Hybrid electric vehicles can provide fuel efficiency improvements over non-hybrid propulsion systems. For example, an electric motor may be operated with electrical energy rather than fuel to supplement or at times entirely supplant operation of the engine. Plug-in hybrid electric vehicles may further increase fuel efficiency by receiving electrical energy from a power source that is external the vehicle rather than relying on generation of the electrical energy on-board the vehicle from the combustion of fuel by the engine. For example, plug-in hybrid electric vehicles have the potential to carry travel between destinations without operating the engine to consume fuel or to consume fuel at even lower rates than may be realized by non-plug-in hybrid electric vehicles.
The inventor herein has recognized several disadvantages with the above approach, particularly in the context of a plug-in hybrid electric vehicle. For example, where the vehicle is frequently operated within a range that does not exceed the electrical storage capacity of the vehicle propulsion system, the engine may not be operated for extended periods of time, and hence fuel stored on-board the vehicle may not be consumed by the engine. Indeed, this is a primary goal of the plug-in hybrid electric vehicle described above. However, where the fuel is not consumed at a sufficient rate, fuel degradation may occur or the degree of fuel degradation may increase as a residence time of the fuel stored on-board the vehicle increases.
Thus, if the vehicle is eventually operated in a manner that exceeds the range afforded by the electrical storage capacity of the vehicle, the engine may be again relied upon to maintain the propulsive capability of the vehicle. Yet, where the fuel is degraded or where degradation of the fuel increases, engine performance may be reduced or engine damage may occur. In some conditions, engine starting may not be achievable or may be difficult to perform as a result of fuel degradation, thereby potentially limiting the vehicle to the range of the electrical storage capacity.
As one approach, the above issues may be addressed by a method that includes producing a motor output by selectively operating the motor to consume electrical energy stored on-board the vehicle; producing an engine output by selectively operating the engine to consume fuel stored on-board the vehicle; and increasing consumption of the fuel by the engine relative to consumption of the electrical energy by the motor as a residence time of the fuel stored on-board the vehicle increases. In some embodiments, the residence time of the fuel stored on-board the vehicle can provide an indication of fuel degradation, enabling the duty cycle of the engine to be selectively increased to enforce a variety of suitable fuel use conditions, particularly in the context of a plug-in hybrid electric vehicle where fuel use may be reduced for extended periods of time even when the vehicle is frequently operated. In this way, fuel degradation may be reduced while enabling the vehicle propulsion system to take advantage of the increased range afforded by a plug-in hybrid electric vehicle.