Control of Hybrid Electric Vehicles (HEVs) is an active research area. Much of the past research focused on one aspect of hybrid vehicle performance–fuel economy. While fuel economy is important for HEVs, reduction of emissions is another major performance of interest, due to ever-tightening emission regulations. Minimization of fuel consumption may have a trickle-down effect but does not guarantee reduced emissions. In fact, over-zealous pursuit of fuel consumption reduction may compromise emission. This paper investigates the emissions formation mechanism, develops an emission model that predicts tail-pipe emissions, and formulates a supervisory control problem of emissions reduction. The Dynamic Programming (DP) technique is employed to solve the optimal control problem of parallel HEVs for both emission reduction and fuel economy. The DP solution of the optimal control problem shows that tail-pipe emissions could be significantly reduced with negligible loss of fuel economy.

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