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Turbochargers boost Ni-resist usage.

Increased use of turbochargers in automotive and other industrial engines is creating new applications for cast irons especially high-nickel alloys. Ni-resist cast irons are particularly suited to these applications because of their excellent corrosion, heat and wear resistance.

One of the problems facing the diesel engineer is the demand for increased power from a basic engine. Once ideal combustion and mechanical efficiencies have been achieved, increased power can be obtained only by burning more fuel in a given time period. This in turn means that the engine speed must be increased or additional fuel must be burned in each cycle.

it is impossible to increase the engine speed beyond a certain point because of internal engine stresses. The amount of fuel injected at each stroke also is limited, since there must always be an excess amount of air in the cylinder to ensure complete and efficient combustion. The volume of air is increased by supercharging, then more fuel can be injected, with a consequent increase in power output.

Although supercharging is usually associated with high maximum output, it also provides the manufacturer an advantage by varying the degree of supercharge; a larger number of varying power ratings can be offered from the same basic engine. The higher pressure and temperature of the cylinder charge reduce ignition delay and promote smoother performance.

Increased torque is made available at lower speeds-a vital factor in automotive and earth-moving applications. On engines required to operate at altitudes sufficient to cause a power drop to less than sea-level output, supercharging can compensate for the loss of air density by forcing a larger volume of air into the cylinder.

Of the possible ways to supercharge a diesel engine, one of the most important is exhaust turbosupercharging, originally developed by Dr. Alfred Buchi in Winterthur, Switzerland, in 1911. This method uses the normally wasted kinetic and thermal energy in the exhaust to drive a gas turbine which in turn drives an air compressor.

Today, tremendous progress has been made in the design and development of turbochargers and they are available in a wide range of sizes for both diesel and gasoline engines. Good Response

Almost all turbochargers are of the same basic design, a centrifugal compressor mounted on the same shaft as a turbine driven by the engine exhaust gases. The exhaust turbocharger is high& efficient and uses a major proportion of the energy in the exhaust to compress the intake air, with only a slight increase in exhaust back pressure.

Since there are no mechanical drives that absorb engine output, the increase with turbocharged power can be regarded as "something for nothing" which, in addition to raising power, simultaneously reduces specific fuel consumption.

In automotive and many industrial engines where power requirements vary, the turbocharger provides good response. As more fuel is injected, the exhaust energy is immediately raised, increasing the compressor output and hence the weight of air in the charge. Output performance is thus closely matched to engine demands at all speeds.

The effects of turbosupercharging on the performance of a diesel depend on the basic engine design, the degree of supercharge, the matching of turbocharger and engine to the duty involved and the amount of intercooling. It is difficult, therefore, to give details of the effects on performance, although certain trends are clearly apparent.

Brake horsepower shows a considerable increase at all speeds and the torque characteristic is well maintained. There is a maximum increase in brake mean effective pressure (BMEP) of 32% at 70% of maximum engine speed while fuel consumption also is improved.

Legislation relating to the limitation of noxious emissions in exhaust gases also has created technical interest in the role turbochargers play in promoting improved combustion to meet emission requirements. Housing Materials

Historically used on commercial diesels, turbosupercharging recently has been applied to cars. Sales of diesel cars in western Europe accounted for 14% of the new car market in 1989 with a 22% increase in Britain, and approximately 40% of these engines are turbocharged. Turbocharging is being applied increasingly to gasoline engines.

Aluminum alloy castings are used for compressor housings and the materials selected for the turbine housings of turbochargers depends primarily on the temperature and oxidizing potential of the exhaust gas from the engine. Turbocharger housing requirements include: * the ability to retain shape during numerous

start/stop thermal cycles and

during rapid fluctuations at operating

temperatures; * the ability to retain adherent oxide

skin in order to avoid oxide particles

flaking off into the turbocharger bearing

or into the converter to smother

catalytic action; * the ability to contain possible wheel

bursts up to maximum operating temperatures.

On diesel engines used in medium-sized trucks to small passenger car applications, low-alloy Si-Mo ductile irons may be acceptable. The silicon promotes improved oxidation resistance compared to unalloyed ductile irons and benefits dimensional stability.

The lower critical temperature at which ferrite begins to transform to austenite is elevated by silicon additions. Provided the maximum temperature of the housing remains below this temperature, dimensional stability is assured. However, for more arduous service it is necessary to use one of the Ni-resist austenitic ductile irons. Alloy Selection

Ni-resists-high nickel cast irons with an austenitic matrix-have excellent corrosion-resisting properties and exhibit good heat and wear resistance. High ductility and toughness are obtainable. Certain grades have special physical and electrical properties.

D-2B Ni-resist (20%Ni;3%Cr) housings are used in certain high-speed diesel engines that, when turbocharged, produce exhaust gas at temperatures in excess of 1472F. The nickel addition stabilizes the high temperature austenite phase over the entire temperature range of the turbine housing, contributing to dimensional stability.

D-2B Ni-resist may be used in turbine housings operating at lower temperatures if the engine application produces exhaust gas with high oxidizing potential, utilizing the alloy's high intrinsic oxidation resistance. However, Ni-resist D5S (36%Ni;5%Si;2%Cr) has become the established material for more demanding applications.

D-5S austenitic iron fills the need for an alloy with outstanding combinations of castability, machinability and useful mechanical and physical properties for thermal cycling conditions. The nominal composition for the alloy included in the ASTM A439 specification is: TC, 2%; Si,

Mn,.07%; Ni, 36%; Cr,2%.

D-5S Ni-resist has a stable austenitic matrix at all temperatures from sub-zero to temperatures of more than 1600F. The alloy contains the maximum amount of silicon and chromium that are soluble at elevated temperatures in the 36%Ni austenitic matrix. Silicon and chromium more than the solubility limit appear as a mixed eutectic of silicide and carbide.

Silicon is vital in improving oxidation resistance, and in the 1650-190OF range D-5S has better resistance than HK40. Moreover, in all oxidation tests the oxidation products were retained as thin, tough, highly-adherent films.

Thermal cycling tests from ambient to 1600F have shown that D-5S has excelilnt dimensional stability. In 75 cycles, from ambient 160OF with a 24-hour hold time at temperature on each cycle, the alloy grew a total of 0.0004 in. per in. during the first eight cycles, then showed no further changes. Availability

D-5S Ni-resist is produced by conventional sandcasting methods and expendable pattern casting (EPC) by John Williams Foundries Ltd, DuportHarper Foundries, West Midlands, and Auto Alloys (Foundries) Ltd, Derbyshire. EPC has been developed mainly to increase the design advantages, minimize stock, reduce machining and lower casting weight.

About 1 000 metric ton of the alloy are being used annually in Britain for housings. The United States usage is 2500 metric ton, mainly for manifolds in 7.31truck gasoline engines with production of turbocharger housings increasing. inc
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Title Annotation:high temperature chrome-nickel steel
Author:Cox, Gordon
Publication:Modern Casting
Date:Dec 1, 1990
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