For Combustion of a fuel, an adequate quantity of air is required. For a Turbocharger
system capacity should be sufficient to ensure that the air demand is met when the
turbocharger is not at its optimum.
In a four stroke diesel engine, this air is induced during a down stroke in one of the two
engine cycles per power stroke. The exhaust gasses are removed by the preceding
For a two stroke no such cycle for scavenging and air replenishment exists. Instead, air
under pressure is supplied at the end of the power stroke providing a new charge of air
and removing the exhaust gasses. The period allowed for scavenging is limited as the
longer the exhaust port or valve remains open so the shorter the travel of piston is
available for compression. The greater the mass of air that can be supplied, the more
efficient the scavenging process will be, and also the greater mass of air will be available
for the combustion of an equally greater mass of fuel. The mass of air is increased by
increasing the pressure at which it is supplied.
Pressure charging can be obtained by a number of means including scavenge pumps,
chain driven rotary blowers and exhaust gas driven blowers.
Exhaust gas driven blowers or Turbochargers make use of gas in the cylinder which
theoretically could be expanded further, the power that would be developed could be used
for driving an engine driven scavenge pump. In practice it is more efficient to use this
exhaust gas in the turbocharger as further expansion of the gas would require an
increased stroke. Increased stroke would mean increased engine height with problems of
crankshaft construction, cylinder lubrication and effective scavenging coming into play.
The work that could be extracted from this low pressure gas would be limited and more
efficiently extracted in a rotary machine.
Centrifugal turbochargers are generally cheaper to produce than axial flow. In addition
for smaller sized radial units the effects of blade leakage are less important They are very
common in automotive systems were lthey are suited to the manufacture of large volumes
of standard design. Axial flow may be selected even when there are centrifugal
alternatives as it is better suited to individual modifiactions and is able to operate better
on heavy fuels.
A turbocharger is made basically in two linked parts, the gas side and the air side.
The gas side is made out of cast iron, is in tow parts and is generally water cooled. The
turbine inlet casing carries the nozzle blade shroud ring and forms the bearing housing.
The turbine outlet casing forms the main part of the blower which includes the
mountings. In addition it forms a shroud for the shaft and contains bled air passageways
for supplying air to the labyrinths seals.
The air side casing is also in two parts but is made of aluminium alloy. The inlet casing
may be arranged to draw air form the engine room or from the deck , both methods via a
filter and silencer arrangement. The advantage of drawing air form outside the engine
room is that it will tend to be cooler and less humid. An advantage of drawing from the
engineroom would be simpler ducting arrangements and that the engine room tends to be
The main parts of the Compressor are the Compressor wheel (made up from a serpate
Inducer and Impeller on larger designs), the diffusor, and the air inlet and outlet casing.
With the wheel rotating a unit of air massin the compressor wheel experiencesa
circumferential velocity (v)at its distance from the wheel centreline (radius r). A radial
velocity is experienced of value v2/r which causes it to move radially outwards. The unit
of air leaves the compressor with a resultant velocity the angle of incidence of which
should, by careful design, match the inducer inlet angle. This leads to maximum
The effects of frictional losses, whether due to surface imperfections or fouling of the
compressor wheel will result in changing the angle of incidence and thus a drop in
Takes place if the air mass delivered by the blower falls at a faster rate than the air
pressure of delivery. With all blowers it is possible to produce a graph showing the effect.
Surging gives an unpleasant noise. The initial action in order to prevent a blower surging
is to reduce engine load. Blower efficiency is highest closer to the surge line and so if a
high efficiency is demanded there is little leeway against surging. In practice the fitting of
blowers is a compromise between a reasonable blower efficiency and an acceptable
degree of safeguard against surging.
Surging is a condition whereby an imbalance in demand and supply of air from the
turbocharger causes a rapid decelleration. This is accompanied by a loud barking noise
and vibration. It was not uncommon on pulse systems in heavy weather, it is less
prevalant in modern constant pressure designs but may begin due to reasons explained
The normal characteristic of a turbocharger running at constant speed is one of reducing
possible pressure ratio for increasing air flow demands. This characteristic is exagerated
when frictional losses are taken into account. As described above from maximum
efficiency the air leaving the compressor wheel should enter the inducer at an optimal
angle. Failure to do so leads to losses and a characteristic shown. It should be noted that
this shows a relationship at a specific instant of Turbocharger speed. It would be possible
to plot many lines of constant speed on the graph. The point at which surging occurs
could be plottd for each and a surge line drawn. Moving the plant operating line towards
the surge line can lead to an increase in turbocharger efficiency.
The stable operating point is at A though which passes the respective engine operating
line ( this line indicates the relationship the engine requires between Air flow and
pressure), the unstable point leading to surging is at B.
If the air flow through the turbocharger reduces The effect would be a decrease in
pressure at the receiver. However the pressure ratio of the turbocharger (running at
constant speed) would Increase. The effect of this is to return the system back to its stable
For an engine operating on the line passing through B then the effects of a reduced air
flow wil be a corresponding reduction in compressor pressure ratio. The engine however
requires increased air flow which the turbocharger cannot supply and the result is
surging. Theroretically this effect begins where the constant pressure line is flat.
Conditions leading to Surging
Turbochargers are generally specified in relation to set ambient operating conditions and
then matched to engine load requirements. Deviation away from this due to such things
as changes in ambient conditions and changes in engine speed/load relationship has to be
taken into account.
It is very unusal for a moden turbocharger to such. However surging may begin after
several years of stable operation.
Some possible reasons are
o For multi blower installations surging can occur due to a difference in
maintenance of cleaning causing one or more to operate at pressure ratio's
above its capability
o Similarly difference in blower component wear, this is particularly true for
such things as increased blade tip clearance
o change in engine speed/ load reltionship- say due to hull fouling
o cylinder power imbalance
o faulty injectors or timing
o dirty air filter
o dirty air cooler (air side)
o High air cooler cooling water temperature
o dirty turbine nozzle ring
o deposits on blades or impeller
o damage to blades
o It is also possible that components downstream from the blower exhasut
such as a fouled exhaust gas boiler can also lead to surging
The above is a very simplified description of the operation of the compressor and how
surging occurs. No doubt I wil be receiving barrages of complaints from turbocharger
specialist. In case you still cannot get it try to understand you are looking at a specific
point in time and looks only at the turbocharger running at a constant speed
This may again be thought of two parts; the gas side and shaft and the air compressor
side. They are usually made of two materials as the conditions that the wheels operate in
is very different. The advantage of making the compressor end of a lighter aluminium
alloy material rather than using the same material throughout, is that it reduces the total
mass of the rotor , is more easily cast into intricate shapes, and the rotational inertia is
Must be capable of maintaining strength at high temperatures so material is usually a
chromium steel. The rotor for a smaller blower may be a single piece forging but for a
larger blower it may consist of two separate sections of shaft and turbine wheel with
The impeller is made of an aluminium alloy and for larger compressors may have a
separate inducer section at the eye. Whatever the form of construction it must preserve
the rotor balance and that means refitting in the same position after removal from the
rotor. This is usually achieved by having one of the connection splines larger than the
The blades shown above are twisted and tapered to allow for the increased blade velocity
with increased radius Blades must be capable of withstanding the high exhaust
temperatures and also the highly corrosive environment of the exhaust gas. Stainless steel
is frequently used. They are mounted axially in the disc using inverted fir tree root or
similar e.g. 'T' piece or bulb roots. Locking strips are provided to prevent axial movement
of the blades in the disc due to the axial gas force.
The blades are not force fit into the disc but are relatively loose.
The blades are made lose fit for the following reasons;
o To allow for thermal expansion
o To prevent force fit stress adding to centrifugal stress (stops the material
o Help dampen vibrations from the gas pulses as the blades pass the nozzles
(especially when partly or wholly blocked)
For larger blades lacing wires are used as a means of dampening vibration by the friction
acting between the wire and the blade material at the hole. The wire is normally fitted
about 1/3 of the way from the tip, it may pass through all blades or batches and is
crimped to hold it in place. Dampening due to friction and stiffening up because of the
connection of a number of blades avoid vibration.
The main problem with lacing wire, usually of wrought iron, is that it breaks and sections
fall out resulting in an unbalanced rotor.
Balance of the rotor is essential in order to avoid vibration and blade damage due to
impact, corrosion, erosion and deposit build up all cause problems.
Blade Wear and its affect of blower Speed
Most main engine turbochargers are water cooled in order to keep temperatures
reasonable. On the most modern of turbochargers this cooling water has been reduced in
quantity to that is required for cooling the bearings. The space between the compressor
and turbine being filled with insulation material.
There are some smaller blower designs which by design can be cooled by air flow. As no
cooling jacket is required it is convenient do place the bearings in between the turbine
and compressor wheels. this allow for better rotor support. The larger blowers have the
bearings placed at the coolest part of the charger, at the ends of the rotor within cooling
jackets. This has the advantage of making them more readily accessible.
Plain white metal bearings may be used , these have an indefinite life but require lube
oil to be supplied at pressure. they also require a header system to supply oil in the event
of the main supply pump failure. A common system is by supplying from the main
engine lube oil system via a header system similar to that employed with steam turbines.
Plain bearing Lube oil system
Care should be taken to ensure that the bearings are adequately protected when the engine
is stopped as the blower is liable to turn due to natural draught (although modern engines
having hydraulic exhaust valve actuation are not susceptible to this as the all valves close
after a short period of inactivity). Locking the blower, isolating the blower from the
scavenge belt by use of a slide valve, putting covers over the blower suction or
continuation of supply of lube oil after engine stoppage may be used.
Ball or Roller bearings require elasto-hydrodynamic lubrication and may be supplied by
means of a shaft driven gear pump from an integral sump. The gear pump is operated by
rotation of the rotator. The bearing housing as a cooling water jacket.
ball and roller bearings have a definite life and must be changed on a running hours
bases, typically every 15,000 Hrs. This means that they should be placed in a readily
accessible position. The transmission of vibration is dampened out by the use of radial
and axial springs between the bearing carrier and the casing.. These can consist of leaf
springs wrapped around the bearing and fitted at the bearing ends.
An axial thrust is generated by the passage of the exhaust gas over the turbine . This must
be balanced out . For turbochargers fitted with plain bearings a double-sided thrust is
fitted at both ends. This takes the form of a collar on the rotor acting on white metalled
'Mitchell' type segments. Double-sided thrusts are fitted to locate the turbine during
rolling and pitching. Generous oil quantities are supplied to bearings in order to allow for
cooling as well as lubrication
These are provided at each ends of the rotor and between the turbine and compressor and
serve to prevent the passage of exhaust gas and also to prevent oil laden air being drawn
into the eye of the impeller from the bearing. Oil seals in the from of thrower plates are
also fitted at the bearings to prevent the passage of oil along the shaft.
Labyrinth seals consist of projections on the rotor which almost touch the casing.
Principle of the Labyrinth Gland
The leakage of steam is reduced by the use of labyrinths, these provide a torturous path
for the gas to follow to exit the turbine reducing the pressure across a series of fine
Within the cavity where the flow is turbulent, the velocity of the gas is increased with an
associated drop in pressure. The kinetic energy is the dissipated by the change in
direction, turbulence and eddy currents.
Air is bled from the compressor end into the middle of the Turbine glands, this air
expands in both directions and provides a very effective seal. The flow of air in the centre
gland also aids cooling and minimises the heat transmission form the turbine wheel.
Care must be taken to ensure that deposits do nit build up in the seals otherwise its
effectiveness is lost. Also there is a possibility of 'rub' occurring
Timing of scavenging on ported liner on two stroke slow speed
The scavenge and exhaust period can be divided into three periods starting as the piston
travels down the cylinder and uncovers first the exhaust port followed by the scavenge air
1, Blowdown-Exhaust port is open and cylinder pressure falls to or below the scavenge
2, Scavenge- Incoming air forces the exhaust gasses and any unburned fuel out
3, Post scavenge- Exhaust only is open, some air is lost during this period. For ported
exhausts this is unavoidable due to design of the liner with the exhaust ports above the
scavenge. Some loss of compression therefore occurs, on the Sulzer RD an attempt was
made to limit the blowdown by the use of a rotary exhaust valve. This proved very
unreliable and was omitted on the later RND.
Modern Slow Speeds make use of exhaust valves, and with the most modern the exhaust
valve timing is variable dependant on load and to some point fuel type.
Blower corrosion can take place on the gas, water or air sides. As most water cooled
blowers make use o the engine cooling systems the same problems and solutions exist as
in the jacket water system. In general with modern systems there are few problems if
treatment quality and quantity is maintained. On the gas side deposits depend upon the
quality of fuel and combustion. Carbon from poor combustion, sulphur products from the
fuel, Vanadium Pentoxide from the fuel and Calcium Sulphate from the alkaline additives
in cylinder oil all result in deposits and/or corrosion. Correct attention to operating
conditions and matching of cylinder oil alkalinity to sulphur content will minimise the
problem. Pitting corrosion and scale formation will lead to imbalance. On the air side
there is a lesser risk but pitting oxidation of aluminium can take place in the prescience of
salt spray. If air is taken from the deck there is greater risk than if it is drawn from the
engine room because the oil mist in the engine room causes a protective film to form on
the aluminium surface. Regular cleaning of parts is essential to maintain efficiency,
minimise corrosion and ensure balance.
Out of service cleaning is relatively straight forward but requires the blower to be
stripped down and time might not allow that. Light deposits on the air side may be easily
wiped away, but gas side deposits require the rotor and nozzle blades to be 'boiled' for
about 12 hours in clean water or water containing chemical; care must be taken in
handling chemicals and 'special shipboard mixtures' should be avoided as they can be
highly corrosive resulting in damage tot he rotor. In service cleaning provides an
For the airside this usually consists of injecting a limited quantity of water into the eye of
the impeller, the water droplets then wipe the oily film from the surface but often deposits
this on the cooler from where it must also be removed. If heavy deposits do form on the
impeller and volute and then the risk of surging will increase. The usual in service
cleaning method for blower gas side employs water but it is also possible to make use of
ground rice or walnut shells, Whichever method is used care must be exercised.
In service water washing of the gas side requires the blower speed to be reduced to half
or below ( 3000 rpm for a medium sized slow speed ), in order to avoid impact damage
by the water droplets
The casing drain must be open and known to be clear. Water is injected via an air
atomiser nozzle into the gas flow. The flow rate is controlled by means of a pressure
gauge and orifice plate. The basic principle is that the water droplets impinging on the
blades has a shot blasting effect. Observation of the water flowing from the drains will
indicate when sufficient water has been injected. On completion the blower speed should
be increased gradually to prevent thermal shocking, ensure all the water in the gas side
casing is removed , and to prevent damage due to any unbalance caused by partially
The injection of nutshells and rice can take place at full load.
Modern trends in Turbocharger design for Large slow speeds
With the search for ever increasing plant efficiency and power/size ratios, greater
demands are made of the Turbocharger. Some manufactures have answered this by the
use of totally water free blowers , these are fitted with plain bearings and supplied from
the main engine lubrication system.
Running the aluminium alloy impeller above the aging temperature (190-200oC)
threatens a reduction in material strength. This temperature can easily be reached at
pressure ratio is of 3.7 and above depending on suction air temperatures.Progressive
creep deformation can occur above 160oC requiring carefull consideration of stress on the
blades. ABB turbos have available an aluminium compressor with a pressure ratio of 4.6
units new designs.
For higher pressure ratios stainless steel or Titanium is used where pressure ratios of up
to 5.2 have been possible.
A typical modern design has plain bearings supplied by oil from the main lubrication
systesm or from a dedicated external system. The casing is entirely uncooled relying
instead on the lubrication oil to be splashed around the generously sized bearing space to
cool the areas adjacent to the bearings
Vairable geometry nozzle rings are available which adjust balde angles depending on
The blades are high chord (thick section) meaning that lacing wires can be omitted.
Special attention has to be made on the shaft fitt arangementalloyed aluminium
compressor wheel as the rotational speeds of 500m/s create high centrifugal stresses. The
number of blades in the volute is matched to the number of blades on the compressor to
The thrust bearing which is subjected to high loading is mounted outside the radial
bearing on the compressor end fo ease of maintenance