Development of a mid-size compound type hybrid electric excavator

Regulations on emission gases and ever-increasing fuel costs call for environmentally friendly and energy efficient machines in industries worldwide. To meet these requirements, a hybrid excavator prototype has been developed where an electric swing motor, an engine assist motor, and an ultra-capacitor module are incorporated into a conventional hydraulic excavator of 22-ton class. The developed hybrid excavator benefits from the regeneration of swing energy when decelerating and the efficient operation of the engine, eventually achieving the enhanced energy efficiency and reduction of emission gases. Based on the authors´ as well as the co-workers´ previous efforts developing the hybrid excavator prototype, this paper will introduce a few techniques to optimize its energy efficiency. These include the 1) engine speed control in proportional to the load torque, 2) pump displacement control when driving the electric swing system, 3) ultra-capacitor voltage management minimizing the electrical energy loss, and 4) cooling fan speed reduction designed for the system with improved energy efficiency. The first three of these techniques are realized into the control algorithm, while the last is implemented through the design modification based on the estimation model of the engine coolant and hydraulic oil temperature according to the fan speed. The gross impact of these techniques on the energy efficiency is validated through the test.