Control of Hybrid Electric Vehicles ANTONIO SCIARRETTA and LINO GUZZELLA OPTIMAL ENERGY - MANAGEMENT STRATEGIES

(HEVs) have existed since before 1900 (Figure 1), interest in these vehicles has grown substantially only in the last 15–20 years. The main reason for this interest is the expectation that HEVs represent a short-term approach to improving fuel efficiency and reducing pollutant emissions of automobiles. In internal-combustion-engine (ICE) systems, the mechanical power needed to drive the vehicle is provided by the engine, whereas in HEVs, an engine and additional energy source (usually, an electric motor fed by an electrochemical battery) combine their output power. How the power split is to be made during vehicle operation comprises a new control task, which is often referred to as the supervisory control or energy management problem. Compared to conventional ICE systems, a hybrid propulsion system can save fuel for the following reasons: an HEV can recover part of the vehicle’s kinetic energy while braking and use this energy at a later time; an HEV can shut down the ICE during idling and low-load phases without compromising the driveability of the vehicle due to the high bandwidth of the electrically generated torque; an HEV can avoid low-efficiency operating points of the ICE by first storing excess power in the batteries and later driving the vehicle in an electric-only mode; and, since the electric motor can provide some of the torque during short acceleration phases, the ICE in an HEV can be designed with a smaller displacement and, thus, a better average efficiency. The achievable improvement in fuel economy depends strongly on the vehicle and the driving cycle; realistic figures range from below 10% for mild hybrids to more than 30% for highly hybridized vehicles. This potential can be realized only with a sophisticated control system that optimizes energy flow within the vehicle. This observation has spurred a considerable amount of research in the last 15 years, as evidenced by the data shown in Figure 2, representing the number of papers published by IEEE from 1990 to 2004 containing the keywords “hybrid vehicle.” Early energy-management controllers were based on heuristic considerations inspired by the expected behavior of the propulsion system. For instance, the maximum torque of an ICE is low at low speeds, while electric motors can usually produce high low-speed torques. Thus, a common control strategy is to run the powertrain in a purely electrical mode from standstill to a chosen vehicle speed. At this speed the electric motor reaches its torque limit, and the engine is turned on. Similar control rules have been derived using heuristic approaches, with the resulting strategies formalized as Boolean or fuzzy rules [2]. Control of Hybrid Electric Vehicles