CHAPTER VIII
IGNITION IN PETROL ENGINES
By J. Ernest Hutton, A.I.E.E.
It has been explained in a previous chapter how the reciprocating piston takes in a charge of explosive mixture and compresses it. It is now necessary to ignite this compressed gas in some manner, in order that an explosion may take place and drive back the piston with great force.
There are two methods of accomplishing this in the petroleum spirit motor, (a) by means of a hot platinum tube, which is known as 'tube ignition'; (b) by an electric spark, known as 'electrical ignition.'
TUBE IGNITION
The Platinum Tube.—At a convenient place in the wall of the explosion chamber a hole is drilled and means provided for attaching about it a nut, in the course of which is drilled a hole of sufficient size to take a platinum tube, closed at one end and provided at the other extremity with a flange. This flange must have a perfectly true face, and fit accurately against the face of the hole in the explosion chamber. Between the flange and the top of the nut is placed a thin asbestos washer; the nut being screwed up, the flange on the platinum tube is held tightly against the hole, so that any gas in the cylinder may have free access the whole way up the platinum tube, but none can escape at the joints.
The position for this tube is defined by the manufacturer so that at the time of greater compression the explosive mixture is forced into the tube and ignited.
The Burner.—To obtain satisfactory ignition, the platinum tube must be heated to a red heat. This is effected by a small Bunsen burner placed directly underneath. The burner consists of a small tube or 'stem' easily detachable from the petrol supply, in which is inserted a wick, composed of cotton rolled up in a sheath of fine brass gauze. The upper extremity of the stem is provided with a removable nipple, in which is drilled a minute hole. The burner is completed by a cowl, with a slot at the top which directs the flame on to the ignition tube. The burner is on the well-known Bunsen system, taking in air through holes at the foot of the cowl, the air being carried upwards with the jet of petrol coming through the small hole in the nipple.
Although there are numerous designs of burners on the market, they differ little from one another save in small details of appearance.
Petrol Supply to Burners.—There are two methods of feeding the burners with petroleum spirit:—
(1) By allowing the spirit to flow by gravity from a small tank fixed in an elevated position—this is known as 'gravity feed.'
(2) By forcing up the spirit from a tank frequently the main supply—placed in a convenient position under the car by means of artificial pressure. This system is known as 'pressure feed.'
The advantages of gravity feed are its greater safety and perfect reliability. A good system of gravity burners will run for months without attention. The flame from a burner should be a bright blue, and directed lengthways on to the ignition tube.
Note.—In small Panhard cars the flame should be across the tube.
Gas-tight Joints.—Should a leakage of gas from the cylinder occur at the joint between the tube and the cylinder wall, it will greatly interfere with the working of the engine.
To detect if there be one, hold a match close against the joint where the tube enters the cylinder, at the same time turning the starting handle. As compression takes place, gas will be forced through any leak, and will show itself by blowing the flame of the match. If a little petrol be poured into the cylinder and the same method of testing employed, the leak will be detected by the escaping petrol becoming ignited.
It is essential that the nut which holds the platinum tube in its place in the wall of the cylinder should be kept perfectly tight, a special 'box' spanner being required for this purpose to give greater leverage.
Cracked Platinum Tubes.—It sometimes happens that the platinum tube becomes cracked, and thus allows the compressed gas to escape.
The same method as above may be employed to test this. If the tube be cracked, a new tube must be inserted, with a fresh asbestos washer. Never use the same washer twice. Care must be taken that the new tube is of the same length and quality as the old tube, to ensure accurate timing of the ignition.
Soot inside Platinum Tube.—Faulty ignition is sometimes caused by the interior of the tube becoming blackened by sooty deposit, which prevents the gas becoming properly ignited.
Take out the tube and clear the interior with petrol and a little waste or rag. If deposit still remains, use a piece of fine emery cloth wrapped round a small stick.
How to Light a Burner.—The burner must be thoroughly heated in order to vaporise the petrol before it is allowed to flow freely through it, and for this purpose small cups are provided at the base in which methylated spirits can be burnt. A small oil-can may be conveniently carried on hooks, provided for this purpose, inside the engine bonnet, to hold the methylated spirit required. Some automobilists dispense with the use of methylated spirits for the preliminary heating, and merely flood the burner with petrol allowed to run through gently, and setting alight to it keep the flame constant until the burner becomes heated. This method is to be deprecated, as very liable to ruin the paint on the engine bonnet.
Burners sometimes 'jump' this usually happens when first lit and not sufficiently heated. Time should be allowed for them to become thoroughly heated before attempting to start the car.
How to Extinguish a Burner.—When putting out burners blow them out with a length of rubber tube and allow them to cool down until the petrol flows freely through them, then turn off tap.
Faulty Burners.—When the flame burns yellow or on one side, it is because the burner is choked by some foreign body lodging in the nipple, and preventing the spirit from having free exit.
The best remedy is to put in a new burner at once, which should always be carried ready for use; the old burner may then be examined at leisure. When it is desired to remedy a faulty burner, the wick should be withdrawn and a 'pricker' run down the stem and out at the hole in the nipple, care being taken not to injure or enlarge the hole. Small particles of soot and dust in petrol are causes of trouble in gravity burners. When cold, and the petrol turned on for a moment, the jet should leap up straight at the tube; if it quivers or is on one side there is something lodged in the nipple. The nipple must be removed and cleaned. Tight wicks, so frequently supplied by manufacturers, prevent the free flow of spirit; loose wicks are useless, as they are at once pushed up to the end of the stem by the pressure of the petrol, and stop the hole; the wick should be a good fit, neither too tight nor too loose.
Leakage in Pressure-fed Burners.—In pressure-fed burners, trouble is sometimes caused by leakage in the system of pipes, allowing the pressure to fall, or by water getting into them which has condensed in the pressure tank. Caution.—Before attempting to put a match near a burner, great care should be taken to be assured that the burner tap has not been inadvertently left on, and the engine box flooded. Many a good car has been burnt this way.
Burners Jumping Out.—Generally caused by too much pressure of petrol. The taps controlling the petrol supply to burners should only be opened a very little way—usually a quarter turn is sufficient.
Sudden bumps in the road will also cause jumping out.
Burners may also blow out. A proper wind shield should be fitted in front of the burner cage.
Spare Parts.—In connection with tube ignition it is necessary always to carry—
Spare platinum tubes.
Spare asbestos washers.
Spare nuts for tubes.
Spare burners.
Spare wicks.
Prickers.
A special spanner for undoing the nuts by which the platinum tubes are screwed into the cylinder.
A spanner for detaching burners from the supply pipe.
A spanner for removing the nipples of burners.
ELECTRICAL IGNITION
The Importance of the Time of Ignition.—As we have explained above, the moment that maximum compression is reached the compressed gases are forced into the hot tube and become ignited. After each explosion a certain proportion of burned gas remains in the tube, and the rising piston causes the fresh gas to mingle partially with this, but not sufficiently to ignite it until the greatest amount of compression is obtained. It is obvious, therefore, that with variable speed of engine, the moment of ignition is not always theoretically correct. In theory, that moment varies with the speed of the engine: many methods of timing 'tube ignition' have been suggested, but up to the present no satisfactory solution appears to have been discovered.
To get over this difficulty, and thereby greatly increase the efficiency of the motor, ignition by means of the electric spark has been devised. By the contrivance known as the shifting 'commutator'—afterwards described—it is possible to alter the moment at which the spark is caused to fire the charge in the combustion chamber, so that whatever the speed of the engine the moment of firing the charge is theoretically accurate.
Two Systems.—There are numerous types of electric ignition on the market, which may be divided up into the following classes:—
(1) With a battery and induction coil.
(2) The magneto system.
(1) Ignition with Battery and Induction Coil. The essential of this system is an electric battery. The function of a battery is to supply the necessary quantity of electricity to create the spark in the combustion chamber. Broadly speaking, there are two kinds of battery used for this purpose on automobiles viz., the 'dry battery' and the 'accumulator' or storage battery.
The Dry Battery. The dry battery, so called because of the absence of any visible fluid, has become exceedingly popular, because it requires little attention. Each battery is usually composed of four separate cells, coupled together by means of small pieces of wire. Each cell consists of a plate of carbon surrounded by a chemical compound, the whole being contained in a thin zinc case. The carbon plate forms one pole of the battery, and is known as the 'positive' (+). The outer zinc case forms the other or 'negative' pole (–).
Poles.—When coupling up a number of cells to form a battery, the carbon of one cell is connected to the zinc of the next, and so on, until all the cells are connected, leaving one free wire at each end of the battery. These wires are known as the positive and negative poles of the battery.
Pressure of Electricity ( Volts). Each cell is capable of giving forth a certain small pressure of electricity. Pressure of electricity may be compared to the pressure of water in a pipe, or steam in a boiler, and is measured in units of pressure (volts) which may be said to correspond to pounds per square inch.
Flow of Electricity (Amperes).—Before, however, any current of electricity can pass out of the cell a complete 'circuit' must be formed between the two 'poles.' A quantity of electricity will then pass round this circuit in proportion to the pressure (volts) in the cell. This quantity or flow is measured in units known as 'amperes.' One ampere flowing for one hour is known as an 'ampere-hour' and the capacity of a battery is measured in ampere-hours.
Coupling in Parallel.—If the capacity of one battery is insufficient, two or more may be joined up in parallel by connecting the positive poles together and the negative poles together. To obtain a sufficient spark a battery must be capable of giving out a pressure of at least four 'volts.' For this reason it is usual to couple up four dry cells together.
Disadvantage of Dry Cells.—As we have explained, the utility of a battery depends on its capacity in ampere-hours. Every dry battery has a rated capacity, and once this quantity of electricity has been drawn from it, it becomes practically worthless, and a new one is required. The cost of these dry cells is very considerable, and if they are left for some time without work it not infrequently happens that the capacity has been seriously impaired owing to electrical leakage, which is nearly always present in dry batteries. For .these reasons the accumulator or storage cell is usually preferred.
Storage Batteries.—The great difference between a dry battery and a storage battery is that in the case of the latter when the battery is exhausted it can be completely re-charged in a few hours. The 'accumulator' or secondary battery may be briefly described as a number of prepared lead plates, immersed in a weak solution of sulphuric acid and water. These lead plates are alternately positive and negative, and are separated from one another by thin strips of ebonite, glass, or other non-conducting material. The whole is contained in a square cell of some suitable substance, which is unaffected by the acid. The positive plates, which are connected together, may be easily detected by their chocolate appearance. The negatives (slate colour) are also connected together, and convenient brass screws (terminals) are fixed, to which wires may be easily attached.
The capacity of each cell depends on the size and number of plates. The pressure of any cell may be taken at two volts when working. Two cells, coupled in series, are therefore required to make a battery of four volts.
Charging Storage Batteries.—Accumulators require before use to be charged with electricity. This can be done by means of a large primary battery, small dynamo, or preferably off any continuous current electric light curcuit. In any case it is absolutely essential that the positive pole of the cell should be connected to the positive pole of the generator, and the negative to the negative. The positive pole of an accumulator is usually painted red and marked thus, (+). An easy way of finding the poles of a generator or accumulator is by placing two small strips of lead connected to the battery wires in a tumbler of acidulated water, and after passing a current of electricity through them, the piece of lead connected to the (+) pole will become chocolate. 'Pole-finding' paper may be also used for this purpose.
How to Charge Storage Batteries from an Electric Light Installation.—If a supply of electricity is available in the house there is a very easy way of charging off any wall switch (see diagram below).
The cell may be left on all night and found charged in the morning. No fear need be entertained of charging the accumulator too long. When fully charged the liquid in the cells assumes a milky appearance, and gives off a sound like gently boiling water.
Avoid Over-discharge of Batteries. Great care should be taken to see that an accumulator is not discharged for a longer
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period than its rated capacity. The pressure of an accumulator keeps practically constant at two volts per cell throughout the period of discharge. Immediately a drop in pressure is observed the cell should be recharged. Nothing ruins accumulators quicker than discharging them after the voltage falls. In no case should they be discharged to such an extent that the pressure of each cell falls below 1·85 or 1·9 volt per cell. The pressure may be conveniently ascertained by means of the instrument known as a 'voltmeter,' which will show at a glance the condition of any cell.
Care should be taken that the liquid in each cell well covers the top of the plates. In process of time the liquid will be found to evaporate; this should be made up by a little clean rain-water, or preferably distilled water.
Switch.—It will be remembered that each battery has two free ends or 'terminals.' From one of these a wire is led to an 'interrupter' or 'switch.' This switch may take one of many forms. The effect of the apparatus is to easily and quickly complete or interrupt the circuit at any desired time in the same way that a tap is used to turn off water.
Induction Coil. From the switch another wire is taken to an apparatus known as the 'induction coil.' The function of this is to greatly intensify the current. On a well-known electrical principle, the current, which is of low pressure (four volts), when it enters the coil, is intensified to a very great degree. The current being required to jump across a considerable gap inside the combustion chamber, a much greater pressure than four volts is essential.
To explain the method of connecting the coil with the battery and engine, it will be necessary to give a brief description of the coil. In the centre lies a bundle of iron wires, known as the 'core,' around which is wound a quantity of thick copper wire, insulated with silk or cotton. This wire is in one piece, and known as the 'primary' winding. On the top of this layer lie laps of very fine wire, likewise carefully insulated. This is known as the 'secondary' circuit. There is also usually contained in the same case an arrangement called a 'condenser,' which we need not describe. Although the two circuits are quite distinct from one another, a current of electricity passing round the primary and suddenly interrupted by means hereafter described will 'induce' a current in the 'secondary' of very great pressure. The ends of the two windings are led to the outside of the case, and terminate in screws or binding posts. These terminals are usually stamped with letters to indicate the method of connection. As many French coils are in use, it may help the novice to mention that the letter p stands for battery, m for commutator, b sparking plug, while in the De Dion coils the brass rings on the outer case should be connected to the framework of the car, called 'earth.'
The Function of the Commutator.—Following the path of the current from the primary circuit of the coil, a wire is taken to the device known as the commutator. This takes many forms, which will be found under the description of the various systems.
The function of the commutator is to automatically make a break in the circuit, with the result that when the moment for firing arrives a flood of electricity at great pressure is induced in the secondary circuit.
The Sparking Plug.—In order to create a spark in the cylinder the wire from the coil is attached to a device known as the 'sparking plug.' This 'sparking plug' may be one of many forms, but all consist of a small central rod or wire, to one end of which is fixed a terminal to which the wire from the coil is attached. The other end takes the form of a knob or is bent at right angles. This conducting core passes through a tube of porcelain, mica, asbestos, or other non-conducting material, capable of resisting the great heat from the combustion chamber. The tube is fastened into a socket, easily screwed into the combustion chamber. The current flowing down the centre conductor finds itself compelled to jump a small gap to a piece of wire or other conductor let into the metal of the sparking plug. This jump gives rise to the spark which ignites the charge.
The Return of the Current to the Coil.—The metal of the sparking plug being in contact with the metal of the engine, the current is conducted from it to the coil.
This is usually done through the metal frame or pipes, which, of course, are good conductors of electricity. These connections are, however, a frequent source of annoyance. The wires are often attached to the frame by small screws, which shake loose owing to the vibration from the engine and uneven surface of the road.
It must be clearly understood that, although the wires, &c. by which the circuit between the commutator and the coil is completed are technically known by the misleading term 'earth,' they are not used to convey the current to the ground but back to the coil.
Thus we have two complete electrical circuits acting in unison with one another. It is obvious that if there is any fault in the primary circuit no spark will be produced in the engine. Faults may arise from many causes.
Insulation. Around wires intended to convey electricity are laid and woven many layers of rubber, cotton, &c. This lapping is to prevent the electricity which is being conducted by the wire from escaping, and is known as 'insulation.' On this 'insulation' the success of electrical ignition to a large extent depends, and the importance of keeping it perfect cannot be too greatly impressed on the novice.
The wires which convey the current from the coil to the commutator and from the commutator to the sparking plug have to be specially insulated, as the current, being at such a high pressure, will take every opportunity of leaving its legitimate path if allowed to.
All wires used for connecting the various parts of the systems should be very flexible, and composed of many strands of fine copper wire. Too much stress cannot be laid on keeping the insulation perfect.
If it be imperfect, the current will leave the wire and jump to the frame and thence back to the battery without performing the work required of it. When this occurs it is known as a 'short circuit.' Electricity always travels by the easiest path, and, if it can avoid doing any work, it will do so.
Possible Defects in Electric Ignition.—Imperfect Insulation.—If a buyer has any doubts as to whether the insulation of the electric system of his car is sufficient under all conditions of weather and to withstand water splashed during washing the car, &c., he would save much trouble in the future by having all the important wires sheathed in fibre or indiarubber piping.
Probably ninety per cent. of ignition troubles arise from faulty insulation.
Insulation Burnt.—A wire placed too close to an exhaust pipe invariably fails after a time, owing to the insulation becoming burnt by the heat of the pipe.
Insulation Cut.—A loose wire hanging against a sharp edge will invariably chafe through in course of time.
Insulation of Coil.—If the insulation of the coil breaks down it cannot be repaired on the road, it must be returned to the makers. A small ticking is usually audible inside when this occurs, when the current is turned on.
Coils placed too near the engine are liable to break down, as the heat is injurious to them. They must be fitted in a cool place.
Insulation Chafed.—Wires laid across moving parts, brake connecting rods, &c., will sooner or later give trouble.
Loose Connections.—All wires when joined together should be carefully soldered, the joints being afterwards insulated with rubber or prepared tapes. Never make a joint in the secondary wire. See that all terminals are tightly screwed up. Special attention should be paid to the 'earth' connections, which are a constant source of trouble. When connecting insulated wire, the insulation must be bared back, so that only the bare wire is attached. Wires sometimes become broken, and being loose make partial contact.
Dirty Connections.—Dirt is a non-conductor, and should be avoided on the electrical system, as on the rest of the car. Battery terminals frequently become corroded from acid fumes; they should be covered with vaseline, and require periodically cleaning. See that all connections at the coil are clean.
Broken or Defective-sparking Plug.—The porcelain may crack and the current jump across the fracture. The points may be sooty and require cleaning. They may be touching and require separating, or they may be too far apart. The usual distance between the points is about one sixty-fourth of an inch, which is approximately the thickness of a thumb nail. An English firm has recently introduced a plug which contains no breakable insulators; this is an undoubted improvement, provided the insulating material employed is found to stand the strain well.
Dirty Commutator.—Clean all contacts from oil and dirt. Most commutators are so placed as to give the maximum possible opportunity to collect oil and dirt. They should always be provided with a cover.
Batteries.—In course of time the batteries will become weak or discharged. Always carry a spare set. A two-way
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switch should be provided on the car so that in a moment the spare set can be brought into use. The diagram shows the method of connecting up the swatch, while both batteries may be charged together without interfering with the connections.
(2) Magnetic Ignition.—From the list of possible failures given in the last section, it will be inferred that there are many faults liable to occur owing to the multiplicity of wires, batteries, coils, and the like. To obviate these difficulties, electricians have designed a little machine known as the 'magneto-generator.'
Simms-Bosch System. Perhaps the best known of this type of machine is the 'Simms-Bosch.' The magneto consists of a number of horseshoe-pattern magnets supported on a metal base, on the inner faces of which are fastened two pieces of metal known as pole-pieces, provided with hollow faces, within which is fastened an H-shaped piece of soft iron (armature). The channels of this armature are filled with insulated wire. In the space between the armature and pole-pieces a 'shield' or tube of soft iron is caused to oscillate. To one end of this shield is attached a crank, operated by a connecting rod from the half-speed shaft on the engine.
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When the shield is caused to oscillate rapidly, currents of electricity are induced in the winding of the armature. These currents are led away through a connected insulated wire to a special device which automatically makes and breaks a circuit in the interior of the combustion chamber. The action is as follows:—The wiper u is normally at rest upon the stud v, which is brought through the wall of the combustion chamber and terminates in a nut to which the wire from the magneto is attached. At the other extremity of u is attached a small rod brought through the flange and connected to s, which is capable of moving about a pivot p. This pivot is in electrical connection with the other end of the armature-winding, through the metal of the engine. When the moment of firing arrives the striker t is caused to drop smartly on s, causing u to separate from v. At the same moment the shield E assumes such a position with regard to a that a current is induced in the windings on a, and being conducted through the connecting wire, a spark is caused to pass between the points of u and v igniting the charge in the engine.
Possible Defects in the Simms-Bosch System. (1) Failure of Insulation. The stud which is brought through the wall of the combustion chamber has to be most carefully insulated from the metal flange in which it is placed. If this fails, the current will jump across to the frame of the motor in a similar manner to that of a broken sparking plug. The insulation is very liable to be burnt, and great care is necessary to make it good again. Thin washers of mica are used, but the intense heat generated in the interior of the combustion chamber appears to quickly affect it.
(2) Failure of Magnets.—After considerable use, the magnets are liable to lose their magnetism, thereby reducing the intensity of the spark. The only remedy is to return them to the makers to be re-magnetised.
(3) Faulty Adjustment.—It is obvious that the position of the shield at the moment of firing must be absolutely accurate, and when it is remembered that the working parts of this whole apparatus are moving at the rate of 350, or more, oscillations per minute, it may be seen that considerable wear is likely to take place, with the result that the parts get out of adjustment. The diagram opposite shows the relative position of the various parts, and, to assist the novice in accurately adjusting them, the following instructions are appended:—
Remove the top plate of the magneto machine by unscrewing tin- screws at the corners; the moving parts of the machine will then be open for inspection. Turn the engine gently round till the ignition point is reached, i.e. when the ignition rod drops, observing carefully the direction in which the oscillating shield e of the magneto machine is moving at this point the side of the envelope moving from the armature a should be clear of the same by about one-sixteenth of an inch. The setting can be done by varying the length of the magneto driving rod if adjustment is provided there, or if not by loosening the nut at the end of the magneto spindle and gently tapping the edge of the armature till the correct setting is obtained. The final adjustment should be made on the tappet t, which should strike the sparking lever's about one-sixteenth of an inch before reaching its lowest point, the exact distance being found by examining the spark (turning the motor smartly round for this purpose) and adjusting the tappet till the best spark is obtained. The lock-nut n should then be screwed up.
The Dawson Ignitor.—As pointed out, in the Simms-Bosch system a mechanical interrupter is necessary in the combustion chamber, owing to the low pressure employed. To obviate this somewhat undesirable adjunct, Messrs. Dawson have introduced a high-pressure magnetic machine. The magneto itself generates electricity at low pressure, which is transformed by means of a special induction coil into a high-pressured current. An ingenious arrangement enables the current to be distributed to any number of cylinders without the necessity of more than one induction coil, and moreover allows the advancement of the spark throughout a complete revolution of the engine.
The machine is worked in conjunction with ordinary sparking plugs on the De Dion principle, and being self-contained can be quickly and easily fitted to any car. The magneto is driven off the main shaft of the engine, and at the same speed, by means of a chain.
The motion being rotary, much less wear should be experienced than with oscillating machines.
Owing to the short time this machine has been on the market, it has been found impossible to obtain data as to its reliability in actual practice.
De Dion Type of Ignition.—We will now describe the ignition fitted to some of the best known types of engines.
De Dion et Bouton.—This well-known firm may be said to have set the fashion of electric ignition in the smaller engines. The system they employ belongs to Class I.
The most notable feature is the commutator. This device consists of a cam or disc c, fastened to the half-speed shaft of the motor, and provided with a wedge-shaped notch. Around the cam is attached a pear-shaped plate f constructed of good insulating material, such as ebonite, to which are attached a
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spring vibrator or 'trembler' t, and a brass pillar in which is fastened a platinum-pointed screw k. The trembler is provided with a platinum stud or 'contact' about the middle.
The Action of the Trembler.—The action of this trembler is very simple. Normally the end of the trembler t presses on the cam c, the platinum contacts on t and k being a little apart. If the engine be now turned round until the time for firing the charge arrives, the trembler will be seen to fall into the notch in the cam, allowing the two platinum points to come into contact. If the distance between the contacts is correctly judged, the trembler will vibrate freely, thereby causing several 'makes and breaks' in the circuit. As previously explained, a stream of sparks will result in the combustion chamber.
And, as also explained, it is necessary to alter the moment of firing the charge.[1] To effect this the plate f is designed to be easily moved backwards or forwards in relation to the cam c. The effect of this is to bring the point of the trembler a little higher up or lower down, causing it to enter the notch earlier or later, so that the moment of contact and consequently the spark is varied according to the will of the operator. The faster the engine runs, the earlier must be the spark.
Hints on Working the De Dion Ignition.—Adjustment of Trembler.—On the correct adjustment of the trembler and the screw k much of the success of the De Dion system depends. The means of adjustment is as follows: Unscrew the sparking plug, and attaching the 'secondary' wire, lay the metal portion of the plug on the top of the engine, care being taken that the terminal is well away from any metal. Now smartly turn the motor starting-handle, when a stream of sparks should be observed to cross between the points of the plug. When the trembler is over the notch in the cam, it should have so far entered it as to be resting on k when it is half in. If the bottom of the trembler be lifted with the finger and allowed to quickly drop there should be a regular hum or buzz. After a little practice the novice will be able to recognise the correct position for the screw k by the hum of the trembler. It should be remembered that, though a stream of sparks may pass between the points of the sparking plug when it is removed from the engine, it does not follow that the same effect will be produced under the conditions of highly compressed gas found in the cylinder.
Moisture is a frequent cause of trouble on motor-cycles. Rain or damp may lodge on the porcelain of the sparkingplug or between the terminals on the ebonite plate on the commutator, or between the terminals of the coil. Remedy. Carefully wipe the affected parts with a dry rag and cover them with a little oil or vaseline.
Battery Short-circuited.—Spanners, oil-cans, tire-pumps, &c., have been known to jump on the top of batteries, thereby connecting the terminals together and causing a 'short-circuit.' Remedy. Always carry the battery in a separate box, away from other things.
Burnt Contacts.—The contacts may become burnt. They should be cleaned up with a smooth file. Loose Contacts. The platinum points on the trembler can become loose. They should be knocked up with a light hammer.
Oil on Contacts.—It frequently happens that oil and dirt accumulate on the platinum contacts, which interrupt the free flow of the current. Care should be taken, therefore, that they are perfectly clean.
Retard Sparking for Starting Engine.—When starting a motor the spark should always be placed as late as possible.
The Benz Ignition.—This system is practically identical with that used by Messrs Panhard-Levassor, Daimler Co., Napier & Son, &c. While the general lines of the De Dion have been followed, one important variation is introduced viz. the alteration in the position of the trembler. In engines running at a lower speed than 1,000 revolutions per minute the De Dion is not found to answer satisfactorily, and a different method of causing a vibratory contact had therefore to be devised. On the Benz system the notched cam is replaced by a round fibre disc a, a brass strip (b) being attached to it over oneeighth its circumference. This strip is connected with the iron axle on which the sleeve revolves. A spring (h) to which is attached a knob, k, rests upon the disc. A wire connects it with the battery. It will be seen then that as the disc revolves the plate b will pass beneath the spring k, thereby completing the electrical circuit from the battery through k b, and on to the metal of the motor, whence it returns to the coil. To effect the rapid interruption of the current a trembler is provided on the coil. The action of this is as follows:—
When a current is allowed to flow round the thick or primary winding of the coil, the iron wires, which it will be remembered composed the core t, become magnetised, and attract the iron knob r of the spring armature. The current for this purpose is led through the screw s to a platinum point
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on r. The moment this occurs the contact-piece on r leaves the point of the screw s, with the result that t instantly loses its power of attraction; this action is repeated with great rapidity so long as the circuit is completed by b and k. The screw's requires to be adjusted in just the same manner as k in the De Dion system, though it must be remembered that the action is reversed, the trembler being in contact with it so long as b and k are apart. In engines provided with two or more cylinders an easy method of testing the spark in each cylinders becomes apparent. In most systems a separate coil is required for each cylinder. Having put the engine in motion, depress with the fingers all the tremblers except one, allowing each trembler to vibrate separately. Should there be a faulty cylinder, it will at once become apparent, and the cause easily located.
The commutator used on the Napier cars has been placed in front of the driver and covered with glass, so that the sparking at the points is easily viewed. A chain driven off the half-speed shaft causes the centre disc to revolve. A special form of contact is used, which possesses the great advantage of being unaffected by oil. In other respects the system closely follows the Benz.
The Canstatt Daimler Co., Messrs. Mors, &c., are at present using the magneto system.
MORS MAGNETO IGNITION
The following notes on the Mors ignition have been kindly supplied by Lord Cairns:—
The magneto ignition system, which is applied to all the new model motor-engines of M. Mors, consists of a magnetoelectric rotary machine, combined with a series of mechanical contact-breakers.
The magneto-electric machine is bolted to the frame, and driven direct by the gear wheel of the valve cam shaft, which engages with a small pinion on the armature spindle. This pinion is a quarter the diameter of the gear wheel of the cam shaft, and the armature therefore revolves four times to each revolution of the cam shaft, or twice to every revolution of the engine. As two cylinders out of the four fire in each revolution, the magneto thus revolves once for each spark obtained, the correct relative position of the revolving armature being geared to agree with the successive breaks of contact, which being operated by a set of cams on the cam shaft, are thus kept in 'time' with the moments of greatest generation of current.
The breaks of contact are produced in the explosion chambers by small rocking arms, called the palettes, of which the inside arms are alternately pressed by springs against the surface of the ignition plugs or 'inflammateurs,' and then removed by the action of the cams, the vertical stalks from which lift and hold lifted the outside arms of the palettes.
The action is as follows:—In each cylinder during nearly two engine revolutions—that is, from the moment of its firing until the moment of its next compression approaches—the cam is holding up the outside arm of the palette, and therefore keeping the inside contact broken. As the piston rises during compression, the cam stalk descends and contact is made inside, but the spark of course does not pass until it is again broken. The cam stalk continues to descend until its top is a certain distance (2 mm.) below the outer arm of the palette. It then rises again and strikes the outer arm sharply, breaking contact within, at the identical moment that the magneto being at its 'tight point,' is prepared to give off most current. This occurs in each cylinder in rotation, and as only one contact and break are made at a time, it is evident that one wire only from the magneto is required, and is clamped to the outer ends of each of the four insulated plugs or inflammateurs, with the usual earth return through the frame.
The magneto and contact-breakers, being both thus 'timed' together, through the medium of the cams and gear wheel on the cam shaft, it is in practice only necessary to observe two important points: First, the magneto pinion has a definite position in gear with the cam shaft gear wheel, and should it at any time be necessary to disconnect the magneto, the teeth should be marked. If this is not done it takes a little trouble to find the timing again, but a few turns of the starting-handle trying the teeth in different positions of gear, soon discovers the best position. Secondly, it is important that the distance that the cam stalks descend below the outside arms of the palettes should be maintained at about 2 mm. (the thickness of two halfpennies). A small difference in this distance will not much affect the magneto spark, but it does affect the advance of the firing; and the shorter the distance the more advanced it becomes, the cam stalks in rising covering the reduced distance and breaking contact the sooner. And the tendency is in this direction, for after running a thousand miles or so, small craters become burnt away on the under surface of the ignition plugs, where contact occurs, and the inside arms of the palettes, in rising to fill these, bring down the outside ends and reduce the distance. In practice this is no particular disadvantage, down to about mm., but below or even at that distance, a sharp back-fire may be experienced on the starting-handle, or if the engine be kept running very slow on the throttle valves, the motor may possibly be stopped. This distance should therefore be verified at intervals, and if necessary restored, which is done either by bending upwards the outside arms of the palette with the pliers, or by putting in a new set of ignition plugs, when the craters may be removed from the old ones by filing flat, and turning a corresponding amount off the flanges. This amount rarely exceeds 12 mm., and the plugs are then as good as new. Even in this case the palette outside arms have generally to be slightly bent, up or down, as the case may be, and it will save a lot of trouble to adjust the distance at 2 mm., and never to go below 112 mm. When a new adjustment has been made, the cocks in the cylinder heads should be opened successively, just to see that each cylinder is firing equally and clean.
Should any difficulty be experienced in starting, and should the other parts of the engine be apparently in good order, the following may be looked at for the cause:—
The armature spindle ends may be examined to see that they are clean, and therefore in good contact.
The ignition plugs may be examined to see that they are not coated with oil, which would prevent a proper contact with the rocking arms. This may occasionally happen if too much oil has been allowed to accumulate in the crank chamber.
The little plug-hole on the dash-board, into which a removable plug is fitted to complete and break the circuit, should be looked at. If it should be very rainy weather, a drop of water may possibly get between the contacts, which are seen held apart when the plug is in place, and so complete the short circuit arranged to take place only when the plug is removed. This can be dried, or a drop of oil as an insulator squirted in between the contacts.
If at starting, one or even two cylinders can be heard missing fire, a few revolutions at a good speed will generally set them going again: if not, it may be necessary to remove and examine the ignition plugs of the defaulters.
The magnetic ignition has been found to work extremely well; and to give very little trouble, if these points are attended to. The magneto machine itself seems, and is, inexhaustible; and the space otherwise required for batteries and cells is all saved. It also gives a fine 'fat' spark, which adds to the efficiency of the explosion. It probably gives a little more noise, if the motor-car is standing still and the engine running very slow on the throttle valves, owing to the firing being always so much advanced; but many of these engines have now the ordinary hit and miss exhaust governor as well, so that the throttles need not be closed so much, and a higher speed allowed. It is, after all, a question of which kind of noise is preferred.
The button on the steering-wheel is arranged to produce a short circuit when pressed, and so stop the firing of the charges in the cylinders. This system in practice is in many ways a great convenience. It can be used in many cases instead of the clutch, and with much less labour, always remembering that the car must not be brought to a standstill by the button, or the motor is stopped. As a brake downhill, it may be used as a slight, or if the gear be on the lower speeds, as a strong compressing brake, and without heating anything; or on a long hill, the engine may be unclutched and stopped altogether and only restarted at the bottom by letting the clutch gently in again. This is not important perhaps, but sometimes a convenience in hot weather, and always some slight economy of spirit, and lastly, in changing gear, the button may be used at the same moment as the clutch with a very quiet change as the result.
The magneto machine seems thoroughly trustworthy. The writer has carried a spare one for many thousands of miles, but never had occasion to use it, the original one having apparently never failed in a single spark.
- ↑ So that it may accord with the speed of the engine, i.e. earlier in the engine stroke for high speed and later for slow speed.