Rolling element bearings are known to give reduced friction losses when compared to the hydrodynamic bearings typically used to support the crankshaft in multi-cylinder engines. This paper describes the design, manufacturing and testing of a modified 4 cylinder light duty Diesel production engine with rolling element bearings applied at the crankshaft main bearings in view of CO2 emission reduction. Selection of the most suitable type of roller bearings for this specific application was made. Technology development through multi-body dynamic simulation and component testing was done to assess the effect on rolling elements performance due to the key challenges inherent to such bearing solution: high instantaneous combustion load, lubrication with low viscosity and contaminated oil, and the cracking process to split the bearing outer raceway.Compared to the baseline production engine with hydrodynamic bearings, a clear cranktrain friction reduction (−58% at ambient and −30% at 90°C) was measured under motored conditions. Both engines were then subjected to fuel consumption testing on a steady state engine dynamometer test bed under a wide range of operating conditions (speed, load, temperature) which indicated 2% fuel economy on average. Clear fuel economy improvement was demonstrated during the transient warm up phase (first fifteen minutes of operation) and typically 7.5% fuel economy was measured during the first five minutes. The fuel consumption over NEDC was calculated from the measured steady state points and leads to a 1.8% fuel economy compared to the best-in-class Euro6 Diesel engine selected.Noise and vibration measurements were also made to support the roller bearings assessment. Performance of the rolling element bearings was validated through full load combustion operation along the whole engine power curve. Finally and in view of mechanical development, the roller bearings engine was subjected to continuous operation at the engine's maximum torque with increasing peak firing pressure.