I recently attended a meeting of the Clean Energy Coalition in Southeastern Michigan to see and learn more about the Eaton Hydraulic Launch Assist garbage truck purchased by the city of Ann Arbor. Hydraulic hybrids are similar to electric hybrids, except instead of storing energy captured from braking in batteries, the energy is stored in hydraulic fluid. The accumulator for the hydraulics stores compressed fluid which when released powers a hydraulic motor to provide power to the wheels of the vehicles during acceleration.
The point was made during the meeting that passenger cars and large SUVs or pickups have also been tested with hydraulic hybrid technology. The main reason that these technologies have not caught on in smaller vehicles is due to the weight of the hydraulic fluid. The amount of fluid needed would substantially increase the weight of a passenger vehicle thereby reducing the overall gains. This has largely left development focused on truck segments in recent years.
In trucks, hydraulic hybrids have a lot of potential for fuel savings. As John Kargul, EPA’s Director of Technology Transfer pointed out during his presentation, hydraulic hybrids are 70% more efficient than traditional Class 6 trucks compared to less than 25% efficiency increase for electric hybrids during a cycle of acceleration to 35 mph and then braking back to 0 mph. This type of operation is well suited to inner-city deliver or garbage collection trucks where vehicles are starting and stopping often and results in 50% fuel savings, according to the EPA.
The hydraulic technology (fluid, pumps, motors, accumulators, etc) used by this type of hybrid is generally well understood and commonplace. Newer accumulator tanks that use carbon fiber to reduce costs are likely the biggest technological breakthrough in recent years, though there have most certainly been other advances in motor and pump efficiency as well. Mass production would help reduce costs, as well.
However, the costs of hydraulic hybrids are not likely fall as dramatically as many anticipate battery prices will in the coming years.
Generally speaking, the hydraulic hybrid cost premium is often similar to that of electric hybrids in medium and heavy duty trucks. This means that the payback period on the premium for hydraulic hybrids is potentially much better if the improvement in efficiency is to be believed. Eaton is claiming a two year payback on their hydraulic launch assist technology in heavy duty trucks (though when I did the math with diesel prices hovering at $3/gallon, I came up with closer to three years). Either way, if these numbers prove valid in the real world, then hydraulics have the potential to be a better bet than electric hybrids for fleets looking to reduce their overall vehicle ownership costs.
Hydraulic hybrids will play a role in the marketplace, but Pike Research anticipates that this role will be with the bigger trucks, Class 6, 7, and 8 in specific niches. Hydraulic hybrids are also likely to be limited to some degree by the job a truck does. For example, a hybrid electric refrigerated truck can run the compressors for the refrigerated box off battery electricity, reducing idle time, but can’t do the same with hydraulic systems. As a result, the hydraulic hybrids are likely to grow within specific niches (garbage trucks, inner-city delivery trucks, shuttle buses), but will likely find difficulty breaking out of those niches.
Article by Dave Hurst, appearing courtesy Matter Network.