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Emerald Group Publishing Limited
Copyright © 2005, Emerald Group Publishing Limited
Put cryogenic power in your piston
Put cryogenic power in your piston
Keywords: Cryogenic equipment, Aircraft engines, Metals
Back in the 1940s scientists discovered that by immersing some metals in liquid nitrogen they could increase the wear resistance of motor parts, particularly in aircraft engines, giving a longer in- service life. At the time this was little more than dipping a part into a flask of liquid nitrogen, leaving it there for an hour or two and then letting it return to room temperature. They managed to get the hardness they wanted but parts became brittle. The applications at this stage were mostly military. A controllable cryogenic treatment of metals and plastics was developed by NASA during the space race and commercialised, through the 1990s, in the USA with many applications.
Andy Priscott, managing director of Cryogenic Treatment Services, explains how the process can improve the performance and power of engines, “Engines have a series of compromises, there are a number of areas where trade-offs have to be made and unfortunately the need for compromise cannot be eliminated. The balance between fuel economy, emissions, and power can be achieved if the octane of the fuel is unlimited. An engine's thermal efficiency has a direct link to the compression ratio, which affects the brake specific fuel consumption (BSFC). The result of higher compression ratios can be abnormal combustion that requires higher-octane fuel. The result is a need to address the design and performance of engine components such as cylinder heads, pistons, and other stressed parts. Having components such as pistons, cylinder heads, valves, blocks and gears treated cryogenically will increase their mechanical properties allowing greater compression, which will result in better performance and wear characteristics.”
Andy continued to explain how the process works, “Using sophisticated cryogenic chambers that are computer controlled we can model a cooling and re-heating curve down to -195°C and up to 300°C. The key to the process is the tight control of the temperature curve. Each process requires a different curve, some remain at -195°C for a number of hours and are slowly brought back to room temperature. Some materials require re-heating to temper the material after cryogenic tempering.
The purpose of this procedure is to remove all residual stress from the metal. When any metal is drilled, machined, welded, cast, or formed, a stress is induced. If these stresses are left unchecked, the part either cracks, fails, or distorts over time,” added Andy.
Co-director, Barry Lomas, commented about the science, “Cryogenics is a mixture of physics and chemistry. In ferrite steels, it's all about the transformation of austenite, a large soft crystal, into martensite, a smaller, harder, more compact crystal. And when we start to get down to -185°C N-carbides start to grow throughout the structure. The net result is that the crystal structure is transformed with the boundary adhesion between the various crystal elements also improved, both delivering better wear properties for the treated metals. We can actually hold temperatures as low as -195°C; hence the company's web and email address are 195below.co.uk,” added Barry.
Andy continued to explain the benefits to treated engine components, “Many benefits will be reaped from the cryogenic heat treatment process. Cooling the component down and then slowly warming alters, the molecular structure making it stronger. The molecular structure is drawn closer but never reaches absolute zero, where there is no movement of electrons. During the computer controlled warming process the molecules are accelerated and the grain structure of the metal takes its natural form. This has the benefit of limiting distortion, adding strength and making the component dimensional more stable. The dimensional stability means that bore distortion in blocks is generally eliminated, maintaining a better piston- ring seal.
The process also thermally stress relieves the components. Stress can be a problem in steel components. Stress boundary areas are susceptible to micro-cracking, which leads to fatigue and eventual failure. Typically, the changes to the structure of steel treated with the cryogenic process are measurable by increases of small carbides on the surface from 33,000 per square millimetre to over 80,000 per square millimetre. This increase in carbides greatly increases the wear resistance of a component. The carbides make a flat super-hard surface on the metal. Think of it as two pieces of smooth glass being rubbed together. They will have a much lower friction than two pieces of sandpaper being rubbed together, thus increasing free movement and reducing the heat generated. The cryogenic process shows increases in performance in all engine parts being treated,” added Andy.
He went on, “The benefits are not only limited to the engine. Treating gears and general transmission components reduces wear and the risk of chipped teeth by relieving the stresses that result in cracking. This reduces overall failure improving durability and performance.”
In conclusion, Andy summarised the benefits. They are as follows:
increased dimensional stability;
increased fatigue resistance;
reduced blow by;
reduced breakage; and
reduced residual stress.