PISTON RINGS

Evening Post, Volume CXV, Issue 147, 24 June 1933, Page 21

PISTON RINGS

FUNCTION AND FITTING

A GASTIGHT CHAMBER

Undoubtedly - the invention of ■ the piston ring was a step which; has gone further towards the development of the internal combustion engine 'than anything else (writos . W.N.5.8.). Essentially, the petrol engine is a compression engine, and its petrol-air mixture is exploded under pressure which is developed by the action of the piston in its cylinder. A reversal of forces; occur, and we find that the tremendous pressure of the burning gases of" the explosion is utilised to d/ive'the piston. Without tho piston ring it would not be possible to design an engine that would' bo at all efficient. The piston cannot' be made to fit tightly in the cylinder, for it would not have room enough to expand and would seize. Yet

the piston must be sealed in its cylinder so as to retain compression and prevent blo_-by of the gases on the firing stroke. It is the piston1 ling wbich saves thajsitoaUoVu*.* . .As every motorist.Tmows, a piston ring is a circular piece of cast iron about one-eighth of an inch in face widtlr, about three-sixteenths of an inch deep, and having a diameter equalling that of tho cylinder when; compressed. The ring is not' solid but "has a split across its circumference, so that it may exert pressure on cylinder walls. On the piston, groove's are cut within about three-quarters of an inch of the piston head to accommodate the rings. The piston itself does not fit tightly in its cylinder, because if it did it would seize on account of the heat of the explosion. Depending on whether the piston is an iron one or an aluminium alloy one, it has a clearance of two to five thousandths of an inch in the cylinder. The piston ring reduces friction to a minimum for the piston itself does not touch the cylinder walls. WHY THE RINGS ARE NARROW. Essentially the piston ring is employed to maintain and seal compression and to prevent the burning gases of the firing stroke from passing down between piston and cylinder walls into the* sump. In order to provide as perfect a fit as possible the rings must bear against the cylinder walls all round with equal pressure. As the heat of the explosion passes from the crown of the piston through the piston head and via tho rings to the cylinder walls and thence to the cooling'water, it is necessary that there is complete contact between the rings and the cylinder walls. A narrow ring can be made to fit the cylinder walls better than a wide one—some of the latest rings are no more than three thirty-seconds of an •inch wide. Rings are either eccentric or concentric in shape. The former are thickest opposite the gap and taper towards the gap, whilst the latter are of equal; thickness all round. Concentric rings are the order of the day. Usually two compression rings and one oil ring are fitted per piston, but some of the latest high efficiency engines have three compression and two oil rings. Compression rings are plain and are nearly all the same as regards the various makes, whereas oil rings are of a variety of designs and types, the most popular being the ventilated type. This type of oil ring has a groove cut circumferentially on its face. and slots communicate with, this groove and the back of the ring. In the oil ring groove on tho piston holes are drilled. Oil finding its way between the ring face and cylinder walls passes via the slots to the holes in the' piston' whence it returns to the crankease. The stepped i oil ring is yet another type and so is

the scraper , oil ring; The functions of an oil ring are to wipe surplus oil] from off the cylinder;walls and to allow just sufficient, to pass to lubricate the upper or compression rings.: Oil lings are either placed' above the gudgeon pin 'and next to the compression 'rings or'below the gudgeon pin and'at-.the bottom of the piston skirt. On account of the almost universal employment of insulated (by means of slots) .and split skirts the oil ring is .placed above the gudgeon pin. WIDTH OF THE GAPS. Since the compression rings (of which there may be two or three) are intended to form a seal between the combustion .chamber and crankcase and to prevent the burning gases of the explosion from passing down between the piston and cylinder walls, they must necessarily be fitted correctly. Under normal working conditions the heat of combustion causes the piston and lings to expand. The width of the gap in the ring is an important factor, and usually a gap of .003 of an inch is given for each inch of diameter of the cylinder, so that a four-inch piston would have a gap of ten to twelve thousandths of-an inch for the rings. If too wide a gap is given an escape is provided for the compression, whereas if the gap is too small there is a likelihood of the end of, the ring butting when expanded and causing the ring to buckle and finally break, besides increasing the pressure exerted by the ring on the cylinder walls.. • Compression rings must exert a certain amount of pressure (about 101b per square inch) on the cylinder walls to j

provide a good seal and to ensure that contact for dissipation of heat is made with the cylinder . block. By haying insufficient pressure the burning gases of combustion are able"!to force their way between the face of the rings and cylinder walls—diminishing power, causing overheating and contamination of the oil. Also, the piston is allowed by the loose .fitting rings to tilt, and as a result the sharp edges of the rings rapidly wear the cylinders. On the other hand, too much pressure is equally bad, for cylinder wear becomes noticeably pronounced and the rings them-' selves quickly wear. away. Just sufficient pressure is required to make certain that the whole surface of the ring bears against the cylinder wall. The fit of the,rings in their grooves is no less important than gap clearance and ring" pressure. In, the first place rings must not be fitted in their grooves so tightly that, they will become wedged in as the metal of the piston around the grooves expands and thereby closes up the grooves. Kings which become jammed.in the grooves must break since they cannot accommodate themselves to the thrust of the piston and contour of the cylinders. Every cylinder wears' oval (due to the side thrust of the piston) and also wears to a taper—wider at the top and narrow,at the bottom of the bore (this is due to the fact that only; a .minimum of oil is permitted~tq reach the upper rings).' For this reason it is essential that the rings be free to move in their grooves. Rings that fit loosely in their grooves cause'much trouble: the force of. the explosion is allowed to pass via the spacious groove into the space between the back of the ring and the inside' of the groove, thus greatly increasing the pressure of , the ' ring on the cylinder walls, which is an undesirable factor. Again, rings that fit loosely in their grooves are conducive to "oil pumping.'-' Oil present between the lower edge of the ring and the bottom edge of the groove is forced by the ring, on reversal of piston motion, into the space behind -the ring and back of tho -groove, from whence it passes into the clearance between the top edge of the groove and upper edge of the ring, when piston motion is again changed, and finally, the oil, now being above the ring, has no difficulty in passing into the combustion chamber. It will be seen that such "oil pumping" allows heavy oil consumption although the rings may be exerting their correct pressure against the cylinder walls and fitting them correctly. TESTING FOR COMPRESSION. Lack of compression is quickly discovered by turning the engine over slowly with the crank, pulling and holding against the compression of each cylinder separately. A flat cylinder is due to leaky,, worn, or broken rings or a, badly .seating valve (its face being pitted, burnt, or warped). Sometimes it is possible to-tell that tho rings are at fault by watching the breather pipe in; the sump, but this test is more or less useless. Assuming that the valves are known to be gas tight, it is fairiy easy to check the piston'rings. Lack of compression will cause bad hill climbing, for the car will not "hold" on a hill, but will gradually drop speed. On decelerating it will bo noticed that absence of compression has its Reflex oh the slowing up of the car. Acceleration without "pinging" or detonation with an advanced spark will-be possible for most of the force of the explosion is escaping down past the cylinder walls. Also, if burning gases can pass between the rings and cylinder walls, the motor will run hot and the cooling water will be at a much higher tern-, peratui'o than normal. Contamination of tho oil in tho sump with carbon is another sure sign that* rings are at fault. When compression is good with a cold engine (and cold oil) and weak when hot, it is a sign of ring trouble, unless gummy oil.is used. , Low oil consumption does not always mean that the rings are sound, nor does car milage prove anything. New rings may be required after 5000 miles with a new car; they may not be required until 30,000 miles are covered. Tho life of a sot of rings depends a great deal on thfe driver and on the mechanic who fitted them. Bad oil, carboned-up

grooves, excessive use of the choke, etc., cause ring wear, but badly fitted rings having too much pressure, loose grooves, or wrong gap clearance will quickly bring troublo to the best b£ cars.