MODELLING AND TESTING OF DYNAMIC PROPERTIES OF C0-45 TURBOCHARGERS

The paper presents modelling process and results of the bench tests of the C0-45, which is a turbocharger for heavy-duty diesel engines. For turbochargers like this, it is necessary to measure the level of vibration. This paper presents the test results for 115 turbochargers. Rotor speed during the tests varied between 25,000 and 42,000 rpm. Rotor speed during the tests on a C0-45 turbocharger varied within three ranges, i.e., Range I: n=25,00032,000 rpm and a=0.5g; Range II: n=33,00038,000 rpm and a=1.0g; Range III: n=39,00042,000 rpm and a=1.5g; (g=9.81m/s). Measurement of vibrations was realized by a sensor located on the body of the turbocharger. Bench tests also included measurement of key charge parameters such as the amount of mass trapped, mean flow velocities, turbulence level, gas pressure, temperature and oil flow. The results are presented in the form of diagrams showing the amplitude of the acceleration function with regard to the rotor’s RPM. Research shows that using the correct parameters in construction and technological processes of assembly are very important for the effective functioning turbochargers.


INTRODUCTION
Turbochargers are mainly used to convert exhaust gases energy into compressed air energy.The air is pumped into a fuel supply system in a combustion engine.
The basic operating parameters of a turbocharger are: compression ratio, airflow, gas temperature, a maximum rotating speed and a level of vibration.The operational accuracy of a rotating set is tested with the use of an engine test stand.

TURBOCHARGER TEST STANDS
Each turbocharger is tested with the use of an engine test stand (Fig. 1).This facility checks the assembly quality and cooperating parts.Operational characteristics and the measurement of vibrations are also determined.
The main elements of an engine test stand are: -turbocharger mounting brackets -combustion chamber with a fuel system -installation of air -lubrication system -measuring and control system -exhaust system Measurement of vibrations is performed by a sensor (Fig. 1), which estimates the level of acceleration of a rotating unit system in a turbocharger.The sensor is located on the body of the turbocharger.
The level of vibrations is a basic requirement that guarantees a proper operation and the usage of a turbocharger.Measurement is performed during a set motion.Rotor speed for 19. a C0-45 turbocharger varies between 25,000 and 42,000 rpm.Commissioning requirements determine the values of maximal acceptable accelerations, depending on the rotating speed of a turbocharger rotor.
Factory acceptance tests have demonstrated that a large number of turbochargers, even when featuring suitable parameters (consumption, compression ratio), is not able to work.
During acceptance testing of 115 turbochargers, it was found that measured accelerations varied between 0.01 and 10.0 g.It should be noted that maximal acceptable accelerations can reach up to 1.5 g.
The reason why turbochargers are not used concerns excessive measured values of acceptable accelerations.Having tested all turbochargers, 33% of them do not meet the requirement concerning boundary vibration values (Fig. 2).

Fig. 2. Percentage of turbochargers according to vibration range level
Turbochargers that meet commissioning requirements can be divided into two groups.The first group includes turbochargers that work in upper ranges of vibrations (0.8g<adop≤1.5g) and the second group includes turbochargers that work in lower ranges (adop≤0.8g).The diagram (Fig. 3) describes vibrations in an rpm function for the first group of turbochargers, whereas the second group is presented in Fig. 4. Having examined vibration measurements for the first group in the range 0≤nw≤42,000 rpm, we can note two resonant zones.The first zone appears during the rotating speed nw=32,000 rpm, while the second zone appears close to a speed equal to nw=38,000 rpm.Ranges of resonant zones agree with the operating ranges of turbochargers.
The second group of turbochargers features small vibrations values.In this case, there is only one resonant speed, which appears at higher rotational speeds, i.e., nw=32,000÷37,000 rpm.

ANALYSIS OF TEST STAND RESULTS
Taking into account the testing results, we can see that the right choice of construction and technological parameters of a rotating set can guarantee the correct operating parameters of turbochargers.
Turbochargers with excessive acceptable vibrations were disassembled in order to find reasons for this situation.Then, rotating units were balanced again.The assembled turbochargers were tested once more.

21.
It was not always possible to achieve the right operating parameters of a turbocharger during another assembly because of vibrations level.Therefore, these actions were repeated to achieve the proper operating parameters.

THEORETICAL MODEL OF A ROTATING SET IN A TURBOCHARGER
To assure the correct work of a turbocharger, it is important to find sources of excessive vibrations and minimize them.Theoretical bases for rotor construction and explanations for vibrations in rotating sets are discussed in [1,4,7].
Test stand studies have shown that the main cause of rotational motion disturbances in a rotating set within a turbocharger is centrifugal fictitious forces, which appear because the axis of rotation does not agree with the main axis of a rotating set.These different axis locations are caused by the imbalance in rotating masses.The disequilibrium allows centrifugal fictitious forces to appear.These forces constitute outer driving forces and could lead to resonance.
It should also be mentioned that gravitational forces, which act upon rotating parts, can be the source of vibrations.
Another cause of vibrations can be related to medium resistance in the area where rotors are working.
The bearing mounting plays an essential part in rotor operations.Slide bearings with an embedded rotor could be the reason for hydrodynamic forces, which in turn contribute to selfactivating vibrations in the oil layer [2,3,4,5,6,8].In the discussed turbocharger, a rotating unit is supported in hydrodynamic bearings with a floating ring bearing.
The following reasons for vibrations have been accepted by the author for the purposes of a design model: -imbalance in turbocharger rotors -the mass of parts in a rotating unit -floating ring bearing masses -radial clearance in a lateral slide bearing with floating ring.
To explain these reasons shown above, the author has designed a dynamic model for the rotating unit of a turbocharger (Fig. 5).Rotors in a turbine and on the compressor side have different masses and different unbalance values.In this model, each bearing has been modelled, including a floating ring bearing mass.Moreover, attention has been paid to rigidity and damping factors, as well as both oil films' ability concerning vibration damping.The motion of the model is examined in two planes, i.e., 0XZ and 0YZ.The rigidity factors (cx, cy) and damping factors (dx, dy) are treated as coupled values.
where matrixes take the form:

CONCLUSION
Table 2 shows that the influence of structural parameters on static and dynamic characteristics can be, on the one hand, contradictory (e.g., the feed oil temperature and the minimal height of a lubricant gap) or, on the other hand, compatible (e.g., the feed oil temperature and the oil film maximum temperature).
The unbalance is an influence factor on displacement values (Table 3).In Task 1, the unbalance was Nw1=0.18•10 - [N•s 2 ]; however, in Task 2, it was Nw1=0,88•10 -4 [N•s 2 ].Having compared these two tasks, it can be seen that in the Knot 4 displacements, the amplitude increases 11 times for the coordinate y4.The smaller values of displacements appear in Task 1.

Fig. 4 .
Fig. 4. Model of a rotating set Values describing the work of a rotating set are shown in Tab.1.The influence of structural parameters on static and dynamic characteristics is described in Tab. 2, while Table3presents published results of the influence of unbalance on displacement amplitude in knots of rotating sets.
3The influence of unbalance on displacement amplitude in knots of rotating sets