Prediction of REESS factor for SOC correction in Hybrid Vehicles

The regulatory mechanisms to measure emissions from automobiles have evolved drastically over the years. Certification of CO2 emissions is one of them. It is not only critical for environmental protection but can also invite heavy fines to OEMs, if not complied with. In homologation test of a Hybrid Vehicle, it is necessary to correct the measured CO2 to account for deviations in measurement from failed Start-Stop phase and difference between start and end State of Charge (SOC) of battery. The correction methodology is also applicable for vehicle simulation in Software-in-Loop environment. The focus of this paper is on the correction of CO2 derived from SOC delta in Homologation cycle. The battery energy delta due to difference in SOC between start and end of drive cycle should be converted to corresponding fuel energy/CO2 expended from Internal Combustion Engine. The resulting correction factor is known as Rechargeable Electrical Energy Storage System (REESS) factor. To provide this factor for one type of engine in a particular car/weight class, at least 8-10 measurements are required. Digitalization of the same will provide a significant cost benefit and a faster prediction of REESS factor. A detailed study of REESS factors for various engines in correlation with engine type, fuel type, operating strategy and mass of car is done and presented here. The variations that have been done through simulation are of 2 types: start SOC variation without optimization and recharge factor variation with fixed start SOC. The simulations are carried out on well validated models of different powertrain types. Additionally, the REESS factor of several carlines were validated too and in good agreement with simulation predictions.