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Air Charging of Engines
Dr. M. Zahurul Haq
ProfessorDepartment of Mechanical Engineering
Bangladesh University of Engineering & Technology (BUET)Dhaka-1000, Bangladesh
zahurul@me.buet.ac.bdhttp://teacher.buet.ac.bd/zahurul/
ME 401: Internal Combustion Engines
c Dr. M. Zahurul Haq (BUET) Air Charging of Engines ME 401 (2011) 1 / 14
Air Charging Methods
Maximum engine power is limited by the amount of fuel that can be
burned efficiently inside cylinder. This is limited by the amount of air
that is introduced into each cylinder as fuel system can always be
designed to provide the required amount of fuel this air flow.
Natural Aspiration: induction process relies solely on thepressure difference that exists between retreating piston and airintake arrangements. Natural charging is enhanced by utilizing:
1 Ram effect of incoming charge2 Pressure wave tuning
Forced Induction: charge is forced to enter into the cylinder at apressure substantially above that of the atmosphere, although themean gas velocities through the intake part remain unchanged forthe same engine speeds.
1 Mechanical supercharging2 Turbocharging3 Pressure wave supercharging
c Dr. M. Zahurul Haq (BUET) Air Charging of Engines ME 401 (2011) 2 / 14
With SI engines, boosting the pressure in the induction system
has the effect of increasing the overall compression ratio. To avoid
knock as the boost pressure increases, the engine compression ratio
must be reduced relative to that required for natural aspiration.
Because only air is compressed in CI engines, problems associated
with knock don’t not arise. Moreover, increasing the density of the
air reduces the delay period, thus actually producing more
favourable rate of pressure rise during combustion.
For every charge temperature rise of l00oC due to pressure
charging a SI engine, the increase in absolute temperature at the
end of compression will be approximately 200oC. In a CI engine,
with its higher compression ratio, the temperature at the end of
compression will be of the order of 300oC higher. In both
instances, these effects increase the rate of heat generation, and
therefore temperature rise, during combustion.
c Dr. M. Zahurul Haq (BUET) Air Charging of Engines ME 401 (2011) 3 / 14
Inertia Ramcharging
At high engine speed charge flowing through the intake system
moves at a high velocity and its momentum keeps it moving
toward the cylinder and can continue to compress the charge into
the cylinder at the end of intake stroke even after the piston has
started upward on the compression stroke (if IV is open).
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Induction work A of the piston
correspond to compression work B.
By properly tuning the intake valve closing, the amount of charge
trapped inside the cylinder can be increased. This phenomenon,
called the ram effect, increases with air velocity and therefore
with engine speed.c Dr. M. Zahurul Haq (BUET) Air Charging of Engines ME 401 (2011) 4 / 14
Pressure Wave Tuning
The high-pressure wave, created when the exhaust valve opens
and rapidly blows down the cylinder contents, travels to the end
of exhaust pipe and is reflected as a low-pressure wave or
rarefaction wave. If this wave is timed to enter the cylinder near
the end of the exhaust stroke it can assist in evacuating the
residual gases and draw in fresh charge as the intake valve opens.
Similarly, rarefaction waves in the intake system are reflected from
the open end of the intake as pressure waves that will force more
charge into the cylinder.
The process, known as Tuning, is highly dependent on the
relationship between valve timing, pipe lengths and the speed of
the sound in intake and exhaust gases. As a results, the benefits of
tuning tend to be concentrated at specific engine speeds and the
effects at other speeds may actually be negative.
c Dr. M. Zahurul Haq (BUET) Air Charging of Engines ME 401 (2011) 5 / 14
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Suction wave moves at sonic speed, Co =
√
γRTo along the pipe
length, L. After time t = L/Co , the wave reaches the open manifold
end and is reflected as pressure wave which returns to intake valve at
same speed. If the intake valve is still open, then this can boost ηv .
Travel time ∆t = 2L/Co must be shorter than valve opening duration.c Dr. M. Zahurul Haq (BUET) Air Charging of Engines ME 401 (2011) 6 / 14
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(1) Resonance chamber (2) Adjuster
(3) Resonance chamber
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Variable-length intake runners
In tuned-intake tube charging, resonance chambers are connected
to the atmosphere or a common chamber by tuned tubes, and act
as Helmholtz resonators.
At low rpm the variable-length intake runners operate in
conjunction with an initial resonance chamber. The length of the
intake runners is adjusted continually as engine speed increases.
c Dr. M. Zahurul Haq (BUET) Air Charging of Engines ME 401 (2011) 7 / 14
Mechanical Supercharging
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In mechanical supercharging, the supercharger is powered directlyby the engine, which usually drives it at a fixed transmission ratio.Mechanical or electromagnetic clutches are used for its activation.
Simple unit on ‘cold side’ of the engine & exhaust is not involved.
Responds immediately to load changes.
Directly engine driven, so causes increased fuel consumption.
c Dr. M. Zahurul Haq (BUET) Air Charging of Engines ME 401 (2011) 8 / 14
Superchargers
Superchargers are air pumps, commonly called blowers. These can
easily produce 50% more power than a normally aspirated engine of
the same size. The crack-shaft usually drives the supercharger with a
belt, but it is sometimes driven by a chain or gears.
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c Dr. M. Zahurul Haq (BUET) Air Charging of Engines ME 401 (2011) 9 / 14
Exhaust-gas Turbocharging
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In turbocharging, exhaust-driven turbine is employed to convertthe energy in the exhaust gases into mechanical energy, making itpossible for the turbocharger to compress the induction gas.
Considerable increase in specific output from a given configuration;
enhanced torque within the effective engine-speed range; significant
reduction in fuel consumption; improvement in exhaust-emissions.
Installation of turbocharger requires materials resistant to high
temperatures; space requirements for turbocharger and intercooler;
low base torque at low engine speed.
c Dr. M. Zahurul Haq (BUET) Air Charging of Engines ME 401 (2011) 10 / 14
Typical Turbocharger Configuration
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A Turbo-Compounded Engine
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With Intercooler & Waste gate
In Turbo-compounding system, a second turbine (power turbine)
in the exhaust is directly geared to the engine drive shaft which
results in higher engine power and efficiency.
Charge cooling prior to entry to the cylinder, can be used to
increase further the air or mixture density. A waste-gate allows a
portion of the engine’s exhaust gas to bypass the turbine.c Dr. M. Zahurul Haq (BUET) Air Charging of Engines ME 401 (2011) 11 / 14
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In turbocharging, the losses due to back-pressure generated in the
exhaust system are more than offset by the effect of the higher
induction pressures in reducing specific fuel consumption and
increasing power.
Turbocharger lag: Owing the inertia of the rotating assembly, it
can take as long as several seconds to respond to higher load
demand.
c Dr. M. Zahurul Haq (BUET) Air Charging of Engines ME 401 (2011) 12 / 14
Wave-Compression Devices (Comprex Supercharger)
Pressure wave superchargers make use of the fact that if two fluids
having different pressures are brought into direct contact in low narrow
channels, equalization of pressure occurs faster than mixing.
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Comprex Supercharger: (1) Engine,
(2) Cell wheel or rotor, (3) Belt drive,
(4) High-pressure Exhaust gas,
(5) High-pressure air, (6) Low-pressure
air, (7) Low-pressure exhaust gas
Because this unit does not have to com-
press the gas it absorbs no more than
between 1 and 2% of the power output
of the engine. Pressure ratios of up to
3:1 are said to be attainable. It is prin-
cipally designed for CI engines.
c Dr. M. Zahurul Haq (BUET) Air Charging of Engines ME 401 (2011) 13 / 14
During each revolution of the drum, one end of each cell in turn passes
the end of the exhaust passage 4 . This allows the exhaust gas, at the
pressure in that passage, to flow along the cell, compressing the air
that it already contains against the closed far end. Further rotation
opens a port at the latter end, which allows the air thus compressed to
flow into the inlet passage 5 , which is then closed again when the cell
passes it. Closure of the port reflects a pressure pulse back to the other
end, which is now open to the exhaust downpipe through 7 .
Consequently, the exhaust gas is discharged to atmosphere, at the same
time generating a suction wave which travels along the cell. When this
reaches the opposite end, the port to the inlet pipe 6 is open, so a
fresh charge of air is drawn into the cell, and the cycle begins again.
c Dr. M. Zahurul Haq (BUET) Air Charging of Engines ME 401 (2011) 14 / 14
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