Light under the bonnet

With composites appearing more and more frequently in cars, will we ever see a 100 per cent plastic engine? Roger Brownlie reviews the move away from metal.

Since the first mass-produced V-8 engine hit the roads in 1932 the basics of the internal combustion engine have not really changed all that much.

This clearly points to both the staying power of what is a classic piece of engineering design, and how in some fundamental areas an otherwise fast-moving industry is slow to advance.

But new materials are constantly evolving – and one area that has attracted increasing interest over the last few years is plastic.

From electrical components to body panels and interior features, its use is growing. The percentage of plastics by weight in cars, for example, has doubled from the early 70s to about 15 per cent today.

With the replacement of more and more metallic components with lighter materials, and advances in polymer technology, the last decade has seen plastic enter the engine compartment itself.

So will the time come when we see the totally plastic combustion engine?

Currently researchers are facing two distinct challenges – how to make the plastic tough enough so that it won’t shatter, and sufficiently resistant to heat.

Plastic has numerous advantages for the car designer. Compared to steel and low-weight alloys, it is light. It also offers designers greater freedom to create the shapes they need for functionality.

Plastic is cheaper, resistant to chemicals and it helps alleviate noise, vibration and harshness. In modern vehicles almost all intake system components – air cleaner housing, resonators, air mass sensor housing and intake manifolds – are plastic.

Jaguar’s XK8, for instance, has a lightweight nylon air intake manifold with a moulded-in fuel rail. According to the company it is the first integrated air-fuel module with two passages into the body of the manifold.

It is made by Siemens Automotive Powertrain Air Induction Operation using BASF Ultramid nylon. Jaguar says the module is half the weight of a similar design made of pressure-cast aluminium.

French automotive expert MGI Coutier recently used the same material to mould a throttle body housing. The firm claims it is able to withstand high mechanical loads and is chemically resistant to car fuel and lubricants.

Elsewhere in today’s vehicles plastic engine rocker arms (made from either compression-moulded vinyl ester or injection-moulded nylon) are now relatively commonplace and the use of plastic oil pans is also becoming popular.

But plastic is an all-embracing term and with so many varieties available it is easy to assume chemists can develop the material for any application including the ability to withstand the mechanical and thermal loads found inside an engine.

Chris Brown, head of polymers and plastics at the National Physical Laboratory in Teddington, explained some of the difficulties involved in adapting polymers for these purposes.

‘There are certain limitations with polymers,’ he said. ‘But you have a few tricks to play with to get the best combination of stiffness and toughness. For example, you can add rubber particles to form a blend to make the polymers less brittle.’

One innovative toughening process along these lines has been developed at The Ohio State University where researchers have mixed plastic with silica to create a material much tougher than plastic alone.

‘While plastic engine parts would mean lighter, more fuel-efficient cars and aircraft, today’s heat-resistant plastics often shatter at the smallest impact,’ said John Lannutti, associate professor of materials science and engineering at the University.

It was military testing of parts made with heat-resistant plastic and reinforced with graphite fibre that illustrated the need for tougher plastics.

‘Because of the way these plastics were made, incidents such as a bird flying into the wing of an aircraft, or even a wrench falling on to a part would shatter them and leave only the woven graphite fibres,’ said Lannutti.

The material created at Ohio State is tougher than plastic alone because it divides the force of a blow into many small interactions involving millions of individual silica particles. ‘As a crack starts to travel through the composite it breaks up into finer and finer cracks, until the material has dissipated all of the energy of the impact,’ said Lannutti.

He and his colleagues call the method synergistic toughening, or ‘toughening across scales’ – because it strengthens material down to the scale of individual molecules.

Fibre-reinforced plastic’s tolerance of temperatures up to 400 degrees C makes it ideal for parts surrounding jet engines. In Lannutti’s laboratory tests, the plastic-silica material retained the heat resistance of fibre-reinforced plastic but showed four to five times the resistance to shattering.

Lannutti said the new composite is not as hard as steel but displays good heat resistance at a fraction of the weight.

Although there is much research on the use of carbon fibre composites for encasing jet engines, Brown believes the usefulness of polymers in a combustion engine is limited.

‘Polymers will degrade above a certain temperature so you don’t stand a hope,’ he said.

The fundamental background to the temperature problems is related to basic chemistry. One of the methods of increasing heat resistance involves processing using traditional polymer techniques: the polymer is transformed through a physical or chemical process into a more temperature-resistant substance.

Alternatively, thermosets like epoxy and phenolic resins can be used.

Using such techniques, BMW, along with moulder Baumgarten, plastic supplier Vyncolit, and system supplier Kolbenschmidt Pierburg, has designed, developed and manufactured the first variable air intake manifold with phenolic composite components for its latest 7-series engines.

Due to the required shape of the parts, Pierburg decided to make the manifold in plastic to save weight and secondary machining costs. The material chosen was Vyncolit X7250, a glass fibre-reinforced phenolic compound. The material promises dimensional stability up to 160 degrees C.

But if polymers have finite limitations due to their chemistry, perhaps other materials could find uses within the combustion engine. For instance, while ceramics, renowned for their resistance to heat, have traditionally been let down by their brittleness, huge improvements in their mechanical properties give them a bright future.

Isuzu Ceramics Research has developed a hybrid ceramic/plastic material highly resistant to cracking and, according to Isuzu, comparable to iron and steel for mechanical parts. This compound consists of silicon nitride and carbon fibre. The company claimed that compared with conventional ceramic materials, cracking resistance is 2.5 times higher.

Isuzu has investigated its use in the manufacture of marine diesel engine cylinder liners and confirmed that this is feasible. They are currently manufactured from steel materials, and have to be replaced regularly due to corrosion and wear.

‘I would not say that mechanical properties are a limitation of polymers.’ said Brown. ‘There are some as stiff and strong as steel. The number one enemy is heat. And that’s a pretty basic rule if you stick to a carbon-chain backbone. You have to do something rather special to improve on that. You have to do some sort of carbon transformation to make it look more like graphite or even diamond.’

The Polish Academy of Sciences UNIPRES High Pressure Research Centre in Warsaw has achieved just that by developing a technology to create super-hard plastics by blending diamond with silicon or metal.

This is done by combining microscopic diamond grains, each one millionth of a millimetre in diameter, with liquid silicon. The two react at 2,000 degrees C and 80,000atm of pressure to create silicon carbide.

The result is a homogeneous substance that has the properties of diamond and can be used for machining blades that are sharper than traditional diamond ones.

While plastic materials clearly offer the car designer a number of attractive advantages, it seems unlikely that in the foreseeable future engines will be composed entirely of plastic.