As the petroleum depletion, some of this demand will probably have to be met by increasing the production of diesel fuels from heavy oil or unconventional oil in the near future. Such fuels may inevitably have a lower cetane number (CN) with a higher concentration of aromatic components. The objective of the present research is to identify the effects of a typical biodiesel fuel as a CN improver for a light-duty diesel engine for passenger cars.Our previous study indicates that methyl oleate (MO), which is an oxygenated fuel representative of major constituents of many biodiesel types, can reduce soot and NOx emissions simultaneously by optimizing performance under exhaust gas recirculation (EGR) when used as a diesel fuel additive. In addition, it was found that MO tends to reduce the ignition delay.We employed a 2.2 L passenger car DI diesel engine complying with the Euro 4 emissions regulation. Four different fuels were compared: JIS#2 diesel fuel, which has a CN of 57 (D100) for a reference fuel, a very low CN diesel fuel (CN45), a blend of CN45 with 40% MO (named CN45+MO), and a blend of CN45 with an alternative CN improver, 2-Ehtyl-Hexyl-Nitrate (EHN), which has the same CN as D100 (named CN45+EHN). These fuels were compared in terms of their thermal efficiency, combustion and exhaust emission characteristics.Experiments were performed using seven engine torque and speed combinations that were selected to reflect typical operating conditions in passenger car diesel engines, focusing on the effects of important parameters such as pilot injection, injection pressure, EGR ratio and the use of premixed charged compression ignition (PCCI).The engine test results showed that CN45 generated longer ignition delays than the other tested fuels under all operating conditions and that pilot injection with this fuel was not sufficient to ensure ignition and effective main combustion at low engine speeds and loads, particularly when using high EGR ratios. This increased fuel consumption and unburned fuel emissions (CO & THC). It was judged that too much amount of MO must be added to improve the deterioration brought by CN45. We therefore compared the performance of CN45+MO to that of CN45+EHN under low temperature and enhanced EGR conditions. CN45+MO produced lower soot and NOx emissions. In addition, the brake specific energy consumption (BSEC), defined on an energy basis, for CN45+MO did not differ significantly from those for CN45+EHN or D100. These effects are due to the enhancement of low temperature reactions brought by oxygenated fuels such as MO. Moreover, MO can reduce maximum pressure rise rates associated with engine noise and vibration observed in case of CN45.In summary, MO is a useful CN improver for light-duty diesel engine.