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