Optimal Control of Engine Warmup in Hybrid Vehicles Vital van Reeven, Theo Hofman, Frank Willems, Rudolf Huisman and Maarten Steinbuch Oil Gas Sci. Technol. – Rev. IFP Energies nouvelles, 71 1 (2016) 14 Published online: 22 January 2016 DOI: 10.2516/ogst/2014042