Identification of parameters of diesel engine crankshaft equivalent dynamictorsion system based on crankshaft variable angular velocity

Trenutna ugaona brzina motora SUS se najčešće koristi za analizu radnih procesa u cilindrima motora, pošto takve informacije u njoj dominiraju. Kod većih motora elastične deformacije kolenastog vratila usled torzionih oscilacija ostavljaju mnogo dublji trag na trenutnu ugaonu brzinu nego što je to slučaj kod manjih, ne mnogo opterećenih motora. Sa stanovištva torzionih oscilacije, dve najvažnije informacije procenjene sa velikom tačnošću, mogu se dobiti jednostavnom harmonijskom analizom trenutne ugaone brzine kolenastog vratila. To su informacije o vrednostima sopstvenih frekvencija oscilovanja i potencijalno kritičnim brojevima obrtaja. Dalji proračuni vezani za probleme torzionog oscilovanja dinamičkog sistema sa motorom kao njegovim delom, zasnivaju se na idealizaciji i uvođenju ekvivalentnog dinamičkotorzionog sistema (EDTS). Parametri EDTS koje je potrebno definisati su inercijalne mase, torzione krutosti i koeficijenti unutrašnjeg i spoljašnjeg prigušenja. Pored toga, potrebno je poznavati momente koji pobuđuju kretanja (uključujući i torzione oscilacije) pojedinih inercijalnih masa dinamičkog sistemu. Za ovakav sistem se mogu postaviti diferencijalne jednačine sa ciljem da opišu kretanja svake inercijalne mase. Ni za jedan od nabrojanih parametara EDTS se ne može tvrditi da je prethodno relativno tačno procenjen, pošto postoje metode koje obezbeđuju samo njihovu orijentacionu procenu. U ovom radu je prikazan postupak identifikacije parametara EDTS na primeru dizelmotora sa dvanaest cilindara u V gradnji, primenom optimizacione metode Markvarta (Marquardt) baziranoj na minimizaciji sume kvadrata razlika izmerene i modelirane trenutne ugaone brzine kolenasatog vratila. Koeficijenti unutrašnjeg prigušenja su poslužili da se proceni ukupni srednji moment trenja u motoru. Momenti gasnih sila su procenjeni na osnovu izmerenih pritisaka u dva cilindra na suprotnim stranama ispitivanog motora. Na ovom primeru se jasno pokazalo da je pretpostavka o ujednačenosti radnih procesa po cilindrima ključni faktor koji utiče na odstupanja izmerene i modelirane trenutne ugaone brzine kolenastog vratila. Procenom pobudnih momenata inercijalnih sila na osnovu razvijene procedure određivanja promenljivog momenta inercije složenog krivajnog mehanizma ispitivanog motora, isključena je mogućnost da se sa te strane unese greška u proračun. Dodatno, momenti koji se od kolenastog vratila oduzimaju za pogon značajnijih podsistema motora su kombinacijom matematičkog modeliranja i neposrednog merenja procenjeni u ugaonom domenu kolenastog vratila i direktno ugrađeni u matematički model. Razvijeni modeli i postupci su iskorišćeni za procenu nivoa torzionih oscilacija u ispitivanom dinamičkom sistemu i mehničkog stepena korisnosti motora.The current angular velocity of IC engines is most commonly used to analyze the work processes in engine cylinders, since such information dominate in it. In larger engines, the crankshaft elastic deformations, due to torsional vibrations, leave a much deeper mark on the current angular velocity than in the case of smaller engines which are not heavily stressed. From the viewpoint of torsional oscillations, the two most important information, estimated with high accuracy, can be obtained by simple harmonic analysis of the crankshaft current angular velocity. Those are information about the values of natural oscillation frequencies and potentially critical rpm. Further calculations related to the problems of torsional oscillation of a dynamical system with an engine as its part, are based on idealization and introduction of an equivalent dynamic-torsional system (EDTS). The EDTS parameters which must be defined are the inertial masses, the torsion stiffnesses and the coefficients of internal and external damping. In addition, it is necessary to know the moments that cause motions (including torsional oscillations) of individual inertial masses of the dynamic system. Differential equations can be set up for such a system in order to describe the motion of each inertial mass. For none of the EDTS parameters mentioned above can be said that it was previously estimated with relative accuracy, since there are only methods that provide their approximate estimation. This paper presents a procedure to identify the parameters of the EDTS, on the example of diesel engine with twelve cylinders in V layout, by applying the Marquardt optimization method based on the minimization of the squares sum of the differences between the measured and modeled current angular velocity of the crankshaft. Internal damping coefficients were used to estimate the total mean friction moment in the engine. Moments of gas forces are estimated based on the measured pressures in two cylinders on opposite sides of the tested engine. This example clearly shows that the assumption of work processes uniformity per cylinders is the key factor that affects the deviations of the measured and modeled current angular velocity of the crankshaft. By evaluating the excitation moments of inertial forces on the basis of developed procedure for determining the variable moment of inertia of the complex crank mechanism of the tested engine, any possibility of introducing an error in the calculus from this side was excluded. In addition, by combining the mathematical modeling and direct measurements, the moments taken from the crankshaft to drive the engine major subsystems are estimated in the crankshaft angular domain and incorporated directly into the mathematical model. The developed models and procedures were used to estimate the level of torsional oscillations in the tested dynamic system and engine mechanical efficiency