Design and manufacture of engines would be greatly simplified if operating conditions were predetermined and uniform. Some applications approach this ideal: constant speed and load, infrequent shutdowns, and controlled ambient conditions. Even then, the engine has to be started at least once. The lubrication system is one of many engine systems expected to perform under a variety of conditions; starting and operating an engine in sub-zero temperatures is particularly demanding.This paper presents details of a design process, outlined in Figure 1, as applied to optimizing lubrication systems for heavy-duty diesel engines. System and component approaches are applied to the design process: Decision Analysis (1) defines objectives and evaluates alternative components for integration to a total lubrication system design. Systems resulting from these analyses are further evaluated by way of Value Analysis (2), network simulation, design layouts, and engine testing. The interaction of these techniques is effective in reducing the resource requirements for development of new designs.Likewise, network simulation and test results are discussed with particular attention afforded pressure control valves, oil filters, and their interaction during cold ambient operation.A typical lubrication system that was studied by these methods is the system for the John Deere 400 Series engine. A schematic of the system is shown in Figure 2. It consists of a gear oil pump, a plate-type oil cooler, full-flow oil filter, and a feedback pressure regulating valve. These componets are part of the “supply side” of the system which pump and condition the oil. Also shown are the journal bearings and piston spray jets. These form the “demand side” of the system which consume the oil.