Efforts to produce lighter vehicles necessarily include engine parts, such as the cylinder casing, which could shed up to 20% of its weight if it were made of fibre-reinforced plastic rather than aluminium – without added costs. Such injection-moulded parts are even suitable for mass production.
Cars must become lighter in order to reduce fuel consumption and for most car designers they are focused on reducing the weight from body parts, but the powertrain system, which includes the engine, also accounts for a large proportion of the vehicle’s weight.
Until now, carmakers 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 , in collaboration with plastics business unit of Sumitomo Bakelite in Japan used fibre-reinforced composite materials to create a cylinder casing for a one-cylinder research engine. The casing weighs around 20% less than its equivalent aluminium counterpart and costs the same to manufacture.
It seems an obvious solution to start replacing the heavier for the light, but getting there involved numerous technical challenges. The materials used have to be able to withstand extreme temperatures, high pressure and vibrations without suffering damage. That fact that plastics possessed these qualities was recognised back in the 80’s, but at that time it was only possible to produce this types of parts in a small volume and by investing a lot of effort in the form of manual labour, a no-go for the automotive industry, in which cylinder blocks are mass-produced in millions of units.
To ensure that their engine would be sufficiently robust they identified the areas subject to high thermal and mechanical loads, using metal inserts to strengthen its wear resistance. 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. 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.
The team uses a glass-fibre-reinforced phenolic composite developed by SBHPP, which fulfils all of these requirements and comprises 55% fibres and 45% resin. A lighter-weight but more expensive alternative is to use a carbon-fibre-reinforced composite – the choice depends on whether the carmaker wishes to optimise the engine in terms of costs or in terms of weight.
The researchers produce these components from granulated thermoset plastics using an injection moulding process. The melted composite material, in which the glass fibres are already mixed with the resin, hardens in the mould into which it was injected. The scientists analysed the process using computer simulations to determine the best method of injecting the material in order to optimise the performance of the finished product. The process is compatible with mass production scenarios and the manufacturing costs are significantly lower than those for aluminium engine parts, not least because it eliminates numerous finishing operations.
Test runs of the new engine have been completed successfully and has proven it’s capable of the same performance as conventionally built engines. It promises to offer further advantages such as lower running noise against engines relying exclusively on metal parts. Initial data also indicates that the amount of heat radiated to the environment is lower than that generated by aluminium-based engines. The scientists intend to take their research further by developing a multi-cylinder plastics-based engine, including the crankshaft bearings.