Monitoring of engine oil aging by diffusion and low-field nuclear magnetic resonance relaxation

Abstract

Time domain, also named lowfield nuclear magnetic resonance is used to monitor oil degradation by measuring relaxation and diffusion. As quality control of oils is indispensable to optimize oil change intervals while simultaneously preventing machinery damage, the technique was applied to detect the degradation state of engine oils as time domain nuclear magnetic resonance is known as a well suited tool to measure quality control parameters for example in food industry. Correlations with commonly applied oil analytics like viscosity measurements and inductively coupled plasma optical emission spec trometry allow to interpret relaxation and diffusion data in detail and finally to deepen the understanding of oil aging processes. Additionally, the measurement temperature was varied to achieve the maximum sensitivity towards oil aging. Low field NMR is not only realized in form of table top instruments, but also in form of field cycling and single sided NMR devices. Fast field cycling as well as single sided NMR were also explored to study oil aging and to provide valuable insight. The latter device was used to obtain information about translational diffusion and transverse relaxation of oils simultaneously.

Dedicated NMR sensor to analyze relaxation and diffusion in liquids and its application to characterize lubricants

“…As in any NMR experiment, the repetition time must be adapted to the longitudinal relaxation time T 1 for fastest but still reliable measurements. In other studies, T 1 was found to depend not significantly on aging at Larmor frequencies above 10 MHz, [ 28 ] and saturation recovery experiments were performed on a gear oil and modeled via the gamma distribution function leading to a longitudinal relaxation distribution. [ 29 ] To transfer the experiments to the sensor, a transmission oil was measured in 5 mm NMR tubes undiluted and additionally diluted with deuterated chloroform (2:1, 99.8%) for showing the feasibility (Figure 4).…”

“…Modeling of magnetization decays by gamma distributions [ 30 ] was shown to describe the magnetization decays of lubricating oils to a high level of numerical accuracy. [ 28,29 ] In the present case, an extension is needed. The noise level here typically amounts to about 0.3% of the maximum signal intensity for 64 acquisitions.…”

“…In previous investigations, NMR diffusion, spectroscopy, relaxation, and single sided NMR were applied to the same engine oils. [ 28 ] The same samples, but here only undiluted, were used to allow for a direct comparison of the new NMR sensor to established methods.…”

“…The sensitivity to oil aging is more pronounced for 〈 R 2,eff 〉 than for σ g , which was also found in the previous measurements using a commercial LF‐TD NMR instrument (Bruker “the minispec” mq20). [ 28 ] Particulate, (super‐)paramagnetic abrasion leading to PRE was shown to mainly cause the increase of 〈 R 2,eff 〉 with run time in all three intervals. [ 29 ] Chemical aging leads to oil sludge, which was reflected in diffusion and in oil viscosity.…”

Low‐field NMR for quality control on oils

“…Summarizing, particulate contamination in oils can be detected by NMR via susceptibility differences (line width or magnetic resonance imaging) or geometrical hindrance (PFG‐NMR). Particles in the small nanometer range and paramagnetic ions cause additionally measurable paramagnetic relaxation enhancement (PRE) …”