Fuel economy and NOx emission potential investigation and trade-off of a hybrid electric vehicle based on dynamic programming

Hybrid electric vehicles (HEVs) combined with more than one power source offer additional flexibility to improve the fuel economy and to reduce pollutant emissions. The dynamic-programming-based supervisory controller (DPSC) presented here investigates the fuel economy improvement and emissions reduction potential and demonstrates the trade-off between fuel economy and the emission of nitrogen oxides (NOx) for a state-of-charge-sustaining parallel HEV. A weighted cost function consisting of fuel economy and emissions is proposed in this paper. Any possible engine-motor power pairs meeting with the power requirement is considered to minimize the weighted cost function over the given driving cycles through this dynamic program algorithm. The fuel-economy-only case, the NOx-only case, and the fuel-NOx case have been achieved by adjusting specific weighting factors, which demonstrates the flexibility and advantages of the DPSC. Compared with operating the engine in the NOx-only case, there is 17.4 per cent potential improvement in the fuel-economy-only case. The fuel-NOx case yields a 15.2 per cent reduction in NOx emission only at the cost of 5.5 per cent increase in fuel consumption compared with the fuel-economy-only case.