A comprehensive overview of hybrid construction machinery

Abstract
With the increasing attention of energy saving and emission reduction technology, the recent application of hybrid powertrain technology affects the development of construction machinery industry. This article reviews these publications and provides comprehensive references. This article reviews the state-of-art for the hybrid wheel loader and excavator, which focuses on powertrain configuration, energy storage devices, and energy management strategies. The basis of classification and characteristic of each powertrain configuration are described. Advantages and disadvantages of batteries, supercapacitors, hydraulic accumulators, and flywheel used in hybrid construction machinery are summarized. The existing energy management strategies for hybrid construction machinery are also elaborated. The technological challenges and developing trends in the near future for hybrid construction machinery are discussed.
Introduction
Electric technology and hybrid powertrain technology are two effective technologies of energy conservation and emissions reduction and have achieved great success in automobile field.1–4 However, because of the heavy-load, low-speed and periodically operation mode, electric technology can not be applied directly in construction machinery. Therefore, in order to decrease the high fuel consumption rate of construction machinery, hybrid powertrain technology is increasingly drawing the attention of manufacturers, government and scholars. Hybrid wheel loaders and excavators, which share the maximum ownership from the global market, have particularly made significant progress.5 Hitachi successfully launched the world’s first hybrid loader in 2003,6 and Komatsu developed the world’s first commercial hybrid excavator in 2008.

With hybrid construction machinery (HCM) attracting more attention, the powertrain configurations, energy management strategies, and energy storage devices have been presented by many scholars for HCM.9–12 Lin et al.13 presented the HCM review in 2010. The paper first analyzed the difference between the hybrid powered automobile and HCM. The hybrid power system applications and energy regeneration system of construction machinery were then summarized. The challenges that face the researchers and construction machinery manufacturers were discussed. However, based on previous researches, further studies on the development history, research progress, existing problems, and developing trends of HCM are overviewed in order to promote research and application of HCM.
Section “HCM powertrain configuration types” reviews the types of HCM powertrain configuration which are currently launched by construction machinery manufacturers. Section “HCM energy storage devices” introduces the advantages and disadvantages of batteries, supercapacitors, hydraulic accumulator and flywheel in application of HCM. Section “HCM energy management strategies” classifies and outlines the control strategies of current hybrid wheel loaders and excavators, although fewer studies are available. Section “Current challenges and trends” presents the main challenges and trends of HCM.
HCM powertrain configuration types
Wheel loaders and excavators are important construction machines that have the maximum ownership. Thus, it is necessary to study the various types of powertrain configuration of hybrid wheel loader and excavator in order to better understand their construction features.
Hybrid wheel loader
The wheel loader mainly requires driving and actuating working devices in V-cycle.14,15 The power requirement has obvious volatility and periodicity. Frequent starts and stops, and larger vehicle weight generates significant amounts of braking energy which is generally wasted by the frictional braking system.16 To effectively use braking energy and control engine in its high-efficiency operating region, many construction machinery manufacturers and researchers have studied hybrid wheel loaders.17 Currently, the hybrid wheel loader has three design options of powertrains: series, parallel, and series–parallel.

Hybrid excavator
The excavator is a type of construction machinery that has a larger weight and higher energy consumption. Statistics show that excavators generally complete 65%–70% of earthwork because of its strong multifunction adaptability, whose amount continues to steady increase.25 Therefore, studying hybrid excavators to promote energy efficiency and decrease gas emissions is significant. A hybrid excavator can typically recycle two energy types, including the braking kinetic energy of swing and gravitational potential energy of booms.