Dispersion Analysis of Ferromagnetic Particles in Used Motor Oils

Studying the content of ferromagnetic wear particles in used motor oil allows us to judge the degree of wear of the rubbing parts of the engine. This paper describes a new method for determining the fractional composition of a system of ferromagnetic wear particles in used motor oil, based on the kinetics of particle sedimentation in a gravity field and the instrumental time of transverse relaxation of NMR protons of hydrocarbons in a polydisperse suspension. Unlike traditional sedimentation analysis by continuous weighing, the proposed method uses a method of continuous measurement of the NMR relaxation time T₂*. It is assumed that suspended particles increase the non-uniformity of the magnetic field of the relaxometer magnet and the apparatus time of spin-spin relaxation proportionally to the mass of the sediment.

An analytical expression for the particle size distribution and for the most probable equivalent radius for a mathematical model of the sedimentation curve is obtained. Two physical models of iron particles of different sizes introduced into fresh motor oils are investigated. It is shown that at the same iron concentration in engine oil, small particles make the magnetic field more non-uniform than large particles. The kinetics of changes in relaxation times T₂^* during sedimentation is studied. It was found that during sedimentation of ferromagnetic particles, the times of spin-lattice T₁ and spin-spin T₂ relaxation remain unchanged. To obtain the distribution of ferromagnetic particles, all necessary transformations of the data array T₂^*(t) were performed numerically. It is shown that, in contrast to the unimodal distributions for model samples of fresh oils with iron particles, M-10G2CS oil used for 420 hours gives a distribution in a wide range of radii from 3 μm to 11 μm. The distributions of particle radii in Shell Rimula 15W40 oils used for 250 hours and 500 hours are, as expected, similar, but the percentage content of fractions in the oil used for 250 hours is less than in the oil used for 500 hours. The nature of these distributions is different from that of M-10G2CS oil (420 hours). The capabilities of the proposed method for highly dispersed systems are limited by the very slow settling of nanosized particles in a gravitational field.

The results obtained in this work can be useful for dispersion analysis of any organic suspensions containing microsized ferromagnetic particles.

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