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Hybrid Electric Vehicle with Flywheel Energy Storage System - page 6 / 11





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The efficiency of ICE varies from 0.15 to 0.35 and the area circled by the equivalent efficiency line of 0.3 is the highest efficiency area of ICE. The efficiency of ISG varies from 0 .4 to 0.85 and is more related with its rotary speed. So in order to improve both the efficiencies of ICE and ISG, relationships among the rotary speed, the output torque of them as well as the energy

storage state ( λ ) of FESS must be harmonized

to the greatest extent. For the complexity of energy flows, the relationships among the influencing factors forementioned are non-linear. Compared with conventional logic control, fuzzy logic control has better capability of solving complicated non-linear problems and is more suitable for the realization of torque distribution. 3.2 Fuzzy logic controller

Fuzzy logic controller uses fuzzy logic to imitate human thought and thus realizes control over non-linear and time-changing systems or systems where math models is difficult to be established [11-17].

The simulation model of hybrid system is established based on MATLAB/SIMULINK and the fuzzy logic controller has total three fuzzy inputs and two fuzzy outputs. First the working plane of hybrid system is divided into 9 zones according to its external characteristic (Fig.7). Driving torque required by HEV is calculated by vehicle model according to the travel of accelerator pedal or brake pedal. The rotary speed of ISG or ICE is also taken for calculation. Then the driving torque obtained by calculation is put into the working plane of hybrid system and the zone that it belongs to is thus determined as the first fuzzy input. The other two fuzzy inputs are the rotary speed of crankshaft of ICE and the energy storage state of FESS.

ISSN: 1109-2777

Jianhui He, Guoqiang Ao, Jinsheng Guo, Ziqiang Chen, Lin Yang

Fig.7 Area division of driving torque required by HEV

Likewise, the working areas of ICE and ISG are divided into 7 zones according to their equivalent efficiency lines (Fig.6). The fuzzy output of ICE (ISG) torque factor denotes which zone the output torque of ICE (ISG) belongs to. The actual output torques of ICE and ISG are obtained by the torque recondition model along with the calculated value of required driving torque. Fig.8 shows the block diagram of fuzzy logic controller.

Fig.8 Block diagram of fuzzy logic controller According to the energy characteristics of

FESS, the energy storage state λ should be

confined in range of 0.1 to 0.4 so that the high energy-loss areas of FESS are avoided (Fig.4).Energy flows during the driving process of HEV are stated as follows: 1) Under situation when vehicle is parking or idle, ISG draws ICE to its minimal working rotary speed (about 900r/min) as soon as HCU receives signal of accelerator pedal; 2Under situation when vehicle is accelerating, ISG assists ICE to offer positive torque if necessary; 3Under situation when vehicle is braking, ISG offers about 0.4, torque of ICE should be


Issue 5, Volume 8, May 2009

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