Unit i Si Engines

8/10/2019 Unit i Si Engines

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UNIT - 1

MIXTURE REQUIREMENTS

SI engines supplied with chemically correct A/F mixture

A/F ratio of weight of air to weight of fuel

Stoichiometric A/F is 14.5:1


Mixture strength

Rich mixture A/F < Theoretical A/F

Lean mixture A/F > Theoretical A/F

Mixture strength is also indicated as equivalence ratio

Equivalence Ratio =

If >1 mixture is rich and if


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Mixture strength for various operating conditions

Cold starting

Gasoline injected cannot vapourise completely as temperature will be lower

-Lesser fuel reaches the cylinder

-Rich A/F of 9:1 has to be supplied

Idling

-A/F of 12:1 is needed

-dilution of fresh charge with residual gases will be more hence rich mixture is needed


Mixture strength for various operating conditions

cruising

- A/F of 15:1 is needed

Acceleration

- More power has to be produced hence rich mixture is needed

Economic & More power operation

- Slightly lean mixture is required for less FC

- Rich mixture is needed for more power generation


Engine performance

Lean mixture poor engine power, mechanical damage, knocking

Slightly lean - Economical, low exhaust emissions

Stoi. Mixture - Best all round performance

Slightly lean - Max power, higher emissions, higher FC


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Too rich - More pollution, uneconomical, oil contamination

COMBUSTION CHAMBER

Basic requirements

1) High power output

- requires high compression ratio

- no excess air supply

- complete utilisation of supplied air

- accommodate large inlet & outlet valves to have higher volumetric efficiency

- streamlining the inlet & exhaust passages to reduce pressure drop & to increase

volumetric efficiency2) High thermal efficiency

- Requires high compression ratio

- Small surface to volume ratio to reduce heat losses

- Smooth engine operation

- Pressure rise should be moderate

- Compact size with shorter flame travel distance to reduce knocking

- Proper location of spark plugs

- Good cooling of charge

3) Smooth Engine operation

- moderate rate of pressure rise during combustion

- proper location spark plug

- shorter flame travel distance

- satisfactory cooling of spark plug and exhaust valves.


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Fig. 4 Types of Combustion Chamber

Types of combustion chamber

T head

- designed to have lower compression ratio to prevent knocking

- has two camshafts for operating inlet and outlet valves individually

F head

- inlet valves located in the cylinder head and outlet valve sin cylinder or vice versa

- improved volumetric efficiency

- reduced width of combustion chamber

Over head

- Higher volumetric efficiency

- Single cam shaft located by the side of the cylinder operating the valves throughtappets, push rods & rocker arms


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Turbulent combustion chamber

- Very small clearance volume provided

- Causes squish turbulence in the mixture, resulting in better mixing & improved

combustionCOMBUSTION IN SI ENGINES

Stages of combustion

Cylinder is supplied with homogeneous A/F mixture by the carburetor

A/F mixture absorbs heat from the hot cylinder, piston, valves

Hydrocarbons of fuel reacts with oxygen and sets up further reactions which ispreflame reactions

Peroxides, aldehydes and even carbon monoxides are formed due to preflamereactions

Combustion initiated by an electric spark in the spark plug

COMBUSTION IN SI ENGINES

Stages of combustion

Spark creates high temperature region around it and the fuel molecules in this

region starts burning which is flame Flame has definite front or boundary called a flame front

Layer by layer combustion occurs as the moving flame separates burnt gases andunburnt gases is normal combustion

Flame started at the spark plug spreads to the farthest part of the cylinder


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COMBUSTION IN SI ENGINES

Fig .3 Stages of Combustion

Stage 1: Preparation period

Duration between spark ignition to the first measurable pressure rise Also called as ignition lag but ignition lag happens only in diesel combustion

Pressure rise will be of 1% of total pressure rise

Depends on temperature, pressure and dilution of residual gases

Stage 2 : Propagation period

Duration between the first measurable pressure rise and peak pressure

Rapid increase in combustion rate

Stage 3 : After burning

Combustion continues after peak pressure is after burning

Combustion reduces

About 10% of heat release occurs


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Factors affecting combustion

Mixture strength

Inlet conditions

Size of the cylinder bore

Ignition timing

Position of the spark plug

Velocity of fresh charge through the inlet

Presence hot sots in the combustion chamber

Dilution of residual gases

Introduction of lubricating oil

Abnormal combustion Knocking

Flame moves in a orderly manner in normal combustion

In knocking combustion Temperature and pressure initiates chemical reaction inthe mixture and this charge gets self ignited

Self ignited flame & normal flame collides forming pressure waves

Pressure waves hits the engine parts creating knocking or pinging sound Abnormal combustion Knocking

Pressure waves travels with velocity of sound back & froth in the combustionchamber until dissipated by friction

Steep pressure rise during knocking sets up vibration in the charge

Vibration increases the heat transfer to the walls, hence more heat loss

FACTORS AFFECTING KNOCK

Molecular structure

- Increase in carbon chain increases knocking tendency

- adding methyl or ethyl group to the carbon decreases knocking tendency

- Napthenes have higher knocking tendency


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Self ignition temperature

- Fuels with higher ignition temperature exhibits less knocking

- Adding tetra ethyl lead increases self ignition temperature

Rate of burning

- Depends on mixture strength

- Increases in burning rate increases knocking

A/F Ratio

- Max power & flame speed resulted in A/F of 13:1

- Moving away from 13:1 gives better results

Charge temperature

- Increase in charge temperature results in increase in end gas temperature,thereby knocking tendency

Charge density

- Reduction in intake pressure reduces knocking tendency

- Supercharging increases knocking

Compression ratio

- Increase in CR results in increase in end gas temperature & pressure, therebyknocking tendency

Dilution of residual gases

- Though it increase the mixture temperature it lowers the knocking tendency

Valve timing

- Late intake closure results in reduction compression ratio compared withtheoretical value

- Late IVC increase overlapping increasing dilution, hence knocking tendencyreduces

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Unit i Si Engines