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).
Cylinder bore wear, focusing on the effects of mixture ratios, carburettor development, and operating temperatures.
| Identifier | ExFiles\Box 132\5\ scan0126 | |
| Date | 1st January 1939 guessed | |
| 6 CYLINDER BORE WEAR well known : alcohol has the least lubricating value of all materials, less than water. The Effect of Mixture Ratios. Before leaving the question of the relation of fuel to bore wear, or at least, that part of the wear for which it may be responsible, it is desirable to consider mixture ratios during operating conditions for maximum wear, namely, starting from cold and during warming up from cold. The mixture ratio for cold starting is about 1/1, the mixture ratio for warming up is from 8 to 9/1. Now, if the difference between 12 and 14/1 is from 3 to 7/1 in bore wear, what must be the result when a 1/1 ratio is used at full choke, and 8 or 9/1 for long warming-up periods? The importance of the careful investigation of starting is obvious as a few administrations of rich mixture would go a long way in destructive effect. Once the engine is started, it should be warmed up on the leanest possible mixture for the shortest possible time. Good cold distribution is essential with as much heat as possible applied to the mixture during its travel from the carburettor to the combustion chamber. To what extent corrosion has been credited with, or exonerated from, wear that should rightly have been ascribed to carburation, the author does not know, but he is certain that, regardless of anything else, destruction of the oil film by wet fuel must be given more consideration. The effect of the mixture ratio of air and fuel represents a considerable difficulty. This phase of the problem will be covered in a later paper on carburation. It would be interesting to examine the mixture ratio of various engines. Fig. 4 shows the results of carburettor development during a year. It will be seen that full throttle and part throttle were originally about the same, an average of 13½/1. This ratio should have been 13/1 for full throttle, and 15/1 for part throttle. Apart from the effect on flexibility and acceleration and potential mileage per gallon at part throttle, these modified carburettors go a substantial way to providing an explanation of the effect of mixture ratio on bore wear. Fig. 5, showing the mixture ratio of the Vauxhall 10 h.p. car, indicates a part-throttle ratio of 17¼/1. This certainly should protect the bores from wear due to rich mixture. Figs. 4 and 5 suggest a promising avenue of approach to the problem of the control of mixture ratio because, with the protection to the oil film, we obtain a gain in fuel consumption. It cannot be argued that all that is required is to modify the carburettor to provide a leaner mixture, because it is necessary first to learn how to burn lean mixtures before they can be offered to the engine as a steady diet. Lean mixtures alone might very easily lead to an increase in bore wear due to an excessive use of the strangler to obtain good operation. A practical solution to this problem is indicated in Fig. 6, a, b, and c, Plate I, showing the thermostatically controlled heated intake manifold. By virtue of this automatic device, the mixture rapidly reaches a stable temperature, and the use of the strangler may be quickly dispensed with. This is most important because a warming-up mixture may have a ratio of about 8/1 and the sooner it is not needed the better. [GRAPH] Fig. 4. Results of a Year’s Carburettor Development Y-Axis: MIXTURE RATIO—LB. OF AIR PER LB. OF FUEL (9 to 17) X-Axis: AIR CONSUMED—LB. PER MIN. (0 to 7) Key: — — Type 30 V.M. from 1936–37 14 h.p. “Vauxhall” car. —x— Type 30 V.I.G. from late 1937 14 h.p. “Vauxhall” car. —o— Type 30 V.I.G.2 from 1938 14 h.p. “Vauxhall” car. Certainly, if the difference between 14 and 12/1 gives a large increase in wear ratio, then a ratio of 8/1 will increase the wear rate dangerously while it lasts. Thus, devices for automatically heating the mixture that can vary the amount of applied heat as required, are of considerable importance. The thermostatically controlled heated air inlet should also be given serious consideration. At this stage, the conditions required to offset bore wear can again be summarized :— (1) Thermostatically controlled coolant temperature at a minimum temperature of 145 deg. F.{Mr Friese}, preferably 160 deg. F.{Mr Friese} (2) Crankcase ventilation to eliminate the effects of blow-by in the oil sump. (3) Adequate oil supply to cylinder bores. (4) Complete control of the lubricant at the piston rings, not at the source of supply of the oil. (5) Control of gas blow-by at the piston rings. (6) Leanest possible mixture ratio during all general operating conditions. DISCUSSION ON CYLINDER BORE WEAR 19 than 0·001 inch in the same period. After 15,800 hours’ running, the wear was 0·013 inch on the average of four cylinders, with no more than 0·003 inch difference between the worst and the best. The cylinders were bigger than those which had so far been discussed—12 inches bore and 18¼ inches stroke—but by a proper selection of cylinder wall material the wear was reduced nearly six times as compared with what was previously considered to be reasonable. It was all-important to know that the factor which resulted in an improvement could be specified quantitatively; without that, the results were not very useful. As all internal combustion engines developed the greatest wear close to the top halting point of the top piston ring, it was difficult to see how lubrication could be controlled by a piston ring situated lower down the piston body. More often than not, the oil-controlling ring did more towards keeping down the oil consumption than it did in lubricating the top ring. It was stated in the paper that the destruction of the oil film by wet fuel must be given more consideration. What quantity of oil should be added to petrol to make up for deficiencies from crankcase lubrication at the top ring? It was notable that in a two-stroke engine the amount of cylinder wear was very small indeed. Tests showed that the addition of oil to the petrol slightly improved the mixture consumption figures; 1 oz. of lubricating oil to the gallon reduced the petrol consumption by about 0·5 per cent, and 2 oz. per gallon reduced it 0·8 per cent; increases up to 7 oz. per gallon, which was the approximate mixture for a two-stroke engine, reduced the petrol consumption appreciably. In this connexion it was well to remember that American and British petrols were not similar. It was true that correct piston rings would do much to improve wear conditions in a cylinder; a piston ring company in this country provided information to show that for 9,000 miles the cylinder bore wear was 0·001 inch, which was not to be despised. The author remarked (p. 10) that cylinder blocks were rather difficult to machine. Such machining, though perhaps a little more difficult than in the case of ordinary grey cast iron, should be easily possible with modern tooling methods. The depression of 1/8 inch water gauge in the crankcase had been referred to. Possibly the author’s intention was to prevent those insidious small leaks which generally took place on bearings and other cases. He recalled the case of an engine in which the bearing next to the flywheel could empty the crankcase of oil within a hour; no doubt this 1/8 inch water gauge depression would prevent such an occurrence. The temperature of the engines was also important. With sea water they could not possibly run at more than 120–130 deg. F.{Mr Friese}, otherwise deposits would occur; but by using an auxiliary cooler and circulating fresh water a temperature of 200 deg. F.{Mr Friese} could be attained, which was more in line with Wing Commander Cave-Browne-Cave’s ideas. Petrol engines of 1,000 h.p. were run at that temperature without ill effects, and indeed it was felt necessary that the running temperature should reach that figure. Mr. ALEX TAUB, in reply, said he wished it to be understood that at no time had he professed in the paper to offer to cure the ills of engines with 20-inch bores. His object was to save the cast iron bore in motor car engines and small trucks. He did not believe in spending money on anything which was unnecessary in industry, and he would state flatly that those who favoured austenitic sleeves and rings would have to produce them at a great deal less than their present cost before they would be used in large quantities. With regard to operating temperatures, he had had a good deal of experience in the days of what was miscalled steam cooling, and he knew that good results were obtained all round by raising the temperature. The nearer it was to 250 deg. F.{Mr Friese} the better the results; but beyond that point there was trouble, due to the temperature affecting the oil in the sump. In America to-day there was a terrific furore about the so-called “varnish” or lacquer on lubricated surfaces. Those who encountered it on Diesel engines called it lacquer. It had reached the motor car stage and it was quite serious. It was contended that the oil companies had overdone refining to get rid of the sludge, which was now worse than before. The factors involved in this sludging were the time the oil stayed in the sump, and the oil temperature. The car manufacturers were responsible for the time, because of their efforts to reduce oil consumption, and to some extent they were responsible for temperature; but they were not responsible for the refining of the oil. One reason why light oils had not been referred to in the paper was that the so-called lacquering was much worse with light oil than with heavy oil and that sufficient reference to this had already been made in other papers. Car engines could be built to-day to run satisfactorily at a compression ratio of about 7/1, and the weakest part of the engine was in the sump. The tapered ring to which he referred in the paper was tapered merely to the extent of 0·001 or 0·005 inch, and it had only been used to offset the retarding of the bedding-in process which followed the | ||