Research On Configuration Analyzing Methodology Of Dual-mode Power-split Hybrid Powertrain

Power-split hybrid electric vehicle which integrated advantages of series hybrid carsand parallel hybrid vehicles has been gained widespread attention. Toyota Prius has made aremarkable success on the worldwide market since1997. Known as Global HybridCooperation, General Motors, BMW and Daimler Chrysler have developed in collaborationa system named "Two-Mode Hybrid". This technology has been employed in cars, such asChevrolet Tahoe Hybrid and Chevrolet Silverado Hybrid. According to power flow types,three categories of power-split modes can be achieved: input-split, output-split andcompound-split mode. According to the different combination of the three modes,power-split hybrid system can be generally classified into two types: single-mode anddual-mode hybrid powertrain. And generally, only the input-split type is feasible for afull-range single mode hybrid system, which is used in Toyota Prius. In general, a dual-modepower-split hybrid system is made of compound-split and input-split mode. Compared tosingle-mode system, this kind of dual-mode system is able to reduce the capacity of powerelectronic machine and offer some improvement in fuel economy. However, the difficultiesin the configuration design of dual-mode system are increased by some factors, such as: thecomplexity and diversity on configuration of dual-mode system and the effect on parametermatching of powertrain elements and system efficiency characteristic. Therefore, in thispaper a systematic analysis methodology is proposed to design dual-mode power split hybridsystem. The main contents include:1. By referring to the researches on configuration characteristics and performance ofpower split hybrid system, the characteristics of power split hybrid system can be listed: inpower-split hybrid system, the engine power is split into two paths: electrical path andmechanical path and engine speed and torque can be decoupled from the wheels. Then, thecontrol of the engine is flexible. The transmitting of engine power through the electrical pathreduces the overall system efficiency. In this case, the powertrain efficiency is higher whenthe power divided into electrical path is smaller. The system efficiency reaches the highestvalue when the full engine power is transmitted to the wheel through the mechanical pathwith no-power shunted to the electrical path. This is called the mechanical point (MP). Thesystem has a better efficiency when the transmission ratio (TR) is nearer MP. Thischaracteristic of power-split hybrid system determines that the motor torque capacity andsystem efficiency is affected by the selection of hybrid powertrain configuration.2. Different from input-split or output-split configuration, compound-split configuration has two MPs. The power divided into electrical path is limited between two MPs, so theefficiency in this area is relatively high. Therefore, compound-split mode is employed intwo-mode hybrid system to increase the TR range of high system efficiency. This paperpresents a method of analysis on compound-split configurations for power-split hybridvehicles which is based on the4-node lever model backward spitting. In this method, thecapacities of motor torque and speed, and the amount of power splitting are estimated to getthe value ranges of lever lengths in the4-node lever model. Then the4-node lever model isbackward split into two single PGs and the inner connection of these two PGs is determined.All the feasible compound-split configurations for power-split HEV can be identified.3. Using the analysis method proposed, the compound-split configuration can beselected easily by setting two mechanical points at the desired transmission ratios. Adual-mode power-split hybrid powertrain system can be designed by adding clutches and thethird planetary gear set to the compound-split configuration selected.4. According to the kinematic characteristics and control principle of power-split hybrid,the point of control strategy for the powertrain system is the selection of engine operatingpoint. The engine operating point determines not only the kinematic relationship of thepowertrain elements, but also the efficiency of the overall powertrain system. Using theengine fuel consumption (FC) as the evaluation index of system efficiency, the distributionof the optimal engine operating point in different conditions is obtained and the advantage ofdual-mode hybrid system is analyzed.