Exhaust-gas-recirculation (EGR) systems have drastically reduced emissions in diesel engines, but the systems can increase oxidation and acid in the lubrication system, thus shortening the useful life of the oil.
At the SAE Commercial Vehicle Engineering Congress, researchers from Honeywell and Afton Chemical presented a lube-filter concept designed to prolong engine life by addressing some challenges of EGR systems.
Three main problems tax a lubrication system that uses EGR. These are soot; increased levels of acids in the engine; and higher temperatures, which bring increased oxidation of the lubricant. Each problem requires a special additive: for the soot, dispersants and antiwear additives; for the oxidation, antioxidants (AO); and for the acids, over-based detergent (OBD) additives to minimize internal wear.
Developing a predictable and reliable release mechanism was key to the project. The use of erosion or diffusion by means of a liquid or gel, as well as chemically treated pellets, to condition the oil were all tried. However, those methods could not produce either predictable or repeatable linear release rates.
A linear release of additives, rather than a high dose early, was determined to be optimum as oxidation rates are irreversible. An extended release allows the acids to be neutralized as they are generated.
The eventual design to combat EGR by-products involved releasing additives through an oil filter with a predictable, repeatable release mechanism. The release mechanism was developed based on two main principals.
First, the stagnation pressure is determined by the density and velocity of the oil through the filter. This varies based on the engine speed and temperature of the oil, but because they remain inside known boundaries, the precise tuning of the release mechanism can be achieved. The fact that higher oil temperatures cause a greater need for the additives in the engine helps to establish the release rate.
Second, the metering tube relies on Darcy's formula to factor the stagnation pressure, additive viscosity, and tube measurements to control the rate at which the additives are released. Taking advantage of flow dynamics available inside the lube filter, researchers used the flow to create stagnation pressure to drive the additive out of an additive basket and then used a metering tube to control the release rate of the additive.
By adjusting the metering tube diameter, the clearance at the stagnation port, additive viscosity, and metering tube length, the design of the release mechanism was adjusted to suit test engines.
The three additives were closely watched, especially the OBD levels, as ash build-up in the engine could lead to piston deposits and shorter diesel particulate filter clean-out intervals. Tests began with laboratory dynamometer trials using Mack T-11 and T-12 dynos and then moved to field validation using eight Volvo D-12 engines and commercially available CI-4 and CJ-4 oils, designed for the 2007 emissions regulations.
The results found that adding OBD and AO products to the oil will prolong total-base-number decay and slow oxidation levels. Although each platform will require a specific balance of the additives, the application of the release system can be adapted for use in other engines.