Power Split Hybrid Configurations for Human-powered Vehicles

This paper presents systematic analysis and design of power split-hybrid configurations using a single planetary gearset and two electric machines for human-powered vehicles. All the 12 possible power split hybrid configurations are investigated, and their performances are compared to a normal (no power assist) bicycle, and two electric bicycles. Several performance indices, including the cyclist’s oxygen consumption, stamina reduction, and pedaling speed preference, are considered in the optimization problem to evaluate the bicycle designs. The dynamic programming technique is used to solve the optimization problem, and the results show that 4 of the 12 power split hybrid configurations are feasible. The optimal design, HyBike-i2, has the pedal connected to the carrier gear and one electric machine to the ring gear on the planetary gearset. The other electric machine and the driven wheel are connected to the sun gear. This design outperforms the normal bicycle and battery electric bicycles, and it achieves substantial reduction in the cyclist’s stamina discharge rate and reduction in oxygen consumption when the vehicle operates in the charge-sustaining mode. INTRODUCTION Human-powered vehicles date back to ancient times and now are used for various purposes, such as urban transportation, recreation, and are frequently realized in the form of bicycles or tricycles. More recently, human-powered vehicles are seen in extreme sports, such as human-powered helicopters [1], aircrafts, or peddle boats [2, 3], and an important consideration is to push for speed and endurance of the cyclist. In developing countries, where bicycles and tricycles are still an important means for commute [4], extending cyclists’ endurance is even more important as it implies longer traveling distances. In one extreme, electric bicycles are used, which can be used without any human peddling, and the common design is to add an electric motor on the crankshaft or the hub of the rear wheel [5]. The “traditional” electric bicycles often use a sizable electric motor and heavy batteries, which become deadweight when the battery is fully depleted and the human has to pedal. Their excessive weight also makes them much more difficult to be moved, e.g., carried a few steps up to reach a charging port, or lifted and transported by buses. Therefore, we are interested in exploring alternative designs that use much smaller electric machines and batteries. In this paper, we investigate the power split hybrid configuration with a planetary gearset on humanpowered vehicles. In our concept, the cyclist is an integrated part of the powertrain, and his/her power output is augmented by the battery power, instead of being completely replaced. This fundamental change in design philosophy makes it possible to have a much lighter bicycle compared with traditional designs. In additional to weight reduction, the power split hybrid configuration has the potential to reduce cost. The derailleur which costs several hundred US dollars [6] can be replaced by a planetary gearset less than 30 USD [7]. Hybrid technology has been applied to a wide range of automotive applications, including automobiles [8] and offroad heavy-duty equipment [9]. Currently, the power split hybrid configuration dominates the hybrid vehicle market because it can take advantage of both series and parallel configurations to achieve better fuel economy when the power management algorithm is properly designed [10-12]. Furthermore, several studies have shown that different power split hybrid configurations should be used on vehicles with different powertrain components and power requirements [9, 13-14]. Since the powertrain characteristics of human cycling are significantly different from those of automobiles with internal combustion engines (ICEs), a different hybrid configuration may be needed for bicycles. Systematic design methodologies for on-road power split hybrid vehicles can be found in the literature [15-17]. The methodology reported in [16] is revised in this study and applied to the human-powered hybrid bicycle. Our goal is to identify the best power split hybrid configuration with smaller electric machines to ease