I.C. ENGINES

LECTURE NO: 09(24 Mar, 2014)

Combustion Formulas

• Combustion is a chemical reaction in which certain elements of the fuel combined with oxygen causing an increase in temperature of the gases

• Main combustion elements are• Carbon • Hydrogen• Sulphur• Oxygen is necessary for combustion and chiefly

diluted by Nitrogen

CHEMICAL SYMBOLS

• Element Symbol Atomic Mass Molecular Mass

• or Compound• HYDROGEN H 1 2• Carbon C 12 24• Nitrogen N 14 28• Oxygen O 16 32• Sulphur S 32 64

Combustion Formulas

• Hydrogen H• Atomic mass of hydrogen = 1• One molecule of hydrogen contains 2 atom of

so it is written as H2

• Since atom of Hydrogen has an atomic mass 1, the molecular mass = 2

• 2 H2 means 2 molecules of hydrogen each consisting of 2 atoms

• Mass would be 2 x 2 = 4

Combustion Formulas

• Oxygen O• Atomic mass of Oxygen = 16• One molecule of contains 2 atom of so it is

written as O2

• Since atom of Oxygen has an atomic mass 16, the molecular mass = 32

• 2 O2 means 2 molecules of Oxygen each consisting of 2 atoms

• Mass would be 2 x 32 = 64

Combustion Formulas

• Water H2 O• Atomic mass of hydrogen = 1• One molecule of hydrogen contains 2 atom of

so it is written as H2

• H2 O • = ( 1 x 2) + ( 16 x 1)• = 18

Combustion Formulas

• C8 H18

• Molecular mass of C8 H18 • = (12 x 8) + (1 x 18) = 114• C6 H18

• Molecular mass of C6 H6 • = (12 x 6) + (1 x 6) = 78• Fuel• Cn H2n+2

Combustion Formulas• H2 + O = H2 O • Two atom of Hydrogen combined with one

atom of Oxygen to form one molecule steam • H2 + O2 = H2O• Two atom of Hydrogen combined with one

atom of Oxygen to form two molecule water• 2H2 + O2 = 2H2O

• + + = =• 1 V + 1 V + 1 Vol = 3 V mixed= 3 V combined

Combustion Calculation

• C8 H18

• Molecular mass of C8 H18 • = (12 x 8) + (1 x 18) = 114• C6 H18

• Molecular mass of C6 H6 • = (12 x 6) + (1 x 6) = 78• Fuel• Cn H2n+2

Combustion Calculation

• Air contain 23 % of O by mass• This means that 1 kg of air contain

0.23 kg of O• If 1 kg of air contain 0.23 kg of O,

then• 1 kg of O will be contained in•

Combustion Calculation

• Air contain 77 % of N by mass• This means that 1 kg of air contain

0.77 kg of N• If 1 kg of air contain 0.77 kg of N,

then• 1 kg of N will be contained in•

Air Required to burn Hydrogen

• Mass of Air required to burn 1 kg of Hydrogen to water is calculated from the equation

• It is usual practice to convenient always to consider 1 kg of fuel

• 2 molecular mass of H + Molecular mass of O = 2 * 2 * 1 + 2 * 16 = 2 ( 2 *1+ 16)

• 4 kg of H + 32 kg O = 36 kg

Air Required to Burn Hydrogen

• 1 kg H + 8 kg O = 9 kg • Since • 1 kg of O is contained in 4.35kg of air• 8 kg of O is contained in 34.8kg of air• Heat liberated by 1 kg H in 144 600 kJ

Air Required for complete Combustion of Carbon

• Equation of the burning of C to CO

• Molecular mass of C + Molecular mass of O = 12 + 2*16 = (12+ 32)

• 12 kg of C + 32 kg O = 44 kg CO• 1 kg of C + 2.66 kg O = 3.66 kg CO• Since • 1 kg of O is contained in 4.35kg of air• 2.66 kg of O is contained in 4.35* 2.66kg of air• Mass of Air required = 11.6 kg• Heat liberated by 1 kg C in 33830 kJ

Air Required to Burn Carbon to Carbon Monoxide

• Equation for incomplete combustion, or burning C to CO

• Molecular mass of C + Molecular mass of O = 12 + 2*16 = (12+ 32)

• 24 kg of C + 32 kg O = 56 kg CO• 1 kg of C + 1.33 kg O = 2.33 kg CO• Since • 1 kg of O is contained in 4.35kg of air• 1.33 kg of O is contained in 4.35* 1.55 kg of air• Mass of Air required = 5.8 kg• Heat liberated by 1 kg C in 10230 kJ

Air Required to Burn Carbon Monoxide to Carbon Dioxide

• Equation for burning CO to CO2

• Molecular mass of CO + Molecular mass of O = (2* 28) + 32 = 2(12+ 32)

• 56 kg of CO + 32 kg O = 88 kg CO• 1 kg of CO + .571 kg O = 1.571 kg CO2

• Since • 1 kg of O is contained in 4.35kg of air• .571 kg of O is contained in 4.35* .571 kg of air• Mass of Air required = 2.52 kg• Heat liberated by 1 kg C in 10130 kJ

Air Required to Burn Sulphur

• Equation

• Molecular mass of S + Molecular mass of O = 32 + 32 = 64

• 32 kg of S + 32 kg O = 64 kg SO2

• 1 kg of S + 1 kg O = 2 kg SO2

• Since • 1 kg of O is contained in 4.35kg of air• Mass of Air required = 4.35 kg• Heat liberated by 1 kg C in 9304 kJ

Air Required to Burn Fuel

• 1 kg of H requires 8 kg O• 1 kg of C requires 2.66kg O• 1 kg of S requires 1kg O • 1 kg of O is contained in 4.35kg air

Problem No 1• A certain petrol consists of 84 % C and 16 % H

by mass. What mass of air is required for complete combustion?

Problem No 2• A fuel oil contains, by mass, 87 % C, 11 % H, 1

% S and 1 % O. What mass of air is required for complete combustion?

Problem No 3• What is mass of air is required to burn

complete 1 kg of benzole C6 H6 ?

Problem No 4• What is mass of air is required to burn

complete 1 kg of ethyl alcohol C6 H6 ?

Determination of Calorific Value• When the composition of a fuel is known. This

method is not so accurate as burning fuel in a calorimeter, so the answers are only approximate.

• Hexane C6 H14 • This fuel contain 84 % C and 16 % H• Heat liberated by 1 kg C = 33830 kJ• Heat liberated by 0.84 kg C = 28417 kJ

Determination of Calorific Value• Heat liberated by 1 kg H = 144600 kJ• Heat liberated by 0.16 kg H = 23136 kJ

• Therefore H.C.V. of fuel = 28417+23136 = 51553 kJ/kg

DIESEL FUEL SYSTEMS

C

A B

E

D

Pressure

VolumeTheoretical Diesel cycle

P V diagram for C.I. Engine / Diesel Cycle EngineD

TDC

BDC

Diesel Cycle

Diesel Cycle

Diesel Cycle• At Constant pressure per kg of air

• At constant volume per kg of air heat rejected

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Example

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Example

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Example

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Example

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THE C.I. ENGINES FUEL SYSTEM• The C.I. engine demands that the fuel supplied is:-• Timed to inject when the piston is near the end of the

compression stroke.• The fuel is atomised (broken up) into fine particles if

efficient combustion and reduced pollution are to be obtained.

• The fuel is forced with sufficient pressure into to the hot air in the combustion engine.

• All C.I. engines require some form of cold start device, this is because when the engine is cold, heat loss can result in the heat generated by the compression stroke insufficient to ignite the fuel. To overcome this, most C.I. engine inject addition fuel when cold (excess fuel) and use some form of electrical heating device, to raise the temperature of the air (glow plugs).

• If theses demands are not met, the engine will produce excessive noise (combustion knock) and pollution.

THE C.I. ENGINES FUEL SYSTEM

• The fuel system consists basically of a • Fuel tank• One or more filters• Low pressure fuel (lift) pump• High pressure injection pump• Injector for each cylinder. • Air must not be allowed to get into the fuel

injection system, the engine will not run, some systems require bleeding if air is present e.g. when the fuel filter is replaced, or the vehicle runs out of fuel.

TYPICAL C.I. FUEL SYSTEM LAYOUT

Fuel Tank

WaterTrap

Lift Pump FuelFilters

FuelInjection

Pump

Fuel InjectorsLeak Off

Engine combustionChamber

FUEL LIFT PUMP (LOW PRESSURE)

• Typically a diaphragm pump driven off of the engine camshaft, or built into the injection pump or electrically driven in later systems• May Have manual priming

• Function is to supply steady flow of fuel to injection pump.

• May incorporate a strainer filter which requires cleaning at service intervals.

MAIN FUEL FILTERS• Maybe of replaceable element or cartridge type.• Filter material must be of large enough surface area to allow for fuel flow, which will stop very small partials of dirt reaching the injection system• Maybe large single units or consist of a ‘primary’ & ‘secondary’ filter.• Must be changed at the correct service interval• Incorporate the water trap

HIGH PRESSURE FUEL INJECTION PUMPS• Can be of ‘Rotary’ or ‘In line’ design. Driven by the engine at half crankshaft speed• Pressurises the fuel and delivers it to the injections at the correct time for combustion• Accurately meters the fuel quantity to match engine load demands• Incorporates a governor to control engine speed and prevent the engine over speeding and damaging itself

FUEL INJECTORS

•Each injector is fitted in the cylinder head above each combustion chamber

•Fuel is delivered to the injectors via thick walled high pressure steel pipes Injector pipes are of equal volume/length to ensure accuracy of timing between cylinders

Each injector sprays atomized fuel in to the combustion chamber to insure complete combustion

COMBUSTION CHAMBERS In the C.I. engine the fuel is injected intothe combustion chamber, it the has to mixthoroughly with the air, ignite and burn all at the same time. To insure this happens, twotypes of combustion chamber have beendeveloped.

DirectInjection

Indirect Injection