The EPA and its partner, Eaton Corporation–Fluid Power, will build the world’s first full diesel-hydraulic series hybrid delivery truck for UPS. The hybrid system is based on numerous EPA pioneering hybrid patents.
A hybrid hydraulic system 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. Earlier EPA prototypes 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.
Benefits of HLA include 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.
Eaton and Peterbilt announced their joint development of refuse trucks using Eaton’s parallel hydraulic hybrid system—Hydraulic Launch Assist (HLA). Peterbilt plans to build and evaluate a production version of the vehicle this year. (Earlier post).
The diesel-hydraulic hybrid under development for UPS, though, is a full series hybrid system that incorporates the new EPA Clean Diesel Combustion engine (earlier post) for further emissions benefits.
The CDC engine (earlier post) powers a hydraulic pump rather than a generator/motor
- A hydraulic drivetrain replaces the conventional drivetrain and eliminates the need for a transmission.
- Regenerative braking captures additional energy for the hydraulic system
Primary hydraulic components consist of two hydraulic accumulator vessels, one engine hydraulic pump, and one integrated rear-drive hydraulic pump-motor assembly
EPA and Eaton are targeting the following results from the combination of the CDC engine with the hydraulic hybrid system:
60%–70% improvement in fuel economy
Meeting the 2010 heavy-duty NOx standard
Recouping the additional cost of the hydraulic hybrid within 3 years
There are related hydraulic hybrid projects underway with the military.