Lightning Hybrids, developer and manufacturer of hydraulic hybrid systems for medium- and heavy-duty trucks and buses (earlier post), has provided some detail and background on a core component of its system: the hydraulic accumulator, or “mechanical battery”.
In the hydraulic hybrid system, hydraulic pumps and the lightweight accumulator store braking energy; upon acceleration the stored energy provides torque to the wheels. Up to a 35% gain in fuel efficiency in heavy drive cycles is achieved by regenerating the braking energy that is normally wasted to heat, and using this energy instead of the internal combustion engine for acceleration. Lightning Hybrids’ patented hydraulic hybrid system includes high-efficiency hydraulic pumps, a power transfer module, the lightweight accumulator system, a valve block, and a proprietary control system.
|The hydraulic hybrid accumulator system. Click to enlarge.|
Lightning Hybrids originally designed its own lightweight composite accumulator made of a thin steel or aluminum vessel wrapped with carbon fiber composite material. In October 2012, Lightning Hybrids sold its accumulator assets, manufacturing equipment and related intellectual property to a CoorsTek group that created a sister company, Steelhead Composites, LLC. (Steelhead has since increased its product line to include other products such as fuel storage vessels for vehicles running on compressed natural gas (CNG)).
|System in place underneath a truck. Click to enlarge.|
Steelhead Composites manufactures accumulators from aluminum tubing and spun carbon fiber. Sections of aluminum tube are formed into the shape of a vessel (the liner). For Lightning’s application, an additional layer of fiber glass composite is applied at the end of the process to provide impact resistance to the accumulators, making them safe to deploy under a truck or bus.
Differentiating features of the Steelhead Composites composite accumulator include:
Steelhead has optimized the size of the composite accumulators to integrate with hydraulic hybrid vehicles and is currently offering these vessels in 10 inch (250 mm) internal diameter. This is different from traditional steel accumulators which have inside diameter of 8 inches (200 mm). Because of their larger internal diameter Steelhead’s accumulators allow for more internal volume and energy storage capability for a given vehicle chassis length. Steelhead has exclusive ownership of the tools and molds required to produce bladders, aluminum liners and port fittings for these larger diameter accumulators.
The operating pressure of Steelhead’s composite accumulators is 6,000 psi (414 bar). This is higher than the industry standard operating pressure of 5,000 psi (350 bar). Traditionally, pressures like this would require a steel vessel that is extremely heavy, compromising the benefits of the hybrid system. The design based on composite material from Steelhead delivers an accumulator that weighs about one-third to one-fourth of its steel equivalent, while safely supporting the maximum operating pressure.
Composite accumulators offered by other vendors are typically adapted from heritage designs of vessels meant for gaseous storage and have relatively small port openings: 2.5-inch (63.5mm) or less. Steelhead’s composite accumulators have been designed with a larger port opening on the hydraulic end: 3.5-inch (89 mm) diameter or larger. A large port opening poses a significant design challenge for the composite vessel. Steelhead has overcome this challenge through extensive design iterations and proprietary technology to strengthen the polar opening. The large port opening allows for better flow rates of hydraulic fluid and easy installation of the bladder. It also accommodates a bladder with higher wall thickness that results in low permeability and longer life span for the bladders.
Since the accumulator has to operate reliably at high pressures repeatedly over many thousands of cycles, manufacturing is carefully controlled, with testing and inspection being key parts of the process. The high pressure accumulators are specifically designed such that a failure over repeated cycles results in a leak and not a catastrophic burst.
The Lightning Hybrids hydraulic hybrid system is that it can easily be installed on a wide range of vehicles. To accommodate vehicles of different rated weights, the company simply uses smaller or larger accumulators, ranging from 10 gallons to 30 gallons (US). (A second pump/motor is added for larger vehicles.)
The Lightning Hybrid System is available now on the following platforms:
Ford: E-350 and E-450 all wheelbases; F-350XT, F-450XT, F-550XT 164-inch wheelbase and greater; F-59 all wheelbases; Transit van/wagon 148-inch wheelbase; Transit Cab Chassis 156-inch wheelbase and greater
Mercedes-Benz: Sprinter 170-inch wheelbase cab chassis; Sprinter 170-inch wheelbase van; Atego all wheelbases (Europe); Vario all wheelbases (Europe); Actros all wheelbases (Europe)
Freightliner: MT-45; MT-55; MB-65; M2 106; Thomas Built Saf-T-Liner C2
Chevrolet Express / Savana: Express/Savana – 2500, 3500 series full body 155-inch wheelbase; Express/Savana – 4500 series cutaway 159-inch wheelbase or greater
Ashok-Leyland: Transit Bus (India)
DAF: LF55 (UK)
Volvo: FL (15 tonne) (UK)