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Aviation History
1950
1950 - 1021.PDF
FLIGHT, 25 May 1950 651 TALKING of IGNITION International Conference Produces a Wealth of Information on Operating Experiences IN these days, when high aircraft-utilization is not merelya desirable factor but often the ultimate one on whichthe continued existence of an operating company depends, the importance of reducing the incidence of minor engine troubles is obvious. In spite of the excellence of modern accessories such faults are still prone to occur on ignition systems in particular, and it was in an attempt to investigate the sources of some of these troubles, and to collate the varied experiences of users all over the world, that Lodge Plugs, Ltd., recently organized a highly successful International Ignition Conference in London. It was attended by some 60 delegates representing such organizations as B.O.A.C., B.E.A., K.L.M., T.C.A., Air France, the R.A.F., the M.O.S. and the A.R.B. Representatives of the Shell Petroleum Company, Wright Air- craft Corporation and Pratt and Whitney Aircraft Division also participated. A. Cdre. F. R. Banks, of the Associated Ethyl Co., Ltd., proved an able chairman. The conference opened with a Paper by Mr. Bernard Hopps, managing director of Lodge Plugs, Ltd., its subject being " Trends in the Design of Aircraft Sparking Plugs," Mr. Hopps said that American airlines were now favouring sparking plugs with heavy nickel electrodes, chiefly, he thought, because they showed a saving in cost as compared with the platinum- electrode type. He had been told that heavy nickel-electrode plugs compared very favourably in performance with the platinum types used hitherto. American airlines, however, had no experience with advanced designs of platinum-type plugs, chiefly owing to the reluctance of American engine designers to approve plugs of foreign make. Mr. Hopps said that it was generally admitted that the heavy-nickel-electrode plug was not as good as its platinum counterpart for lean mixtures and cold starting and was also difficult to clean by sand-blasting. It was to be hoped, he said, that in future engine manufacturers would give some consideration to the plug designer and not restrict him to dimensions which were not in the best interests of good design. Lead Fouling On the subject of failures, he thought that one of the prin- cipal troubles was still lead fouling on all types of plugs and engines. There were two main categories, namely, the bridging of the spark gap with lead deposits, and lead fouling of the insulator nose. The former was practically always associated with in-line engines, particularly of the lower-duty types, where platinum electrodes were not operating at a temperature high enough to vaporize the lead deposits. On the higher-duty in-line engines, a type of erosion known as "necking" was being experienced, in which the platinum earth-electrodes were eroding approximately mid-way between the actual firing tip^nd the steel body. From the design point of view, the speaker said that his own firm had obtained some success with a plug designed to run on the cold side, but principally by providing sufficient clearance round the nose of the insulator to utilize the scaveng- ing action of the gases. He thought that the plug designer, however, could do very little to eliminate lead fouling; the responsibility lay with the engine manufacturer ' o improve his distribution, and with the fuel supplier to produce fuel con- taining less lead. The problem of waterproofing the harness and the plug still appeared to be a source of serious trouble, but the plug manufacturer could do little apart from supplying a ceramic insulator screen-tube and ensuring that the plugs were accu- rately machined. Mr. Hopps demonstrated, however, an ex- ample of a new American method of waterproofing which incorporated a rubber flange in the cable sleeve so arranged that it formed a seal at the top of the screen-tube insulator. In conclusion, he said that his company was also studying the possible production of a cheap expendable plug which would fly for 300-400 hr and could then be scrapped, thus eliminating the cost of servicing altogether. In the subsequent discussion, Mr. G. R. Shaw, of B.O.A.C, spoke of ignition difficulties which had been experienced in Hercules engines used in Solents. Out of a total of 700 services operated by ten aircraft in one year 550 had produced instances of ignition trouble; of this number 250 cases involved plug changes, 80 harness changes and 50 magneto changes. The main difficulty, lie thought, lay in condensation when engines cooled rapidly in air conditions of high humidity. On the subject of what the operator needed, Mr. Farren, of T.C.A., thought that the cost of the ignition operation as a whole must be drastically reduced. The life of the plug was net so important as its consistency in operation. The operator should be able to "fit and forget." Mr. West, of Rolls-Royce, Ltd., said that although there had been many promises of a fuel additive to eliminate lead fouling and tests had proved highly successful, there was still no supply. It was generally agreed, however, that no operator could yet afford to pay the increased price for his fuel which would result if improvements in lead-free qualities were made. A. Cdre. Hutton, Royal Air Force, thought that the solution to the problem of moisture in the harness was to be found in controlled ventilation. The second paper, by Messrs A. R. Griffin and J. G. Dawson of Shell Petroleum Refining and Marketing Co., Ltd.. waa entitled '' Spark Plug Performance as Affected by Fuel Factors." It pointed out that the use of tetra-ethyl lead as a scavenger in aviation fuel, although a virtue in some respects, was also a vice in that lead fouling of the plugs was increased. Three types of lead fouling were recognized. The first, known as the "spark-gap bridging failure," caused by molten lead compounds lodging across the electrodes. In modern civil engines, however, it had, to a certain extent, been overcome. The second (and main) trouble was now due to what was known as the "shunt resistance" failure, caused by lead deposits on the insulators forming an alternative path for the high-tension current. It was thought that this type of failure usually occurred when the plug insulator temperature was raised by an alteration in the engine operating conditions. The third type of failure, known as "tracking," was a surface discharge which took place down the insulator rather than across the electrodes; it was produced by conductive deposits on the insulator itself. The main factors which influenced lead fouling were engine operating temperatures in the cylinder and at the plug. At higher temperatures the deposits were con- siderably smaller. The method of introduction of the fuel was also thought to have an important bearing on the rate of deposit formation. As compared with results of similar tests carried out with a carburettor (which gives comparatively poor atomization of the fuel) use of fuel injection systems reduced the rate of deposition. Although it was accepted that the influence of fuel on plug performance was due to the lead, bromine and sulphur which it contained, the elimination of any of these, or even their reduction to negligible proportions, was not possible under present conditions. Deposits were also increased by low engine and plug temperatures so that improvement could only be brought about by using the hottest running-plug compatible with the demands of high power operation. . ; Trouble-Shooting .;*-;--> Mr. Welch of the British Thomson-Houston Co., Ltd., de- livered a paper on " Fault Diagnosis and Location in Aircraft- engine Ignition Apparatus." He described the basic operation of an electronic fault tester, and, with the aid of lantern slides, demonstrated typical faults as revealed on the oscillograph of a B.T-H. tester. He said that fault location was even simpler than diagnosis because the instrument always commenced the cathode-ray picture at one definite point in the engine cycle and then showed each spark successively in the order of firing. Power from an aircraft battery or from the normal 50 cycle mains supply could be used for the B.T.-H. tester. The apparatus was compactly enclosed in an easily transportable case, and weighed less than 421b. Considerable discussion followed Mr. Welch's paper as to the relative merits of air- borne and ground test installations. From the trend of the general discussion on the conference the chairman said he gathered that the opinion of the confer- ence was that non-detachable plugs to last from 800 to 1,000 hr were required, but remarked that if they got one to last even 500 hr it would take three years to develop. On the subject of fuel:air ratios it was generally agreed that all operators should attempt to determine the correct cruising ratios, as many of the present troubles were due to using weaker mixtures than modern engine design demanded. Of the effect of water/ methanol injection, Mr. Farren, of T.C.A., said that Champions had reported increased fouling, but the A.B.A. representative, whose aircraft used water in- jection on every take-off, stated that no adverse effects had been noticed. As far as magnetos were concerned the opinion of the meeting seemed to be that failures were mainly mchani- cal and were usually due to faulty bearing and cam lubrication.
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