PISTON DESIGN

Evening Post, Volume CXV, Issue 135, 10 June 1933, Page 17

PISTON DESIGN

USE OF LIGHT ALLOYS

PROBLEMS THAT ARE FACED

(By W.N.5.8.) Improved engine performance, higher speeds, and increased output have called for close attention . being given to piston design. Light alloys, besides reducing the mass of reciprocating parts, have better heat conductivity than cast iron pistons, but their strength and resistance to wear is inferior to arid their rato of expansion greater than cast iron. < The disadvantages of aluminium and similar alloys -would seriously outweigh their greater qualities, namely low weight and rapid heat conductivity, were it not for the ingenuity of designers who Jiavo made tlie light alloy piston more or less completely oust the cast iroi\ typo. Th© requirements in a satisfactory piston made of light alloys and tho design and problems thereof are interesting:— 1. Low Weight.—Kcduecd bearing loads and vibration, arid increased mechanical efficiency result from tho use of light alloys in pistons. Power output and better performance follow when inertia forces are reduced, since these forces vary directly with the load, and as the square of tho speed. The acceleration qualities.of a motor with aluminium alloy pistons arc far superior to those of a motor fitted with heavy iron pistons, as every motorist knows. . ' 2. Thermal Conductivity. — ingn speeds and high efficiency demand a piston which'will conduct heat away as quickly as possible. If the temperature of tho crown of the piston-, is not kept down pre-ignition, volumetric efficiency, and other troubles will manifest themselves. Take two pistons, one cast iron, the other aluminium alloy, operating under full load and at maximum rovs. The temperature in the centre- of the crown of the cast iron piston will bo 450deg C, and that of the aluminium one 250deg 0. It will bo seen how the aluminium piston permits dissipation of the heat of the explosion, whilst th© iron piston retains it. Design plays an important part in aluminium alloy pistons for care must be taken that differences in temperature in the piston are kept at a minimum. The webs extending from the crown to the gudgeon pin bosses encourage the heat -of the crown to flow towards the piston skirt in the vicinity of the gudgeon pin bosses. An undue amount of' heat being transferred to the skirt in these two places causes distortion of the piston skirt and head. GUDGEON PIN LOADS. 3. Strength.—The thickness of the crown and ability, of the head to carry the piston rings must be carefully gone into in the designing of a piston. Gudgeon pin loads are, however, a most serious problem.to deal with. Unless the gudgeon pin itself is very stiff, it will bend under ■ tho explosion. load (6001b per sq in) and consequently distort the piston skirt if it is not made exceptionally rigid. Tho strength of all aluminium alloys rapidly falls as temperature increases, but for normal operating conditions there is a sufficiently wide margin of safety. The use of an alloy which is known to retain its strength to a greater extent at high temperatures will result in a saving of weight, as gudgeon pin bosses and piston head can be made lighter. Heat treatment during manufacture will give either ductility or hardness,, depending on whether the temperaturo of the operations is high or low. (d) Expansion.—The great disadvantage of alloy pistons is tho high coefficient of expansion.' Iron pistons with a relatively low.; co-efficient of expansion require large, enough clearances in the cylinder, but tho clear^ ances of alloy pistons must be very generous. Piston slap at low speeds and light load and when, ; starting from cold cannot be avoided with alloy pistons. Unless ample clearance is given, an alloy piston will seize in its cylinder upon reaching normal operating temperature. The expansion of an alloy piston is affected by the shapo of the casting arid temperature flow from the piston crown. As it is the skirt whioh guides the piston in its cylinder caro is taken that tho heat of the explosion is not transferred to the skirt. Consequently most piston skirts aro insulated from the piston head and crown by means of slots cut circumferential!}'. However, as. the gudgeon pin bosses are always incorporated into tho skirt a certain amount of tho heat of the piston head flows through tho webs linking tho bosses with the piston head, it is here that design plays an important part, since it is necessary to prevent local distortion of tho skirt in these places. If the temperature of tho skirt can be kept low the degree of expansion

I will bo correspondingly reduced, allow- ; ing a closer clearance with tho cylinder and absence of piston slap, together with lower oil consumption. (o) Resistance to Wear.—Pure aluminium will not stand up to wear on account of its extreme softness, and its lack of strength at high temperatures makes it valueless. When copper, silicon, etc., are added to aluminium an alloy is produced which has an increased resistance to wear, but such a piston cannot be compared to an iron one. Forged alloy pistons wear better than cast alloy ones, as the matrix of the former is closer. Friction is related to the hardness of an aluminium alloy and the co-efficient of friction with forged alloy pistons ie again lower. Friction, too, is increased with temperature. AVith so many obstacles in the way designers have had a difficult battle in order to produce an alloy piston which will not distort or seize, which requires a small clearance, which will not lose strength or increase friction during the range of normal operating temperatures, and which will improve the general efficiency of the.engine. DISSIPATION OP HEAT. (f) Stability of Structure.-rThis means freedom from changes in physical or. mechanical properties and absence of distortion—the latter may be guarded against by the designer. Aluminium expands regularly up to 250 degrees Centigrade, after which its rate of expansion is decidedly rapid. Tho crown is the only portion to reach this temperature, and care is taken that ..the expansion sets up no stresses on the gudgeon pin bosses or skirt leading to distortion and -failure. The heat of the crown must be so regulated that no portion of the piston expands more than another portion. When designing' a modern alloy piston, heat flow, mechanical strength, and minimum friction aro the three essentials to be considered. By having the crown thinnest in the centre no_ excessive temperature difference will develop between the centre and edge of the crown. In a': well-dosigncd piston hent is mostly transferred to the cylinder walls (and so to the cooling water) via the two top piston rings. The skirt, being more or less insulated to prevent distortion and expansion, is not called upon to dissipate very much heat, its temperature never beingabove lOOdeg Centigrade. Piston rings, in order to maintain full contact with the cylinder walls bo that the heat from the piston crown may escape, must be as narrow as possible, for it is difficult for a wide ring to make full contact. j The latest design of alloy pistons are are most interesting, great ingenuity being shown in the- webbing between the crown and gudgeon pin bosses, in the shape of the skirt, and control of heat flow. A piston is also made having an alloy head and a cast iron skirt, and it is probably the best design so far produced. , i Cylinder wear is usually greater with alloy pistons than with cast iron. On account of the softness _of the metal in an aluminium alloy piston particles of abrasive material such as dust metallic particles, etc., become embedded in the piston and rapidly score and wear away tho cylinder walls. Sixty per cent, of tho latest cars have alloy pistons of various types, and J the balance have very light cast iron pistons. In many ways a light cast iron piston is superior to an alloy one, but as far as heat conductivity and lightness are concerned the alloy piston leads, and its gseat success is due to design more than anything else.