No part of an engine is stressed more than its pistons. When a piston rises in compression, the pressure of the air-fuel mixture confined above rises to around 15 bars. Then a spark ignites the mixture. Over the next 45 degrees of crankshaft rotation, the mixture burns, reaching a maximum pressure of perhaps 90 bar. The gas temperature rises by 2500°C. The melting point of pure aluminum is slightly lower than 650°C, and the alloy for greater strength reduces it to 500 degrees.
How can metal survive? First, the pistons are exposed to combustion for only a tenth of the engine's rotation time, or 60 to 80 degrees on the 720 of the four-stroke cycle. Second, the surface of the piston is protected by a thin “boundary layer” of stagnant gas that clings to it, providing effective insulation. Additionally, aluminum conducts heat three times faster than iron or steel, while having just over a third of the weight. This means that the heat absorbed by the piston crown is quickly carried away to a cooler part of the piston. Because aluminum is light, more of it can be used to carry away the heat of combustion.
Pistons in brief:
– The key to fast, efficient combustion is a flat, featureless combustion chamber with at least one recess area in the shape of the valve heads.
– To minimize bearing friction, pistons should be as light as possible. Forging combines lightness and improved resistance to stress fatigue.
– So that the gas pressure can keep the piston rings always at the bottom of their grooves, the rings are lightened by being ever thinner.
As engine performance increases, with larger bores and shorter strokes, piston weight must be reduced to reduce vibration and stress on bearings and mechanical parts. Traditional pistons conduct their heat to the cooler cylinder wall through thick piston crowns, but such weight is impossible to bring to high revs. Therefore, today's light, thin, short-skirted pistons are also cooled by engine oil jets.
People used to talk about a limiting piston speed (usually given between 20 and 25m/s), but what really limits the rpm is the extreme constraint of piston acceleration. While the pistons of a Formula 1 engine from the V-10 era reached maximum accelerations of up to 10.000 G. The pistons in a 600cc sports car engine capable of reaching 15.000 rpm experienced 7.000 G. More the acceleration is high, the greater the forces of the connecting rod on the piston pin and the more cracks can form in the material.
Because they are closest to the heat, the upper piston rings should be plated or filled with a high melting point metal such as chromium (1850°C) or molybdenum (2600°C). This delays wear by limiting localized welding and subsequent tearing of surface metal particles. The upper rings generally have a cylindrical face to ensure sealing even with some piston tilt. The second so-called sealing or compression segments (rarely used in racing engines) generally have a tapered face. The oil scraper ring consists of two very thin, flexible rails exerting a high specific pressure, pressed outwards by an elastic expander. Durable oil control is essential to the long life of exhaust catalysts.
Although some heavy-duty diesel engines have steel pistons, those used in spark-ignition engines are cast or forged from two basic types of aluminum alloys: either a low expansion aluminum-silicon alloy and wear-resistant, i.e. a high-strength aluminum-copper-nickel-magnesium alloy.
Pistons have evolved from a high-domed, bucket-shaped shape of the 1960s to today's flat-topped "ashtray" proportions, with every detail of their underside having received notable lightening thanks to quite a few materials strength studies. pushes. The pieces shaped in this way closely to the requirements of nature have a fascinating beauty. They cannot have other forms and functions as well.
