The energy efficiency of the rangeextended electric bus (REEB) developed by Tsinghua University must be improved; currently, the energy management strategy is a chargedepletechargesustain (CDCS) strategy, which exhibits low energy efficiency on the demonstration model. To improve the energy efficiency and reduce the operating cost, a rulebased control strategy derived from the dynamic programming (DP) strategy is obtained for the Chinese urban bus driving cycle (CUBDC). This rule is extracted by the powersplitratio (PSR) from the simulation results of the dynamic powertrain model using the DP strategy. By establishing the REEB dynamic models in Matlab/Simulink, the control rule can be achieved, and the power characteristic of powertrain, energy efficiency, operating cost, and computing time are analyzed. The simulation results show that the performance of the rulebased strategy presented in this paper is similar to that of the DP strategy. The energy efficiency can be improved greatly compared with that of the CDCS strategy, and the operating cost can also be reduced.
Environmental concerns and increasing fuel cost have motivated manufacturers and governments to develop alternative technologies to replace conventional internal combustion engine (ICE) vehicles [
A rangeextended electric vehicle (REEV) provides a platform to overcome the BEV’s drawbacks and reduce fuel consumption and energy waste [
The abovementioned optimal control strategies are all global optimal resolution approaches using the DP algorithm, which are difficult to implement onboard because of the computational burden. In contrast, the rulebased control strategy is easy to apply in realtime. This paper combines the advantages of the DP strategy and the rulebased strategy and presents a rulebased strategy derived from the DP strategy to reduce the energy consumption for THU REEB.
The schematic diagram of electric powertrain is shown in Figure
Electric powertrain configuration of the THU REEB.
The dynamic characteristics of the engine and the generator models are ignored to reduce the computational burden of the DP strategy. Instead, the brake specific fuel consumption (BSFC) map, which is indexed by engine speed and torque, is employed for the fuel consumption calculation, as shown in Figure
Engine BSFC map.
Generator efficiency map.
Given that generator is mechanically coupled to the output shaft of the engine, the generator and engine are at the same working points. The ideal fuel economy curve of the range extender is obtained using the method described in Chen et al. [
Range extender fuel consumption map.
Because the RC battery model is too complex for use in the DP process, the battery model is simplified as an equivalent circuit with a voltage source and resistance, as shown in Figure
Equivalent circuit of simplified battery model.
The battery charging efficiencies
Targeting the fuel consumption, a backward simulation model is established based on the DP strategy. The resistance force of powertrain is expressed as the following state equation:
The driving power of powertrain is expressed as follows:
The parameters of the rangeextended electric bus are shown in Table
Parameters of rangeextended electric bus.
Bus  Curb mass/kg  13400 
Passenger mass/kg  2760  
Frontal area/A/m^{2}  7.83  

0.75  




0.512  

6.2  

2.18  


Motor  Rated power/kW  100 
Peak power/kW  180  
Maximum torque/N⋅m  860  
Maximum speed/r/min  4500  
Voltage/V  300  


Engine  Displacement/L  1.9 
Maximum output power/kW  82 (4000 r/min)  


Generator and generator controller  Rated power/kW  50 
Rated torque/N⋅m  220  


Power battery  Capacity/Ah  180 
Voltage/V  350 
As the driving cycle is an important factor influencing the energy consumption of electric vehicles, the simulation of the rangeextended electric bus is conducted based on the Chinese urban bus driving cycle (CUBDC) shown in Figure
Chinese typical urban bus driving cycle.
The DP strategy is suitable for the optimizing energy management system only if the driving cycle is known in advance. The application scope of the preset rule, which is derived from the DP strategy based on CUBDC, should be defined. It is assumed that if the driving cycles are similar to CUDBC, then the abovementioned preset rule by DP can be applied directly. The eigenvalues that can distinguish different types of driving cycles should be selected as applicable conditions of the control strategy for different driving cycles. In the research of MontazeriGh and Fotouhi [
DP is a mathematical method to design optimal energy management controllers of hybrid powertrain [
The state equation for DP strategy in the discrete form is given as follows:
The key of the DP strategy is to present a reasonable cost function. In this paper, the electricity consumption is converted to the equivalent fuel consumption, so minimum fuel consumption is regarded as the only optimal objective. The cost function
To develop the control rule derived from DP strategy, the parameter of powersplitratio (PSR) is proposed indicating the ratio between range extender and motor. PSR is defined as follows:
Distribution of the PSR points using the DP strategy.
In Figure
Power feature of the PSR points in the dead band.
The PSRRB strategy is formulated based on the PSR points outside of the dead band, which can be fitted by the curve shown in Figure
Fitting curve according to the PSR points outside of the dead band.
To verify the validity of the aforementioned control strategies, the rangeextended electric bus is simulated to analyze the energy efficiency using the DP and PSRRB strategies on the basis of the CUBDC. Moreover, for comparison to the present strategy of the THU REEB, the CDCS strategy is conducted in the simulation model. When the SOC value decreases to 0.3, the range extender is forced to charge the battery sometimes in the charge sustaining stage [
SOC decrease curves using the DP, PSRRB, and CDCS strategies.
Figure
Power features of the components based on the different strategies.
The electricity and fuel consumption of the REEB powertrain are shown in Table
Energy consumption comparison.

DP  PSRRB  CDCS 

Electricity consumption/kWh  59.25  59.17  52.22 
Fuel consumption/L  61.12  61.39  67.01 
Fuel saving rate compared with the CDCS strategy/%  8.7  8.4  — 
Energy saving rate compared with the CDCS strategy/%  7.3  6.9  — 
Operating cost/RMB per day  356.92  358.38  386.83 
The energy flow diagram of the PSRRB strategy and the CDCS strategy is presented in Figure
The energy flow diagram of the REEB powertrain for different strategies.
PSRRB strategy
CDCS strategy
The operating cost is also analyzed. Because the REEB is charged at night, the offpeak electricity price is used for the calculation. The operating cost of the PSRRB strategy is in agreement with that of the DP strategy; however, compared with the CDCS strategy, the operating cost savings of more than 28 RMB per day are achieved, according to the current electricity and fuel prices in typical cities of China. For the traveled distance (1,000,000 km) of urban buses during the lifetime of the buses [
To present a new type of energy management strategy that can greatly improve the energy efficiency, reduce operating cost, and meet the realtime range application requirement for the THU REEB, a rulebased control strategy with power splitting characteristic (PSRRB), which is derived from the DP global control optimal strategy, was investigated. The simulations were conducted to analyze the power characteristic of powertrain, energy efficiency, operating cost, and computing time by different strategies under the China urban bus driving cycle.
The PSRRB strategy was found to achieve lower fuel and energy consumption compared with those of the CDCS solution. An inherent difference between the proposed strategy and the CDCS strategy was demonstrated in this research. Compared with the CDCS strategy, the fuel savings rate and the energy savings rate can reach approximately 8.4% and 6.9%, respectively, because PSRRB strategy balances the energy loss between the engine energy efficiency and charging the battery via the range extender to improve the powertrain energy efficiency.
The costeffectiveness was also demonstrated to be improved by using the PSRRB strategy. Comparing to the CDCS strategy, the operating cost can be reduced by over 140,000 RMB during the lifetime of the vehicle, according to the current electricity and fuel prices in typical Chinese cities.
Based on the aforementioned simulation results, the rangeextended electric vehicle using the PSRRB strategy exhibits excellent performance, in terms of both energy efficiency and realtime ability; thus, the PSRRB strategy is a feasible onboard energy management strategy for use in the rangeextended electric bus designed by THU.
The authors declare that they have no competing interests.
This study is sponsored by new energy vehicles strategic projects of Chinese Academy of Engineering (2015XZ360303).