MICE I - Module 2 Notes.pptx

Scavenging
• Scavenging is the process whereby air at a
pressure greater than that of atmospheric
pressure is used to push the exhaust gas out of
the cylinder of an engine.
• Unlike the 4 stroke engine, a two stroke diesel
engine does not use the piston to push out the
exhaust gas, instead, air enters the cylinder
around bottom dead center and sweeps or
scavenges the exhaust gas from the cylinder.

Scavenging
• Removal of exhaust gas from the cylinder after
combustion and its replenishment with air for
subsequent combustion.
• Efficient scavenging is necessary for good
combustion
• The passage of scavenge air will also assist
cooling of piston and cylinder

Scavenging
• Two-stroke engines rely upon a charge of
scavenge air under slight pressure sweeping
through the cylinder and expelling the exhaust
gas in front of it
• This process takes place while both scavenge
and exhaust connections are open and the
piston is near the bottom of the cylinder
• Hence a very short period of time is available
for scavenging to take place.

Scavenging
• Air must be supplied at a higher pressure than that in
the exhaust manifold using any of the below means:
- Reciprocating scavenge pumps
- Rotary blowers – electric or engine driven
- Turbochargers
Scavenge air enters through ports near the bottom of the
cylinder liner when these are uncovered by the piston
moving down to the BDC until the piston again covers
the ports during its upward stroke.
The directional flow of air within the cylinder is decided
by the engine design and the exhaust arrangements.

Scavenging
• A scavenging system must fulfill the following
requirements:
- The exhaust ports / valve must open earlier to
provide a lead to the exhaust. During this
period the pressure in the cylinder falls below
the scavenge pressure.
- The scavenge ports must be closed after the
process of gas exchange is completed
- Loss of fresh air charge escaping through the
exhaust ports should be minimised

Scavenging
A scavenging process may be considered to take
place in a number of stages.
1. First period or the blow down period:
- This period begins at the moment the exhaust
ports are uncovered.
- The exhaust gases are blown down in the exhaust
manifold where a lower pressure exists
- The gas is compressed at the vicinity of exhaust
ports inside the cylinder leaving a rarefied area
immediately behind it. The pressure drops below
the scavenge air box pressure.

Scavenging
2. Second period or the scavenging period proper.
- The period begins after the scavenge ports are
opened.
- Scavenge air enters the main cylinder, sweeps the
residual combustion gases out of the cylinder and
charges the cylinder with fresh air.
- The mass of air drawn in the cylinder depends on
the difference in pressure between scavenge
trunk and exhaust system.

Scavenging
3. Third period
- A further period in which an effort is made to
contain the air taken in the cylinder already.
The process of charging the cylinder is
associated with a degree of inter-mixing with
exhaust gases which affects the purity of charge
and increases the charge temperature.

Scavenging
There are two basic methods of scavenging:
1. Uniflow or through-scavenge - air passes straight up
through the length of the cylinder forcing the exhaust
through ports or valves at the top of the cylinder.
2. Reverse flow scavenging - in which air passes over the
piston crown and rises to form a loop within the cylinder,
expelling gas through exhaust ports.
Depending on the relative positions of exhaust and air ports,
the reversed flow system are divided into:
1. Full loop scavenging with exhaust on top of air ports at
the same side of the engine
2. Cross scavenging with scavenge ports facing the exhaust
ports

Uniflow Scavenging
- Air flows in streams with slight induced rotational motion.
- The charge is not allowed to change direction and hence
intermixing is minimum.
- Due to absence of eddies and turbulence it is easier to push the
products of combustion without mixing and short circuiting.
- Scavenge efficiency is the highest.
- The system is particularly suitable in slow speed engines with long
stroke and large area of escape for exhaust gases.
- Uniflow scavenging is achieved by:
1. By two pistons working in one cylinder as in opposed piston
engine
2. By a poppet valve which provides large area for escape of exhaust
gases so that the desired pressure drop in the cylinder is achieved
without turbulence at exhaust
3. By an exhaust piston controlling the exhaust ports, while the air
inlet ports are controlled by the power piston.

Reversed flow Scavenging
- The engines employing a reversed flow system
of scavenging are structurally simpler.
- Owing to the absence of cams, valves and
valve gear, engines are simple and sturdy.
- Cylinder head can better withstand the
thermal stresses.

Reversed flow Scavenging
The disadvantages with the system are:
- The piston skirt has to be much longer than that for a
uniflow scavenged engine. This is because the skirt has
to seal the scavenge and exhaust ports when the piston
is at TDC.
- There is a greater possibility of intermixing air and
exhaust. As a result the purity of charge is less and the
charge temperature is higher
- A sharp temperature difference exists within a small
area around the scavenge and exhaust ports. Hence
the possibility of thermal cracks at the bars and the
chance of thermal distortion of the liner are greater.
- The exhaust back pressure may rise due to narrowing
of exhaust passage by deposit of unburnt carbon.
- The piston rings will wear out unevenly

Cross Scavenging
Disadvantages:
- The scavenging air is not able to get rid of the
layer of exhaust gas near the wall resulting in
poor scavenging
- Some of the fresh charge also goes directly
into the exhaust port resulting in poor bmep.

Loop Scavenging
- Loop scavenging avoids the short-circuiting of
the cross scavenged engine and thus improves
the scavenging efficiency.

Scavenging pumps
Since the pumping action is not carried out by the
piston of a two-stroke engine, a separate pumping
mechanism, called the scavenging pump, is
required to supply scavenging air to the cylinder.
Types of scavenging pumps range from crankcase
compression, piston type blowers to roots blower.
Careful selection of scavenging pump is important
since the design of a two-stroke engine is affected
by the type of scavenging pump used.

Crankcase scavenging
• Crankcase is used for compressing the incoming air and
then transferring it to the cylinder through a transfer port.
• Cheap in initial cost
• Uneconomical and inefficient in operation
• Amount of air which can be used for scavenging is less than
the swept volume of the cylinder due to low volumetric
efficiency of the crankcase which contains a large dead
space.
• Since the delivery ratio is less it is not possible to scavenge
the cylinder completely of the products of combustion and
some residual gases always remain in the cylinder resulting
in low mep.
• Also oil vapours from the crankcase mixes with the
scavenging air resulting in high oil consumption.

Piston, Roots and Centrifugal Blowers
• Piston type blowers are used only for low
speed engines.
• Roots blower is preferred for small and
medium output engines
• Centrifugal blower is preferred for large and
high output engines

Piston, Roots and Centrifugal Blowers

Exhaust gas turbocharging system

Compression Ratio (CR)
• Thermal efficiency of a Diesel engine increases
with increase in CR. But a very high CR gives a
very high peak pressure and temperature. The
crankshaft and other components are to be
designed to withstand the peak load. Hence too
high CR would involve higher weight and cost of
engine.
• The upper limit of CR is therefore fixed by the
strength of the cylinder, the bearings and other
parts whose stresses are determined by peak
mechanical and thermal loading.

Compression Ratio (CR)
• Increase in CR in the lower range gives a
proportionate gain in thermal efficiency. But in
the higher range the gain becomes
progressively less.
• Higher CR helps easy starting from cold
condition.

CR and Supercharging
• Mep can be increased by burning more fuel,
but this increases the fuel consumption and
maximum pressure.
• Maximum pressure rise is a major factor in the
mechanical design of the engine.
• Hence if it is desired not to increase the
maximum pressure, a further boost of the
mep is possible by reducing the CR with
simultaneous increase in the charge air
pressure at the intake. This principle is
adopted in high pressure turbo-charged
engines.

Supercharging
• A supercharger is an air compressor that increases the pressure or density
of air supplied to an internal combustion engine. This gives each intake
cycle of the engine more oxygen, letting it burn more fuel and do more
work, thus increasing power.
• Power can be increased by increasing the compression ratio which
increases the maximum cylinder pressure. But with super charging, the
rate of increase in max pressure is less than the rate of increase of brake
mean effective pressure. The rate of increase of max temperature is also
low in case of supercharging.
• Power for the supercharger can be provided mechanically by means of a
belt, gear, shaft, or chain connected to the engine's crankshaft. When
power is provided by a turbine powered by exhaust gas, a supercharger is
known as a turbosupercharger – typically referred to simply as a
turbocharger.

Supercharging
• A naturally aspirated engine draws air of the same
density as the ambient atmosphere.
• Since this air density determines the maximum weight
of fuel that can be effectively burned per working
stroke in the cylinder, it also determines the maximum
power that can be developed by the engine.
• Increasing the density of the charge air by applying a
suitable compressor between the air intake and the
cylinder increases the weight of air induced per
working stroke, thereby allowing a greater weight of
fuel to be burned with a consequent rise in specific
power output.

Supercharging
• Supercharging is meant for increase in output by
burning more fuel, it has no relation with thermal
efficiency. However a slight gain in thermal efficiency
results because of better combustion of fuel.
• The power is a function of average pressure but the
engine dimensions are ascertained on the basis of
maximum pressure. Supercharging increases the
average pressure without appreciable increasing the
maximum pressure.
• Thus it is a means of improving power without
increasing the weight of the engine. Hence
supercharging improves the power to weight ratio of
the engine.

Supercharging
• The mass of fuel required to be injected for
generation of a larger power has to be more than
that compared to a non-supercharged engine.
• Supercharged engines must have a faster rate of
heat release within almost the same period of
injection for better thermal efficiency. Hence
supercharging is associated with high maximum
pressure also.
• But the rate of increase in max pressure is much
less than the rate of increase in power output.

Supercharging
• The power expended in driving the
compressor has an important influence on the
operating efficiency of the engine.
• It is relatively uneconomical to drive the
compressor direct from the engine by chain or
gear drive because some of the additional
power is thereby absorbed and there is an
increase in specific fuel consumption for the
extra power obtained.

Supercharging
• About 35 per cent of the total heat energy in the
fuel is wasted to the exhaust gases, so by using
the energy in these gases to drive the compressor
an increase in power is obtained in proportion to
the increase in the charge air density.
• The turbocharger comprises a gas turbine driven
by the engine exhaust gases mounted on the
same spindle as a blower, with the power
generated in the turbine equal to that required
by the compressor.

Supercharging
Advantages of exhaust gas turbo-blower system:
• A substantial increase in engine power output for
any stated size and piston speed, or conversely a
substantial reduction in engine dimensions and
weight for any stated horsepower.
• An appreciable reduction in the specific fuel
consumption rate at all engine loads.
• Increased reliability and reduced maintenance
costs

Pulse System
• A certain mass of a gas at a higher pressure
and temperature can be made to work by
expanding to a lower pressure and
temperature.
• The temperature and pressure that prevails in
a cylinder at the point of exhaust will be 500
to 600 deg C and 3.5 to 4 bar

Pulse System
• The expansion that takes place in the diesel
cylinder is limited.
• Complete expansion of the gases from the
ignition pressure to the atmospheric pressure
cannot be achieved.
• Pulse system aims at further expansion of gas
beyond that had taken place in the engine
cylinder.
• The loss of work in the cylinder due to
incomplete expansion appears as work in the
turbine and thus input to the compressor shaft.

Pulse System
• The first phase of exhausting is a blowdown
process.
• The exhaust ports act as nozzles which
produce a high velocity stream of gas down to
the exhaust pipe. The pipe constructed in
small diameter, is quickly pressurised to form
a pressure pulse. The pulsating pressure wave
reaches up to the nozzle of the turbine where
further expansion of the gas takes place.

Pulse System
• When the turbine works in this manner by
expanding the unutilised part of expansion
taking place in the nozzles and blades of the
turbine it is called pulse system of
turbocharging.

Pulse System
• The speed of the compressor changes according
to the exhaust temperature and pressure and the
demand of the engine.
• It supplies air in a matching relationship
automatically starting from part load to full load
operation. It also responds well to quick load
fluctuation.
• It is necessary for a multi-cylinder engine to
arrange the exhaust pipes in divided ducts up to
the turbocharger.
• The turbocharger may have gas inlet at two to
four points, each point supplying one segment
containing a number of nozzles.

Pulse System
The design of exhaust ducting should meet the
following requirements:
- To preserve the kinetic energy of blowdown by
interposing resistance in the line as the gas is
taken through narrow, short and straight pipe
lines up to the entry of the turbine
- To prevent the interference in the scavenging of
one by the exhausting of the other
- To distribute energy of the exhaust gas equally
amongst the number of turbochargers
- To tune the exhaust system so that the manifold
pressure pulsation is not reflected back to the
engine

Pulse System
Exhaust grouping:
If the exhaust period of one cylinder overlaps with the
scavenge of the other, the exhaust pressure from the
cylinder which is exhausting interferes with the
scavenging of the other cylinder.
In order to avoid occurrence of such combinations in
multi cylinder two or four stroke cycle engines, the
cylinders are selectively grouped with connections to two
or more exhaust pipes.
The pipes are arranged in smaller diameters to preserve
the pressure pulse due to blow down and in short straight
lengths to prevent any loss of energy.

Pulse System
Exhaust grouping:
• A three-cylinder four stroke cycle engine will
have the firing sequence separated by 2400.
• Hence a four stroke supercharged engine
having number of cylinders four and above
will require exhaust grouping.

Pulse System
• Makes full use of the higher pressure and temperature
of the exhaust gas during the blow down period
• While rapidly opening the exhaust valves, exhaust gas
leave the cylinder at high velocity as pressure energy is
converted into kinetic energy to create the pressure
wave or pulse in exhaust
• These pressure waves or pulses are lead directly to the
turbocharger
• Exhaust pipe, so constructed in small diameter, is
quickly pressurized and boosted up to form pressure
pulse or wave
• Pressure waves reach the turbine nozzles and further
expansion takes place.

Pulse System
Turbocharger Arrangement in Pulse System:
• Interference exists between exhausting and
scavenging among cylinders
• To prevent this, cylinders are grouped relatively
with connections to two or more exhaust pipes
• Pipes are arranged, in small diameter to boost up
pressure pulse and in short, straight length to
prevent energy loss
• Number of exhaust branch depends upon firing
order, number of cylinders and turbocharger
design

Pulse System
Advantages:
• At low load and low speed it is more efficient
(Still efficient when Bmep is < 8 bar)
• No need of assistance with scavenge pump and
blower at any load change.
• It is highly responsive to change in engine
condition giving good performance of all speed
of engines.
• High available energy at turbine
• Good turbocharger acceleration

Pulse System
Disadvantages:
• The exhaust grouping is complicated.
• Different sizes of exhaust pipes are needed for
spare.
• High pressure exhaust from one cylinder
would pass back into another cylinder during
the low pressure scavenging period thus
adversely effecting the combustion efficiency.

Constant Pressure System
• In this system the admission of gas to turbine
blades is at constant velocity distributed
uniformly over the entire blade area
• All cylinders exhaust into a pipe of large
diameter which is common to all cylinders.
• The pressure pulses are first damped out by
expanding the gas in this chamber which is
then maintained at a constant pressure.
• The exhaust manifold acts as a reservoir and
supply the turbocharger at a steady pressure
through one entry point.

• In the constant pressure system no attempt is
made to recover work due to unutilised
expansion in the cylinder.
• The exhaust is allowed to throttle from the
cylinder through the exhaust valve (or port)
without doing any work into the exhaust
manifold which is of larger diameter.
• The work transfer takes place solely by virtue of
enthalpy drop as the exhaust gases expand
through the nozzles and over the blades of the
turbine.
• The turbine operation is more efficient.

• As the diesel cylinder is uprated, the mep is
increased and the enthalpy content of the
exhaust gas is considerably more. But the
pulse system fails to boost up the turbine to
supply the additional air now required.
• As an improvement over pulse system, the
constant pressure turbocharging has been
developed and successfully employed.

• Exhaust gas from all cylinders into a common
large manifold where pulse energy is
largely dissipated.
• The gas flow will steady rather
than intermittent and at a constant pressure at
turbine inlet.
Turbocharger Arrangement in Constant Pressure
System:
• No exhaust grouping
• Exhaust gases enter into large common manifold
and then to turbine
• Firing order not considered

Disadvantages:
• For successful operation of constant pressure
system, there must always be a higher pressure in
the compressor outlet than the exhaust pipe
after the cylinder. This condition is difficult to
achieve at part load range of operation or during
acceleration period, when the energy level in the
exhaust gas through put to the turbine is low.
• The resultant delay in turbo-blower acceleration,
or deceleration, results in poor combustion
during transition periods.

Advantages:
- Better and more rational use of exhaust heat
- A steady pressure before the turbine and hence
an efficient turbine operation
- The work transfer at the turbine wheel is smooth
- The compressor capacity can be increased as
more energy is available for utilisation according
to power
- Since pressure pulses are not required, expansion
in the cylinder can be carried longer in the stroke
to a lower temperature and pressure, thus
attaining a gain in output.

Advantages:
- A reduction in specific fuel consumption due
to better scavenging
- Since exhaust grouping is not necessary, the
exhaust piping is made simpler.
- The lack of restriction on exhaust pipe length
permits greater flexibility in positioning the
turbo-blower relative to the engine.
- Typical positions are at either or both ends of
the engine, at one side above the air manifold
or on a flat adjacent to the engine.

Turbocharging in Four stroke engine
• Exhaust gas turbocharged single-acting four-
stroke marine engines can deliver three times
or more power than naturally aspirated
engines of the same speed and dimensions.
• At one time almost all four-stroke engines
operated on the pulse system, though
constant pressure turbocharging has since
become more common as it provides greater
fuel economy while considerably simplifying
the arrangement of exhaust piping.

• In matching the turboblower to the engine, a
free air quantity in excess of the swept volume
is required to allow for the increased density
of the charge air and to provide sufficient air
for through-scavenging of the cylinders after
combustion.
• Modern engines carry bmeps up to 28 bar in
some cases, requiring greater proportions of
excess air which is made possible by the latest
design of turbocharger with pressure ratios as
high as 5:1.

• Optimum values of power output and specific fuel
consumption can be achieved only by utilization of the
high energy engine exhaust pulses.
• The engine exhaust system should be so designed that
it is impossible for gases from one cylinder to
contaminate the charge air in another cylinder, either
by blowing back through the exhaust valve or by
interfering with the discharge of gases from the
cylinder.
• During the period of valve overlap it follows that the
exhaust pressure must be less than air charging
pressure to ensure effective scavenging of the cylinder
to remove residual gases and cooling purposes.

Turbocharging in Two stroke engine
• Compared with four-stroke engines, the
application of pressure charging to two-stroke
engines is more complicated because, until a
certain level of speed and power is reached, the
turbo-blower is not self-supporting.
• At low engine loads there is insufficient energy in
the exhaust gases to drive the turbo-blower at
the speed required for the necessary air-mass
flow.
• In addition, the small piston movement during
the through scavenge period does nothing to
assist the flow of air, as in the four-stroke engine.

Turbocharging in Two stroke engine
• Accordingly, starting is made very difficult and
off-load running can be very inefficient; below
certain loads it may even be impossible.
• A solution was found by having mechanical
scavenge pumps driven from the engine
arranged to operate in series with the turbo-
blowers.
• Standard on modern engines, however, are
electrically driven auxiliary blowers.

Supercharging of SI engines
• Supercharging increases the tendency to knock
and pre-ignite.
• As the flame front propagates from spark plug
towards BDC , the flame front exerts pressure on
the end gases at bottom and this pressure if
sustains for time equal to the ignition lag of end
gases and temperature rises above self-ignition
temp of fuel, then end gases would ignite before
the flame reaches there and another flame front
propagates in the direction towards TDC and
when it will collide with original flame front,
shock wave will generate which leads to
enormous pressure rise .

• This means a reduction in the ignition delay is
a favorable condition for knocking.
• Supercharging will cause increased intake
pressure and temperature which will reduce
the ignition delay, thus increasing the
tendency to knock.

• Hence the supercharged SI engines employ lower
compression ratios.
• This along with increased heat losses result in
lower thermal efficiency.
• Thus supercharged SI engines have a greater fuel
consumption than naturally aspirated engines.
• Injection of water into the combustion chamber
and intercooling of charge air before feeding to
the engine are the methods used to overcome
the issues.

• Because of poor fuel economy supercharging
is used only when a large amount of power is
needed or when more power is needed to
compensate altitude loss.

Supercharging in CI engines
• Detonation occurs because of increased
ignition delay - if ignition delay is more then
more fuel will accumulate till ignition starts
hence ignition of more fuel instantaneously
leads to pressure rise called detonation.
• Supercharging will cause increased intake
pressure and temperature which will reduce
the ignition delay thus reducing the tendency
for detonation.

Supercharging in CI engines
• The increase in pressure and temperature of
intake air reduces ignition delay and hence the
rate of pressure rise during combustion,
resulting in better and smoother combustion.
This allows a poor quality fuel to be used.
• The increase in intake air temperature reduces
volumetric and thermal efficiency but the
increase in density compensates for this.

Charge air cooling
• The increased weight or density of air
introduced into the cylinder by pressure
charging enables a greater weight of fuel to be
burned, and this in turn brings about an
increase in power output.
• The increase in air density is fractionally offset
by the increase of air temperature resulting
from adiabatic compression in the turbo-
blower, the amount of which is dependent on
compressor efficiency.

Charge air cooling
• For a charge air pressure of, say, 0.35 bar, the
temperature rise is of the order of 330C—
equivalent to a 10% reduction in charge air
density.
• For a charge air pressure of 0.7 bar, the
temperature rise is some 600C, which is
equivalent to a reduction of 17% in the charge
air density.
• Much of this potential loss can be recovered
by the use of charge air coolers.

Charge air cooling
Charge air cooling has a double effect on engine
performance -
• By increasing the charge air density it thereby
increases the weight of air flowing into the
cylinders, and by lowering the air temperature it
reduces the exhaust temperature and the engine
thermal loading. The increased power is obtained
without loss, and with an improvement in fuel
economy.
It is important that charge air coolers should be
designed for low pressure drop on the air side;
otherwise, to obtain the required air pressure the
turbo-blower speed must be increased.

Charge air cooling
• A typical construction has cooling surface that
consists of two banks of rolled-in aluminium-
brass finned tubes which are expanded in
tube plates.
• The top tube plate is firmly held while the
bottom plate can slide to take up expansion of
tubes.
• Air is passed over the fins and cooled down.
• Cooling water passes through the tubes in two
straight passes.

Charge air cooling
• The air must not be cooled below the dew
point of the vapour at the condition of
pressure, temperature and humidity existing
in the charge air pipe, since this will cause
condensation.

Radial flow turbocharger
• The diesel engine exhaust gases enter through the water-
cooled gas inlet casing (50), expand in the nozzle ring (30) and
supply energy to the shaft (20) by flowing through the blading
(21).
• The gases exhaust to the open air through the gas outlet
casing (60), which is also water cooled, and the exhaust
piping.
• The charge air enters the compressor through an inlet stub
(82) and the silencer filter (80).
• It is then compressed in the inducer and the impeller (25),
flows through the diffuser (28) and is fed to the cooler via the
pressure stubs on the compressor casing (74).

• Air and gas spaces are separated by the heat
insulating bulkhead (70).
• In order to prevent exhaust gases from flowing
into the balance channel (Z) and the turbine
side reservoir, barrier air is fed from the
compressor to the turbine rotor labyrinth seal
via channel X.

• The rotor (20) has easily accessible bearings (32, 38) at
both ends, which are supported in the casing with
vibration damping spring elements.
• Either roller or plain bearings are used but for the most
common construction using roller bearings a closed
loop lubrication system with an oil pump directly driven
from the rotor is used (47, 48).
• The bearing covers are each fitted with an oil filter, an
oil drain opening and an oil gauge glass.
• On models with plain bearings, where the quantity of
oil required is large, these are fed from the main engine
lubricating oil system.

• A turbocharger consists of two sections:
- Compressor
- Turbine

Compressor
A centrifugal compressor which has a large
capacity to handle air and is simple in
construction is the best choice where a high
mass flow with low delivery pressure is
demanded.
It consists of a rotor or impeller containing radial
vanes mounted on a shaft and enclosed in a
casing with fine clearances.

Compressor
• A single-stage centrifugal compressor will have
the following divisions:
- an intake system
- the impeller channel
- a diffusion system

Intake system
• The intake system consists of an intake
silencer, guide-ways and impeller intake guide
vanes or inducers.
• The function of the system is to admit air to
the hub of the impeller without shock and
friction.

Impeller channel
• The impeller channel begins from the center
of the hub and extends radially outwards to
the tip.
• Air is given a centrifugal force so that it leaves
the impeller vane at a high velocity.
• The displacement of air creates a suction
inducting more air through the inducer.

Impeller channel
• The impeller is usually made by forging of light
alloy of aluminium-silicon.
• The physical properties of such material are
lightness, strength together with toughness,
and its capacity for a smooth surface finish.

Diffusion system
• Comprises of a stationary vaned diffuser and a
vaneless space of gradually increasing area.
• Diffuser is the name given to the fixed vanes
surrounding the impeller.
• The high velocity of air leaving the impeller tip enters
the fixed diverging vanes of the diffuser.
• The air stream is slowed down in the diffusion
process where some of the kinetic energy is
converted to a pressure head.
• By the diffusion process the air is slowed down to
intake velocity with a rise in pressure.
• The process involves compression and consequently
a rise in temperature.

Turbine
• The turbine is driven by the engine exhaust gas,
which enters via the gas inlet casing.
• The gas expands through a nozzle ring where the
pressure energy of the gas is converted to kinetic
energy.
• This high velocity gas is directed onto the turbine
blades where it drives the turbine wheel, and
thus the compressor at high speeds (10 -15000
rpm).
• The exhaust gas then passes through the outlet
casing to the exhaust uptakes.

Turbine
• The nozzle ring is where the energy in the
exhaust gas is converted into kinetic energy.
• It is fabricated from a creep resistant chromium
nickel alloy, heat resisting moly-chrome nickel
steel or a nimonic alloy which will withstand the
high temperatures and be resistant to corrosion.
• Nimonic alloys typically
consist of more than 50% nickel
and 20% chromium with additives
such as titanium and aluminium.

Turbine
• Turbine blades are usually a nickel chrome alloy or a
nimonic material (a nickel alloy containing chrome,
titanium, aluminium, molybdenum and tungsten) which
has good resistance to creep, fatigue and corrosion.
• Blade roots are of fir tree shape which give positive fixing
and minimum stress concentration at the conjunction of
root and blade.
• The root is usually a slack fit to allow for differential
expansion of the rotor and blade and to assist damping
vibration.
• On small turbochargers and the latest designs of modern
turbochargers the blades are a tight fit in the wheel.

Turbine
• Lacing wire is used to dampen vibration, which
can be a problem.
• The wire passes through holes in the blades and
damps the vibration due to friction between the
wire and blade.
• It is not fixed to each individual blade.
• The wire can pass through all the blades, crimped
between individual blades to keep it located, or it
can be fitted in shorter sections, fixed at one end,
joining groups of about six blades.

Turbine
• A problem with lacing wire is that it can be
damaged by foreign matter, it can be
subjected to corrosion, and can accelerate
fouling by products of combustion when
burning residual fuels.
• Failure of blading due to cracks emanating
from lacing wire holes can also be a problem.
• All the above can cause imbalance of the
rotor.

Turbine
• The turbine casing is of cast iron.
• Some casings are water cooled which complicates the
casting.
• Water cooled casings are necessary for turbochargers
with ball and roller bearings with their own integral LO
supply (to keep the LO cool).
• Modern turbochargers with externally lubricated
journal bearings have uncooled casings.
• This leads to greater overall efficiency as less heat
energy is rejected to cooling water and is available for
the exhaust gas boiler.

Turbine
• Labyrinth seals or glands are fitted to the shaft and
casing to prevent the leakage of exhaust gas into the
turbine end bearing.
• To assist in the sealing effect, air from the compressor
volute casing is led into a space within the gland.
• A vent to atmosphere at the end of the labyrinth gives
a guide to the efficiency of the turbine end gland.
• Discoloring of the oil on a rotor fitted with a roller
bearing will also indicate a failure in the turbine end
gland.

Turbine
• A labyrinth arrangement is also fitted to the
back of the compressor impeller to restrict the
leakage of air to the gas side.

Turbocharger
NOZZLE RING
TURBINE BLADING

Turbocharger
• The horizontal rotor is constructed in parts and made
hollow out of high alloy nickel chromium steel.
• The shaft carries a single stage axial gas turbine wheel at
one end and a radial single stage compressor at the other
end.
• The shaft rests on sleeve bearings.
• At the compressor end there is a thrust bearing for
keeping the rotor assembly in its true axial alignment
while it is free to expand at the other end.
• The rotor shaft is provided with shrunk on bushes for
bearings and shrunk on labyrinth seal.
• The labyrinth seals are supplied with sealing air taken from
the scroll housing through outside tubes, which are easy
to inspect and clean.

Turbocharger
• The casing is divided to form the turbine and the
compressor housings at two ends.
• The subdivision wall is provided with water
cooling arrangement.
• The air is admitted axially through the impeller
and passes radially outwards to the tips.
• The blower side is equipped with a silencer.

Turbocharger
• The turbine end housing contains one or two gas
inlets. A grid is provided before the inlet to arrest
any large piece of metal from entering the turbine
and causing damage to the blades.
• The turbine bearings are lubricated from an
overhead tank with a constant static pressure head
above the bearings.
• This ensures the lubrication is continued unaffected
in the event of lubricating oil pump failure.
• The overflow sight glass provides means for
inspection of the oil quantity and flow at all times.

Compressor Characteristics
• The relationship between speed of rotation,
mass flow and pressure ratio is termed as
compressor characteristics.

• A typical characteristic curve for one particular
speed is shown here.

• At point C the valve is fully open, hence no
obstruction for the flow to move ahead.
• The static pressure developed is negligible but
the velocity is maximum and the shaft power is
high.
• As the resistance to flow is increased the mass
flow is reduced causing the pressure to rise.
• A characteristic curve is obtained when the
pressure ratio is plotted for each position of valve
opening.

• The pressure ratio attains a maximum value at B.
• If the flow is still reduced the pressure suddenly
drops below the delivery pressure initiating
surging or pumping of air back to the compressor.
• Surging is defined as excessive aerodynamic
pulsation in the air stream.
• When the pressure ratio drops the air will flow
back or surge to the compressor due to higher
pressure downstream.
• The next moment the compressor regains its
pressure ratio and delivers.

• Due to this characteristic of compressor at low
mass flow rate, it cannot be worked at point left
of maximum pressure ratio.
• A compressor is most efficient within a narrow
range on the right of the operating point B.
• At every speed the compressor will have a
different characteristic curve.
• A family of such curves will show the
characteristic of the compressor over a certain
operating speed range.

• That portion of the curve which remains on
the left of the maximum pressure line is
inoperable due to surge.
• The line A
joining
these points
is called
the surge limit.

Blower Surging
• Too low an air-mass flow at a given speed, or pressure ratio, will
cause the blower to surge, while too high a mass flow causes the
blower to choke, resulting in loss of pressure ratio and efficiency at
a given speed.
• The blower impeller, as it rotates, accelerates the air flow through
the impeller, and the air leaves the blower with a velocity that is
convertible into a pressure at the diffuser.
• If, for any reason, the rate of air flow decreases, then its velocity at
the blower discharge will also decrease; thus there will come a time
when the air pressure that has been generated in the turbo-blower
will fall below the delivery pressure.
• There will then occur a sudden breakdown of air delivery, followed
immediately by a backward wave of air through the blower which
will continue until the delivery resistance has decreased sufficiently
for air discharge to be resumed.
• ‘Surging’ is the periodic breakdown of air delivery.

Blower surging
• In the lower speed ranges surging is
manifested variously as humming, snorting
and howling.
• If its incidence is limited to spells of short
duration it may be harmless and bearable. In
the higher speed ranges, however, prolonged
surging may cause damage to the blower, as
well as being most annoying to engine room
personnel.

Problems associated with T/C
Turbine blading:
- Vibration of blades due to unbalance of the
rotor as a result of deposits or fouling.
- Vibration of blades due to the pulsating flow
of exhaust gases admitted partially.
- Fatigue failure may occur at the blade root.
- Deposits on the blade surface

Nozzle Ring:
- Deposits from engine exhaust may clog the
nozzles partially
- Nozzles of small turbochargers with four
stroke engines are narrow. Further narrowing
will affect the performance causing high back
pressure, loss of power, increase in thermal
loading and frequent turbocharger surging.

Bearings:
- Erosion of the bearing balls or rollers caused
by impurities present in lubricating oil may
reduce their service life.
- Heavy momentary loading due to vibration,
bearing running dry or by contaminated oil,
overheating, etc can also cause damage to the
bearings.

Turbine casing:
- Thinning of the wall due to corrosion on the
exhaust side
- Loss of metal due to erosion by particles of
impurities in the gas
- Thermal cracks due to rapid load changes or
due to cooling system failure
- Thinning of wall due to corrosion in the
cooling water space

Maintenance on turbocharger
• Regular checking of oil level in the bearing sump
• Changing of oil every 500 to 1000 running hours
• Cleaning of air filter every 1000 running hours
• Water washing of the turbine side every 48
running hours (A/Es) / leaving a port (M/E)
• Dry / grit cleaning of the turbine side daily
• Water washing of blower side every 24 / 48
running hrs
• Renewal of bearings
• Cleaning of cooling water chamber

Water washing of the turbine side
• Accumulation of dirt on turbine will cause imbalance
and lead to higher stresses of the bearings.
• The water must be injected into the exhaust system
ahead of the protecting grids with the engine running
at low power.
• The amount of water to be used depends upon the
volume of gas and its temperature.
• The flow rate must be so that 50% to 70% of the water
is evaporated and escapes through the exhaust, while
the remaining water is drained through the tap in the
exhaust casing.

• The cleaning effect is based on the water
solubility of the deposits, as well as on the
mechanical effect of the striking water
droplets.

Cleaning procedure:
- Reduce power until the turbocharger runs at the
speed recommended by the maker
- The temperature of the gas entering the turbine
must be less than 300deg C
- Open drain valve and ensure it is clear
- Supply water at the recommended pressure and
flow rate
- Check water draining through the drain. Cleaning
can be stopped when the water becomes clear.
- Continue running for 3 more minutes to ensure
all parts are dry

MICE I - Module 2 Notes.pptx

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MICE I - Module 2 Notes.pptx