Preventing Excessive Cylinder Wear
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How the Use of Alloy Irons has Overcome a Difficult Problem .
ONE of the most important problems facing the operator of motor vehicles and the manufacturer who sumilies them is that of cylinder wear. The modern high-speed, highefficiency engine appears to cause difficulties in this respect which were hardly appreciated in the old days, or perhaps the effects are more apparent in later engines.
Apart from this, modern engines are, in general, so quiet that piston' slap, if it develops to any marked degree, is immediately noticeable.
Undue wear causes rapid loss of efficiency, an increase in oil consumplion and considerable expense in reboring or relining, and probably the fitting of new pistons. Cast iron is used for cylinders in the majority of cases.
A Serious Problem.
Formerly, this would develop a hard, glazed surface, which almost arrested wear, but the employment of aluminium pistons, higher compression ratios and greater loading of tha piston rings seems to inhibit this condition and, in many cases, has led to wear so rapid as to constitute a most serious problem.
Interesting research by the Institution of Automobile Engineers has thrown considerable light on the subject, and the conclusion reached is that wear is due to a combination of two major causes—mechanical abrasion and corrosion, the latter being due to the products of combustion.
The factor of abrasion becomes more apparent in engines which are required to be run for long, periods under heavy load, this applying particularly to buses, coaches and longdistance lorries. Corrosion, on the other band, becomes of greater importance where engines tend to run cold, as in the case of delivery vehicles with long stops between short periods of running. Various methods have been devised to limit these troubles. They have been arrested to a certain extent by , developments in the clas.ses of lubricant employed, but, in the main, by attention to the metallurgical side.
A study of the ordinary type of cylinder block shows that its design militates against the production of the best wearing quality in the walls. The upper sections, comprising mainly the water jacket, are generally thin; the cylinder walls are-relatively thick, and since the hardness and density of cast iron depend greatly on the rate of cooling, it necessarily follows that an iron of special quality must be used, or great care taken to prevent either the thin section being too hard or the thick ones too open in structure and consequently ;weak. The problem is not eased by the fact that the cylinders are. located in the centre of the casting, where cooling is further retarded.
Faced With Two Evils.
The manufacturer is thus faced with alternative evils—cylinder walls which are weaker than desirable, or outer sections difficult to machine. The latter factor adds to the difficulties of the production engineer, with the result that, iti many mass-produced cylinders, the wall hardness may be as low as 160 Brinell, compared with the 220 which is claimed to be the ideal.
A solution to this problem has been found in the development of nickel cast irons, by which the structure in the thin and thick sections can be, to a considerable extent, equalized and the big difference in hardness reduced to 15 points or 20 points by the addition of only 1 per cent. or 2 per cent. of nickel to a suitable base iron.
Utilizing such material, it has been found possible to obtain cylinder castings which are readily machineable in all sections and yet have a Brinell hardness of between 200 and 220 in the walls.
Further advantages of this material are that it ensures pressure tightness and maximum strength. It is generally necessary to use either a lowsilicon iron or to add a small proportion of chromium at the same time as the nickel. An additional good feature is that the valve seats stand up to their work much better.
. Nickel Cast Iron for Liners.
Nickel cast iron can also be employed for liners, whether these be cast in sand or centrifugally. These liners can, if required, be heat treated in a manner impossible with ordinary block castings. Liner castings, even with only a small content of nickel and chromium, can be satisfactorily heat treated by oil quenching and tempering, or a larger nickel content can be used without heat treatment.
Recent results have shown that, where abrasive wear predominates, hard iron in, a cast condition gives better results than where the material is heat treated to produce an equivalent hardness.
Liners for both petrol and oil engines are now being regularly made in irons containing 4-5 per cent. of nickel, with or without proportions of chromium or molybdenum. Sometimes they are machined as cast; in others the process is facilitated by slowly cooling from a temperature of about 650 degrees C.
When this latter course is adopted, the original hardness may be restored after machining by air cooling the iron from a temperature of between 800 degrees C. and 900 degrees C.