WSEAS TRANSACTIONS on SYSTEMS
If output or input power were zero, the rotary speed of flywheel would be reduced very slowly to its minimal value under the effect of friction
resistance. The energy storage state ( λ ) would
also fall gradually to zero (Fig.4). The process of energy loss will last more than eight hours. Along with the falling of rotary speed, friction resistance falls as well. In the first hour, value of
λ drops quickly to 0.5 and then descends
slowly to zero. For the flywheel cannot hold a charge for long, it is suitable for exterior systems with demands of quick charging and discharging. HEV is just such a system.
Fig.4 Energy loss of FESS
3 Energy management
3.1 Characteristics of hybrid electric system
ISG is coupled coaxially with engine crankshaft and the rotary speed of ICE crankshaft is equal to that of ISG revolution axis. So the purpose of energy management is mainly to determine the torque distribution between ICE and ISG. At the beginning when HEV starts up, ISG offers positive torque and draws ICE to its minimal rotary speed for ignition. During the driving process, when greater torque for acceleration is needed, ISG offers positive torque and assist ICE to meet the demands of vehicle. Fig.5 presents the maximal driving torque curve of hybrid system.
Jianhui He, Guoqiang Ao, Jinsheng Guo, Ziqiang Chen, Lin Yang
Fig.5 External characteristic of hybrid system Efficiencies of ICE and ISG are the main factors that influence the performance of HEV. So the energy management strategy ought to improve both the efficiencies of ICE and ISG so that the full potential of hybrid technology could be exhumed. According to the data of experiments, the working torque areas of ICE and ISG with equivalent efficiency lines are presented in Fig.6.
(a) Working torque area of ICE and its efficiency map
(b) Working torque area of ISG and its efficiency map Fig.6 Working torque areas of ICE and ISG with equivalent efficiency lines
Issue 5, Volume 8, May 2009