Abstract This paper is a review of theturbocharger lubrication system in order to further analyze the tribology andprovide solutions towards minimizing the wear and frictional effects to improvethe turbo efficiency. In particular, theconditions under which the lubricant runs through the turbocharger from itsentrance up to its exit from the bearing housing.
Turbochargers shaft support development using semi-floating bush bearings has a significant impact on turbochargermanufacturing cost, as well as on their operational features. Further benefitmay be had, when ball bearings are used (usually angular contact bearings areused) for turbocharger shaft support as they provide: A) a reduction of the kinetic frictioncoefficientB) reduction of turbocharger lag duringturbocharger shaft acceleration from low to high speeds The failures that occur inturbochargers, their majority ( up to 50%) are caused by faulty lubrication .Other reasons for failures in turbochargers account for human errors (misuse)and external factors (foreign objects disturbing any component) Keywords : Turbocharger , lubrication , bearing , floating , semifloating 1.0 INTRODUCTION The basicfunction of lubricants other than reducing friction over surfaces is cooling ofthose surfaces too. However lubricating fluids work under unfavorableconditions where there are high temperatures due to friction between movingsurfaces .Another factor that causes this rise of oil temperature is themigration of heat from the neighboring components which operate at hightemperatures. All three components of heat transfer contribute to higheroperating temperature in the turbocharger bearing housing: heat conductionbetween adjacent turbine and bearing housings, as well as compressor andbearing housings; heat convection – directly from the hot casings onto the lubricatingoil, and radiative heat transfer primarily from the very hot exhaust turbinehousing but, also from the engine depending on the proximity of theturbocharger installation and exhaust manifold to the main engine block.
Aschematic of the primary heat transfer routes in a turbocharger are indicatedin Figure 1. The oil exposure to very high temperatures may cause deteriorationof its structure, degradation of its lubricating properties and even,eventually, lubrication failure. Thedesigners of turbochargers insist on ever improved lubrication system designs,which simultaneously provide turbocharger cooling, aiming at a more effectivelubrication and cooling process. On one hand, oil passes through the bearingwhich is located next to the hot turbine housing.
Because of the significantheat levels transferred from the turbine housing into the bearing housing(sometimes the exhaust gas temperature exceeds 900°C), oil temperature in thissection is increased significantly. On the other hand, the oil passes throughthe bearing which is located next to the compressor housing. This section ismuch cooler comparitively to the section next to the turbine housing. Oiltemperature is typically between 40-50 °C (depending on the environmentalconditions in which the turbocharger has to operate).
For this reason theindustry is replacing mineral lubricants with synthetic lubricants because ogtheir higher resistance towards wear and better availability of temperaturefluctuations In spite of this, a significant number of turbochargers getdamaged and failures occur due to the lack of appropriate lubrication. The useof improper oil type, the high temperatures that occur in the bearing housingsection next to the turbine housing and the turbocharger misuse by the operator,are factors that contribute to the oil choking (Figure 2). As already mentionedoil choking results in a significant amount of charred oil accumulating in theturbocharger bearing section, where it can block the oil flow and therefore,cause turbocharger failure due to the lack of appropriate lubricating oil massflow rate. · THE TURBOCHARGER BEARING HOUSING The bearinghousing is located between the turbine and compressor The bearing housingencloses the bearings of the turbocharger shaft with the lubrication-coolingcircuit, the shaft connecting the turbine rotor and compressor impeller and thelubricant sealing rings . Lubricating oil reaches the bearings, from the internalcavities and engravings inside the housing manufactured for this purpose. Underfull load, lubricating oil reaches the turbocharger bearings at typical ratingsof 2 bar pressure and 1.
9 l/min of volume flow rate for conventionalturbochargers in this power output range . The bearings and the shaft aredesigned with adequate tolerances such that the lubricant penetrates betweenthe bearing and the shaft, lubricating the operating surfaces and at the sametime reducing friction, while avoiding direct and boundary contact between thesurfaces. After lubricating the bearings, the lubricantis gravity guided towards the engine sump, having lost all the pressurebuilt-up before its entrance to the body of the turbocharger and then towardsthe bearings. 1.
1 SINGLEBUSHING BEARING Thesebearings are stable inside the bearing housing bore , simplifying the bearing manufacturingand reducing cost. Lubricating oil forms an oil film between the bearing innersurface and shaft surface, reducing the friction coefficient between the twosurfaces and thus avoiding direct-boundary contact between the surfaces (metalto metal contact). Roller element bearings support radial shaft loads but notthrust loads. This is the reason why a thrust bearing is used to support the thrustloads, which is lubricated by the engine lubricating system (Figure 3).
Figure (3) 1.2 FULLFLOATING BUSHING Fullfloating bushing bearings are not attached to the turbocharger but there is atolerance between the bearing and the bearing housing bore surface (Figure 4).In between these two surfaces an oil film is provided by the oil supply system.
The lubricant circulates between the shaft and the bearing (lubricant flowstowards the inner clearance through holes drilled on the circumference of thebearing), but also between the outer surface of the bearing and the surface ofthe bearing housing bore (outer clearance). These bearings are manufactured insuch a manner that the inner tolerance (between shaft and bearing) is smallerthan the respective outer tolerance (between the bearing and bearing housingbore). During turbocharger operation these bearings are forced by hydrodynamicfriction of the lubricant to rotate with the shaft. Rotational speed of thebearings is determined by inner and outer lubricant film resistance forces,acting on the bearing. In theory, bearing rotational speed can reach up to 50 %of the shaft rotational speed.
In reality, this speed is smaller, because for agiven rotational speed, their speed increases proportionally until it reachesan equilibrium point. Measurements showed that the inner oil film temperatureis higher than the outer film temperature, resulting in lubricant viscosityalteration which limits bearing rotational speed The lubricant film between thebearing and the bearing housing bore functions, also, as a shock absorber(damper); the shocks coming from inaccurate shaft alignment or due to shaftresonance. In full-floating bushing bearings, friction between the shaft andthe bearing is reduced by half, resulting in increased turbocharger life aswear between the shaft and the bearing is reduced. Choosing the correct size oftolerance enables optimization of hydrodynamic lubrication, as well asoptimization of anti-shock lubricant film behavior. This is the reason why theexisting tolerance between the shaft and the bearing is calculated by theturbocharger load carrying capacity, while tolerance between the bearing and thebearing housing bore is calculated so that the anti-shock lubricant filmproperties are optimized . Whenthe bearing rotates at half the shaft rotational speed, the holes in thebearing circumference, which enable lubricant flow to the inner shaft-bearing.
tolerance, are operating as acentrifugal pump resulting in a pressure difference between the inner and theouter clearance formed by the bearing, the shaft and the bearing housing bore,respectively. When this pressure difference occurs, it can exceed the enginelubrication system pressure, causing oil starvation to the inner clearance. Asa consequence, bearing and shaft wear can result very quickly due to (dry)boundary contact between the two co-operating surfaces Additionally, anotherproblem appearing in these types of turbocharger is increased wear at thebearing locations in the bearing housing due to their rotation. This problem ispartially solved by specially made cast iron cases, while the bearing housingis made of aluminum and the bearing supports are treated with surfacehardening. 1.3 SEMIFLOATING BUSHING This mostrecent turbocharger shaft support method relies on sliding bush bearings, andis achieved by the use of semi-floating bushing bearings (Figure 5). Thesebearings are attached to the bearing housing and their rotation as well astheir axial motion, are prevented by a flange which is pinned on the end of thebearing. They are only allowed to move perpendicularly to their shaftdirection, operating as a damper.
The fact that these bearings do not rotateimplies that lower oil pressure supply from the engine lubrication system isrequired compared to full floating bushing bearings. Additionally, when theturbocharger body is manufactured from aluminum, the bearing supports are notrequired to be treated with surface hardening, resulting in lower manufacturingcost. 2.
0 TURBOCHARGERFAILURES Metallurgyis used for the manufacturing of turbocharger parts.. Some of the qualitystandards that must be met are related to the dimensioning of their parts(dimensional tolerance control) as well as to their operational behavior.
Turbocharger parts have very small tolerance ratios after their assembly, andapart from individual alignment they must, also, be aligned as an assembly(rotor system balancing). If there are any alignment or balancing imperfections existent in the shaft assembly,vibrations will result occur turbocharger operation at high speed, resulting ina noisy turbocharger and in most severe cases, will cause bearing failure. If any abnormality existsafter turbocharger parts assembly or during their operation (for instance lackof lubrication or foreign objects entering any of the housings), apart from theeventual partial damage or total failure which can result in the turbochargerdue to poor manufacturing quality, will, also, have a detrimental effect onturbocharger efficiency. 2.
1 Due To Lack Of LubricationWhen the turbocharger operating conditions change (for example after anincrease in rotational speed due to a change in engine load), both thelubrication and cooling demands change (increase) as well. In the case that amomentary interruption of the oil flow through the turbocharger occurs,particularly when the turbocharger operates under high load and high speed, theconsequences could be catastrophic for the turbocharger shaft, for its bearingsand generally for the whole turbocharger system. The delayed or reducedquantity of oil flow (oil feeding lag) through the turbocharger can berecognized by the discoloration of the shaft or the bearing surfaces at thepoints of contact 2.
2Due to Foreign object presence in the lubricating circuit The presenceof any foreign objects present inside the lubricating circuit is dangerous forturbocharger operation . Such particles , if drift through the pathways , canenter or get trapped within the inside or outside clearances of the shaftbearings . In such cases they can cause bearing and shaft damage . Thisconsequently causes increase in vibrations which then in turn causes reductionin turbocharger efficiency and longetivity . In the case that particles arelarge enough, they can block the internal oil passages, as well as thecircumferential holes of the bearings, causing a reduction or interruption ofthe oil flow through the shaft–bearing clearances. All these can result in anexcessive increase of temperature in the bearing housing which can initiallylead to melting of the operating parts and thereafter to their welding andeventual turbocharger failure 2.3 Due to Foreign object presence in the housings As far as a compressor housing isconcerned , when a foreign object enters into it either because of the lack of airfiltering or because of the use of an inappropriate air filter while turbooperation , it causes the compressor blades to wear out before time .
If thesize of foreign object is large , it may cause bending of the blades and evenmay cause fracture On the turbine side, due to the extremelyhigh thermal stresses under which the engine operates, disintegration of smallparticles from engine parts (such as fragments of the valves, the piston rings)may occur. When these particles enter the turbine housing, they hit againstthe tips of the turbine blades, are knockedback into the turbine housing, until their size is reduced to levels such thatthey are incapable of causing further wear. This kind of wear does not usuallycause the turbocharger rotor to lose its alignment and balancing qualities(except for the case where these particles cause a structural blade failure),and it will continue to operate for many hours. In this case, turbochargerfailure occurs when one of the turbine blades breaks down and the rotor becomesunbalanced causing in this way the bearings to wear out rather quickly 2.
4 Due to Over Speeding Engine-Turbochargercompatibility is a really important factor . If mismatched , it can causecatastrophic effects which are caused due to over speeding of the turbocharger. This usually occurs while overcoming turbo lag . 2.5 Due to Misalignment in impeller-shaft assembly Alignment defects of the rotor,cause an increase in the centrifugal force applied to the rotating parts.
The resultof these conditions is the increase of rotor vibrations which result in evengreater bearing wear. Another malfunction which may occur by an unbalancedrotor is the contact of the compressor blades with the internal surface of thevolute resulting in a reduction in turbocharger efficiency. Additionally , thevibrations on the shaft are increased and this may lead to structural failuresand possible failure of the compressor impeller blades in addition to the totalwear of other rotating parts. 3.0 CONCLUSIONS Turbochargeris a rotor dynamic turbomachinery which is capable of boosting the power of anaturally aspirated engine to higher standards , however at the cost of lowfuel efficiency and a greater environmental hazard . It comprises of two mainsections, a shaft with an overhung turbine rotor at one end and a compressorimpeller at the other end , both connected by the same shaft as if one’srotation causes the other to rotate also . This rotating shaft can go at veryhigh rpms ( usually up to 150000 rpm ) and needs a cleverly designed bearingsystem and a lubricating system to support The selection of bearing type isvery important and several such types are available for use.
Among them full floating bushing bearings is thetype of bearing that is most effective. The reduced friction offered by thesebearings, contributes to fewer lubricant flow towards the turbocharger, as wellas to an improved mechanical efficiency of the turbocharger. In addition, therelative ease of rotation reduces turbocharger lag leading to improved engineperformance Turbocharger failures are usuallyexperienced due to faulty maintenance , poorly managed lubrication circuits andno ” housekeeping ” (cleaning) of the turbo assembly at regular intervals .Poorly selected compatibility and casually designed bearing systems also causefailures in turbomachinery REFERENCES: 1- Hugh Maclnnes, Turbochargers, Editor Bill Fisher 2- Geralis A. and Gasparakis, E. Turbochargers-SuperchargersLubrication 3- Moran, Michael, and Howard Shapiro.
Fundamentalsof Engineering Thermodynamics. 5th ed. John Wiley & Sons, 2004