This work formed part of the North American Space Agency /US Department of Energy project to produce a 55% thermodynamic efficient diesel engine, which required the construction of a "low heat rejection" engine. Work was carried out on a Caterpillar 3406B, a commercial, multi-cylinder, 'low emission' diesel engine. The work evaluated oil mass and gas flow, the chemical and physical degradation of commercial and experimental lubricants, the effects the sulphur content in diesel fuels had on the particulate size in lubricants, lubricant transport through the piston ring pack of the Caterpillar 3406B, and the effects different piston ring packs had on both lubricant degradation and lubricant transport. Piston ring zone sampling was used throughout this work. Lubricant samples were taken from four positions on the Caterpillar 3406B piston, namely the 2nd compression ring, accumulator groove, 1 st compression ring and base of the crown land. A duel sampling system was developed which enabled lubricant samples to be taken from two sampling positions on the piston simultaneously. This sampling system proved to be reliable with over 650 hours of operation, at various engine speeds and loads. Oil mass and gas flow experiments showed complex behaviour. Oil mass and gas flow generally increased as engine speed and load increased, however 'peaks' of oil mass and gas flow were observed at various engine speed and load combinations, which may be a reflection of the piston ring pack design. Examination of oil mass obtained from 1991 and 1994 US Emission control piston ring packs, under conditions of low engine speed and low load, demonstrated that the 1994 piston rings allowed more lubricant to enter the ring pack. Significantly less oil was obtained from the base of the crown land with the 1994 piston ring pack, highlighting their superiority with regards to reducing lubricant related emissions. Measurement of the chemical and physical degradation of various lubricants demonstrated that degradation increased as the sampling position moved closer to the piston crown. Different degradation levels were noted whether the sampling position was on the piston face or behind the piston rings. Analysis of lubricant degradation using statistical techniques demonstrated that lubricant type, sampling position, fuel sulphur content and piston ring pack significantly affected lubricant degradation The work showed that engine speed and load did not effect lubricant degradation The use of statistical analysis demonstrated that lubricant samples could be graded according to their level of degradation, which highlights the potential use of piston ring zone sampling as a quick screening method for evaluating experimental lubricants. Lowering the sulphur content in diesel fuels had the effect of increasing the size of particulates in the lubricant. This questions the effectiveness of the dispersant additives in the lubricant formulation, and highlights the need for more effective dispersant additives to be used with lower sulphur diesel fuel Lubricant transport to the 1!1t compression ring was found to be considerably slower in a modem engine, when compared to older engines. This demonstrates that there may be a need for engine manufacturers to recommend the use of synthetic lubricants over the use of mineral based lubricants. A lubricant flow model was developed which showed that the piston rings act as secondary oil control rings and that 19«110 of the lubricant that reaches the 2nd compression ring would be transported to the base of the crown land. Comparison of lubricant degradation in the 1991 and 1994 US Emission control piston ring packs demonstrated that the 1994 piston ring pack degraded lubricants to a lesser extent, which is thought to be due to there being more oil present in this piston ring pack. The lubricant transport measurements showed that the transport time to the 191 compression ring was less for the 1994 piston rings than the 1991 piston rings, and that the 1994 rings allowed more lubricant to be passed from the 2nd compression ring to the 191 compression ring.