Fig. 54.—Wiring Diagram for Four Cylinder Engine with Trembler Coil Ignition.
When the switch in the low tension circuit is opened the ignition is off, because the current is then permanently interrupted; when the switch is closed the ignition is on. To economize current a quick make-and-break device should be used instead of the wipe form of contact breaker illustrated, and a non-trembler coil used. It is very important to fully retard the ignition lever when starting an engine having coil ignition, because it is very liable to backfire and injure the operator’s wrist; with magneto ignition this is less liable to happen.
Timing the Ignition.—Various instructions are given from time to time for correctly timing magneto ignition, but the following will be found to give satisfactory results. First ascertain the firing order of the cylinders as explained above, and then bring No. 1 piston on to the top dead-centre. Rotate the driving spindle of the magneto until the carbon brush H of the high tension distributor makes contact on the segment connected to the lead marked (1). If the leads are not marked it will be necessary to determine which is No. 1 by observing the direction of rotation of the brush. Next adjust the position of the driving spindle very carefully by turning it to and fro, so that when the ignition lever K (see Fig. [48]) is in its mid position the platinum contacts Fl, F2 are fully separated, the brush H still being on segment No. 1. Then push the magneto gear wheel into mesh with the engine gear wheel which is to drive it, and firmly bolt down the magneto to its bracket. Similar instructions may be followed out for the coil ignition system.
[CHAPTER VII]
LUBRICATION
Properties of Oils.—Owing to the very high speed at which the modern petrol engine runs great attention must be paid to lubricating the moving parts, otherwise undue wear or even seizure will result. We must be very careful to choose a suitable oil, one which is chemically pure and retains its lubricating properties at high temperatures. A considerable amount of oil finds its way into the cylinder, where it comes into direct contact with the hot gases. If an oil is heated a temperature will sooner or later be attained, when the oil will give off an inflammable vapour, i.e., one which will burn. This temperature is called the flash point of the oil. If the oil is likely to get into the cylinder of a petrol engine it should have a very high flash point; in fact, most of these oils do not flash until well over 400° Fahrenheit. Also when the oil is burnt it must not leave any appreciable residue. Some oils are very defective in this respect, and leave large quantities of carbon deposit on the metal walls of the cylinder and the valves; others again are gummy or too viscous even at high temperatures. Such oils must be avoided equally with those which lose their viscosity too much under heat.
Splash System of Lubrication.—One method of lubricating the working parts is known as the splash system. In this system oil is poured into the crankchamber and the moving parts dip into it, splashing it all over the interior of the crankcase and the lower portions of the cylinder walls. Oil holes are drilled in such positions that as the oil drops down again after being splashed upwards some of it will fall into these holes and lubricate the bearings. This is a very cheap method of lubrication in first cost, but very wasteful and unsatisfactory in regular use, hence it has practically died out. As the oil is used up a fresh supply must be admitted by some form of continuous drip-feed arrangement, the oil being forced over very often from a small tank on the footboard by means of air pressure or the pressure of the exhaust gases from the engine. It is very difficult under these circumstances to estimate how much oil is present in the crankchamber at any given instant, so that there was usually alternately too much or too little. Too little oil meant undue wear on bearings (perhaps seizure), and too much oil meant a smoky exhaust which became very obnoxious when the engine was suddenly accelerated.
Fig. 55.—Improved System of Splash Lubrication.