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 𝑡𝑕𝑒 𝑖𝑛𝑑𝑖𝑐𝑎𝑡𝑜𝑟 𝑑𝑖𝑎𝑔𝑟𝑎𝑚

𝐿𝑒𝑛𝑔𝑡𝑕 𝑜𝑓 𝑡𝑕𝑒 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