Contamination and Wear

Oil is the life blood of any engine and clean oil is essential if an engine is to operate effectively throughout its working life. During its specified period of use, either time or distance related, oil is subject to millions of thermal cycles, extreme mechanical loads and the continuous addition of engine generated contaminant. At the same time, the oil is expected to maintain its viscosity and lubricity whilst cooling and protecting sophisticated mechanical components operating at the extremes of their design parameters. The centrifuge as an effective oil cleaning system will increase engine durability, make possible extended oil drain intervals and assist the engine to sustain emission standards throughout its working life. With no replacement elements it will also assist in minimising the enviromental costs associated with engine operation since landfilling of oil filter elements is prohibited and components must be recycled to save the worlds non replenishable natural resources.

The sources of oil bound contamination may be summarised as follows:

• combustion by-products

• contamination of the ingested air

• dirt found in fuels

• bad maintenance

• dirt entering through seals

• dirt built-in on assembly

• debris created by wear of engine componets

This process is a “chain reaction” and can lead to catastrophic wear failure. The distribution of contaminant particle sizes in used engine oils has been analysed and the results showed that above 0.5μm the number of particles present varies approximately according to an inverse cube law with particle size. Thus there are many more small particles than large (one 100μm particle can be broken down into 1000 10μm particles). Through the accumulation over many years of laboratory and in-service experimental data, it has been shown that significant wear in modern engines results from the presence in circulation of contamination particles smaller than ten microns in section. The size of particular contamination is important when one considers the typical values of oil film thickness between sliding surfaces of the various engine components:

• between piston ring and cylinder wall: 0-1.5μm

• between cam and finger follower: 0-0.5μm

• camshaft oil films: 0-7μm

• big end bearings: min 1-2μm

Abrasive particles which are larger than the oil film thickness interfere with the lubricating process causing exacerbating wear of the engine components. Although all modern engines are fitted with full flow filters, they are not designed to remove particles of this size from circulation but instead prevent large debris from causing catastrophic engine failure.

The following figure shows the traces along the surfaces of the piston pins from a standard engine, an engine equipped with bypass filter and an engine with equipped with a centrifuge. The traces taken over these pins before the tests were all straight lines. The traces below show the shape taken by the pins as they became worn during the tests.

Tiny metallic particles catalyse the process of oil oxidation. It is recognised that the removal of these tiny metallic particles extends the useful life of the engine oil by slowing down oil oxidation.

Engine lube oil is also contaminated by the progressive addition of carbon soot which, when present in volume, increases viscosity and absorbs anti-wear additives. As manufacturers increase fuel injection pressures and raise the top piston ring to meet emission requirements, more soot is formed. When subjected to high temperatures such as in the ring pack area, the carbon soot becomes a hard abrasive deposit. Sub-micron nonmetallic particles ,mainly as soot, form the bulk of insolubles in engine lubricating oil. By keeping the level of insolubles low the centrifuge prevents the blocking of full flow filters thus extending the oil and full flow filter change intervals. Removal of abrasive dirt from an engine’s oil inhibits component wear and consequently extends engine life.