German researchers have developed a prototype engine with plastic parts that could lead to light-weight cars with reduced fuel consumption.
Efforts to produce lighter vehicles necessarily include engine parts, such as the cylinder casing, which could shed up to 20 per cent of its weight if it were made of fibre-reinforced plastic rather than aluminium.
Such injection-molded parts are even suitable for mass production, researchers said.
Until now, car-makers have relied on aluminium to reduce the weight of engine components such as the cylinder block.
In the future, car manufacturers will be able to achieve further weight savings by designing cylinder blocks in which certain parts are made of fibre-reinforced plastics.
An experimental engine developed by the Fraunhofer project group for new drive systems (NAS), which forms part of the Fraunhofer Institute for Chemical Technology ICT, demonstrates this principle.
"We used a fibre-reinforced composite material to build a cylinder casing for a one-cylinder research engine," said Dr Lars-Fredrik Berg, who is the project leader.
"The cylinder casing weighs around 20 per cent less than the equivalent aluminium component, and costs the same," said Berg.
The materials used have to withstand extreme temperatures, high pressure and vibrations without suffering damage.
"First we looked at the engine design and identified the areas subject to high thermal and mechanical loads. Here we use metal inserts to strengthen their wear resistance," said Berg.
One example is the cylinder liner, inside which the piston moves up and down millions of times during the life of the vehicle.
The researchers also modified the geometry of these parts to ensure that the plastic is exposed to as little heat as possible.
The characteristics of the plastic material also play an important role. It needs to be sufficiently hard and rigid, and resistant to oil, gasoline and glycol in the cooling water, researchers said.
It must also demonstrate good adherence to the metal inserts and not have a higher thermal expansion coefficient than the metal – otherwise the inserts would separate from the substrate.
Berg's team uses a glass-fibre-reinforced phenolic composite, which fulfils all of these requirements and comprises 55 per cent fibres and 45 per cent resin.
A lighter-weight but more expensive alternative is to use a carbon-fibre-reinforced composite – the choice depends on whether the car-maker wishes to optimise the engine in terms of costs or in terms of weight.