Despite high potentials of power-split hybrid electric vehicles (PS-HEV), their design and control problems are nontrivial. For instance, there exist twenty-four ways of connecting four components (two electric machines, an engine, and a vehicle wheel) with a planetary gear (PG), and more than thousand ways with two PGs. Furthermore, when PG and final drive ratios are considered design variables, finding an optimal design that fulfills both high fuel economy and short acceleration time is a challenge. In this paper, a systematic configuration searching methodology is proposed to find an optimal single PG PS-HEV configuration for both performance metrics. First, by identifying all the possible single PG configurations and reorganizing them into a compound lever design space, the performance metrics are explored in the continuous design space. Then, the designs are mapped onto the 'fuel economy -acceleration performance' plane to solve the multi-objective configuration selection problem. Thus, a highly-promising configuration ('o6'), which outperforms Prius design in the acceleration performance, is selected among Pareto Frontier. A case study has been conducted on a sport utility vehicle specification. The study illustrates that the performance metrics of candidate configurations change significantly, and thus selecting a proper configuration is crucial to evoke full potential of the given powertrain components.
Most of the prior design studies on compound split hybrids focused on the selection of optimal configurations through evaluating their performance within the physical design space, i.e., powertrain configurations. However, the authors revealed that using the compound lever for the performance analysis dramatically reduces the design space as redundant configurations exist for a single compound lever design, resulting in computational load reduction. Nevertheless, using the compound lever results in the loss of information required to realize the given configurations as these two configurations are represented by two different sets of variables. The powertrain configuration is defined by two physical design variables, i.e., gear ratios of the two planetary gears. However, the compound lever design is defined by two nonphysical design variables, α and β, which are the vertical bar lengths between the output node (vehicle) and the two motor/generators' (MG) nodes. Thus, if the compound lever is used as a design tool, the selected designs should be converted into powertrain configurations. This paper introduces an automatic methodology to generate feasible powertrain configurations for any given compound lever using generic conversion equations that express the relationship between the nonphysical design variables, α and β, and the physical design variables, gear ratios. Conversion maps relating the 252 powertrain configurations to the compound lever design space were generated, and the results confirmed that the compound lever removes the redundancy existing in the physical design space.
Powertrain configurations described with elementary (physical) levers can intuitively depict the connections between planetary gear (PG) shafts and powertrain components. However, finding optimal compound-split hybrid configurations using the elementary lever is practically impossible due to the large design space. In fact, each of the existing 252 compound-split configurations has three design variables: two PG gear ratios and a final drive gear ratio. In this paper, a compound (virtual) lever-based design methodology that eliminates redundant elementary lever designs is proposed to enable a full compound-split hybrid electric vehicles design domain search. The performance metrics were assessed in the compound lever design space. Later, the designs were converted back to elementary lever configurations by applying a design space conversion map, and their performance metrics were plotted on a fuel economy versus acceleration performance plane to compare the potential of the 252 compound-split configurations. Finally, an optimal configuration that can reach 0-160 kph in 15.36 sec, which is 5.90 sec faster than that of the Prius configuration, while maintaining a competitive fuel economy, was selected. The proposed method revealed that there are still many configurations that are potentially better than the commercially available split hybrids.INDEX TERMS Compound lever, compound-split, design methodology, multi-objective configuration selection, power-split hybrid electric vehicle.
Nowadays, power-split hybrid electric vehicles (PS-HEV) are very popular mainly thanks to the success of Toyota Prius. Despite their superior performance, the design and control of PS-HEVs are non-trivial due to the large number of design candidates and the complex control problems. For instance, there exist twelve ways to connect the four components (two motor/generators, an engine, and a driving wheel) with a single planetary gear-set (PG), and the number increases to 1152 possible configurations when using two PGs. Moreover, if we consider the final drive (FD) and PG ratios as design variables, finding the best design becomes intractable. In this study, we introduce a simple yet powerful way to find the optimal designs of single PG PS-HEVs. The suggested method consists of two parts — full-load analysis and light-load analysis. The full-load analysis computes 0–100kph times to evaluate acceleration performance of all designs using instantaneous optimization approach. The light-load analysis evaluates the fuel economy of selected designs (designs with acceptable acceleration performance) using equivalent consumption minimization strategy (ECMS). Note that the sun-to-ring (SR) gear ratio and the FD ratio are considered design variables, and thus one can see how fuel economy and acceleration performance of each configuration vary with SR and FD ratios. Based on these analyses, the optimal design that balances full-load and light-load performances can be selected.
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