The lube oil filtration system for diesel engines is becoming increasingly important. Oil filtration systems typically consist of full-flow and bypass filters. A well-designed system will reduce engine wear and oil usage and may play a role in controlling engine emissions. Thus, both economic and environmental benefits are realized. Past work has dealt with the benefits of using finer full-flow filters and with the use of bypass filters; however, the critical issue of optimization of the entire system has largely been ignored. This is the subject of this paper.The removal and dust-holding capacity characteristics of a variety of commercially available and prototype filters, including cellulose, synthetic and “centrifugal” filters, were determined in the laboratory. The performance of these filters in various full-flow/bypass combinations were further evaluated in the field on city buses. The results are in qualitative agreement with the predictions of our filtration system optimization model. Based on the laboratory, field and model results, guidelines are discussed for filter selection and system optimization.The lube oil filtration system is critical to the smooth operation and maintenance of diesel engines. Lube oil filtration systems play a major role in reducing contamination levels in oil and in reducing engine wear [1, 2, 3 and 4]. The use of finer filtration has been shown to reduce wear [1-2] and can result in extended oil life, decreased oil consumption, and increased fuel economy [3, 5]. As a result, finer filtration can produce substantial maintenance cost savings [6, 7 and 8]. Thus, system optimization can determine future operating costs. The optimization and design of lube oil filtration systems are the subjects of this paper.There are environmental implications associated with the design of oil filtration systems. Finer filtration maintains oil cleanliness; hence, oil life can be extended and the amount of waste oil generated reduced. In recent years, the disposal of used oil filters has been identified as an environmental concern. Through regulation, the U.S. Environmental Protection Agency (EPA) strongly encourages the recycling of used filters , and a number of states strictly regulate their disposal. Filter disposal, whether by recycling or other, means, costs money. In some areas, the cost of disposal approaches the cost of purchasing a new filter. Thus, there is an economic incentive to increase filter life.In the past, bypass filtration used in conjunction with full-flow filtration, was used to increase filter element life [6-7, 11]. However, this did not always result in less waste since the systems were often not optimized and since two filters instead of one were used. With the advent of new synthetic filter media and optimization strategies, waste can be minimized. Diesel exhaust emissions have also been found to correlate to suspended solids concentrations in the oil . The use of finer filtration to maintain oil cleanliness should facilitate control of noxious diesel emissions. Thus, there are environmental and economic incentives for the use of filtration systems which yield cleaner oil.Diesel engine lube oil filtration systems typically consist of full-flow and bypass filters. The full-flow filter cleans all of the oil flow before it enters the engine's oil galleries. The bypass filter cleans only a portion of the total flow in a side-loop arrangement. Usually the bypass filter is considered to be a polishing filter providing finer filtration than the full-flow. Though past experience has demonstrated the importance of bypass filters [2, 5, 6 and 7, 11], they are not required for every system . This has been particularly true in recent years with the development of high-efficiency, high dust-holding capacity filter media. However, bypass filters can compliment full-flow filters by removing fine particles not efficiently removed by the full-flow. Soot is a major factor limiting oil life, and bypass filters are particularly suitable for controlling soot levels in oil .In some applications, depth filters with large surface area have been found to be useful for controlling the accumulation of dissolved contaminants, such as acids, in the oil . Optimization should consider the sizes and types of contaminants which need to be controlled, as well as the types of full-flow and bypass filters available. Additional factors, including the restriction across the filters and flow rate through the bypass filter, also need to be considered .A number of studies have discussed specific full-flow/bypass filtration systems. A field study involving 6V71 engines concluded that the combination of a pleated paper full-flow and depth bypass filters yielded longer oil and filter life than a full-flow filter alone . Another study  compared the use of 40μm full-flow filters with and without 15μm cellulose paper, 8μm stacked disk or cotton depth bypass filters. It was concluded that bypass filters reduced wear to one-half the levels found with the full-flow filter alone and that the finer bypass filters provided even better protection. In these studies, the effects of the addition of a bypass filter were explained, but generalizations useful for system optimization were not developed. In this paper, laboratory and field studies have been used to develop guidelines to aid lube oil filtration system design.