Chargeboard claims it is the first Chinese company to develop hydraulic hybrid technology. Last year, the company signed a contract with the Beijing Bus Company to jointly promote the technology in the city.

A hydraulic hybrid (earlier post) uses an accumulator (which stores energy as highly compressed nitrogen gas) and one or more hydraulic pump/motors rather than the battery pack, electric generator/motor and power electronics used in electric hybrids.

Like their electric cousins, hydraulic hybrids can come in different configurations with different benefits.

A milder form of hydraulic hybrid is termed Hydraulic Launch Assist (HLA). With HLA, a reversible hydraulic pump/motor and accumulators are added to optimize fuel economy, while keeping the vehicle’s conventional engine and transmission.

Regenerative braking captures the braking kinetic energy. Hydraulic hybrids developed by the US EPA stored and re-delivered about 80% of braking energy back to the wheels. The efficiency of this regenerative braking makes a hybrid hydraulic design very attractive for vehicles operating in stop-and-go conditions.

Work being done in the US by Eaton and Peterbilt points to HLA benefits of a 25%–45% improvement in city fuel economy, with a concomitant reduction of emissions by 20%–30%; better acceleration, less brake maintenance and reduced operating costs.

A full diesel-hydraulic series-hybrid under development for UPS by Eaton and the EPA (which uses the new EPA Clean Diesel Combustion engine—earlier post) promises a 60% to 70% improvement in fuel economy.

The hydraulic series-hybrid uses the clean-diesel engine to power a hydraulic pump rather than a generator/motor. A hydraulic drivetrain replaces the conventional drivetrain and eliminates the need for a transmission.