Heavy duty trucks account for about 50 percent of the NOx burden in urban areas and consume about 20 percent of the national transportation fuel in the United States. There is a continuing need to reduce emissions and fuel consumption. Much of the focus of current work is on engine development as a stand-alone subsystem. While this has yielded impressive gains so far, further improvement in emissions or engine efficiency is unlikely in a cost effective manner. Consequently, an integrated approach looking at the whole powertrain is required.A computer model of the heavy duty truck system was built and evaluated. The model includes both conventional and hybrid powertrains. It uses a series of interacting sub-models for the vehicle, transmission, engine, exhaust aftertreatment and braking energy recovery/storage devices. A specified driving cycle is used to calculate the power requirements at the wheels and energy flow and inefficiencies throughout the drivetrain. The total energy consumed and pollutants emitted is accumulated over the driving cycle.It is shown that for an urban heavy duty driving cycle, up to 20 percent improvement in vehicle fuel economy can be achieved with up to 30 percent possible with more advanced controls. Fuel savings over a representative driving cycle for interstate long haul trucks have not been evaluated yet, but are likely to be substantial. Global positioning systems and known route and terrain can be used to implement a “look ahead” control strategy. Further fuel economy improvements are therefore possible. A typical long haul truck consumes about 75 to 150,000 liters of diesel fuel a year. At 1995 fuel prices, a 10 percent improvement in fuel economy results in savings of about $6,000 to $9,000 over a 2 to 3 year period. The added costs of hybrid powertrain should be kept within this target value.It is shown that with the use of a hybrid powertrain, the engine can be run at selected, narrow ranges of speed and load. Such a strategy allows both nitric oxide and particulate emissions to be reduced by over 50 percent compared to current levels. Thus, with a systems approach (rather than concentrating on the engine alone), it is possible to simultaneously reduce emissions and fuel consumption by 50 percent and 30 percent respectively over current levels.