Recent government requirements for greater fuel efficiency have led to lighter cars hitting the market, but there's only so much that can be shaved off the body and chassis. To find further weight reductions, a Fraunhofer project group is developing ways of building engine cylinder blocks that are partly plastic.
Lighter engines that can still do the job of their heavier cousins have been around since the first aluminum engine blocks were introduced in the 1960s. Going one step further with plastic has been on the drawing boards since the 1980s, but plastic parts able to withstand engine heat and stress could only be made in small volumes and at great cost.
The approach taken by the Fraunhofer project group for new drive systems (NAS) was to create an experimental engine using fiber-reinforced plastic suitable for injection molding instead of aluminum.
"We used a fiber-reinforced composite material to build a cylinder casing for a one-cylinder research engine," says Lars-Fredrik Berg, project leader and manager of the research area Lightweight Powertrain Design at NAS. "The cylinder casing weighs around 20 percent less than the equivalent aluminum component, and costs the same."
Fraunhofer says that using plastic not only allows for lighter vehicles, but provides the added advantages of reduced fuel consumption, less noise, and less heat radiated from the engine. However, achieving this meant overcoming problems with heat and vibrations.
"First we looked at the engine design and identified the areas subject to high thermal and mechanical loads," says Berg. "Here we use metal inserts to strengthen their wear resistance."
Other problems the project faced was getting the plastic to bond well with and expand like metal, redesigning engine parts to keep heat away from the plastic, and making the plastic hard and rigid, yet also capable of withstanding contact with oil, petrol, glycol, and water coolants.
The project eventually settled on a glass-fiber-reinforced phenolic resin of 55 percent fibers and 45 percent resin that uses granulated thermoset plastics in an injection molding process. The glass fibers are already mixed with the liquid resin, which is injected and hardens in the mold. The trick was sorting out the details of the process to obtain the best results while eliminating many of the finishing operations needed for metal parts. Carbon composites can also work and are lighter, but are more expensive.
Fraunhofer says that a prototype of this engine will be presented at this year’s Hannover Messe this month, and that the project is now working on a multi-cylinder engine.
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