N.C. Division of Pollution Prevention and Environmental Assistance
Re-refined Oil

Engine Oil By-Pass Filters


A major source of soil, groundwater and surface water pollution is used engine lubricating oil. As part of a total program to decrease waste oil generation and lengthen the life of engine oil, oil bypass filters can play a significant role.  Engine oil filters are designed to remove particles down to an average of 25 microns in size.  However, a wear study by a major engine manufacturer concluded that the most damaging particles to an engine are smaller than 10 microns in size.  Oil bypass filters, installed in parallel with the engine oil filter, remove engine particles down to an average of 1 micron in diameter.  The result is decreased generation of used oil, longer intervals between oil changes, increased engine life, and money saved. 

Full-Flow vs. Bypass Filters

Vehicle engines are usually equipped with "full-flow" filters to remove particles from the oil that otherwise could damage sensitive engine parts, such as the bearings. These filters remove only particles of grit, metal shavings, etc., that average about 25 microns and larger in size. They do not remove other contaminants such as water, fuel and gases that become present in engine lube oil systems as the engines operate.

The "bypass" filter does not replace the full-flow filter; it is installed in addition to and parallel to it. About 10 percent of the total oil flow is diverted through the bypass filter in a continuous cycle. The bypass filter cleans the oil by removing particles larger than 1 micron, as well as the detrimental water, fuel and gases. This 10 percent oil flow is then returned to the oil pan. As a result, most of the contaminants in the oil are filtered out, leaving engine oil that will not need to be changed.  Oil may need to be added as necessary to replace the amount lost to volatilization during engine combustion. The bypass filter not only eliminates the need for oil changes, but it also extends engine life as these contaminants are removed.


Bypass Filters in Use

The Department of Defense, some truck fleets, and several school systems have investigated the use of bypass filters. The Miami-Dade County, Fla., school system--the third largest school district in the nation--reports fewer oil changes and money saved after equipping its buses with bypass filters. From 1997 - 1999 DPPEA conducted a joint two-year study with the N.C. Department of Public Instruction to install engine oil bypass filters in 60 school buses in 10 counties (six in each county). By the study's completion, all of the effected county managers and lead people felt the filters had performed very well. There were significant savings in time and money that otherwise would have been spent on oil or in getting rid of waste oil. In addition, the Society of Automotive Engineers conducted a 12-year study on the use of bypass filters in school buses. It, too, yielded positive results.

It is important to note that these filters have been used in critical-service marine engines for several years, including those of the major oil companies. Although the oil itself will no longer need to be changed, the filters--both the full-flow and the bypass--will need to be changed periodically. A current test on one diesel fleet shows that the full-flow filter on each vehicle will need to be changed/cleaned only about every 40,000 miles, and the bypass filter per vehicle should be changed/
cleaned only about every 15,000 miles.

Typical Bypass Filter

oil bypass filter


Warranty Concerns

A primary concern with the use of either type filter is the effect it would have on vehicle warranties. Filter manufacturers and vehicle manufacturers have discussed this concern extensively. The position of the vehicle manufacturers is usually that if an engine problem cannot be directly attributed to an oil filter, the warranties remain in effect.

Cost and Environmental Benefits of Bypass Filters

  • Reduced demand for new oil, leading to increased costs savings.
  • Aid in reducing engine problems and contribute to extending engine life.
  • Reduced or eliminated oil changes.
  • Reduced oil disposal costs.
  • Less ground and surface water contamination due to less oil waste.
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