Air Charging of Engines - Bangladesh University of...

Air Charging of Engines

Dr. M. Zahurul Haq

ProfessorDepartment of Mechanical Engineering

Bangladesh University of Engineering & Technology (BUET)Dhaka-1000, Bangladesh

[email protected]://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


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.


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.


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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.


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.


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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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.


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.


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.


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.


Air Charging of Engines - Bangladesh University of...

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