5 Gas Exchange Processes

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55TOPICTOPIC

Gas exchangeGas exchangeprocessesprocesses

Chapter 6Chapter 6Sections 6.1 6.3Sections 6.1 6.3


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Purpose & GoalsPurpose & Goals

Purpose of gas exchange processes is to remove

the burned gas at the end of the power stroke and

to admit the fresh charge for the next cycle Goals

! Inducting of the maximum air mass "power is

proportional to air mass flow rate#

! mixture preparation "in $I engine#! setting up the flow field within the engine cylinders that

will give a fast combustion for satisfactory engine

operation


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Four-stroke SI EngineFour-stroke SI EngineIntake system and averageIntake system and average

pressure within itpressure within it

Pressure drops depends on

%engine speed&

%flow resistance of the

elements in the system&

%their cross%sectional area&

%charge density


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Four-stroke SI EngineFour-stroke SI Engine

Valve timing and pressure-Valve timing and pressure-volume diagramvolume diagram

wide%open throttle

part throttle

'alve open phases are extended to

%improve emptying an charging of cylinder

%make use of inertia of gases in the intake

and exhaust systems

('O )*%+*, before -TC

('C ./%0*, after T1C

I'O .*%2*, before T1C

I'C /*%3*, after -1C

overlap


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Four-stroke SI EngineFour-stroke SI EngineCylinder pressure and valve lift vs rank angleCylinder pressure and valve lift vs rank angle


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!ur"oharged four-stroke CI Engine!ur"oharged four-stroke CI Engine

Intake and e#haustIntake and e#haustproessesproesses


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Volumetri effiienyVolumetri effiieny

Is a measure of the effectiveness of the engine and itsintake and exhaust systems as an air pumping device

1efined for )%stroke engines only as

If a,*is evaluated at atmospheric condition& then vis

the overallvolumetric efficiency

If it is evaluated at inlet manifold conditions4 vthen

measures the pumping performance of the cylinder&inlet port& and valve alone

1iscussion will cover unthrottled engine operation

0

2

,

a

v

a d

m

V N

=

&


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Volumetri effiienyVolumetri effiieny

Is affected byIs affected by

5uel type& 567 ratio& fraction of fuel vapori8ed in the

intake system& fuel heat of vapori8ation

9ixture temperature as influenced by heat transfer

:atio of exhaust to inlet manifold pressures

Compression ratio

(ngine speed

Intake and exhaust manifold and port design

Intake and exhaust valve geometry& si8e& lift& and

timings


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Volumetri effiieny vs$ mean piston speedVolumetri effiieny vs$ mean piston speed

'olumetric efficiency of $I (ngine is lowerthan that of diesel due to

% flow losses in carburetor and throttle4

%intake manifold heating4

%presence of fuel vapor4

%higher residual gas fraction


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Effet on volumetri effiieny of different phenomenaEffet on volumetri effiieny of different phenomena


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Flow through valvesFlow through valves

'olumetric efficiency is affected by intake and

exhaust valve geometry& si8e& lift& and timings


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9ass flow rate through a poppet valve is describedby the e;uation for compressible flow through a flow

restriction

This e;uation is derived for one%dimensional

isentropic flow& and corrected by experimentallydetermined discharge coefficient CD

7ir flow rate depends on the upstream stagnation

pressurep*

and temperature T*

& static pressuredownstream of the flow "assumed e;ual to the

pressure at the valve throatpT# and a reference area

ARcharacteristic of the valve design


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9ass flow rate of subsonic flow


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5or flow into the cylinder through an intake valve&p*is the intake system pressure andpT is the cylinder

pressure

5or flow out of the cylinder through an exhaust

valve&p*is the cylinder pressure andpT is theexhaust system pressure

The value of CDand the choice of reference area are

linked together4 their product is the effective flowareaAEof the valve

Geometric minimum flow area is one of the choice

for reference area



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Poppet valve geometryPoppet valve geometry

Parameters definingpoppet valve geometry


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Valve open areaValve open area

Instantaneous valve flow area depends on the valve lift andgeometric details of the valve head& seat& and stem

9inimum valve flow area is calculated for three stages of

valve lift


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5or first stageminimum flow area is between the valve andthe seat& perpendicular to the seat= 5or this stage

cos 2 sin 22

v

m v v

L

A L D w

= +


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5or second stageminimum flow area is still between thevalve and the seat& but not longer perpendicular to the seat=

5or this stage

( )

1

2 2 2

tanm m vA D L w w = +


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5or third stageminimum flow area is the port flow areaminus the sectional area of the valve stem

( )2 2

4m p sA D D

=


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'alve%lift curve and valves

open area as a function of

camshaft angle


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%isharge oeffiient

of a typial inlet

poppet valve


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Valve timingValve timing

Typical valve timingdiagram for high%speed

$I engine

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5 Gas Exchange Processes