INTRODUCTION U.S. highway petroleum consumption is nearly 12 million barrels per day, and more than 24% of this is due to heavy- duty (HD) trucks [ 1]. Thus, HD truck hybridization should be seriously considered in the national energy strategy to increase fuel economy, reduce greenhouse gases, and reduce our dependence on imported oil. If fuel consumption by HD trucks can be reduced just 5%-10% using hybridization, this would result in a very significant reduction in annual fuel consumption by the nearly two million highway tractors on U.S. roads. It has been clearly demonstrated that hybridization can be beneficial for light-duty (LD) vehicles [ 2, 3, 4]. Studies have shown that hybridization can boost LD fuel economy by more than 30% on the highway and 50% in the city [ 5]. Substantial efforts are now underway to demonstrate the benefits of hybridization in medium-duty (MD) and heavy- duty (HD) vehicles also (e.g., buses, delivery trucks, and utility trucks), especially where stop-and-go driving offers many opportunities for regenerative braking [ 6, 7, 8, 9, 10, 11, 12]. An earlier result reports a 20% fuel economyimprovement for a MD hybrid truck in urban use [ 7]. The fuel economy of a recently tested hybrid city bus was estimated to have improved about 36%, and chassis dynamometer measurements have indicated even higher fuel efficiency benefits of 42%-54% [ 6]. Engine-out emissions from the hybrid bus were also reported to be significantly less [6]. Hybridization is emerging in the HD truck market, and several manufacturers currently offer class 8 hybrid vehicles for specific applications [ 13-14]. However, basic information on the potential benefits of HD hybridization in class 8 trucks is still limited in the open literature [ 12]. This is particularly true regarding emissions, emission controls, and their relationship to fuel economy. Developing a better understanding of the fuel efficiency-emissions control interactions for HD hybrids is critical because of the rapidly evolving aftertreatment technologies and the simultaneous constraints imposed by evolving emissions regulations [ 15]. Like other diesel-powered vehicles in the U.S., class 8 HD trucks are typically required to remove carbon monoxide (CO), hydrocarbons (HCs), nitrogen oxides (NOx), and particulate matter (PM) from their exhaust [ 15-16]. The 2013-01-1033 Published 04/08/2013 doi:10.4271/2013-01-1033 saecomveh.saejournals.org Simulated Fuel Economy and Emissions Performance during City and Interstate Driving for a Heavy-Duty Hybrid Truck C. Stuart Daw, Zhiming Gao, David E. Smith, Tim J. Laclair, Josh A. Pihl and K. Dean Edwards Oak Ridge National Laboratory ABSTRACT We compare the simulated fuel economy and emissions for both conventional and hybrid class 8 heavy-duty diesel trucks operating over multiple urban and highway driving cycles. Both light and heavy freight loads were considered, and all simulations included full aftertreatment for NOx and particulate emissions controls. The aftertreatment components included a diesel oxidation catalyst (DOC), urea-selective catalytic NOx reduction (SCR), and a catalyzed diesel particulate filter (DPF). Our simulated hybrid powertrain was configured with a pre-transmission parallel drive, with a single electric motor between the clutch and gearbox. A conventional heavy duty (HD) truck with equivalent diesel engine and aftertreatment was also simulated for comparison. Our results indicate that hybridization can significantly increase HD fuel economy and improve emissions control in city driving. However, there is less potential benefit for HD hybrid vehicles during highway driving. A major factor behind the reduced hybridization benefit for highway driving is that there are fewer opportunities to utilize regenerative braking. Our aftertreatment simulations indicate that opportunities for passive DPF regeneration are much greater for both hybrid and conventional trucks during highway driving due to higher sus