Dispersion Analysis of Ferromagnetic Particles in Used Motor Oils

The study of the content of ferromagnetic wear particles in used motor oil allows for an assessment of the degree of wear of the engine’s friction parts. This work describes a new method for determining the fractional composition of the system of ferromagnetic wear particles in used motor oil, based on the sedimentation kinetics of particles in the gravitational field and the instrumental time of transverse proton nuclear magnetic resonance (NMR) relaxation ( $$T₂^*$$ ) of hydrocarbons in a polydisperse suspension. Unlike the traditional sedimentation analysis by continuous weighing, the proposed method employs continuous measurement of the NMR relaxation time $$T₂^*$$ . It is assumed that suspended particles increase the magnetic field inhomogeneity of the relaxometer magnet, and the instrumental spin–spin relaxation time is proportional to the mass of the sediment. An analytical expression for the particle size distribution and the most probable equivalent radius for the mathematical model of the sedimentation curve was obtained. Two physical models of iron particles of different sizes introduced into fresh motor oils were studied. It was shown that at the same iron concentration in the oil, smaller particles cause greater magnetic field inhomogeneity than larger particles. The kinetics of changes in $$T₂^*$$ relaxation times during sedimentation were investigated. It was established that during sedimentation of ferromagnetic particles, the spin-lattice relaxation time T1 and the spin-spin relaxation time T2 remain unchanged. To obtain the distribution of ferromagnetic particles, all necessary transformations of the $$T₂^*$$ (t) data array were performed numerically. It was demonstrated that, unlike the unimodal distributions observed for model samples of fresh oils with iron particles, the used M-10G2CS oil after 420 h of operation exhibits a broad radius distribution ranging from 3 to 11 µm. The particle radius distributions in Shell Rimula 15W40 oils used for 250 and 500 h, as expected, are similar; however, the percentage content of fractions in the oil used for 250 hours is lower than that in the oil used for 500 hours. The nature of these distributions differs from that of the M-10G2CS oil (420 h). The applicability of the proposed method to highly dispersed systems is limited by the very slow sedimentation of nanosized particles in the gravitational field. The results in this work may be useful for dispersion analysis of any organic suspensions containing microsized ferromagnetic particles.