New hybrid electric bus takes advantage of composites
Mobile Energy Solutions LLC (MES, Golden, Colo.) will soon premiere a “green” transit bus, featuring a fuel cell/hybrid permanent-magnet electric propulsion system (built by UQM Technologies, Frederick, Colo.) optimized for fuel efficiency with an all-composite body. The low-floor, 37-passenger vehicle is being developed under two R&D contracts from the U.S. Department of Transportation Federal Transit Admin.’s (FTA) National Fuel Cell Bus Program. The 35-ft/10.7m long bus bodies will be infusion molded by Martin Marietta Composites (MMC, Raleigh, N.C.). MMC designed the body per MES’ load cases and executed laminate design and finite element analysis (FEA), which was then reviewed by Golden-based MES’ subsidiary CompositeTek.
The design will be fabricated in two sections: an upper shell (the roof from the window line up) and a lower shell comprising four separately molded subcomponents that will be bonded together and then joined to the upper shell. The structure incorporates primarily monolithic solid laminate, but has balsa-cored sandwich construction in the engine compartment and in the floor pan that supports the rechargeable batteries. MMC’s VP Grant Godwin says the completed structure forms a monocoque that will function as body and chassis. The body’s reduced weight will make the bus more efficient than a standard bus retrofitted with hybrid propulsion, says MES’ VP of operations Phil Sweesy.
For now, tooling is simple plywood-and-fiberglass female molds made from CNC-machined foam plugs. Godwin says more robust composite or, possibly, metallic molds will be developed for rate production, assuming sufficient demand. Through the integration of mold inserts, MMC will be able to use the same tools to produce bodies ranging from 25 ft/7.6m to 45 ft/13.7m in length.
The predominately E-glass/vinyl ester layups include wovens and stitched multiaxials, with a small amount of uni carbon fiber at strength-critical suspension attachment points, engine mounts and window frames. Laminate thickness varies from 0.375 inch/9.4 mm to 5.5 inches/137.5 mm in cored sections. During development, MMC used resins from Ashland Performance Materials, Composite Polymers (Columbus, Ohio) and reinforcements from Vectorply Corp. (Phenix City, Ala.) and Owens Corning (Toledo, Ohio), but Godwin notes that MMC has not yet selected the production suppliers.