User:Razeenn

This is my Material Science A244 project on the ICE Piston. Feel free to browse it.

-Razeen Nieftagodien, 16031741

Introduction to the piston
The piston is a component in a piston-cylinder set up, which involves a solid, flat topped, plate (i.e. a piston) moving within a cylindrical sleeve (i.e. a cylinder), as shown the figure. The piston-cylinder combination is used commonly in the internal combustion engine (ICE), although it not used in ICE’s exclusively. Alternatives to pistons are used, although rarely, such as rotors in the Wankel rotary engine. The most commonly known application of the ICE is the automotive engine.

The main functions of the piston in the ICE are to contain or seal in air fuel mixture and to transmit the energy developed by the combustion to the crank shaft via the conrods. The piston is connected to the conrod by a gudgeon pin.

There are two main types of pistons, forged and cast, getting their names from the fabricating process used to produce them. Cast pistons are the most common type of piston, because they are more economical to manufacture.

The materials and mechanical properties
As stated before, cast-iron and aluminium silicon alloy are the materials of mostly used. In modern times aluminium silicon alloy is the preferred material and therefore we will only be looking at pistons made from this. The aluminium is commonly alloyed with about 12% - 16% silicon which makes it a eutectic material. Pistons with a higher percentage silicon are used to a lesser extent and are hypereutectic. Alloying with silicon allows the piston to have a lower thermal expansion which is of high importance in an ICE as the engine starts at a relatively cold temperature and the increase in heat can cause a significant thermal expansion. The silicon addition means that instead of having a low tolerance on the piston and cylinder, tolerances can be made higher, increasing the efficiency of the design. On the downside, the added silicon makes the piston very brittle. If fact, it is so brittle, dropping a piston could cause it to crack. Due to the processes of forging and cast, forged piston of the same material are more ductile that their cast counterparts and have a higher ultimate strength but are heavier and more expensive. The higher ductility and strength of forged pistons makes it optimal for high performance application where there is a higher level of detonation (ignition of uncombusted air-fuel mixture outside of the power stroke) and the higher manufacturing price makes it more practical in higher end application. The ability for forged pistons to take a higher level of detonation means the piston it can last longer before failing. With the former noted, the cast pistons are more than adequate for standard application and cheaper to produce, which makes it a much more practical choice. It can be said that using a forged piston in place of a cast piston when not necessary is simply a waste of money. The heavier forged piston also deters non-high end engine manufacturers from using it due the weight requiring additional energy and thereby reducing output power.

The manufacturing process
The production of cast pistons begins by heating the alloy to 700˚C and melting it. The molten alloy is then poured into the casting die in the shape of the piston and left to cool. Hot water is used after the piston is solid enough to further cool the piston. Once the piston has cooled, they are tempered overnight to improve strength and ductility as well as improving machinability. After the tempering, the runner (the device used to pour the material into the casting) is removed. The gudgeon pin and lubrication holes are then drilled and the piston is sent to a computer numeric controlled lathe and the outer layer of material is taken off the piston, in case there are any imperfections from the casting. The slots are made at this time for the compression and oil rings. The piston goes on to be ground to its final size.

The forged piston starts of as billet (single long rod) alloy that are cut to size and heated. The forging process is done by placing the material into a punch and pressing it with 2 000 tons of pressure into the shape of a piston. The piston cools after this for an hour as the punching process causes the piston to become extremely hot. Once cooled, the piston is sent to a computer numeric controlled lathe to remove excess material and the slots for the compression and oil rings. The gudgeon pin and lubrication holes are drilled. Excess metal is milled off after the drilling. Lastly the piston has its rough edges removed and cleaned.

Improvements to pistons
The current method of piston manufacturing seems to be the optimum method, however, the late 1980’s experiments were done by Mercedes-Benz on a car using carbon pistons. It was believed that carbon would be a better material for producing pistons. The hypothesis was that carbon has a lower density, low thermal expansion, higher damping capacity and low coefficient of friction, which would (for the above mentioned properties) decrease weight, allow for higher tolerancing, reduce noise and vibration, and reduce wear respectively. The tests were first done in a lab and then in real life situations. The result were astonishing, emissions were reduced significantly, the was a slight increase in power and reduction in fuel consumption, but most of all, a 56% reduction in the amount of oil the engine used up with hardly any wear to the piston. All this was achieved with a car that only had its pistons changed and no optimisation done to the engine to work more efficiently. The only problem was that the tensile strength of graphite is 70MPa compared to the 225MPa of the aluminium alloy used, and this restricted engine speed of the car to 4000rpm. There is no further document on the developments from the experiment. There are, however, companies today who manufacture carbon pistons such as, Schunk Kohlenstofftechnik GmbH.