## 2 General Principle and Operation of Hydraulic Energy Regeneration and Conversion Technologies
### Technical Routes and Technical Indicators  
The earliest hydraulic power energy recovery and conversion technology was studied by the EPA, researchers and other institutions in the 1970s, and in 1988, various hybrid options were investigated by the EPA in order to explore the cost-effectiveness potential for hydraulic hybrid vehicles. A diesel full-series hydraulic hybrid test chassis was first demonstrated in 2000, and then a parallel hydraulic hybrid was developed in a Ford F-550 work truck, which achieved a 20-30% improvement in fuel efficiency in 2003. In subsequent projects, the world's first full-series hydraulic drivetrain of the delivery truck was designed and manufactured in Washington in June 2006; the revolutionary yard hostler with a series hydraulic hybrid in April 2009; and a series HHV minivan in January 2011. The EPA's HHV technology provides the possibility for multiple drivetrains or hardware configurations, depending on which will be the most durable, efficient, clean and cost-effective option. The development of the EPA's hydraulic hybrid vehicle technology represents, to a certain extent, the process of early hydraulic hybrid vehicles, which is related to the research and development of hydraulic hybrid vehicles, as shown in Table 3. Therefore, it can be concluded that HHVs have the advantages of a lower cost and higher power density; the latter is particularly advantageous as it makes HHVs recover much more of the available kinetic energy [69], which makes them more competitive than HEVs in certain application scenarios, as mentioned before and in Table 3.  
The related hydraulic hybrid vehicle technology has also been extensively studied, as can be confirmed by the following studies: passenger cars [38], buses [40], heavy-duty trucks [42], and special trucks [43]. However, hydraulic accumulators have a lower \begin{table}
\begin{tabular}{c c c c} \hline
**Item** & **Series** & **Parallel** & **Series–Parallel** \\ \hline Power element number & 1 & \(\leq\)2 & \(\geq\)2 \\ Hydraulic accumulator & \(\surd\) & \(\surd\) & \(\surd\) \\ Hydraulic actuator & \(\surd\) & \(\surd\) & \(\surd\) \\ Hybrid degree & Light & Medium–full & Medium–full \\ \hline \multirow{3}{*}{Strengths} & High engine efficiency & High technical maturity & Good emission reduction \\  & Easy control & Increased transmission efficiency & Increased transmission efficiency \\  & Simple construction & Elevated reliability & Increased engine efficiency \\ \hline \multirow{3}{*}{Weaknesses} & Interior efficiency & Complicated management & High cost \\  & Inadequate reliability & Inferior efficiency & Complex management \\ \cline{1-1}  & High cost & Finite energy-saving & Complex structure \\ \hline \end{tabular}
\end{table} 
Table 2: Summary of the configuration characteristics of the hybrid hydraulic vehicles. \begin{table}
\begin{tabular}{c c c c} \hline
**Year** & **Types** & **Configuration** & **Effect or Improvement** \\ \hline
2000 & Passenger vehicle test chassis & Full-series HHV & Uses a small 1.9 L diesel engine. \\  & & & Improves fuel economy without expensive lightweight materials. \\ \hline
2003 & Ford F-550 truck & Parallel HHV & Improves energy efficiency by 20–30\%. \\
2004 & SUV & Full-series HHV & Achieves 85\% better fuel economy. \\ \hline
2006 & Delivery truck & Full-series HHV & Achieves 60–70\% improvement in fuel economy. \\ \hline
2009 & Yard hostler & Series HHV & Improves fuel economy from 50\% to over 60\%. \\
2011 & Light-duty vehicles & Series HHV & Increases fuel economy by up to 60\% in city. \\  & & & Drives with an overall improvement of 30 to 35\%. \\ \hline \end{tabular}
\end{table} 
Table 3: Types, configurations and main features of hydraulic hybrid vehicles over time in EPA. energy density than HEVs, which makes some vehicle configurations, such as BEVs, more difficult [70].