MODERN LANDINGS OF EFFICIENCY OF AIR COMPRESSORS PISTON RING - CYLINDER TIGHTENING

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1 MODERN LNDNGS OF EFFCENCY OF R COMPRESSORS PSTON RNG - CYLNDER TGHTENNG FLTCENU Constantin, TURCNU Ludmila, BOLOG Octavian, MNE Mitica, "DUNRE DE JOS" UNVERSTY, GLT, ROMN S u m m a r y The paper advances a methodology o approaching the tribological processes related to the compressor piston tightening system, based on the tribomodeling by similitude which is an analytical experimental research method the associated outits or the experimental research and some results regarding tribological parameters and the inluence o these on reliability prediction. Keywords: piston ring, tightening, reliability prediction 1. NTRODUCTON The piston ring-cylinder has a large area o spreading in many engineering ields. One o these is the piston air compressors. This system has a very important role in the case a compressor, to play assuring the energy conversion eiciency. The eiciency o this assembly is based on tightening assuring or a long time o his lie with minimal energy consumption. n act, the eiciency and reliability o this tribosystem are deining or the energy consumption characteristic o the compressor. 2. GENERLS The literature provides data regarding the tribology o this tribosystem and ways or optimizing and durability increasing, bat applicability is limited by the narrow experimental domain perspective o the phenomenon that occur. n this way, the research program developed by scientists rom, Dunarea de Jos University o Galati, has pursued the ollowing objectives. Determination o the gas leaing-low trough scaling by theoretical and practical methods as well. Designing o a physical model, which replicate the tribologycal processes o the tribosystems. Elaboration o an prognosis algorithm or the reliability o the system trough optimal tribological al thermodynamical datas. n the paper some aspects regarding the research techniques testing rigs and some inal results are presented. Considering the eects o the gas low loss by the lac o proper sealing o the compressor inside [2] it is convenient to diminish such losses (the ideal case would be to eliminate them but this is impossible). One o the discontinuity zone o the compressor inside is the piston sealing system where tightness is assured by the tight contact between the sealing elements-piston segments and the two alternative translation moving parts between which there are the piston and the cylinder jacet. s regards the sliding contact between the piston ring and the cylinder jacet, this is tighter th nternational Tribology Conerence, Otober 2003., Belgrade, Serbia

2 when the radial contact pressure between the two parts is higher. This, however, increase the riction between the two parts resulting in a number o undesired eect such as wear, higher temperature in the area, higher mechanical energy consumption to overcome the riction. ny approach to the optimization o the piston sealing system should consider: provision o the admissible degree o tightness to obtain some better indices o perormance o the thermodynamic process and as low as possible consummations to overcome riction in the contact zone. s the two objectives have an opposite character they should be investigated separately and rom the result interpretations. 3. THEORETCL ND EXPERMENTL METHODOLOGY Due to the large number o actor aecting the piston cylinder assembly [2], it was ound convenient to employ the analyticalexperimental method o tribo-modeling by similarity. The relations between the physical parameters on with the tribological processes depend: the contact orce N[N], the riction orce F[N], the relative velocity w [m/s], the shape actor K[m -1 ], the mechanical wor consumed or removing the material volume by wear [J/m 3 ], the ultrasound pulse coeicient α[db/m], the oil low Q[m 3 /s], time τ[s], volumetric wear intensity v[m 3 /m], the unction: F( N, F, w, K,, α, Q, τ, v ) = ct. (1) have all been expressed by means o the dimensional analysis by V.. Voitov [5] as a criterion equation (2): Π Π F Π τ = ct. (2) where: Π is the invariant o the volumetric use intensity: v Q K α = N w Π (3) Π F is the invariant o the riction orce: F K α F = N w Q Π (4) Π τ, is the time invariant: 7 / 3 τ N w α Π τ = (5) Q K ccording to the undamental properties o the similitude and modeling, each invariant should tae this value both on the model and on the ull scale: Π ( m) = Π ( n); Π F ( m) = Π F ( n); (6) relations rom which the parameters values are determined v, F and τ, or the ull scale tribosystem depending on the values o the same parameters on the model and the scales o the physical parameters determined: Vn Vn = = Vm Vm.. K K α N α N w w Q Q (7) 7 / 3 N w α τn = τm. N w Q where i represents the scale o the physical parameter. The research team used or the irst time a combination o Voitov's relationships or modeling using the computer assisting technique. This because much dierence is observed i is not considerable the similarity laws. For completing and checing the analytical results o mathematical model based on the similarity theory, it is necessary the experimental research results. 4. RESULTS ND DSCUSSONS n order to determine the tribological parameters or the piston-cylinder system a tribological research stand was designed by the research team, based on a sliding riction tribomodel. The scheme o this stand is illustrated in Figure.1. 8 th nternational Tribology Conerence, Otober 2003., Belgrade, Serbia 301

3 On the bedplate the ollowing are assembled/dismounted: the electrical engine 2, the shat 3, the upper plate 11. The moving triboelement, the dis 6, receives rotation movement rom the engine across a speed reduction gear. The driver when o the assembly being part o the piece where the moving dis is rotating around the shat 3. The samples 7 are leaning against the mobile dis; they stand or the ixed tribo-element and they are embedded into the radial grooves in dis 8 which is part o the ixed plate 11 across a subassembly which contains a tensometric couple 9. Rotation around the axis o this subassembly is hindered by two bolts, 12. controlled orce is axially acting upon the above subassembly. The size o the axial orce can be varied by varying the length o the lever 10 and the weight suspended at the end o it. Fig.1 The scheme o stand to determine the tribological parameters or the piston cylinder system The two rotating and ixed diss are inside a vessel, which is part o the speed reduction gear thus being possible to control the amount o lubricating oil in the riction area. Figure 2 schematically illustrates the tribo-model: 1-the rotating dis (mobile tribo-model); 2 eight samples, 3 ixed dis where the samples are slightly screwed into over circles o radius r, 5 the tensometric couple, 6 the electronic potentiometer. Experimental researches have been conducted on the above stand to determine the tribological parameters o the piston ring cylinder tribosystem o the air compressor K 902 [6]. For the experiment purpose the dis o the wheels 1 was made rom the same material as the cylinder jacet rom the ull-scale tribo-system and the samples acting as ixed tribo-elements, were made rom the material o the piston ring. The contact suraces o the two tribo-elements had the same machining quality as in nature. lso the same lubricating material was used. Fig. 2. The oversimpliied illustration o the tribomodel n the tribo-model, the sliding contact to axial direction between two cylindrical suraces is replaced by the sliding contact between lat suraces and the rotation direction is reversed. With the tribo-system the segment perorms the alternate shiting movement to the cylinder at variable sliding speed, while with the tribo-model, the tribo-element simulating the jacet o the cylinder perorms the relative movement o continuing rotation at constant speed against the ixed samples, which represent the piston ring. n order to mae the joint between the outputs resulted by thermodynamic and tribological approaches we will enumerate the thermodynamic determinations and emphasis on the tribologycal. The tribological research made on the mathematical and physical models, had the aim the ollows: - determination o the gas low leaage; - inluence o these leaage on the low coeicient o the compression stage; - inluence o the gas leaage on the compression mechanic wor; - inluence o the gas leaage on the general eiciency o the compressor. The major aims o the tribological research was to: determination o the riction coeicient (µ) and its variation in dierent wor regimes and parameters. determination o the wear in various conditions. The riction coeicient µ calculated with Coulomb relation was determined or a case o th nternational Tribology Conerence, Otober 2003., Belgrade, Serbia

4 limit and dry lubrication, cases well nown as worst possible. m m = [g/m] (8) s where s is riction way calculated with relation: s = u τ [m] (9) where u is the speed and τ is variation o the time. With the wear parameters determinate is obtained inormation regarding reliability o the system, using a prediction parameter o the time between the reparation, the relative leaage low measuring piston-ring joint. The length o piston ring joint ater the time τ is: Fig.3 t is observed that µ is constant with variation o speed within the speed limit or the compressor. ter the limit o 1,5 m/s, a small variation is observed but less important in this case. The igure 4 present the variation o the wear with speed or bought limit and dry riction. 3 = a + 2π b 10 L [mm] (10) a = a 0 a - distance between the ends o ring, a0 this initial distance a variation o a in time τ. = δ - aria o ring s window, δc- L c Variation o radial dimension o pistonring is: b = Vhn τ (11) Distance between the ends o ring is: a = a 0 a (12) ria o ring s window is: = L δc (13) Practical leaage low trough piston sealing is: 1 m p = az + b (14) Fig. 4 The variations are positive and linear the biggest pitch being in the case o unlubricated (dry) regime. The wear intensity by mass has been calculated as it ollows: Theoretical leaage-low trough piston sealing is: pa vc n m t = [g/s] (15) R Ta 60 8 th nternational Tribology Conerence, Otober 2003., Belgrade, Serbia 303

5 Was choused the qualitative indicator o piston sealing system unction, the relative leaage: m p p = mt ν (16) n these relations δ c is clearance o piston, a and b are the experimental coeicients depending by area o ring's window, z is the order number o the ring. The igure 5 presents the variations o relative leaage-low in time or various compression levels. was established to 0,035 and did not have visible variations with speed increasing in limit riction; riction coeicient has drastically variations at change o lubricating regime. t can arrive at a value o 0,27-0,3. wear intensity presents an easy increase with increasing at speed. This rate o increasing is the biggest at dry riction regime; it was elaborated a methodology by study prediction time between the reparation, using the ratio o variation o ring window; the variation o "relative leaage" in time can be used in designing o compressors and in maintenance ield. 6. REFERENCES Fig. 5 The diagram ν(τ) can be used in designing o compressors or the reliability orecasting. 5. CONCLUSONS the value o riction coeicient ater measurements and statistic interpretation [1] Falticeanu C.: Elemente de tribologie, Editura Fundatiei Universitatea "Dunarea de Jos", Galati, [2] Turcanu L.: Contributions about the Study o Piston Compressors Tightening, Ph. D. These, Galati, [3] Turcanu L., Falticeanu C.: ncercari experimentale privind uzura in sistemul de etansare al pistonului de compresoare, Conerinta nationala a Termotehnicienilor, Craiova, [4] Turcanu L., Falticeanu C.: Methodology and Systems or nvestigating the TriboSystem Piston Ring-Cylinder rom Compressors Machine, nalele Universitatii "Dunarea de Jos", Fasc. Tribology, [5] Voitov V., saov D.: Modelirovanie geanicinogo trenia v tribosistemah, Rev. Trenie i znosti, 3/1996. [6] Voitov V.., Veniov G.: Teoria podobia i modelirovania, Vissaia sola Mosva, [7] Falticeanu C., Turcanu Ludmila, Falticeanu C. L.: Some Researches Regarding the Behaviour o Piston Ring-Cylinder Tribo- System rom the Piston Compressor, Balantrib, Turey, th nternational Tribology Conerence, Otober 2003., Belgrade, Serbia