In the first article in this series, we talked about the composition of sand and dust in harsh, arid environments around the world, established that the particles were mainly silica-based, irregular in shape and significantly harder than steel and aluminium alloys found in modern mechanical machinery and ascertained that particles less than 100 microns are able to pass through the air filter and become dispersed in the engine lubricant—consequently damaging engine components.
Having validated the presence of sand and dust particles in an engine, the next stage was to scientifically understand the extent and nature of the damage caused. A bespoke test rig was designed and developed which incorporated the cylinder head of a globally recognized 2.2 liter diesel engine. The cylinder head was directly driven by an electric motor to ensure high levels of test accuracy and repeatability (see Figure 1).
The cylinder head was initially run using a standard SAE 15W-40 engine lubricant without silica contamination; the wear observed was negligible. Prior to running a test with silica contamination, a short experiment was conducted to ensure the contamination was representative of real-life. Reference-quality silica in a broad size range was evenly dispersed into an engine oil and passed through a variety of real engine oil filters. The particle size range post-filter was measured, therefore defining the particle size for testing.
The test on the cylinder head was repeated, this time with silica contamination, and revealed some surprising results.
Conventional wisdom suggests that severe high-pressure contacts with minimal oil film, such as the cam-tappet interface, would suffer the greatest amount of wear when presented with abrasive particles. The study, however, showed minimal wear in these contacts since the majority of particles are actually too large to be drawn into the clearance between the cam and tappet and are simply pushed aside.
It therefore stands to reason that components which are separated by larger oil films are more at risk to wear, since abrasive particles are able to enter into the contact zone. Measurement of the camshaft bearings proved this theory to be true. These components showed high levels of wear after being run for the equivalent of only a few hundred miles (see Figure 2).
This behavior can be further substantiated by analyzing the entrainment mechanisms and component dimensions involved.
As is common in journal bearing design, the lubricant is fed directly into the center of the contact. In this experiment, the lubricant contains dispersed silica particles between 2.4 and 32 microns in size and, given that the oil film thickness in a camshaft journal bearing is between 0.5 and 20 microns, it is inevitable that a large proportion of the silica particle sizes will be forced into this contact (see Figure 3). The dynamic nature of the valvetrain means that the gap will fluctuate during normal operation, closing the gap and pushing the silica particles into the relatively soft metal surface. At the same time, the shaft is also rotating at high speed, causing the particles to cut into the metal, resulting in high wear rates.
The issue is further exacerbated because journal bearings are made from soft metal (in this case aluminium) meaning that the hardness difference between the silica particles and components is even greater, resulting in accelerated wear. In this experiment, the wear was so extreme that it caused a significant widening of the gaps between the journals and bearings. This, in turn, resulted in reduced resistance to oil flow within the cylinder head, causing the oil pressure to drop by over 30%.
Of course, the cylinder head is not the only area in an engine to contain journal bearings. Crankshaft and big end bearings can also suffer excessive wear from these types of contaminants, leading to increased oil consumption, power loss and potential engine failure.
Harmful silica becomes dispersed within the engine lubricant and has the potential to cause damage to specific components.
The concluding article of this series will investigate the remedial actions which can be undertaken to reduce contaminant-related component wear in engines to avoid expensive repair bills and unscheduled downtime.
For more information, please contact your Lubrizol representative.