Lubricating Oil Consumption

Hannu Jääskeläinen, Kent Froelund

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Abstract: The main sources of engine lubricating oil consumption include the piston-ring-liner system, turbocharger, valve stems, and crankcase ventilation. Lubrication oil consumption is also affected by engine operation, such as by transients, and by the formulation of the lubricating oil. Significant reductions in engine-out PM emissions in diesel engines were achieved through the control of lubricating oil consumption. The control of lubricating oil consumption in engines equipped with sophisticated aftertreatment systems is even more critical than it is in cases where it is done solely to reduce engine-out PM emissions.

Lubricating Oil Balance

Lubricating oil consumption can have significant impacts on a number of aspects related to engine emissions, performance and maintenance. In diesel engines with exhaust aftertreatment, lubricating oil can have a number of negative effects on aftertreatment systems and components. Some reasons for controlling lubricating oil consumption are outlined in Table 1.

Table 1
Main Reasons for Controlling Lubricating Oil Consumption
Reduce engine maintenance costs Replacing oil consumed by the engine is an added maintenance item that increases the cost of engine service. In some cases, excessive oil consumption can lead to shortened oil change intervals. Lubricating oil consumption can also affect the rate of formation of deposits in the engine.
Reduce emissions Lubricating oil can contribute to increased engine-out emissions. In particular, lubricating oil can be a significant source of:
  • HC and CO emissions, especially in smaller engines, and
  • total PM mass and particle number emissions.
Reduce impacts on aftertreatment system cost and performanceLubricating oil consumption ash accumulation can have a significant impact on the diesel particulate filters (DPF). Most ash accumulating in the DPF is attributed to lubricating oil consumed in the engine. Keeping lubricating oil consumption low allows:
  • smaller DPFs to be fitted to the engine due to lower ash accumulation rate,
  • less frequent regeneration (and fuel economy penalty) to avoid excessive pressure drop,
  • less deterioration of DPF ceramic substrates.

Excessive accumulation of oil derived hydrocarbons in the DPF can also lead to uncontrolled regeneration and subsequent DPF damage.

All emission control catalysts must be designed—in terms of sizing and precious metal loading—to account for catalyst activity loss due to exposure to oil derived catalyst poisons and, in the case of SCR catalysts, to oil derived hydrocarbons.

All engine manufacturers understand the need to provide products with low oil consumption. However, ensuring that all the necessary steps are taken by the engine manufacturer and their parts suppliers to achieve this goal is a major challenge. While the technical knowledge exists to provide engineers with an understanding of the technology necessary to design a product with low oil consumption, a myriad of other steps beyond acquiring the technical knowledge must be successfully implemented to mass produce an engine with consistent and reliably low oil consumption. Oversights in the design process, selection of materials with inadequate wear properties, problems in production quality control and unanticipated operating conditions are just a few of the possible reasons that can cause products to experience excessive oil consumption. In some cases, the source of the problem may be totally unrelated to engine lubrication but through a series of events, it can ultimately lead to excessive oil consumption. The scope of the challenge to produce an engine with consistent and reliably low oil consumption is evident from the fact that some late vehicle models are the source of relatively widespread customer complaints [1971][1972].

In some cases, engine manufacturers have intentionally increased lubricating oil consumption with devices that blend small amounts of oil with fuel on-board the vehicle. This technique is usually part of an automatic oil changing system that replenishes the used oil blended with the fuel with fresh oil. This can significantly extend the oil change interval and reduce maintenance costs. This practice is subject to some restrictions for heavy-duty diesel vehicles certified to US 2007 and later onroad emission standards.