Tests have been performed on a single-cylinder medium-speed diesel research engine (SCRE-251) to investigate the possibility of using an in-line oil-cleaning device (IOD) on diesel locomotive engines for improving engine performance and exhaust emissions. The engine was operated at full load for both the baseline and with IOD test. Engine fuel consumption and emission data were obtained at different engine operating time. The effects of the device on engine combustion process were determined by analyzing recorded engine cylinder pressure data. Oil samples were taken during the test to monitor changes in oil properties. The engine test results showed that the device reduced engine fuel consumption by up to 2% of the baseline results. Average CO and smoke of the device were lower and average NOx of the device was higher than that of baseline. The trends of changes of fuel consumption and emissions are consistent with Taylor’s  investigations. Engine oil analysis indicated that the viscosity, TAN and TBN data of the IOD test changed slightly with increasing oil-aging time.