今日自省：

今日习得：

ECMS（equivalent consumption minimization strategy）

The essence problem of the hybrid electric vehicle is to optimize the instantaneous power split between the two power source while meeting the driver demand at the wheel. 

At any time and for any vehicle speed, the control strategy has to determine the best distribution of this motive power between both engine and motor by setting operating points. 

1. the first constraint is that the motive power requested by the diver must always be satisfied.

2. the second constraint is that the battery SOC is nominally maintained.

Within these constraints, the objective is to operate the powertrain in order to achieve the maximum fuel economy. Ideally the motive power must be split at each time to minimize the overall fuel consumption over a given trip. such as:

The main problem with this global minimization criterion is that to solve such optimization problem the whole driving schedule has to be known a prior, thus real-time control cannot be readily implemented. To avoid this drawback, the ECMS original approach is to replace the global criterion by a local one, reducing the problem to a minimization of an equivalent fuel consumption at each time. the local criteria becomes at all times:

where  is the equivalent fuel flow rate at time. we define the equivalent fuel cost simply as the sum of actual consumption rate of the engine  and the equivalent fuel use rate due to the electric motor :

the global minimization problem and the local one are not strictly equivalent. However, the local one above can be easily used for real-time control and use of the equivalent fuel flow rate indirectly accounts for the non-local of the problem.

as for a optimization problems, the overall control strategy is charge-sustaining, there is no net usage of electricity from the battery. this means that the electricity used during a battery discharge phase must be replenished at a later phase using the fuel from the engine. in other words, any instantaneous usage of the electric machine is converted in terms of a future fuel cost. Thus, an average or typical efficiency for these energy paths is considered in converting electrical use to equivalent fuel consumption. These efficiencies take into account not only the electrical motor efficiency, but also the battery charging and discharge efficiencies.

For any vehicle operating point (speed, power required from the powertrain) the entire range of possible power splits is examined, and the equivalent fuel flows are determined for every combination. The combination with the lowest instantaneous fuel cost is selected. This can be pre-computed and stored as an optimum power split map using the actual efficiency maps of the IC and EM machines.