combustion-in-si-engine.ppt

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COMBUSTION COMBUSTION IN SI IN SI

ENGINESENGINES

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Conditions of combustion in SI Conditions of combustion in SI enginesengines

The angular speed in SI engines is in the range of 500-5000 min-1,The angular speed in SI engines is in the range of 500-5000 min-1, so the whole cycle is 10-100 ms. During this time the mixture mustso the whole cycle is 10-100 ms. During this time the mixture must be ignited, burnt and reburnt.be ignited, burnt and reburnt.

Laminar flame is too slow, however the mixture in the cylinder is turbulized and flame is turbulent, which makes the rate of combustion very high.

In the process of combustion in SI can be divided into three phases a) Ignition lagb) Flame propagationc) afterburning.

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Idle 4-stroke S I engineIdle 4-stroke S I engine

0 360

a

b

c

d

Compression

180Cranke angle(deg)

Pressure

Expansion

com

bust

ion

TDC

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Stages of combustion in SI Stages of combustion in SI enginesengines

Changes of the pressure Changes of the pressure in the cylinder of SI in the cylinder of SI engineengine

Phases of combustion:I – ignition lag II – Flame PropagationIII – Afterburning

A→B- Ignition lagB →C- Flame propagation C→D- After burning

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Rate OF Pressure RISERate OF Pressure RISE

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Optimisation of combustion Optimisation of combustion in SI enginesin SI engines

There is a dominating tendency to burn lean There is a dominating tendency to burn lean mixturesmixtures

((øø>>1), because it improves efficiency of the >>1), because it improves efficiency of the engine and reduces pollutant emission.engine and reduces pollutant emission.

Combustion of lean mixtures in SI engines Combustion of lean mixtures in SI engines causes some problems:causes some problems:

a) Speed of laminar flame propagation is lower,a) Speed of laminar flame propagation is lower,

b) Ignition is more difficult.b) Ignition is more difficult.

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Fuel Knock ScaleFuel Knock Scale

To provide a standard measure of a fuel’s ability To provide a standard measure of a fuel’s ability to resist knock, a scale has been devised by to resist knock, a scale has been devised by which fuels are assigned an which fuels are assigned an octane number octane number ON.ON.

The octane number determines whether or not a The octane number determines whether or not a fuel will knock in a given engine under given fuel will knock in a given engine under given operating conditions. operating conditions.

By definition, normal heptane (n-C7H16) has an By definition, normal heptane (n-C7H16) has an octane value of zero andisooctane (C8H18) has a octane value of zero andisooctane (C8H18) has a value of 100.value of 100.

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Fuel Knock ScaleFuel Knock Scale

The higher the octane number, the higher the The higher the octane number, the higher the resistance to knock.resistance to knock.

Blends of these two hydrocarbons define the knock Blends of these two hydrocarbons define the knock resistance of intermediate octane numbers: e.g.,resistance of intermediate octane numbers: e.g.,

a blend of 10% n-heptane and 90% isooctane has a blend of 10% n-heptane and 90% isooctane has an octane number of 90.an octane number of 90.

A fuel’s octane number is determined by A fuel’s octane number is determined by measuring what blend of these two hydrocarbons measuring what blend of these two hydrocarbons matches the test fuel’s knock resistance.matches the test fuel’s knock resistance.

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combustion-in-si-engine.ppt

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