Internal Combustion Engine Part 2
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Transcript of Internal Combustion Engine Part 2
1. The working fluid is air, which continuously
circulates in a closed loop (cycle).
2. Air is considered as ideal gas.
3. All the processes in (ideal) power cycles
are internally reversible
4. Combustion process is modelled by a heat-
addition process from an external source
5. The exhaust process is modelled by a heat-
rejection process that restores the
6. Working fluid (air) at its initial state
7. There is no heat loss from the system to
the surroundings
8. The working medium has constant specific
heats through out the cycle
9. The physical constants viz., Cp, Cv, γ and M
of the working medium are the same as
those of air at standard atmospheric
conditions.
For Eg. Cp = 1.005 kJ/kg K
Cv = 0.717kJ/kg K
γ = 1.4 and M = 29kg/kmol
W = 𝑃1𝑉1
γ−1[(𝑟γ−1)(𝑟𝑝 − 1)]
MEP = 𝐴𝑟𝑒𝑎 𝑜𝑓 𝑡𝑒 𝑖𝑛𝑑𝑖𝑐𝑎𝑡𝑜𝑟 𝑑𝑖𝑎𝑔𝑟𝑎𝑚
𝐿𝑒𝑛𝑔𝑡 𝑜𝑓 𝑡𝑒 𝑖𝑛𝑑𝑖𝑐𝑎𝑡𝑜𝑟 𝑑𝑖𝑎𝑔𝑟𝑎𝑚
Pm= 𝑃1𝑟[(𝑟γ−1)(𝑟𝑝−1)]
(γ−1)(𝑟−1)
η = 1- 1
γ(𝑟)γ−1 [
(ργ−1)
ρ −1]
W = 𝑃1𝑉1𝑟γ−1 [γ ρ−1 −𝑟1−γ ργ −1 ]
(γ −1)
Pm = 𝑃1𝑟γ
(𝑟−1)
[γ ρ−1 −𝑟1−γ ργ −1 ]
(γ −1)
Fuel-Air cycle is defined as the
theoretical cycle that is based
on the actual properties of the
cylinder gases.
The actual composition of the cylinder gases (air
+ fuel + water vapour + residual gases).
The variation of the specific heat of these gases
with temperature.
The incomplete mixing (in-homogeneous) of fuel
and air at higher temperatures (@ above 1600
K).
The variations in the number of molecules
present in the cylinder as the temperature and
pressure change.
No change in the fuel or air chemical
composition before combustion.
The process is frictionless and
adiabatic.
Charge is in chemical equilibrium
after combustion.
Combustion process is instantaneous.
Fuel is completely vaporized and
perfectly mixed with the air (for SI
only).
Dissociation is defined as the
disintegration of burnt gases at high
temperatures. Disintegration increases
with temperatures as shown below.
The general effect of dissociation can be
explained as follows: As the temperature
increases considerable amount of heat
will be absorbed by the elements that
undergoes dissociation. This heat will be
liberated when these elements re-
combine as the temperature falls
Thus we se that the effect of dissociation is a
suppression of part of the heat during the
combustion process and the liberation of this
heat during the expansion process. Though
looks similar to that of variation of specific
heat, its effect is much smaller that it.
The dissociation mainly is of CO2 into CO and
O2 : 2CO + O2 ⇔ 2CO2 + Heat
This process commences at about 1000 0C and
by the time it reaches 1500 0C it reaches 1%.
There is also a very little dissociation of H2O :
2H2 + O2 ⇔ 2H2O + Heat
Air-standard analysis predicts no variation of thermal efficiency with mixture strength. Fuel-air analysis, however, suggests that the thermal efficiency will deteriorate as the mixture supplied is enriched.
This can be explained by the increased losses due to dissociation and variable specific heat as the engine temperature is raised due to enrichment of fuel towards the chemically correct mixture. Further, enrichment beyond the chemically correct mixture will result in the supply of unusable excess fuel hence the thermal efficiency will drop rapidly. This implies that the thermal efficiency would increase as the mixture is weakened. This is true up to certain limit beyond which thermal efficiency drops again due to erratic combustion of the fuel. Thus the best thermal efficiency would be near the chemically correct ratio toward the weak side.
Engine running at constant engine speed and throttle opening with variable fuel supply
The F/A cycle efficiency increases with CR in the same manner as that for air standard cycle. This is because of the increased scope for expansion work. and also the increased in the end-of-compression pressure and temperature which causes the end-of-combustion pressure and temperature also to rise
We see that as the mixture is made lean (lesser fuel) the thermal efficiency increases. This is because of the lesser thermal energy released which results in the lowering of the cylinder temperature and pressure hence reducing the specific heat and dissociation losses. This is valid up to certain limit beyond which it again drops down due to erratic burning of the fuel.
For a given compression ratio, the maximum cycle temperature is reached when mixture is slightly rich (about 6% rich) and that for maximum cycle pressure is at about 10% rich. This is because at chemically correct mixture, due to the chemical equilibrium losses, there is still some oxygen present at state point 3, this will cause more fuel to combine with oxygen and burn raising the temperature of the cylinder. Further, this increment in the number of molecules in the cylinder allows for higher peak pressure as the gas law states: P*V = N*R*T. This also helps in increasing the cycle MEP
In SI engines combustion process is initiated by a spark between electrodes of spark plug
Energy Requirement - 10 millijoules for A/F of 12-13:1
Duration of few micro-seconds is sufficient Break down voltage – critical voltage
below which no park will occur Pressure, temperature and density have
influence on voltage required to cause spark
In practice spark energy to the tune of 40 millijoules and duration of about 0.5 millisecond is sufficient over entire range of operation
1. Battery Ignition system –
conventional transistor assisted
2. Magneto Ignition system - Low
tension, high tension
3. Electronic
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Fuels may be chemical or nuclear
A chemical fuel is a substance which releases
heat energy on combustion
The principle combustible elements of each
fuel are carbon and hydrogen.
Sulphur is a combustible element but its
presence is undesirable because sulphur
upon combustion gives rise to sulphur
dioxide, which is a harmful gas and also a
pollutant. Sulphur dioxide is considered as a
greenhouse gas and also results in acid rain
Primary fuels and Secondary
Solid Liquid and gas
High energy density (kJ/kg)
Good combustion qualities
High thermal stability
Low toxicity
Low pollution
Easy transportation/transferability and storage
Compatibility with engine hardware
Low deposit forming tendency
Economically viable in large quantities
Easy mixing with oxygen and Low heat of
evaporation (hfg)
No Chemical reaction with engine components
Coal – There are various types of coal according
to their chemical and physical properties
(Lignite, bituminous, anthracite )
Ultimate Analysis – Chemical analysis of the presence of carbon, hydrogen nitrogen and sulphur
Proximate Analysis – It gives percentage of moisture, volatile matter (water derived from chemical decomposition of the coal, combustible gas like H2, CH4,C2H6 and tar), combustible solid (called fixed carbon), and ash
Most of the liquid fuels are hydrocarbons.
1. Petroleum and its Derivatives - petroleum
oils are complex mixtures of hundreds of
different fuels. The necessary information
to us is the relative proportions of C, H2
given by ultimate analysis
2. Synthetic fuels - Usually called synfuels,
which are liquid and gases mainly produced
from coal, oil shale(is an organic-rich fine-
grained sedimentary rock containing
kerogen), tar sands (oil sands, tar sands or,
more technically, bituminous sands are a
type of unconventional petroleum deposit)
and also from various wastes and biomass
3.Alcohols
The alcohols are a partial oxidation product of petroleum, and are not found to any extent in the crude oil. The compounds are saturated, with a chain structure with the general formula R.OH. Here the radical R is the paraffin group attached to the hydroxyl radical OH. Alcohols are designated by the name of the radical:
1. CH3OH: Methyl alcohol or Methanol.
2. C2H5OH: Ethyl alcohol or Ethanol.
3. C3H7OH: Propyl alcohol or Propanol.
4. C4H9OH: Butyl alcohol or Butanol
Gaseous fuels are may be either natural or
manufactured; natural gas: is a mixture of
components, consisting mainly of methane (60-
98%) with small amount of other hydrocarbons.
In addition, it consists; N2, CO2, H2 and traces
of other gases. Its sulphur content ranges from
very little (sweet) to larger amounts (sour). It is
classified as associated or non-associated
depending on whether it is associated with oil
or not. It is stored as compressed natural gas
(CNG) or as liquid (LNG) at pressure ranging
between 70 to 210 kPa.
Liquid petroleum gas (LPG): It is
mixture of propane and butane and
some other light hydrocarbons.
Propane and butane are the main
constituent of LPG
Coal gas: It is a by-product obtained
during the destructive distillation of
coal. Its main approximate
composition is CH4 = 25%, H = 53%,
CO = 9%, N = 6%, CO2 = 2% and other
hydrocarbons 1%
Coke oven Gas: It is produced during the manufacture of coke from new coal in a coke oven where the volatile matter is distilled off and the coke-oven separated from liquids and solids in the volatile matter by cooling and extraction. It consists about 50% H2, about 30% methane(CH4), and the remainder of various other gases. Its heating value ranges between 14200 to21300 kJ/m3
Blast-furnace gas: It is produced as by-product from blast furnaces used in iron reduction process, has about 30%, CO, 2% H2, 11% CO2 and about 60% N2
Other gaseous fuels : producer gas, water gas and town gas