US2955886A - Protective system - Google Patents

Description

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United States Patent PROTECTIVE SYSTEM Filed Apr. 2, 1959, Ser. No. 803,591

No Drawing.

7 Claims.

This invention relates to coated aluminum pistons for internal combustion engines, particularly engines having aluminum alloy cylinder walls.

Aluminum alloy pistons have long been successfully used in internal combustion engines, including both light and heavy two-cycle and four-cycle gasoline engines, and also in diesel engines. Pistons operate at relatively high temperatures, and reciprocate at high speeds, so that aluminum alloys are particularly useful in this application, because of their high heat conductivity and low weight relative to their strength and volume. Another established use of aluminum alloys is in the heads of internal combustion engines, where the low weight of aluminum alloys is useful, and high heat conductivity is also important. While these applications of aluminum alloys have been helpful in reducing engine weight, and in increasing engine efiiciency, the application of aluminum alloys to engine blocks has been retarded by difiiculties which have been encountered in connection with the cylinder walls. Aluminum alloy pistons present difficult problems of scoring and scufiing when used in aluminum alloy cylinder walls, unless looser fit is used than would be acceptable for automotive and like purposes. This problem has been overcome in many commercially accepted engines by inserting cast iron or like cylinder wall liners in an aluminum block. However, this solution of the problem is complicated and expensive and less efficient in operation, and for many years an extensive search has continued for a practical and economical means for producing an engine, particularly of the type used in conventional automobiles, having aluminum pistons operating successfully over a standard service life in a cast aluminum alloy block having cylinder walls of the same alloy, unlined and uncoated.

This object has heretofore been achieved to a large extent, but the one test which has not heretofore been met is repeated cold-starting at zero degrees Fahrenheit, which is a requirement in some areas of the United States and other countries. The insertion of special liners is not acceptable, for reasons of cost as pointed out above, and it is not practicable to plate, spray or otherwise coat the cylinder walls, because of the technical problems of working inside such small spaces. Various coatings for the pistons have been tried, without success, includ ing chrome plating. Furthermore, analysis of the problem has been complicated by the presence of the piston rings, which have to be made of special materials, such as cast iron or steel, frequently with a chrome plating.

Since the coatings tried for the pistons were not successful, there was no assurance whether the problem stemmed solely from the coatings tried on the pistons, or whether it might be seriously influenced by the operation of the rings.

The present invention overcomes these difficultics by providing a coating on an aluminum alloy piston which has proved successful in repeated cold-starts and general operating tests when used in a cast aluminum alloy block having no liner or coating on the aluminum alloy cylinder walls. The success of the coating has demonstrated, furthermore, that the conventional chrome-plate piston rings work successfully in this combination, without any special modification.

More particularly, we have discovered that aluminum bronze can be coated on aluminum alloy pistons to provide a hard, tough coating which has a low coefiicient of friction against aluminum alloy cylinder Walls, and which can be applied in such manner as to give sufiiciently porous surface to retain lubricant for good long-wearing operation against aluminum alloy cylinder walls, even when tested under severe cold-starting conditions. Aluminum bronze is so hard and tough that it is a particularly difficult metal to work metallurgically, but nevertheless it is readily coated on aluminum alloy pistons by flame spraying, after the piston surface to be coated has first been roughened, as by shot blasting or the like. The roughened surface has an extremely tenacious adherence to the aluminum bronze, as evidenced, for example, by resistance to scraping with a knife point. On the other hand, if the piston surface is smooth when coated, the aluminum bronze coating is readily detached.

The portion of the piston which requires coating is the portion below the rings which is in sliding contact with the cylinder wall, generally referred to as the skirt of the piston. Some pistons have split skirts, and others have skirts extending solidly all of the way around the piston. Some pistons have special steel struts embedded in them, and others do not. It is conventional to cam-grind pistons to an oval shape, which expands to a generally circular shape to fit the circular cylinder walls after the engine has reached full operating temperature. The aluminum bronze coating of the invention is applicable to any of these piston designs, and, although particularly useful in connection with the close tolerance engines of the kind used in automobiles, is also useful for coating the skirts of aluminum alloy pistons of other internal combustion engines having aluminum alloy cylinder walls. Such cylinder walls are preferably of aluminum alloy in which the principal alloying element is silicon in high proportions (e.g., up to 25% or more), for good wearing properties, but the invention is also applicable in connection with cylinder walls of aluminum alloys generally.

The particular aluminum alloy of which the piston is made, including the skirt, is not critical for purposes of the invention. Piston alloys conventionally have silicon as the principal minor alloying element, in relatively high proportions, such as about 12% or more, but the amount or presence of silicon is not critical with respect to operation of the coating of the invention. It may be observed, however, that the proportions of constituents of the piston alloy affect its thermal expansion characteristics, and the proportions of the constituents of the aluminum bronze coating alloys similarly affect the coefiicient of thermal expansion of the coating. For best adhesion under all operating conditions, it is desirable to select a combination of piston alloy and aluminum bronze alloy having comparable coeiiicients of thermal expansion, although it is not essential that there be an exact identity of these characteristics.

For the purposes of the invention, aluminum bronze is an alloy consisting of about 4% to about 11% aluminum, substantially the whole balance consisting of copper. Minor amounts of other metals may be present in the alloy for convenience of manufacture, but these other metals are not significant for purposes of the invention, in connection with coating aluminum alloy piston skirts for improved performance in aluminum alloy cylinders. More specifically, iron in the amount of 0.5% to about 5% is generally preferred. Nickel may be present in amounts up to about 7%, but, although usable, it is not 3 preferred in amounts over about 1%. Silicon may be present in amounts up to about 3%, manganese up to about 3%, tin up to about 1%, Zinc up to about 1%, tellurium up 'toabout 1% andthe total of: others upto about 1%. Preferably, the total of all alloying'elemen-ts' other than aluminum and-copper'should notexceed about 10%. The alloy which is presently preferredfrom' the point of view of good performance and availability typically containsabout 90% copper, 8% aluminum and 2% Iron.

The low coeflicient of friction-of an aluminum bronze coatingonan aluminum alloy piston skirt operating inan aluminum alloy cylinder helps to prevent scoring and scufling, but another important feature is the semi-porous nature ofth'e-coating'when it is flame sprayedonto the skirt. The degree of porosity depends principally upon the flame spraying. conditions, which can be varied to obtain the desired porosity, as will be understood by those familiar with conventional flame spraying techniques. The advantage of semi-porosity of the coating is the ability to retain lubricating oil after the motor has beenrun up. to operating temperature and then stopped for severalhours, when the'hot oil has a tendency to drain away from the piston and cylinder walls. Theretained oil helps to lubricate the piston. and cylinder walls during the next start, before normal circulation of oil is efiective. The desired semi-porosity of the aluminum bronze coating of the invention is not lost during operation ofthe engine, because the aluminum bronze is hard andtough enough to retain its shape and hold the pores open for oil'retention. Consequently, when the motor is subjected to severe cold-start testing, the combination of lubricant retained on'the coating, and the naturally low coefficient of friction of the coating, successfully prevent scoring and scufiing of the cylinder wall, and any corresponding injury to the piston skirt.

In order to demonstrate the invention, theengine of a Renault 4 CV automobile, 1956' model, was torn down, and several of the cast iron cylinder sleeves were replaced with aluminum alloy cylinder sleeves (about 15% to 20% silicon). The aluminum pistons were-duplicated with aluminum pistons of the same aluminum alloy about 12% silicon) and identical construction. The test pistons were provided with chrome-plated steel piston -rings-of the same size and shape'asthe original rings. All critical dimensions were duplicated as exactly as possible, in each of the replacement parts before they were installed.

One set of pistons was prepared in accordance with the invention, by turning them to approximately the desired final outer diameter (a little over two inches), and then grit blasting them (using No. 30 angular steel grit supplied by National Carborundum Company) to roughen the outer type surface of the solid-type skirt of each of the pistons in this set. Each piston was then placed about 3 inches away from a standard flame sprayer which was supplied with air at 70 p.s.i. and a stream of burning oxygen and acetylene gas, which converted a feed wire of aluminum bronze (about 90% copper, 8%, aluminum and 2% iron) into fine molten droplets. These droplets were sprayed against the roughened surface of the piston skirt until it had a substantially uniform coating about 0.02" thick all around thepiston. This coating was then cam-ground down to the final standard dimensions of the piston, and the pistons thus prepared were installed in the aluminum alloy sleeves, and the rebuilt engine was installed in the automobile, which was driven 500 miles at 30 mph. to break in the pistons before the tests began.

The broken-in rebuilt engine was supplied with the standard amount of oil (1030 weight motor oil, Shell Oil Companys X1'00 Multi-Vis). A special oil pan was installed, with means for pumping refrigerant through it, andrefrigerant was pumped through the oil pan and engine block until the 'wholeen'gine had reached a temperature of 0 F. This temperature was maintained for three hours at the beginning of each test cycle, and then the engine was started and idled for one minute. It was then driven on the road for ten minutes at 25 mph, and the throttle was then opened wide for about 15 minutes, which caused 'the automobile to run at about 65 to 70 mph, and the oil temperature to reach about 180 F. The automobile was then stopped and allowed to stand-idle at normal temperature for about 3 /2 hours, which completed the first cycle of the cold-start test. This test cycle was'repeated 18 times, and then the engine was torn down for inspection af parts. The pistons having the aluminum bronze coating, and the surrounding aluminum alloyicylinder Walls, were found to be in excellent condition, with no evidence of scufiing, which refers to Wide and deep destruction of the surface finish of the cylinder wall, and no significant scoring, which refers-to narrow, shallow lines indented into the'su'rface finish of the cylinder wall.

For purposes of comparison, Babbitt metal was similarly sprayed on the skirts of like pistons, and these pistons were similarly installed and run in the engine. It was found thatthepiston skirt became scuffed, and the aluminum alloy cylinder lining became scored and scuifed,-.-after three cycles of cold starts. A set of uncoated pistons: caused excessive scufiing and scoring even before'the first icold-sta-rt cycle was reached. Coatings of a silicone resin on some'of the pistons, of a polytetrafiuoroethyleneresin (Teflon of du Pont de Nernours) on other pistons, and a coating of molybdenum sulfide on' stillothe'r pistons, applied by other methods than flamespraying, produced excessive sending and scoring after three cold-start cycles, in each case. Sets of aluminum alloy pistons plated with chrome were tried and foundto scufi excessively during the cold-start tests.

The oil retaining capacity of the sprayed aluminum bronze coating of the invention is illustrated by preparing one test specimen of uncoated aluminum alloy having apolished surface, and a second specimen of the same alloy roughened and sprayed with the aluminu-m' bronze of the invention (e.g., copper, 8% aluminum and 2% iron), which is polished to the same extent as the coated skirts used in the above-mentioned tests. Both specimens are immersed in oil (Shell Rotella, SAE 20" weight) for half an hour at room temperature, and are then hung with the test surfaces in vertical-position and at a temperature of F. for 4 hours. The firstspecin'ten (regardless of which aluminum alloy is used) shows an oil retention capacity of 1.5 milligrams per-square inch, as against 6.5 milligrams per square inch for the specimen coated in accordance with the invention, based on figures obtained by weighing the test'specimens' dry before immersion in the oil, and weighing them after being immersed and hung as described.

While present preferred embodiments of the invention, and methods of practicing the same, have been illustrated and described, it will be understood that the invention is not limited thereto, but may be otherwise variously embodied and practiced within the scope of the following claims.

We claim:

1. A piston for an internal combustion engine, said piston having an aluminum alloy skirt, and a coating of aluminum bronze adhering to said skirt.

2. A piston for an internal combustion engine, said piston having an aluminum alloy skirt, and a coating of aluminum bronze adhen'ng to said skirt, said coating being'semi-porous, so that it is capable of absorbing a substantial quantity of oil.

3. A piston for an internal combustion engine, said piston having an aluminum alloy skirt, a coating of aluminum bronze adhering to said skirt, said coating being semi-porous, so that it is capable of absorbing a substantial quantity of oil, and said aluminum bronze coating adhering directly to the aluminum alloy of the skirt, with a roughened interface therebetween.

4. A piston for an internal combustion engine, said piston having an aluminum alloy skirt, and a coating of aluminum bronze adhering to said skirt, said aluminum alloy of the skirt having silicon as its principal alloying element.

5. A piston for an internal combustion engine, said piston having an aluminum alloy skirt, and a coating of aluminum bronze adhering to said skirt, said aluminum bronze coating consisting of about 90% copper, 8% aluminum and 2% iron.

6. An internal combustion engine comprising an alu- No references cited.