ic engine fuels
TRANSCRIPT
-
8/9/2019 Ic Engine FUELS
1/44
THERMAL ENGINEERINGUNIT-1
-
8/9/2019 Ic Engine FUELS
2/44
FUELS
Fuels can be in solid, liquid or gaseous
state.
Principle constituent of any fuel are
carbon and hydrogen.
-
8/9/2019 Ic Engine FUELS
3/44
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
-
8/9/2019 Ic Engine FUELS
4/44
FORMATIONOF COAL
PEAT
LIGNITE
BITUMINOUS COAL
ANTHRACITE COAL
-
8/9/2019 Ic Engine FUELS
5/44
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.
-
8/9/2019 Ic Engine FUELS
6/44
LIQUID FUELS (Typical oil pool
formation)
-
8/9/2019 Ic Engine FUELS
7/44
LIQUID FUELS (Typical oil pool refining)
-
8/9/2019 Ic Engine FUELS
8/44
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.
-
8/9/2019 Ic Engine FUELS
9/44
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.
-
8/9/2019 Ic Engine FUELS
10/44
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.
-
8/9/2019 Ic Engine FUELS
11/44
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
-
8/9/2019 Ic Engine FUELS
12/44
INTERNAL COMBUSTION ENGINE
-
8/9/2019 Ic Engine FUELS
13/44
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.
-
8/9/2019 Ic Engine FUELS
14/44
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.
-
8/9/2019 Ic Engine FUELS
15/44
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.
-
8/9/2019 Ic Engine FUELS
16/44
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.
-
8/9/2019 Ic Engine FUELS
17/44
THEORETICAL PRESSURE Vs CRANK
ANGLE DIAGRAM
-
8/9/2019 Ic Engine FUELS
18/44
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.
-
8/9/2019 Ic Engine FUELS
19/44
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.
-
8/9/2019 Ic Engine FUELS
20/44
COMBUSTIONWITH DETONATION
-
8/9/2019 Ic Engine FUELS
21/44
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.
-
8/9/2019 Ic Engine FUELS
22/44
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.
-
8/9/2019 Ic Engine FUELS
23/44
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.
-
8/9/2019 Ic Engine FUELS
24/44
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)
-
8/9/2019 Ic Engine FUELS
25/44
-
8/9/2019 Ic Engine FUELS
26/44
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.
-
8/9/2019 Ic Engine FUELS
27/44
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 :
-
8/9/2019 Ic Engine FUELS
28/44
TURBOCHARGING
-
8/9/2019 Ic Engine FUELS
29/44
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 :
-
8/9/2019 Ic Engine FUELS
30/44
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.
-
8/9/2019 Ic Engine FUELS
31/44
T-HEAD TYPE COMBUSTION CHAMBER
Combustion Chamber
-
8/9/2019 Ic Engine FUELS
32/44
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
-
8/9/2019 Ic Engine FUELS
33/44
L-head or side valve combustion
chamber
-
8/9/2019 Ic Engine FUELS
34/44
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.
-
8/9/2019 Ic Engine FUELS
35/44
RICHARDO TURBULENT HEADSIDE VALVE
COMBUSTION CHAMBER
COMBUSTION CHAMBER
EX
INLET VALVE
EXHAUST
VALVE
EXHAUST
VALVE
PISTON
CYLINDER
-
8/9/2019 Ic Engine FUELS
36/44
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.
-
8/9/2019 Ic Engine FUELS
37/44
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.
-
8/9/2019 Ic Engine FUELS
38/44
BATH-TUB TYPE OF COMBUSTION CHAMBER
PISTON
CYLINDER
COMBUSTION
CXHAMBER
EXHAUST VALVE
SPARK
PLUG
-
8/9/2019 Ic Engine FUELS
39/44
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 :
-
8/9/2019 Ic Engine FUELS
40/44
WEDGE-SHAPED COMBUSTION CHAMBER
PISTON
CYLINDER
SPARK
PLUG
EXHAUST
VALVE
EXHAUST
MANIFOLD
-
8/9/2019 Ic Engine FUELS
41/44
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.
-
8/9/2019 Ic Engine FUELS
42/44
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.
-
8/9/2019 Ic Engine FUELS
43/44
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.
-
8/9/2019 Ic Engine FUELS
44/44
END