Hybrid Electric Vehicle Energy Management and Systems Control
A hybrid electric vehicle (HEV) has two or more sources of onboard power. In current production vehicles, these power sources are an electric motor and a gasoline or diesel internal combustion engine (ICE). The integration of these power-producing components allows for many different types of HEV designs. A strategy is needed to control the flow of power and to maintain adequate reserves of energy in the storage devices. Although this is an added complexity not found in conventional vehicles, it allows the components to work together in an optimal manner to achieve multiple design objectives, such as high fuel economy and low emissions.
Hybrid Electric Vehicle Configurations
The biggest distinction between hybrid designs is whether the electric and power producing components operate in parallel, series, or a combination of the two. In a parallel design, the auxiliary power unit or APU (a diesel or gasoline ICE in current production hybrid vehicles) can mechanically drive the wheels. In a series design, the APU generates electricity and does not directly drive the wheels. A third type combines the best aspects of both and is sometimes called a combined or series/parallel design. A combined design allows the APU to directly drive the wheels but also has the ability to charge the energy storage device through a generator. The combined hybrid is a subset of the parallel design because it can directly drive the wheels from the APU. The way the hardware components are connected (parallel, series, or combination) is referred to here as the "hardware configuration," and the management of the power flow among the components is referred to as the "control strategy" or more generally "energy management."
A secondary distinction between hybrids is charge-sustaining versus charge-depleting hardware configurations and control strategies. Charge-depleting vehicles allow their batteries to become depleted and cannot recharge them at the same rate they are being discharged. The common charge-depleting vehicle is often referred to as a "range-extender," because it provides enough energy to extend the driving range of the vehicle but is not recharged quickly enough to power the vehicle completely, unless the APU is larger than the average power load of the vehicle. A charge-sustaining HEV has an APU that is adequately sized to meet the average power load and, if operated under the expected conditions, will be able to maintain adequate electrical energy storage reserves indefinitely.
Hybrid Electric Vehicle Control Strategies
The flexibility in HEV design comes from the ability of the control strategy to manage how much power is flowing to or from each component. This way, the components can be integrated with a control strategy to achieve the optimal design for a given set of design constraints. There are many (often conflicting) objectives desirable for HEVs. The primary ones are to:
- Maximize fuel economy
- Minimize emissions
- Minimize propulsion system cost to keep the vehicles affordable to the consumer market
- Do all of the above while maintaining or improving on acceptable performance (acceleration, range, handling, noise, etc.)
To achieve these objectives, the hardware configuration and the power control strategy are designed together. The hardware configuration dictates to some extent what control strategies make sense.