gas exchange process

8/14/2019 Gas Exchange Process

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Gas Exchange Process


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VOLUMETRIC EFFICIENCY


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Volumetric efficiency is affected by the follow

fuel, engine design, and engine operating varia

I. Fuel type, fuellair ratio, fraction of fuel vaporized in the intake system, and

fuel heat of vaporization

2. Mixture temperature as influenced by heat transfer

3. Ratio of exhaust to inlet manifold pressures4. Compression ratio

5. Engine speed

6. Intake and exhaust manifold and port design

7. Intake and exhaust valve geometry, size, lift, and timings


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VOLUMETRIC EFFICIENCY OF A

IDEAL CYCLE


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EFFECT OF FUEL COMPOSITION

PHASE, AND FUEL AIR RATIO


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EFFECT OF INLET AND EXHAUST

PRESSURE RATIO AND COMPRESSIO

RATIO


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FRICITONAL LOSSES

Combined Quasi-Static and

Dynamic Effects


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Pressure variation

due to engine load


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Ram Effect

The pressure in the inlet manifold varies during each cylinders intake process the piston velocity variation, valve open area variation, and the unsteady gas-feffects that result from these geometric variations. The mass of air inducted icylinder, and hence the volumetric efficiency, is almost entirely determined bypressure level in the inlet port during the short period before the inlet valve is

At higher engine speeds, the inertia of the gas in the intake system as the intais closing increases the pressure in the port and continues the charging procespiston slows down around BC and starts the compression stroke. This effect progressively greater as engine speed is increased. The inlet valve is closed som60" after BC, in part to take advantage to ram phenomenon.


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REVERSE FLOW INTO THE INTA

Because the inlet valve closes after the start of the compression stroke,

reverse flow of fresh charge from the cylinder back into the intake can

the cylinder pressure rises due to piston motion toward TC. This revers

largest at the lowest engine speeds. It is an inevitable consequence of th

valve closing time chosen to take advantage of the ram effect at high sp


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TUNING

The pulsating flow from each cylinder's exhaust process sets up pressure wav

exhaust system. These pressure waves propagate at the local sound speed rela

the moving exhaust gas. The pressure waves interact with the pipe junctions a

in the exhaust manifold and pipe. These interactions cause pressure waves to

reflected back toward the engine cylinder .

These pressure waves may aid or inhibit the gas exchange processes. When th

process by reducing the pressure in the exhaust port toward the end of the

exhaust process, the exhaust system is said to be tuned.


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Instantaneous

pressures in the

intake and exhaust

manifold


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Variation with Speed, and Valve

Area, Lift, and Timing


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FLOW THROUGH VALVES


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Poppet Valve Geometry


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Flow Rate and Discharge

Coefficients


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RESIDUAL

GAS

FRACTION


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EXHAUST GAS FLOW RATE AND

TEMPERATURE VARIATION


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SCAVENGING IN TWO-STROKE

CYCLE ENGINES


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Scavenging

Parameters and

Models


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FLOW THROUGH PORTS


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SUPERCHARGING AND

TURBOCHARGING

gas exchange process

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