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).
Technical bulletin on recent aircraft ignition developments, focusing on magnetos and the Liberty-12 Delco ignition-distributor.
| Identifier | ExFiles\Box 37\1\ scan 168 | |
| Date | 1st July 1927 | |
| Vol. XXI July, 1927 No. 1 RECENT AIRCRAFT-IGNITION DEVELOPMENTS 37 any adjustment whatever and were apparently good for at least 500 hr. more before an adjustment would be necessary to correct the timing. At the end of this test, the magneto was run up to 8000 r.p.m. without misfiring or any chattering of the contacts. Development of the details of this type of breaker is still under way but the fundamental principles have already been established as entirely satisfactory. All the Splitdorf double-magnetos have been equipped with tungsten contacts and their operation has been entirely satisfactory. The principal difficulty previously experienced with tungsten contacts has been hard starting, because of a high-resistance coating that forms on the contacts if they happen to stand apart for several days. This apparently causes no trouble on the double magnetos, as the contacts are “made” several times while the booster magneto is furnishing the ignition during starting, and this breaks down any surface resistance. The use of tungsten contacts greatly reduces the cost of the magneto. BATTERY IGNITION DEVELOPMENT As a matter of fact, comparatively little development in battery ignition for aircraft engines has taken place since the war. Under the pressure of wartime demands, the Liberty-12 Delco ignition-distributor was developed in a very short time, and practically the entire supply was produced without any changes in design. It is a tribute to the manufacturer that this type of distributor has given good service ever since. It is only within the last 2 years that any new types have been designed, chiefly on account of a different angle between the cylinders and of higher engine-speeds. The Liberty Delco distributor assembly is shown in Fig. 12 and is characterized by the following features: (1) Direct drive from the camshaft (2) Ignition-coil integral with the distributor-head (3) Double breaker-arms (4) Auxiliary contact-arm to prevent sparking on reverse rotation (5) Carbon-brush-type distributor (6) Spark-advance effected by shifting the entire distributor The use of a carbon brush in the distributor necessitates frequent cleaning of the distributor track to remove the oil and carbon. It has also been found that moisture in the coil insulation that had not been entirely removed during the process of manufacture greatly reduces the energy of the spark, when the coil reaches a temperature of more than 150 deg. fahr., sometimes causing scattered missing and roughness of the engine. This has been a very obscure trouble and, as usual, the condenser has been blamed for much of it. The usual method of cure has been to put on a new distributor. More or less trouble has always been caused by oil leakage, particularly on inverted engines. To overcome this trouble and to improve the reliability of the distributor, the Materiel Division, with the cooperation of the Delco Co., has worked out the following modifications, for converting the carbon-brush type of distributor into what is called the Liberty Delco air-gap type of distributor: (1) Carbon brush replaced with metal pin to give an air-gap distributor (2) All moisture removed from coils by careful Fig. 13—DELCO IGNITION-DISTRIBUTOR FOR NINE-CYLINDER RADIAL AIRCRAFT-ENGINE This Is an Emergency Magneto-Replacement Unit and Is Not To Be Continued. All Parts Are Enclosed for Radio Shielding FIG. 12—LIBERTY-12 DELCO IGNITION-DISTRIBUTOR ASSEMBLY COMPONENTS This Distributor Has Direct Drive from the Camshaft, an Ignition-Coil Integral with the Distributor-Head, Double Breaker-Arms, an Auxiliary Contact-Arm To Prevent Sparking on Reverse Rotation, a Distributor of the Carbon-Brush Type, and the Entire Distributor Is Shifted To Effect Spark Advance | ||