Since the term "oil consumption" [14, 15, 16, 17, 18, 19, 20, 21] as used in the internal combustion industry often covers a number of phenomena, it would be wise first to provide an explicit definition. Oil consumption denotes the change in the amount of lube oil in the sump over time, usually measured gravimetrically, and is affected by three criteria (Fig. 9):
1. Lube oil loss through the exhaust system
2. Lube oil deposits in the engine (extreme case: oil sludge)
3. Drag-in of products from the combustion chamber (extreme case: cold operation in gasoline engine with growing lube oil content in the sump)
The term "oil emission" [18, 19] is understood as the loss of lube oil solely through the exhaust system, determined e.g. based on the correlated mass of a tracer in the fresh oil in the exhaust gas stream. Oil emission is therefore only a greater or lesser fraction of the above defined oil consumption and is useful especially in determining the potential for improving engine emissions. Oil consumption, on the other hand, is more significant as an indicator for oil change intervals and, if large amounts of combustion products are dragged into the oil, it can even assume negative values. Consequently measurements of both parameters often do not correlate, as can be seen in Fig. 10.
For purposes of assessing the oil scraping effect of piston rings, both of these parameters can yield valuable information. Currently it is not possible to determine directly the lube oil loss solely through the piston ring system, since oil losses / oil emissions also occur through blowby and the lube oil elements it contains, and through other lubricated components of the engine such as the valves and turbocharger. Fig. 11 indicates these different paths schematically.
The diagram additionally indicates a special overflow construction which is part of the "Movan" engine oil consumption measurement system developed by GOETZE. This system is capable of high-precision measurement with statistical analysis.
Oil consumption of the piston / piston ring / cylinder system can be reduced by making adjustments to all of the three components. Briefly, the adjustments to the piston rings can be summarized as follows:
1. Increase contact pressures
2. Increase conformability
3. Design sharp bottom scraping edges for the downstroke
4. Design a hydrodynamic running face geometry for the upstroke / avoiding top scraping edges
5. Reduce the pumping action in the piston grooves - more stable contact conditions
On the whole, however, when considering any of these options the effect on the tribology and self-cleaning characteristics of the complete system must be taken into account. Too thin an oil film may cause wear problems and/or increased friction losses, while insufficient scavenging can lead to excessive deposits with adverse effects on performance after long operating times.
Fig. 9: Definitions of Oil Consumption
Fig. 10: Comparison of Oil Emission and Oil Consumption
Fig. 11: Oil Consumption Paths