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).
Article explaining the operation of a magneto, including fault-finding and procedures for removal and re-fitting.
| Identifier | ExFiles\Box 4\5\ 05-page043 | |
| Date | 9th November 1912 | |
| 628 THE AUTOCAR, November 9th, 1912. How the Magneto Works. current passes from this armature along the central screw, through the brush and spring bow on the contact breaker cover, and so along the wire to the switch. From the switch it goes to the metal frame of the car. Thus the current, instead of going to the distributor and the sparking plugs, runs idly to the frame of the car, and no ignition takes place in the cylinders. "Well, when you switch off you waste current?" "That is true, but it is only until the engine stops, as directly the engine stops the magneto ceases to generate current. One of the weakest parts of the magneto system is the switch, it is rarely as well made as the magneto, and the wire from the magneto to the switch often becomes defective by rubbing against part of the engine, so allowing the current to leak when it is all required for ignition purposes. If the engine will not start, or if for any reason you suspect the switch is defective, disconnect the wire from the contact breaker cover and see if the engine runs all right. If so, you will know that it is either the wire or the switch, generally the former, which is defective. In the case of the Simms and the Bosch the arrangement for the switch is slightly different. A terminal is provided on the adjustable ring, and to this the switch wire is attached. In all cases the action is the same, this terminal being connected by a spring blade to the central screw on the contact breaker. When you switch off you switch the current from the armature to 'earth,' or some metal part of the car frame, instead of allowing it to go to the distributor." "Supposing the wires are not connected up to the sparking plugs, or the sparking plugs are not in place, and the magneto revolves, would any damage be done, as apparently the current from the distributor would have nowhere to go?" "In all probability no trouble would ensue, but to guard against any possibility of damage the magneto is fitted with what is termed a 'safety spark gap' device. This is arranged underneath the conductor and on the top of the cover which roofs in the armature (figs. 24 and 25). If you remove your conductor you will see the safety spark gap, which in the case of the Eisemann is black and the Bosch white. This black (or white) top is formed with a central brass button which makes contact with the conductor in the case of the Eisemann magneto, or by means of a blade with the terminal on the carbon brush holder in the case of the Bosch magneto. Beneath the brass button is a peg. If the current can find no outlet along the high tension wires it jumps from the button to the peg. This peg is fixed to the cover which roofs in the armature, and the current therefore finds its way through the magnets to the base plate or 'earth,' and shorts back to the armature winding." "In the case of the Simms (fig. 26) the safety spark gap is arranged inside the magnets and surrounds the high tension conductor. It is covered by a glass lid{A. J. Lidsey} to prevent the sparks, which would take place when the safety spark gap operates, from causing any damage. If there were any petrol vapour near the magneto these sparks might cause an explosion. If the spark gap is enclosed, as it is in each case, no damage can be done." "If I take my magneto off the engine to examine and clean it, how shall I replace it correctly?" "Before you disconnect the magneto take off the distributor cover and ascertain the position of the distributor brush. A simple method of remembering this is to consider the background of the distributor as a clock face, and imagine that the distributor arm or bush is the hour hand. It is then easy to remember that the distributor is in the four o'clock, eight o'clock, or some other position. You can now disconnect the magneto, remove it, and tinker about with it as much as you like, but you must not turn the engine. When replacing the magneto turn the armature shaft till the distributor is in the same position as before, and then couple up." "Suppose that by any chance I should turn the engine while the magneto is off. How am I to get it back again correctly?" "Turn the distributor until it is on the segment which supplies No. 1 cylinder. Now turn the engine round, watching the inlet valve of No. 1 cylinder. During one pull of the starting handle the inlet valve will lift, showing that No. 1 cylinder is on the suction stroke. During the next pull of the starting handle the gas will be compressed by the piston rising to the top of its stroke. Directly you have pulled No. 1 cylinder over compression it will be on the ignition stroke, but the piston has passed over the dead centre. It is therefore necessary on the compression stroke only to pull the handle about half way. The piston is therefore nearly in the firing position, and you can then attempt to couple up the magneto. You will find that the coupling part on the armature drives it will not be in quite and that the armature shaft will have to be turned slightly to get the coupling parts to engage properly. This brings the distributor on the magneto to the exact and proper position in relation to the piston. When this is right for No. 1 cylinder the timing of the remainder is correct. Then connect your wires up properly, remembering that the order of firing is 1, 3, 4, 2 on most engines. If you have any doubt on this point, open the compression taps, turn the engine by hand, and note the order in which the inlet valves lift, commencing with No. 1." "When an adjustable magneto coupling is used, the driving and driven portions should be marked before they are uncoupled, so that they may be replaced in the same relative positions, but the distributor position must be noted as well, for the magneto shaft can be put back one complete revolution wrong." Fig. 22.--Bosch distributor exposed. (Image labels: COVER, SEGMENTS, LUBRICATOR, SECURING CLIPS, LEAD WIRE, CARBON BRUSH) Fig. 23.--View of exterior of Eisemann contact breaker cover. | ||