Impact of Engine Oil on Emissions and Fuel Economy

Hannu Jääskeläinen

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Abstract: Heavy hydrocarbons derived from the engine oil are a significant contributor to the organic carbon portion of diesel particulates. In addition, lube oil additives are the main source of metallic ash that becomes accumulated in diesel particulate filters. One of the main drivers in the development of oil formulations for diesel engines with exhaust aftertreatment is the reduction of sulfated ash, phosphorus and sulfur. Through their effect on friction, engine lubricants can also affect engine fuel consumption.

Emissions Performance

The properties of engine oils can affect engine emissions in a number of ways. As discussed under Exhaust Particulate Matter, heavy hydrocarbons derived from the engine oil are a known significant contributor to the organic fraction (OF) or organic carbon (OC) portion of diesel particulates [429]. Engine oil hydrocarbons have also been suspected of contributing to the nucleation mode OF and, therefore, to particle number emissions [828]. Another possible contributor to particle number emissions are high molecular mass polymers, such as viscosity modifiers, a single molecule of which could in theory be counted as a particle by particle counting instruments.

These lube oil derived emissions are controlled through lowering the amount of oil consumed by the engine. This can be achieved through (1) engine design and/or by (2) modifying oil properties, such as lower volatility, better seal compatibility to minimize leakage, and better detergency/oxidative stability to minimize cylinder deposits.

The oil consumption from heavy-duty diesel engines meeting US EPA 2004 on-highway emission standards can be estimated from a benchmarking study carried out on 4 heavy heavy-duty and 3 light heavy-duty diesel engines and operated over a cycle based on the AVL 8-mode cycle [1270]. Based on this work, the brake-specific oil consumption of these engines is estimated to vary between 0.09 to 0.45 g/kWh. This represents about 0.2% or less of fuel consumption.

For low emission diesel engines using aftertreatment, the most important emission aspect of diesel engine oil is its compatibility with exhaust aftertreatment technologies. There are three major mechanisms for possible interference between lube oil components and aftertreatment devices: