat exchanger after

compression and prior to entry to the cylinder, cooling of the charge further

increases density.

The above combination are shown the figure below :


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TURBOCHARGING


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COMBUSTION CHAMBER TYPES

The combustion chamber is important as it

affects the engine performance, its knocking

tendencies and exhaust pollutants.

Brief description of a few important types ofcombustion chambers, with location of valves

and spark plug, showing their developments

are given below :


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T-HEAD TYPE COMBUSTION CHAMBER

This was the earlier type used by Ford Motor Corporation duringearly stages of engine development in 1908.

The T-head suffers following disadvantages :

There was a violent knocking even at compression ratio 4:1, this wasalso because of low octane number of the petrol available at that

time, which varied from 45 to 50.

-The distance across the combustion chamber is very long. The spark plug is

located near the exhaust valve, so the flame travel distance from the spark plug tothe end-gas (near the inlet valve) increases. Therefore, knocking tendency is

increased.

-The configuration provides two valves on either side of combustion chamber,

requiring two camshafts.


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T-HEAD TYPE COMBUSTION CHAMBER

Combustion Chamber


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L-head or side valve combustion

chamber

The combustion chamber is in the form of more or less flat slab, extending over thepiston.

The disadvantage of T-head combustion chamber forced the development ofL-

head, in which two valves are placed on the same side of the combustion chamber,

thus reducing the flame distance, and the valve are operated by a single camshaft.

During 1910-1930 this type of combustion chamber were commonly used in SI

engines, in which, the valves are placed side by side in a detachable block.

Manufacturing and maintenance of this type of combustion chamber are both easy.

The detachable head can easily be removed for decarbonising without disturbing

either the valve gear or the main pipe work.

In its original form, this type of engine gave poor performance because of the

following limitation : lack of turbulence, extremely prone to knock, extremely

sensitive to ignition timing.

The side valve engines are not preferred for higher compression ratio on account of

inadequate volumetric efficiency, no compactness and additional requirement of

cooling.

This engine do not compare well with overhead valve engines


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L-head or side valve combustion

chamber


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RICHARDO TURBULENT HEADSIDE VALVE

COMBUSTION CHAMBER

The main objective of this design was to increase turbulence in order to obtain a

higher flame speed and to reduce the knocking tendency.

During compression stroke the gases were forced back to the main body through

restricted passage-way that creates additional turbulence

By varying the throat area of the restricted passage it was possible to achieve any

desired degree of turbulence. The rate of combustion during the second stage ofcombustion was improved and this resulted in improved performance.

In order to reduce knocking tendency to minimum, the distance of effective flame

travel was shortened by forming a very thin layer of entrapped gas between the

piston crown and portion of chamber at end gas region, where the piston was at

TDC. The flame distance is further reduced by placing the spark plug in the center of

effective combustion space with a slight shift towards exhaust valve.

With the relatively high octane petrol available today, the compression ratio has

been increased resulting in lack of space to accommodate the engine. Hence this

engine can no longer compete with the overhead design.


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RICHARDO TURBULENT HEADSIDE VALVE

COMBUSTION CHAMBER

COMBUSTION CHAMBER

EX

INLET VALVE

EXHAUST

VALVE

EXHAUST

VALVE

PISTON

CYLINDER


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OVERHEADVALVE ORI-HEAD TYPE

COMBUSTION CHAMBER

The two important arrangements of this type of chamber are :bath-tub typeand wedge type combustion chamber.

ADVANTAGES : The volumetric efficiency is higher because of better breathing of engine from

larger valves and the pumping losses are less because of more direct passageways

with less pressure drop through valves. The average flame travel distance is reduced and the engine is less prone to knock,

resulting in less octane requirement.

The surface to volume ratio is decreased which results in an increase in thermalefficiency of the engine. Its also provides more complete combustion of fuel, thusproducing more power and reducing air pollution.

Hot exhaust valve is placed over the head instead of in the cylinder block, thusconfirming thermal failures only to cylinder head which can be easily removed andreplaced.

The possibility of leakage of compression gases and jacket water is reduced as inthis type the cylinder head bolts are subjected to less force.

The casting process is easier, thus leading to reduction in costs.


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BATH-TUB TYPE OF COMBUSTION CHAMBER

It is the most simple and convenient form of overhead valvecombustion chamber with an oval-shaped combustion chamber

bolted over the main cylinder in such a way that some part of oval

portion overhangs the cylinder. This part maybe used for squish

(squish is the rapid injection of gas trapped between the piston and

some flat or corresponding surface in the cylinder head).

Both the valves are mounted vertically overhead, with spark plug at

side.

The main disadvantage is that the valves are placed in a single row

along the cylinder block, resulting in less space to locate valves forlarge diameter. It reduces volumetric efficiency.

More space for valves within the bore diameter can be made

available if the stroke/bore ratio is kept unity or less.


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BATH-TUB TYPE OF COMBUSTION CHAMBER

PISTON

CYLINDER

COMBUSTION

CXHAMBER

EXHAUST VALVE

SPARK

PLUG


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WEDGE-SHAPED COMBUSTION CHAMBER

The combustion chamber is wedge-shaped with slightly inclined

valves .

This type has also given satisfactory performance.

The wedge-shaped combustion chamber is shown below :


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WEDGE-SHAPED COMBUSTION CHAMBER

PISTON

CYLINDER

SPARK

PLUG

EXHAUST

VALVE

EXHAUST

MANIFOLD


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F-HEAD TYPE COMBUSTION CHAMBER

The type of combustion chamber in which one valve is located in the

head and the other in the block is known as F-head combustionchamber.

It has a well shaped piston crown with a correspondingly matched

sloping cylinder head.

The inlet and exhaust valve are inclined. The inlet valve is located inthe head and the exhaust valve in the block.

The plug is in an excellent position in the flat roof of the chamber.

The flat roof allows the larger use of a larger size of the inlet valve

than the exhaust valve. The cooling of spark plug and exhaust valve is efficient.

The flame travel distance is short and the end-gas is reduced to a thin

layer, so the knocking tendency is reduced.

The operation of the valves involves a complex mechanism.


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Hemispherical combustion chamber

A hemispherical chamber with inclined valves is the best design to

use where maximum specific output is required, involving pistonspeeds exceeding 15m/s.

Nearly all racing cars have used the hemispherical cylinder head with

domed piston.

ADVANTAGES :-The combustion chamber is very compact.

-The surface-to-volume ratio is small which reduces the heat loss of

the cylinder wall, thus providing a higher thermal efficiency.

-the larger diameter valves employed increases the volumetricefficiency.

However, the operation of the valves and placing of the spark plugs in

a multi-cylinder engine present difficulties unless a twin overhead

camshaft mechanism is used.


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PISTON CAVITY COMBUSTION CHAMBER

The combustion chamber comprises a bowl in the piston crown in

conjunction with a flat cylinder head.

It resembles the combustion chamber of the normal direct-injection

compression ignition engine.

Here an almost ideal chamber shape is provided with all surface

machined to give an accurately defined volume.

Such a design was not possible in past when long strokes and low

compression ratios were used, but now with the use of high

compression ratios and stroke/bore ratios near one less, this

configuration has become practical and likely to appear more infuture.


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END

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