Abstract improve the turbo efficiency. In particular, the

Abstract

 

This paper is a review of the
turbocharger lubrication system in order to further analyze the tribology and
provide solutions towards minimizing the wear and frictional effects to improve
the turbo efficiency.  In particular, the
conditions under which the lubricant runs through the turbocharger from its
entrance up to its exit from the bearing housing.
Turbochargers shaft support development using semi-floating bush bearings  has a significant impact on turbocharger
manufacturing cost, as well as on their operational features. Further benefit
may be had, when ball bearings are used (usually angular contact bearings are
used) for turbocharger shaft support as they provide:

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A)   
a reduction of the kinetic friction
coefficient

B)   
reduction of turbocharger lag during
turbocharger shaft acceleration from low to high speeds

 

The failures that occur in
turbochargers, 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 , semi
floating

 

 

 

1.0  INTRODUCTION

 

The basic
function of lubricants other than reducing friction over surfaces is cooling of
those surfaces too. However lubricating fluids work under unfavorable
conditions where there are high temperatures due to friction between moving
surfaces .Another factor that causes this rise of oil temperature is the
migration of heat from the neighboring components which operate at high
temperatures. All three components of heat transfer contribute to higher
operating temperature in the turbocharger bearing housing: heat conduction
between adjacent turbine and bearing housings, as well as compressor and
bearing housings; heat convection – directly from the hot casings onto the lubricating
oil, and radiative heat transfer primarily from the very hot exhaust turbine
housing but, also from the engine depending on the proximity of the
turbocharger installation and exhaust manifold to the main engine block. A
schematic of the primary heat transfer routes in a turbocharger are indicated
in Figure 1. The oil exposure to very high temperatures may cause deterioration
of its structure, degradation of its lubricating properties and even,
eventually, lubrication failure.

 

 The
designers of turbochargers insist on ever improved lubrication system designs,
which simultaneously provide turbocharger cooling, aiming at a more effective
lubrication and cooling process. On one hand, oil passes through the bearing
which is located next to the hot turbine housing. Because of the significant
heat levels transferred from the turbine housing into the bearing housing
(sometimes the exhaust gas temperature exceeds 900°C), oil temperature in this
section is increased significantly. On the other hand, the oil passes through
the bearing which is located next to the compressor housing. This section is
much cooler comparitively to the section next to the turbine housing. Oil
temperature is typically between 40-50 °C (depending on the environmental
conditions in which the turbocharger has to operate). For this reason the
industry is replacing mineral lubricants with synthetic lubricants because og
their higher resistance towards wear and better availability of temperature
fluctuations In spite of this, a significant number of turbochargers get
damaged and failures occur due to the lack of appropriate lubrication. The use
of improper oil type, the high temperatures that occur in the bearing housing
section next to the turbine housing and the turbocharger misuse by the operator,
are factors that contribute to the oil choking (Figure 2). As already mentioned
oil choking results in a significant amount of charred oil accumulating in the
turbocharger bearing section, where it can block the oil flow and therefore,
cause turbocharger failure due to the lack of appropriate lubricating oil mass
flow rate.

 

 

·        
THE TURBOCHARGER BEARING HOUSING

 

 

The bearing
housing is located between the turbine and compressor The bearing housing
encloses the bearings of the turbocharger shaft with the lubrication-cooling
circuit, the shaft connecting the turbine rotor and compressor impeller and the
lubricant sealing rings . Lubricating oil reaches the bearings, from the internal
cavities and engravings inside the housing manufactured for this purpose. Under
full load, lubricating oil reaches the turbocharger bearings at typical ratings
of 2 bar pressure and 1.9 l/min of volume flow rate for conventional
turbochargers in this power output range . The bearings and the shaft are
designed with adequate tolerances such that the lubricant penetrates between
the bearing and the shaft, lubricating the operating surfaces and at the same
time reducing friction, while avoiding direct and boundary contact between the
surfaces.  After lubricating the bearings, the lubricant
is gravity guided towards the engine sump, having lost all the pressure
built-up before its entrance to the body of the turbocharger and then towards
the bearings.

 

1.1  SINGLE
BUSHING BEARING

 

These
bearings are stable inside the bearing housing bore , simplifying the bearing manufacturing
and reducing cost. Lubricating oil forms an oil film between the bearing inner
surface and shaft surface, reducing the friction coefficient between the two
surfaces and thus avoiding direct-boundary contact between the surfaces (metal
to metal contact). Roller element bearings support radial shaft loads but not
thrust loads. This is the reason why a thrust bearing is used to support the thrust
loads, which is lubricated by the engine lubricating system (Figure 3).

 

               

                                                                   
Figure (3)

 

 

 

 

 

1.2  FULL
FLOATING BUSHING

 

 

Full
floating bushing bearings are not attached to the turbocharger but there is a
tolerance 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 flows
towards the inner clearance through holes drilled on the circumference of the
bearing), but also between the outer surface of the bearing and the surface of
the bearing housing bore (outer clearance). These bearings are manufactured in
such a manner that the inner tolerance (between shaft and bearing) is smaller
than the respective outer tolerance (between the bearing and bearing housing
bore). During turbocharger operation these bearings are forced by hydrodynamic
friction of the lubricant to rotate with the shaft. Rotational speed of the
bearings 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 a
given rotational speed, their speed increases proportionally until it reaches
an equilibrium point. Measurements showed that the inner oil film temperature
is higher than the outer film temperature, resulting in lubricant viscosity
alteration which limits bearing rotational speed The lubricant film between the
bearing and the bearing housing bore functions, also, as a shock absorber
(damper); the shocks coming from inaccurate shaft alignment or due to shaft
resonance. In full-floating bushing bearings, friction between the shaft and
the bearing is reduced by half, resulting in increased turbocharger life as
wear between the shaft and the bearing is reduced. Choosing the correct size of
tolerance enables optimization of hydrodynamic lubrication, as well as
optimization of anti-shock lubricant film behavior. This is the reason why the
existing tolerance between the shaft and the bearing is calculated by the
turbocharger load carrying capacity, while tolerance between the bearing and the
bearing housing bore is calculated so that the anti-shock lubricant film
properties are optimized . When
the bearing rotates at half the shaft rotational speed, the holes in the
bearing circumference, which enable lubricant flow to the inner shaft-bearing.
tolerance, are operating as a
centrifugal pump resulting in a pressure difference between the inner and the
outer clearance formed by the bearing, the shaft and the bearing housing bore,
respectively. When this pressure difference occurs, it can exceed the engine
lubrication system pressure, causing oil starvation to the inner clearance. As
a consequence, bearing and shaft wear can result very quickly due to (dry)
boundary contact between the two co-operating surfaces Additionally, another
problem appearing in these types of turbocharger is increased wear at the
bearing locations in the bearing housing due to their rotation. This problem is
partially solved by specially made cast iron cases, while the bearing housing
is made of aluminum and the bearing supports are treated with surface
hardening.

 

 

1.3  SEMI
FLOATING BUSHING

 

This most
recent turbocharger shaft support method relies on sliding bush bearings, and
is achieved by the use of semi-floating bushing bearings (Figure 5). These
bearings are attached to the bearing housing and their rotation as well as
their axial motion, are prevented by a flange which is pinned on the end of the
bearing. They are only allowed to move perpendicularly to their shaft
direction, operating as a damper. The fact that these bearings do not rotate
implies that lower oil pressure supply from the engine lubrication system is
required compared to full floating bushing bearings. Additionally, when the
turbocharger body is manufactured from aluminum, the bearing supports are not
required to be treated with surface hardening, resulting in lower manufacturing
cost.

 

 

2.0  TURBOCHARGER
FAILURES

 

 

Metallurgy
is used for the manufacturing of turbocharger parts.. Some of the quality
standards 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, and
apart 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 in
a noisy turbocharger and in most severe cases, will cause  bearing failure. If any abnormality exists
after turbocharger parts assembly or during their operation (for instance lack
of lubrication or foreign objects entering any of the housings), apart from the
eventual partial damage or total failure which can result in the turbocharger
due to poor manufacturing quality, will, also, have a detrimental effect on
turbocharger efficiency.

 

2.1 Due To Lack Of Lubrication

When the turbocharger operating conditions change (for example after an
increase in rotational speed due to a change in engine load), both the
lubrication and cooling demands change (increase) as well. In the case that a
momentary interruption of the oil flow through the turbocharger occurs,
particularly when the turbocharger operates under high load and high speed, the
consequences could be catastrophic for the turbocharger shaft, for its bearings
and generally for the whole turbocharger system. The delayed or reduced
quantity of oil flow (oil feeding lag) through the turbocharger can be
recognized by the discoloration of the shaft or the bearing surfaces at the
points of contact

 

         2.2
Due to Foreign object presence in the lubricating circuit

 

The presence
of any foreign objects present inside the lubricating circuit is dangerous for
turbocharger operation . Such particles , if drift through the pathways , can
enter or get trapped within the inside or outside clearances of the shaft
bearings . In such cases they can cause bearing and shaft damage . This
consequently causes increase in vibrations which then in turn causes reduction
in turbocharger efficiency and longetivity . In the case that particles are
large enough, they can block the internal oil passages, as well as the
circumferential holes of the bearings, causing a reduction or interruption of
the oil flow through the shaft–bearing clearances. All these can result in an
excessive increase of temperature in the bearing housing which can initially
lead to melting of the operating parts and thereafter to their welding and
eventual turbocharger failure

 

 

2.3 Due to Foreign object presence in the housings

 

As far as a compressor housing is
concerned , when a foreign object enters into it either because of the lack of air
filtering or because of the use of an inappropriate air filter while turbo
operation , it causes the compressor blades to wear out before time . If the
size of foreign object is large , it may cause bending of the blades and even
may cause fracture

 

On the turbine side, due to the extremely
high thermal stresses under which the engine operates, disintegration of small
particles from engine parts (such as fragments of the valves, the piston rings
)may occur. When these particles enter the turbine housing, they hit against
the tips

of the turbine blades, are knocked
back into the turbine housing, until their size is reduced to levels such that
they are incapable of causing further wear. This kind of wear does not usually
cause 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, turbocharger
failure occurs when one of the turbine blades breaks down and the rotor becomes
unbalanced causing in this way the bearings to wear out rather quickly

 

2.4 Due to Over Speeding

 

Engine-Turbocharger
compatibility is a really important factor . If mismatched , it can cause
catastrophic 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 result
of these conditions is the increase of rotor vibrations which result in even
greater bearing wear. Another malfunction which may occur by an unbalanced
rotor is the contact of the compressor blades with the internal surface of the
volute resulting in a reduction in turbocharger efficiency. Additionally , the
vibrations on the shaft are increased and this may lead to structural failures
and possible failure of the compressor impeller blades in addition to the total
wear of other rotating parts.

 

 

 

3.0                  
CONCLUSIONS

 

 

Turbocharger
is a rotor dynamic turbomachinery which is capable of boosting the power of a
naturally aspirated engine to higher standards , however at the cost of low
fuel efficiency and a greater environmental hazard . It comprises of two main
sections, a shaft with an overhung turbine rotor at one end and a compressor
impeller at the other end , both connected by the same shaft as if one’s
rotation causes the other to rotate also . This rotating shaft can go at very
high rpms ( usually up to 150000 rpm ) and needs a cleverly designed bearing
system and a lubricating system to support

 

The selection of bearing type is
very important and several such types are available for use. Among them full floating bushing bearings is the
type of bearing that is most effective. The reduced friction offered by these
bearings, contributes to fewer lubricant flow towards the turbocharger, as well
as to an improved mechanical efficiency of the turbocharger. In addition, the
relative ease of rotation reduces turbocharger lag leading to improved engine
performance

 

Turbocharger failures are usually
experienced due to faulty maintenance , poorly managed lubrication circuits and
no ” housekeeping ” (cleaning) of the turbo assembly at regular intervals .
Poorly selected compatibility and casually designed bearing systems also cause
failures in turbomachinery

 

 

 

 

 

REFERENCES
:

 

 

1-      Hugh 
Maclnnes,  Turbochargers,  Editor

&
Publisher Bill Fisher

 

2-      Geralis 
A.  and  Gasparakis, 
E.

Turbochargers-Superchargers

Lubrication

 

 

3-      Moran, Michael, and Howard Shapiro. Fundamentals
of Engineering Thermodynamics. 5th ed. John Wiley & Sons, 2004

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