Engineering Insights: Composites Lessen Load in Fuel-Cell Demonstrator
CFRP decklid helps offset fuel-cell system weight to meet vehicle weight target.
By Peggy Malnati, Contributing Writer | February 2008
Fuel-cell systems might eventually revolutionize propulsion for ground transportation with energy-efficient power that produces little or no pollution. However, significant technical challenges must be overcome before fuel-cell vehicles (FCVs) are cost-competitive with internal-combustion, battery-electric, and hybrid-electric vehicles. Automakers are using small fleets of demonstration vehicles to work out these kinks in the hope of being first to market with commercial FCVs. Ford Motor Co. (Dearborn, Mich.) was among the first to put a fleet of FCVs on the road. In 2005, 30 units — heavily modified versions of Ford’s production compact, the Focus — were deployed to city governments and research organizations for real-world testing.
Engineering Challenge: Find a materials/process solution that minimizes decklid weight and can be fabricated cost-effectively in extremely low volumes yet accommodates carryover components and duplicates exterior appearance.
Design solution: Replace stock steel decklid with solid and sandwich CFRP constructions processed via vacuum bag/autoclave on carbon composite tools using an in-mold primer.
One challenge is offsetting the higher mass of the fuel-cell and hydrogen storage systems (approaching 400 kg/900 lb) to meet vehicle weight targets. To achieve the desired curb weight of 1,600 kg/3,527 lb for the Focus FCV, Ford responded with an aggressive lightweight-ing strategy that included aluminum hoods, front fenders, wheels and suspension com-ponents; lighter steel body panels; titanium suspension springs; lightweight front and rear glass; polycarbonate side windows; and a carbon/aramid composite underbody shield and fuel-cell cover. Ford also replaced the steel decklid with on of lighter and cost-effective carbon fiber-reinforced plastic (CFRP).
THE ROAD TO CARBON FIBER
Before the Focus FCV, automotive CFRP was used primarily on sports cars and supercars, usually driven by the need for aggressive styling, low mass (to boost performance), and tooling cost reductions on these low-to-moderate volume vehicles. Although styling and speed were unimportant for the Focus FCV, mass was critical to optimizing driving range and cost savings that, if they could be reaped, would help offset the investment for the fuel-cell system.
Ford asked Multimatic Inc. (Markham, Ontario, Canada), an experienced automotive body/chassis systems supplier, to design and produce several components for the FCV, including the decklid. Goals for the latter were that it be produced in low volume at reasonable cost, yet meet all OEM performance and appearance requirements at less than 4.75 kg/10.5 lb — 56 percent less mass than the production steel part. While the decklid could be altered, design changes would be limited by the fact that the exterior styling surfaces were to remain essentially unchanged and hardware and trim components from the steel decklid would carry over into the new component.
The benchmark was a multipiece steel decklid weighing 10.8 kg/23.8 lb and measuring 430 mm long by 1,260 mm wide by 450 mm high (17 inches by 50 inches by 18 inches). Components targeted for conversion included the Class A outer panel, the inner panel, and local reinforcements for latch, hinges and rubber bumpers that mounted on the inner panel. Carryover items included the wire harness and clips; emergency release and fasteners; latch hardware and fasteners; bumper stops; hinges and reinforcements; and the interior trunk liner, all of which attached to the inner panel and therefore would impact its design. Additionally, the outer panel provided attachments for the carryover stop lamp, lid handle and exterior trim.
Initially, Ford considered the decklid in aluminum because, at that time, other closure panels were being converted to aluminum, which could be stamped using the steel part’s legacy production tooling. However, the steel decklid’s tools did not reproduce its complex geometry properly in aluminum. The cost of new tools could not be amortized over 30 vehicles. Given the design criteria, composites were the logical next choice.
DESIGNING WITHIN CONSTRAINTS
Source: Ford Motor Co.
In 2005, Ford Motor Co. deployed a fleet of 30 fuel cell vehicles (FCVs) based on its Focus production passenger car, to city governments and research organizations for real-world testing.
The panel geometry was carried over from the steel design to maintain functionality. This permitted bonding of the latch reinforcement to both inner and outer panels. The design also accepts the stock trunk liner, although it was subsequently eliminated. While the preliminary composite design was similar to the steel decklid, several factors prompted alterations. A hydrogen vent duct was added to the outer panel to bleed off excess pressure from the hydrogen-storage tank. This change altered the local geometry of the inner panel’s seal plane as well. Additionally, the inner panel geometry was greatly simplified to aid manufacturing feasibility of the composite design.
