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Turbocharger lubrication

Thanks to the constant push from legislators towards greater fuel economy and lower emissions, turbo-supercharging is back in vogue with mainstream vehicle manufacturers. The result is that forced-induction engines such as the Ford EcoBoost range are seeing a growing use in production-engined competition vehicles, providing (relatively) low-cost power in a compact package; thanks to the fact most of these new-generation motors follow an I4 configuration.

The use of turbo-supercharging though brings some considerations regarding lubrication systems, above and beyond those required with naturally aspirated engines.

Turbo-superchargers have various types of internal bearing, with either rolling element or journal bearings being commonplace. Regardless of their type, however, they all are subject to very high load and temperature variations. A turbo compressor can spin at anything up to 100,000 rpm under race conditions, so consistent lubrication is vital. Invariably, a turbo-supercharger will receive its oil supply from the engine, although there is at least one unit on the market with a self-contained lubrication system.

In this unit the main bearings are located in the cold-side compressor housing rather than between the compressor and turbine housings. A cavity is built into the front portion of the compressor housing in which specially formulated high-speed bearing oil resides. This oil is transferred to the ceramic ball bearings via a pair of wicks through which a mist of oil is drawn; as the bearings are not flooded in lubricant, drag is reduced, and due to the bearings’ location, lubricant temperatures are also kept low.

However, in most conventional turbos the bearings are located between the hot and cold sides of the turbo, and are subject to much greater heat transfer from the exhaust gases, so a constant flow of oil is needed. This flow lubricates the bearing and contributes to cooling. Many modern turbos often feature additional water cooling, but some still rely solely on the oil. Regarding the plumbing of turbos, it is widely known that the bore of the exit pipework for the turbo oil supply must be far larger than that of the inlet, to accommodate any aerated oil. Similarly, unless a scavenge pump is fitted, the oil exit must be above the level of the main engine sump to allow the lubricant to drain.

One key factor that is often overlooked is filtration of the turbo’s oil supply. The three main causes of turbo failure are excessive heat, ingestion of foreign objects into the compressor wheel and oil contamination. Most engine oil filters will trap particles larger than 100 microns, which is not ideal for the longevity of a turbo’s bearings, so most turbo manufacturers recommend that the lubricating oil is filtered down to 10 microns, particularly if a turbo uses rolling element bearings.

Given the high shaft speeds, even the slightest contamination can lead to degradation of the bearing surfaces – even carbon particles deposited in the oil from the combustion process can cause problems. The solution is to run a secondary filter element in the turbo oil feed lines, providing the necessary supply of clean lubricant.

However, it is not a straightforward case of ensuring that the filter element can trap particles of the required size; it must also still allow sufficient oil flow to prevent starvation of the bearing. With packaging space often at a premium, filter manufacturers have had to develop specific high-flow filters to ensure that filtration as well as flow requirements are met.

On a final point, it should be noted that because of the generally small size of turbo oil supply lines and filters, regular inspection is imperative. Any build-up of deposits in these lines can restrict flow, so the measures put in place to protect the turbo bearings can actually become the reason for their failure.

Written by Lawrence Butcher

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