Labyrinth seals are fitted just inboard of the bearings to seal the shaft against air leakage. To assist this seal and to help cool the shaft, particularly at the turbine end, the labyrinth is supplied with pressure air bled from the compressor discharge volume.
The compressor impeller has its leading edges machined into blades which induce a flow onto the main compressor radial vanes. The steepness and overall width of these vanes dictates the compression ratio and capacity respectively. Modern, superbly machined rotors are capable of providing compression ratios (delivery pressure/inlet pressure) of 4:1 or more. Such a value is adequate for the foreseeable future so ‘two stage’ turbocharging, with all its associated complexities, is not essential.
The impeller discharges high velocity air into the ‘diffuser’ ring, where the kinetic energy is converted into pressure energy. Diffuser rings are important components within the pressure development section of the turbocharger. Simply by changing one diffuser ring for another with different characteristics, the output pressure form a turbocharger can be altered to suit a particular engine so the diffuser is one of the simplest ways of matching the output pressure from a turbocharger to that required by a particular engine. Such matching can therefore be achieved without recourse to changing the shape or size of impeller or indeed the frame size of a given turbocharger. There are limits as to how much the diffuserring can vary the turbocharger performance, but its capabilities should be borne in mind should any trouble be experienced with continuous surging or lack of scavenge efficiency.
The annular space between the impeller tips and the diffuser wing will be pressurised from the impeller discharge itself, so the air will attempt to ‘leak’ down the back plate of the impeller to gain access to the exhaust gas passages. To limit this, rather than stop it, a series of concentric grooves are machined into the back of the impeller forming a labyrinth seal which regulates the leakage to an acceptable level.
The air passing down the back of the impeller is usually guided by a sleeve through which the shaft runs to the turbine disc. Thus, not only does the air seal the passage of exhaust gas, but the leakage also cools the disc. The disc forged into, or bolted onto, the shaft carries the turbine blanding, which is usually fitted into the disc using the inverted ‘fir tree’ root method. This method of fitting allows the roots of the blades to expand into the disc (small clearances below the fir tree ‘branches’), whilst still resisting the enormous centrifugal forces to which the blades are subjected.
The blades should be very slightly loose in their housing so that there are no residual stresses imposed on the root by fitting. Any such stresses imposed on the root by fitting. Any such stress could cause early failure of the blade since the stresses created whilst running, including the thermal stress, are very substantial. Thermal stress develops as a result of the alternate flow over the blades of hot exhaust gas and then cool scavenge air. The same gases could contain corrosive elements as well as abrasive or scale forming substances. To reduce as far as possible the development of scale or deposits of any type, ‘water washing’ of the blades is frequently adopted. This process, injecting water into the gas flow just prior to the nozzle ring, is an attempt to dissolve the scale and at the same time clean the nozzles and blades by the impact effect of the water droplets. Such a procedure should be carried out to the guidelines set down by the manufacturer, the frequency of washing being dictated by the quality of the fuel and the effectiveness of combustion. Above all it should be appreciated that water washing should not be applied to a turbocharger whose previous history is unknown. It is possible, in such a case, that the removal of the soluble deposits could lead to the rotor being thrown out of balance. Thus water washing should be established from the outset, from new or after each overhaul, and the frequency between washings maintained, so that the chances of an unbalanced situation developing are diminished. Water washing is not an alternative to stripping the unit down for manual cleaning; it simply means that the operating efficiency of the unit can be maintained at a higher level between scheduled overhauls. It is interesting to note that some units use nut shells (graded, and toasted) for this purpose, their impingement effect taking off more than just the water soluble compounds.
The effectiveness of the above can be monitored from readings of pressures and temperatures taken throughout the system, both before and after cleaning. With clean fuels and good combustion the cleaning process is not as important, as the blades and nozzles will not foul up so quickly. The blades, made from highly corrosion resistant materials, are usually of the ‘taper twisted’ type. The taper reduces the mass of material towards the tips and in so doing reduces the stress on the blade root. This is subjected to high centrifugal forces as well as bending, the bending being occasioned by the variations in gas pressures flowing over the blades. To withstand these, the bottom of the blade is substantially radiussed into the root. To further support the blades, against both bending and vibration, they may be held together by a lacing wire or wires. This wire is not secured to each blade, but is threaded through neat fitting holes so that the blade warms up into its ‘desired’ operating position, the wire, through expansion and centrifugal force, locks onto the blade.
The blades give each other mutual support and in so doing dampen down the vibrational tendencies. The ‘twist’ of the blade is intended to give it a better chance of matching to the gas inlet velocity and direction. The velocity from the nozzle will be fairy uniform across the radial depth of the nozzle, but the linear speed of the blade increases with radius so that the tips are travelling at a greater speed than the roots. The twist therefore helps to match the blade inlet angle to the relative velocity between gas and blade. This aids smooth gas entry to the blades with consequent improvement in their operating efficiency.
The casings of turbochargers are made to match their service demands. They are of aluminium alloy, lightweight and corrosion resistant at air side, with cast iron at the higher temperature and water cooled turbine end.
Non-water cooled units have recently appeared. Higher thermal efficiencies of constant pressure charged engines have resulted in less heat being released from the engines to the turbochargers. In addition, the removal of heat from the turbocharger reduces its thermal efficiency and thus the heat available to the ‘waste heat’ boilers. The non-water cooled unit counteracts these and eliminates the corrosion problems of the inside surfaces of some of the water cooled units. Corrosion occurred where the circulating water cooled the plating down to such an extent that the internal surfaces were being cooled to below their dew point, with the resultant acidic attack.
Another development, concurrent with the new wave of non-water cooled units, is the location of the support bearing between the compressor and turbine wheels. This is a better position for talking the rotor weight than the extremities of the shaft, where there is the potential roar shaft whirl. The original reason for locating the bearings at the ends of the shaft was accessibility; much reduced in the new location. However improvements in design have improved accessibility again and, at the same time, extended the time periods required for the overhaul of such bearings.