From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Analysis of cylinder bore wear, oil consumption, and piston ring performance during engine running-in, with comparative data on blow-by over time.
| Identifier | ExFiles\Box 132\5\ scan0133 | |
| Date | 25th April 1940 guessed | |
| 12 CYLINDER BORE WEAR represented its true characteristics. After further running, two other readings were made and improvement followed up to stabilization. Obviously if the natural frequency of the ring was a controlling factor, this condition could hardly exist. It does indicate that the degree of bedding-in influences the speed at which rings can vibrate freely. Any factor that frees the ring from the damping influence of contact with the cylinder walls at the ring points, will start the rings vibrating. Thus, the greatest factors preventing flutter become high tension, high points, quick and perfect bedding-in. Oil Consumption. When bore wear is considered from a general point of view, oil consumption is found to be the criterion by which it is judged by the [Graph with labels: BLOW-BY—CU. FT. PER MIN. on Y-axis, SPEED—R.P.M. on X-axis. Curves for 7 HOURS, 29 HOURS, 40 HOURS] Fig. 14. Comparative Blow-by during Running-in Time public. The user is not so much interested in the reason why his oil is consumed in large quantities as in the fact that it is being so consumed. For this reason, it is important that rings should be effective under adverse conditions, and it has been shown above that properly designed rings will do well, even with mediocre bores. The author believes that this is important for any combination of materials for rings, bores, or liners. Rings must maintain their ability to seal, so that their edges, their freedom in the grooves, and the conditions of the ring lands may all be maintained. Contamination of the oil must be minimized and radial pressure characteristics represent an important element in this study. The fact that heavy expenditure has been incurred for special rings and liners is hardly sufficient. Overheating of the piston and rings and, possibly, liners will follow blow-by, and must therefore be averted. Returning to oil consumption, since it is the over-all criterion, leaks must not be overlooked. Oil leak at the rate of one teaspoonful per mile will cause loss at the rate of 1 gallon per 1,000 miles. When rates of consumption of 2,500–5,000 miles per gallon of oil are expected, such a rate of leakage destroys the result of painstaking engine develop-ment. The design of flanges for the gasket must be given adequate consideration to ensure a good seal. Leaks at the front and rear bearings must be considered. A crankcase ventilator that will maintain a slight vacuum is the best possible aid to the elimination of leaks. The author has repeatedly recommended a copious supply of oil for the bores. This must not be confused with high oil consumption. Although careless striving for oil mileage has brought with it considerable engine trouble, yet it is still possible to operate for 20,000 miles at an average of 5,000 miles per gallon of oil. This is being done regularly with thousands of cars and trucks. Good oil mileage must be the result of good design and must follow the bedding-in of the parts involved. This is a normal consequence unless disintegration is going on during bedding-in. A copious oil supply in an engine in which the necessary control cannot be maintained on account of improper design of piston or rings, or disintegration from blow-by, will soon become a hazard. For instance, let it be assumed that the supply of oil is copious and that the ring combina-tion includes two low-pressure compression rings and two low-compression oil rings. When new, the two oil rings will hold back a considerable quantity of oil and the compression rings will do their share of the control. As soon as the compression rings have been subjected to blow-by they soften, and become covered with carbon; the carbon and heat then affect the upper oil ring which loses tension and also becomes covered with carbon. The oil control has now been reduced to a minimum, but the oil supply remains at a maximum, and the result is that even should no serious wear be present in the bores. Thus if the life of piston rings is to be a maximum, they must have the highest resistance to blow-by.{R.W. Bailey - Chief Engineer} Factors that serve to make good cast iron rings apply to rings made of other materials. In the majority of the automobile engines of the world, cast iron rings, individually cast to form, have been found to maintain their characteristics over the longest period, the important characteristic being a pre-determined radial pressure pattern. The fact that a ring may be austenitic does not mean that liberties CYLINDER BORE WEAR 13 can be taken with blow-by.{R.W. Bailey - Chief Engineer} The author believes that if 90 per cent of the motor cars and light trucks were equipped with engines having cast iron bores and cast iron rings, then better results might be achieved at no more cost. A private communication from Mr. C. H.{Arthur M. Hanbury - Head Complaints} Schlesman, of the General Laboratory of the Vacuum Oil Company in America, confirms the author’s state-ment that the proportion of oil supply does not in itself control oil consumption. Mr. Schlesman finds that the piston will control the amount of oil passed to the rings, the optimum thus being set by the piston. This is undoubtedly true, but some pistons will give an optimum three times as much as others. Further, the ability of the piston to control oil, changes with age and handling. A piston that has been overheated and then overloaded will dis-tort and lose its ability to limit the optimum amount of oil passed to the rings. The piston must be good, but it must be assisted by good rings. Mr. Schlesman further suggests that light oil is the most necessary first aid to good engine life. It is known that a flat viscosity index implies a light oil when cold but not necessarily when hot. The author’s firm, by giving adequate consideration to lubricant characteristics, are able to recommend one oil for use in England all the year round. This is an important decision for a climate such as that of England. Laboratory Testing of Piston Rings. Since the author believes that piston rings, to control blow-by and oil consumption, are of prime importance, first consideration is here given to the procedure for laboratory checking of piston rings in operation. Correlated road and laboratory data in abundance are necessary before a fair judgement can be arrived at from indoor tests, and an accurate prediction made of the probable results of a given combination. From experience, the Vauxhall engineering laboratories have selected an engine speed of 2,200 r.p.m. for bench tests for oil consumption. When an engine, which is completely run-in, shows a certain oil consumption at 2,200 r.p.m., it is fairly certain that the figure will not be exceeded on the road. How long that maximum will be maintained and how long it will take to arrive at this peak, are matters to be determined by road test. At least it is possible to eliminate the “unsatisfactory” combinations without road test. The methods used to measure oil consumption at the test bed at Vauxhall are capable of giving results which are closely repeatable. Oil consumption must be measured during actual operation and not by or after the engine is stopped as will give uncontrollable variations. Fig. 15, Plate 1, shows the arrangement used, a dry sump with an outside oil container set on an accurate weighing scale. When the engine is running and the oil is at the proper tem-perature, the weight of oil consumed in an hour is easily determined. No readings are taken until the oil temperature is stabilized and the engine has been run long enough to ensure that the oil in the con-tainer is representative of the oil in the sump. The amount of oil in the container will change as the oil is consumed. Oil consumption for any speed is con-sidered stable if, after three hourly readings, no change is recorded. At Vauxhall a special oil cooler is used to control the oil temperature, as this is ex-tremely important for accurate tests. A separate pumping system circulates oil from the sump to the cooler and back to the sump. This amount of oil is considered as part of the oil that is in circulation. A fair test temperature is taken to be 185 deg. F.{Mr Friese}, since this is a good average road temperature. With such a layout, it is possible to determine the condition of a new engine and to learn whether or not the oil control is excessive during its early life, a common fault. The schedule for run-in will vary with different makes of engine. The objective of the run-in is to arrive at the road load at 2,200 r.p.m. as quickly as possible, without injury to bores or rings. When the initial oil consumption has become stable, the engine is run in completely, to the same extent as is required to obtain a complete stabilized horse-power curve. The oil consumption is again checked at 2,200 r.p.m., road loads and readings of oil consumption being taken until three similar consumption readings are obtained. The figures thus arrived at will be com-parable and for all practical purposes will be found to be equal to the road figures for the engine under test when run in. In the early stages of this bench test at 1,000 r.p.m. and road load, quick deteriora-tion follows if an oil consumption such as 30,000 miles per gallon is indicated. More moderate figures for initial oil consumption are usually satisfactory: 6,000 to 8,000 miles per gallon of oil is a good indication for 1,000 r.p.m. car driving speed. After complete run-in, the oil consumption test should be followed by the blow-by test. In fact, this is the most favourable time for this test. If there are indications that the rings have not attained complete blow-by control, then the break-in period may be extended. A slow sealing combination is indicated under such conditions. Methods of measuring blow-by have been the subject of controversy. Many laboratories have developed numerous methods which give widely varying results. Those piston ring manufacturers engaged in proving small increments of improvement have been forced to develop reliable methods of their own for the measurement of blow- | ||