*2.2. A Novel Engine Model Based on BPNN*

The majority of engine models adopted in HEV simulation research include a kind of simple experimental look-up-table [9,10,48–53], which is acquired from an engine steady-state universal characteristic experiment. This approach can only reflect the static fuel economy, rather than the transient fuel consumption. Therefore, some researchers noticed the difference between steady and transient fuel consumption, began researching transient fuel properties, and attempted to build transient fuel consumption models [56–59]. In [60], it was found that the transient fuel consumption would increase with the increase of the loading rate, as shown in Figure 2.

**Figure 2.** Comparison of the specific fuel consumption of the engine in transient and steady-state conditions. Reproduced from [60], China national knowledge infrastructure, 2015.

Therefore, the increased fuel consumption in a transient state could be more inevitably obvious in the bulldozer's severely fluctuant load. To provide the fuel consumption in this situation with a greater accuracy, a novel engine model with a transient fuel consumption sub-model based on BPNN is proposed in this paper. The modeling approach of key sub-models of the engine model is briefly described in the following.
