Naked Science Forum
Non Life Sciences => Technology => Topic started by: neilep on 06/10/2007 22:48:01
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Dearest Engineers!
See this engine ?
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nice eh ?
Engines are my all time favourite piece of mechanical powering object of engineering thing that gives power to things that need it !!
[:)]
But...do they have to be made of metal ?...what about some sexy carbon fiber ?....or anything else ?
whajafink ?....cos I dunno !!
hugs et les shmishes
Neil
mwah mwah mwah
xxxxxx
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Firstly, carbon fibre is rarely used as a material on its own - when people talk about carbon fibre, they are usually referring to carbon reinforced plastic, which is a composite (they key issue here is that the properties of the material depend both on the properties of the fibre, and the properties of the matrix it is embedded in).
More specifically, there is no particular reason why metal must be used, but metal is a convenient material that can cope with high temperatures, has good thermal conduction (thus making it easy to cool) and good shock resistance.
Carbon reinforced plastic would not survive the temperatures for all the conventional plastics we normally use. Whether carbon reinforced materials other than plastic might be used is another matter.
Ceramics certainly could manage the heat, but don't handle the shock resistance easily (I believe the high temperature fans of some modern jet engines may be fabricated using ceramics).
Ofcourse, it is probably wrong even to regard the engine above as being fabricated of metal. It is fabricated using many different materials, although the cylinder block, pistons, and most load bearing components, are indeed metal. The seals, electronics, and many other components are not metal at all, and the engine uses a whole myriad of materials for different purposes.
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Yes, ceramics can't still be used also because of their low resistance to traction, flexion, torsion; so it would be particular problematic to make ceramic rods, cogs, gears, crankshafts, cramshafts, at a reasonable cost.
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Anyway - what's wrong with metals?
There's one available for almost every job you might want to do - except insulation..
Composite rules!
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Anyway - what's wrong with metals?
There's one available for almost every job you might want to do - except insulation..
...and friction; ceramics are harder than metals, in general.
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Some parts have to be made of metal, but thermo-plastics and ceramics have already been utilised in engines.
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There has been work on composite carbon matrix metals, but the casting and machining is to say the least "difficult". Many parts on high performance engines can be made out of things other than metal, but nothing is at the same time easy to machine, strong, ductile and long lasting in temperature extremes and high shock loads.
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Have ceramic engines become significantly more realistic since 2007 when this thread was started?
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Cheaper to make yes, but the parts makeup has not changed much, you still need metal parts.
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Is tungsten carbide a ceramic?
No doubt we will be seeing more composites in the engines, although often the covers we see on some new engines are just pretty plastic hiding all the important bits.
I could imagine a plastic oil pan, but one of the reasons that steel is often chosen over aluminum for the oil pan is that it is cheap and durable. Nobody likes a rock knocking a hole in the bottom of their oil pan.
One could likely use a polymer engine block with steel alloy or ceramic cylinder liners. Thermal expansion may be an issue, trying to match the thermal expansion profiles of all the parts in the engine so one doesn't get pieces coming loose, seizing, or cracking.
No doubt we'll see more composites in the automotive industry over the next few decades. However, things like plastic bumper covers often crack with the slightest ding, especially after aging for a decade or so, and are a pain to replace.
A car company would have serious publicity issues if they used, say a plastic oil pan that experienced embrittlement after a decade or so, and required a very expensive replacement.
Engines are heavy, although a lot of weight is saved by using aluminum (with cylinder sleeves). I could imagine someone making a carbon fiber engine block for race cars. While the stresses in a race car can be extreme, many components are designed to last a single race before complete tear-down and rebuild.
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Here's a link that discusses the advantages of ceramic (rotary) engines and the various types of ceramics that might be suitable: Why Ceramic Engines? (http://ceramicrotaryengines.com/).
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The above discussion is mostly about fuel-burning engines (internal combustion & jet engines) where the strength and temperature resistance of metals is an advantage.
Other categories of engine are:
- Electric motors: We can use ceramic magnets, but conducting electricity at room temperature is best done by metals like copper. (You could use superconducting ceramics at very low temperatures, and plasma at very high temperatures...)
- Flywheels: These could use composites
- Compressed air engines: This could in principle use plastics or composites, but the energy density is not high.
- Rubber Bands: When I was a kid we had model airplanes that had a rubber-band energy storage system
Overall, while there are alternatives to metals for some parts of an engine, overall it's pretty hard to beat the strength, conductivity, malleability and wide temperature range of metals for a variety of applications...
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You can build rocket engines completely out of fiber reinforced plastics; particularly phenolics for the nozzle and other hot bits. They don't last all that long, but then again, they don't have to!
The Apollo capsule used phenolics for the reentry shield, the plastic chars and forms a layer that protects the capsule; it gradually erodes, but lasts long enough. It also works in engines provided the chamber pressure isn't too high.
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If a gun can be knocked up in plastic with a 3d printer - http://www.thenakedscientists.com/HTML/news/news/1000170/ - then I wouldn't be surprised if an engine couldn't be similarly manufactured...