Construction
Axial
Centrifugal
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.
Compressor
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 slightly pressurised.
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 compressor efficiency.
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 efficiency
Surging
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 later.
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 point A.
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
- 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
- Similarly difference in blower component wear, this is particularly true for such things as increased blade tip clearance
- change in engine speed/ load reltionship- say due to hull fouling
- cylinder power imbalance
- faulty injectors or timing
- dirty air filter
- dirty air cooler (air side)
- High air cooler cooling water temperature
- dirty turbine nozzle ring
- deposits on blades or impeller
- damage to blades
- It is also possible that components downstream from the blower exhasut such as a fouled exhaust gas boiler can also lead to surging
Rotor
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 reduced.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 bolted connection.
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 others.
Blades
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.
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The blades are made lose fit for the following reasons;
- To allow for thermal expansion
- To prevent force fit stress adding to centrifugal stress (stops the material 'yielding')
- 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
Bearings
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.
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
Labyrinth seals consist of projections on the rotor which almost touch the casing.
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
1, Blowdown-Exhaust port is open and cylinder pressure falls to or below the scavenge pressurePlain bearing Lube oil system
Labyrinth Seals
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.
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 clearances 
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 port.
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.
