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The automotive engineer's modern quest to reduce weight and thereby the fuel consumption of cars has almost exclusively focused on body parts. More efficient engine technologies have been developed, but it is the stronger yet thinner steel alloys, composite and aluminium body panels by which a car mainly sheds the pounds.
Ironically, probably the single heaviest part of every car, the powertrain, including the engine, has remained largely untouched. Materials within an internal combustion engine have to be able to withstand extreme temperatures, high pressure and vibrations over a long period of time. This has meant carmakers have relied on aluminium to reduce the weight of engine components, such as the cylinder block.
There have been exceptions. Notably, a US firm called Polimotor developed an engine that used a reinforced plastic for the engine block and various other components, including piston skirts, connecting rods and the oil pan. Some surfaces, the piston crowns and the combustion chambers were reinforced with with metal. A version of the engine based on a Cosworth design was developed and successfully raced in a Lola car in the US.
The Polimotor engine in the Lola car
The idea was never developed beyond proof-of-concept. The barrier in the 1980's was mainly production and the associated high costs. It was only possible to produce these type of plastic parts in a small volumes with a high amount of manual labour. But this month a group of German researchers from the Fraunhofer Institute displayed a prototype engine at the Hannover Messe exhibition made of fibre-reinforced plastics that they say overcomes these production barriers.
Dr. Lars-Fredrik Berg, project leader and research manager of Lightweight Powertrain Design at the Fraunhofer Institute, says: “We used a fibre-reinforced composite material to build a cylinder casing for a one-cylinder research engine. The cylinder casing weighs around 20% less than the equivalent aluminium component, and costs the same.”
The research team used methods similar to Polimotor's to protect the engine's plastic components from the extreme conditions in an internal combustion engine. For example the cylinder liner, inside which the piston moves up and down millions of times during the life of the vehicle contains is metal. The researchers also modified the geometry of these parts to ensure that the plastic is exposed to as little heat as possible, says Berg: “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.”
The prototype engine developed by the Fraunhofer team
Composite materials have also improved over the last 30 years. The plastic used in the engine needs to be hard and rigid, and resistant to oil, gasoline and glycol in the cooling water. It also has to adhere to the metal inserts while not having a higher thermal expansion coefficient than the metal. Otherwise the inserts would separate from the substrate.
The glass-fibre reinforced phenolic composite the Fraunhofer team used was developed by the high-performance plastics business unit of Japanese firm Sumitomo Bakelite. It comprises 55% fibres and 45% resin. A carbon-fibre reinforced composite could be used but would be more expensive. The choice would depend on whether the carmaker wants to optimise the engine in terms of costs or in terms of weight.
The researchers produced 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. Computer simulations were used to determine the best method of injecting the material and optimise the performance of the finished product. The process is compatible with mass production scenarios. In addition the manufacturing costs are lower than those for aluminium engine parts because it eliminates numerous finishing operations.
Test runs of the new engine have been completed successfully. “We have proved that it is capable of the same performance as conventionally built engines,” says Berg. The engine also offers further advantages such as lower running noise. Initial data also indicates that the amount of heat radiated to the environment is lower than that generated by aluminium-based engines.
Encouraged by the results the researchers are now working on a multi-cylinder plastics-based engine, including the crankshaft bearings. Time will tell if plastics-based engines will enter the mainstream, but the prototype engine would seem a definite step forward.