Lubrication is used for both cooling and lubricating in marine diesel engines which are used for producing electricity and propulsion system on ships. Furthermore, lubrication oil is cooled with both sea water and fresh water in conventional and central cooling systems, respectively. In this study, friction behaviours are experimentally investigated between piston ring and cylinder liner pairs in marine diesel engines when the cooling sea water or fresh water are mixed into the lubrication oil in tubular and plate type heat exchangers. Experiments are carried out with marine type crankcase lubricant and under varied engine speeds, temperature and sea/fresh water mixture ratios of 5% and 10% on the reciprocating test engine. Frictional forces are measured by the three-axis Kistler 9027C force sensor. Furthermore, Taguchi L16 orthogonal arrays experimental design method is used in the analysis of the results.