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6th August 1983, Page 66
6th August 1983
Page 66
Page 67
Page 66, 6th August 1983 — n Do
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Which of the following most accurately describes the problem?

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PISTON ringliner combinais one which is ignored by .ators until something goes lg. Considering the thermal mechanical stresses and ns that the piston has to put vith, it is perhaps surprising as a component it should such a high degree of reliaf. After all, peak pressures wrong side of 140 bar lOpsi) are not uncommon.

uch reliability does not hapovernight, however. A lot of :Hence and hard cash is tssary as I found out when I ad the Lymington factory of worthy to talk to technical :tor Dr Robert (Bob) Munro. ellworthy supplies all the UK ufacturers and also has an ciate company in France to )Iy French manufacturers. export side of the corni's piston business tends tois the bigger (Marine) type igine although on the piston side Wellworthy sells rings 3imler-Benz and Daf.

Iting this in perspective, Ly3ton produces 50,000 pis tons a week in addition to 400,000 rings in the same period. These are all for diesel engine use with petrol engine components being made by other parts of the Associated Engineering empire.

The pistons range in bore size from 75 to 600mm (2.9 to 23.6in) with rings being produced in diameters up to 1,0 0 Omm (39.3in).

An aluminium alloy incorporating silicon is the most widely used material for pistons because its numerous advantages outweigh its only significant disadvantage. It has good thermal conductivity bearing and rubbing properties and is easy to cast and machine. Against these must be set its rapid fall-off in strength at temperatures above 200°C (390°F) The most recent development in piston technology has been squeeze casting which eliminates micro-porosity and improves fatigue strength. Bob Munro is certainly a fan of the process. "It is the thing for the future. In 20 years I know of no development more exciting."

The technique involves applying pressure to a metered amount of molten aluminium alloy in a die by means of a hydraulically actuated die plug. The casting is then allowed to solidify while still maintaining the pressure.

Apart from the advantages already mentioned, the technique enables Wellworthy to incorporate other materials for local strengthening in critical areas. The company is at present looking at a wide range of potential materials, concentrating on ceramic fibres to increase the permissible crown temperature thus allowing an increase in rating for the engine. It is anticipated that the fatigue strength of the piston should be at least twice that of the conventional alloy.

Bob Munro is confident of the ability of the squeeze casting technique to solve severe cases of bowl cracking. Bowl cracking has in fact been more of a problem than generally realised, with manufacturers tending to underrate the specification of their engines as a safety precaution. Wellworthy believes that squeeze casting will provide the opportunity to have specific ratings higher than before.

There is no limit to the bore size which can be accommodated with squeeze casting other than the standard production limitations.

Squeeze casting offers scope for reinforcement in other areas as well. It has been proved as being effective for bowl rein forcement and Bob Munro thinks it has great potential for improving the boss strength particularly in the case of squat pistons running at high cylinder pressures.

Wellworthy has its own computer which is used solely for piston analysis concentrating on photo-elastic stress patterns. Bob Munro said that the company went "very firmly" along the finite element path and he believes that "we are ahead of the game in predicting temperature stresses and strains."

A few years ago Wellworthy was still using the "conventional" development methods for new pistons. This involved manufacturing a prototype, testing it, looking at the bedding patterns, then modifying the design and repeating the exercise. It was decided that this was taking far too long — hence the computer.

According to Bob Munro, the ultimate aim is to build up a complete predictive package on stresses and strains and to do it quickly. With experience, Wellworthy has already halved the time taken for a piston analysis.

Piston analysis is, in fact, the big problem as it combines thermal and mechanical loadings and it has been proved that it is far too easy to overload the computer by the sheer quantity of data fed into it.

Proof that the stress analysis technique works is shown in the accompanying set of diagrams. When the predicted stresses due to thermal and mechanical loading are added together in this particular case, the factor of safety falls below one at the five o'clock position in the oil gallery thus indicating the likely point of failure. Testing a piston to such a design to the same loadings results in a crack coinciding with the prediction.

Wellworthy has a very innovative electronics department at Lymington which, among other things, has developed an ultrasonic machine for testing the bond of an insert. The company also sells such equipment with the ultra-sonic tester for example being sold in the USA, USSR, Brazil, South Africa, India, Japan, China and Mexico.

A typical application for ultrasonic inspection is to assess the metallurgical bond between a ring insert and the aluminium alloy of the basic piston structure. It can also check on porosity and internal cracks.

Many engine manufacturers are working on some form of insulation within the engine itself to reduce the heat to water. In the Wellworthy part of the equation this means restricting the heat flow from the combustion chamber by means of an insulating layer.

Such insulation raises the temperature of the combustion chamber wall, which many people believe will automatically produce an improvement in fuel economy. Wellworthy research, however, has shown that an increase in piston and cylinder head temperature of 31 5°C (600°F) will only improve the work output by less than three per cent, with most of this being evident in an increase in exhaust gas energy.

A variety of insulating materials has been tried, including ceramic, sprayed ceramic coating, composite metal/ceramic, thin section high strength cast iron, and air gap.

Bob Munro suggests that the most reliable option to date is the air gap. Ceramics are fine from the insulation point of view but are less satisfactory for impact loading.

There are a number of advantages associated with insulation with the primary one being improved fuel economy. There are other benefits, however, which include the ability to get away with a smaller cooling system owing to the low heat loss into the coolant. A further benefit which is likely to achieve even more significance in the future is that an insulated engine is less sensitive to fuel quality.

Adding up the theoretical advantages has convinced Bob Munro of the urgent need for a cost-effective insulated piston available on a regular production basis.

Wellworthy is involved in the design and manufacture of the complete piston/ring/liner package and is currently working on a range of ring coatings with the financial assistance of the Department of Industry. Coatings were developed initially to with the greater durability quired by the engine merit turers with electro-pla chrome being perhaps the r well known.

The problem is to develc coating which improves the rability of the ring but not a. expense of bore wear. It is r great deal of use developit coating which results in ring wear over a 100-hour tt the effect on the other side o fence is five thou taken off bore.

Wellworthy has now licensed by Laystall to use latter's silicon carbide impre tion technique, which is USE harden the surface of pa bores.

It is no good developing a design if the resultant con nent cannot be produced production basis. The mato dustry is full of tales about component which wor perfectly when produced c one-off prototype basis which fell apart when prodt in quantity on the line. Munro is nothing if conscious of this.

He believes that the latest duction techniques which being developed so quickly be an important feature of Wellworthy attack on the wo markets. "Unless your pro tion work is up to scratch, , design work is useless."

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