CHAPTER VIIGNITION AND IGNITION DEVICES

Fig. 38.—General arrangement of the Carburetting Plant, showing Petrol Tank (A), Petrol Filter (B), Carburettor (C), and Extra-air Valve (E).

The carburettor proper may be constructed in a variety of forms, but the elements of which it is composed are: (1) the float chamber A, (2) the petroljet B, (3) the choke tube C, (4) the mixing chamber D, and (5) the throttle valve E, as shown in Fig.39.

Fig. 39.—Sectional Drawing of a Carburettor of theJetType.

The Float Chamberis generally cylindrical in form and the liquid enters at the bottom, the flow being regulated by a pointed rod called aneedle valve. A hollow metal float which can slide freely up and down the needle valve stem operates two levers which are pivoted on thefloat chamber cover. It is well known that when a body is immersed in a liquid the liquid exerts an upward pressure on the body equal to the weight of liquid displaced by the body. The float being hollow and made of very thin sheet metal, displaces a very large quantity of liquid in proportion to its own weight, and is therefore verybuoyant. The buoyancy of the float will, of course, depend on thedensityof the liquid in the float chamber, and it will naturally sink deeper down into petrol than it would into a heavier spirit such as paraffin or benzol. The action of the float is as follows:—Supposing the petrol to be turned off and the needle valve lifted up off its seating, then on turning on the petrol supply the petrol will run into the float chamber, and as the level of the liquid rises the float will rise too, lifting up the outer ends of the levers and depressing the needle valve down on to its seating by means of the collar which is rigidly attached to the spindle of the needle valve. If at any time the level of the liquid in the chamber falls, the float will fall also, thus allowing the outer ends of the levers to drop and raise up the needle valve from its seating; this allows more petrol to enter the chamber and raises the float again, thus keeping a constant level in the chamber.

The height of the orifice in the top of the petrol jet above the bottom of the float chamber determines the height at which we require the liquid to stand in the chamber. As a general rule the level of the liquid in the float chamber should be slightly below the top of the jet orifice to prevent the liquid oozing over and causingfloodingor continuous dripping of petrol from the jet, even when the engine is not running. The height of the collar on the needle valve spindle must be adjusted until the float closes the valve down on its seating when the liquid has risen to the desired height in the float chamber. Hence, if a carburettor has been adjusted to work with petrol, it will require to have some slight extra weight added to the float when workingwith heavier spirits to cause it to sink to the required depth in these denser spirits.

The Petrol Jet and Choke Tube.—Thepetrol jetgenerally consists of a short tube of fine bore, one end of which contains a very small orifice for the purpose of spraying the petrol into the choke tube. When the engine is at rest it is easily seen that the pressure of the air in the choke tube is atmospheric, and that the pressure above the liquid in the float chamber is also atmospheric, but when the engine is running it draws air up the choke tube at a very high speed and thus causes a partial vacuum round the petrol jet, and therefore the petrol spurts out of the jet under the pressure difference which then exists and issues in the form of a fine spray which is readily vaporized. The choke tube is purposely made of rather small diameter, in order to get a high air speed, which results in a low pressure round the jet and ensures a good driving force to spray the petrol out of the jet. The speed of the engine is controlled by the position of the throttle valve or disc E, which regulates the amount of air flowing up the choke tube, and therefore incidentally checks the quantity of petrol issuing from the jet by regulating the vacuum in the neighbourhood of the jet orifice. At low engine speeds there is very little suction or vacuum effect on the jet, but at high engine speeds with full throttle opening the maximum suction of the engine is exerted upon the jet. Thus at low speeds with this type of carburettor we do not get enough petrol out of the jet, and at high speeds we get too much, which results intoo weaka mixture at low speeds andtoo richa mixture at high speeds. One reason for this is that the air flows out of the choke tubefasterthan it flows into it, owing to the fact that its volume increases as the pressure decreases, and hence the pressure round the jet falls very rapidly indeed as the air velocity increases and causes too much petrol to issue from the jet in proportion to the quantity of air flowing through the tube.Thechoke tubeis often a plain piece of pipe, as shown in Fig.40, instead of being tapered as in Fig.39.

The Mixing Chamber and Throttle Valve.—The throttle valve is usually a plain flat disc of metal mounted on a spindle which can be rotated and thus regulate the size of the air passage to the engine. It is placed above the petrol jet and situated in the mixing chamber, which is simply a short length of pipe (of the same bore as the engine induction pipe) surrounded by a hot-water jacket, the supply of hot water being drawn from the engine cooling system. The heat from this jacket should be sufficient to make up for the fall in temperature that would otherwise result due to the vaporization of the petrol as explained above.

Fig. 40.—Plain formof Choke Tube.

Recent Improvements in Carburettors.—Another defect of this simple type of carburettor becomes apparent in the larger sizes required for multi-cylinder engines. To pass the requisite quantity of petrol to keep the engine running at high speeds without creating too great a suction effort and thereby hampering the engine, necessitates the use of a jet of larger calibre, so that the liquid is no longersprayedbut issues in the form of a fine stream which is not readily vaporized. This has been overcome by the use ofmultiple-jet carburettorswhich have several jets each surrounded by its own choke tube, but all controlled by one throttle valve and supplied from one common float chamber. In this case the total cross-sectional area of all the jet orifices together could be made sufficient to pass the necessary quantity of fuel, but the bore of each individual jet orifice would be comparatively small and spraying would result as before. Another very successful device is shown in Fig.41, in which A is the petrol jet which, in this case, has no special orifice and is surrounded by a larger tube B containing small holes for the inlet of air and outflow of petrol. As the petrol issues from the jet it strikes against the pointed cone on the end of the screw C, and is thus very successfullyatomizedand broken into small particles which can be readily vaporized.

Fig. 41.—Petrol Jet,specially arranged forAtomisingthe Petrol.

Fig. 42.—Compensated Petrol Jet.A is the Main Jet and B theCompensating Jet suppliedhrough the Orifice C.

There are several devices for keeping the strength of the mixture constant at all engine speeds irrespective of the amount of vacuum in the choke tube. One of the best of these is illustrated in Fig.42, and consists in the use of acompensating jet. The main petrol jet A is of sufficient size to supply the requirements of the engine under full speed and with the resulting high vacuum; it is fed directly from the float chamber in the usual manner. The compensating jet B surrounds the main jet and is supplied with petrol through an orifice C, so arranged that it offers a greater resistance to flow than the passage up the centre of the main jet. At all engine speeds up to a certain predetermined maximum the compensating jet will supply most of the petrol, but as the demand increases the main jet will also begin to supply, and simultaneously the compensating jet will commence to go out of action owing to its supply of petrol becoming partly or wholly exhausted due to the restriction of the orifice C.

The simple jet-in-tube carburettor has been greatly improved by the addition of anautomatic extra-air valve, of which a simple form is shown in Figs.43and44. It consists of a small mushroom type valve A, with its seating B so arranged that it can be screwed into the induction pipe of the engine. The valve is held up against its seating by a light spring C, so that at high engine speeds when there is a good vacuum in the induction pipe the pressure of the atmosphere will open the valve against the tension of the spring and allow air to pass into the induction pipe, thus reducing the amount of vacuum and simultaneously weakening the mixture.

Fig. 43.—Automatic Springcontrolled Extra-air Valve.

Fig. 44.—Plan View ofAutomatic Extra-air Valve.

The points of agood carburettor:—

These may be set out in the following order—

(1) Completeatomizationandvaporizationof the liquid fuel at all engine speeds.(2) The supply of anadequate quantityof gas of thecorrect proportionswith all throttle openings and at all temperatures.(3) Sufficientmechanical strengthanddurabilityto withstand road shocks and to ensure freedom from breakdowns without undueweightorcomplications.(4) Ability to continue working correctly when the car is on an incline or affected by the camber of the highway.(5) Moderate first cost.

(1) Completeatomizationandvaporizationof the liquid fuel at all engine speeds.

(2) The supply of anadequate quantityof gas of thecorrect proportionswith all throttle openings and at all temperatures.

(3) Sufficientmechanical strengthanddurabilityto withstand road shocks and to ensure freedom from breakdowns without undueweightorcomplications.

(4) Ability to continue working correctly when the car is on an incline or affected by the camber of the highway.

(5) Moderate first cost.

Pressure Feed and Gravity Feed.—In Fig.38we showed a gravity-fed system or one in which the petrol is fed from the tank to the float chamber of the carburettor by the action of gravity only. For this system to be successful at all times the carburettor must be placed low down to obtain a goodheadfor the flow of petrol in the connecting pipes, as there is a practical limit to the height at which the petrol tank can be fixed. Also the pipes must have a continuous run down towards the float chamber to prevent air-locks in them, and they must be kept away from the hot exhaust system. When all these points can be secured this system is perfect. An alternative system is to force the petrol into the float chamber by maintaining an air pressure (of 2 or 3 lb. per square inch) on the surface of the liquid in the petrol tank. With this arrangement the carburettor may, if desired, be situatedabovethe level of the petrol tank in a moreaccessibleposition, but it necessitates the fitting of a smallair pumpon the engine and the use of ahand air pumpfor starting.

We have already stated that the charge of explosive mixture is ignited in the cylinder at the end of the compression stroke by means of an electric spark. The electric spark takes place as the result of an electric discharge across thegapbetween theelectrodesof thesparking plug.

The Sparking Plug.—Two views of a typical sparking plug are shown in Figs.45and46, in which A is thehigh tensionelectrode which is periodically charged with electricity at high voltage (or electrical pressure) from a high tension magneto or a high tension coil, and B1, B2are electrodes which, being in metallic contact with the cylinders and framework of the engine, are thus at zero potential. The electric discharge occurs across the gap C1, C2in the form of a spark or flash. The electrode A is heavilyinsulatedfrom the metal casing D of the sparking plug by porcelain insulators E and F. The locknuts G and H serve to keep the plug gas-tight and hold the several portions together mechanically. The terminal K is used for clamping the wire (orlead) which brings the supply of high tension electricity. The high tension electric current may be supplied either by (1) a magneto machine or (2), a coil and accumulator ignition system.

Fig. 45.—Sectional Drawingof a Sparking Plug.

Fig. 46.—A Sparking Plug.

Fig. 47.—Outside View of a High Tension Magneto.

Fig. 48.—End View of a High Tension Magneto, showing High Tension Distributor and Low Tension Contact Breaker.

The High Tension Magneto.—In Figs. 47, 48 and 49 we show a modern high tension magneto suitable for a four-cylinder engine. It consists of the stationary magnets A, the driving spindle B, the high tension electrode D, the high tension distributor C, and the low tension contact breaker E. The armature, condenser, and distributor gear wheels are not shown in the drawings, but are situatedinsidethe machine in the space between the high tension electrode D and the low tension contact breaker E. As the spindle B is rotated by gearing driven from the engine crankshaft the armature attached to it generates a high tension current and a low tension current. The high tension current passes to the high tension electrode D and thence across the machine to the carbon brush H of the high tension distributor C. The low tension current passes through the platinum-tipped contact screws F1, F2of the low tension contact breaker. Twice during each revolution of the armature these contacts are separated owing to the fibre block attached to the bell crank lever G passing over the stationary cams T1, T2; this constitutes themake-and-breakdevice for interrupting the primary current. The momentary interruption of the primary current in this way causes a very great increase in the electrical pressure (or voltage) of the secondary or high tension current which is sufficientto bring about the spark discharge across the gap between the electrodes of the sparking plug. Since there are two of these cams on the low tension contact breaker it will be understood that the armature can supply current for two sparks in every revolution it makes. If we bear this fact in mind we will have no difficulty in determining the relative speeds of the magneto armature and the engine crankshaft for any type of engine. A four-stroke engine requires one spark in every two revolutions made by the crankshaft, so that a four-cylinder engine of this type requires two sparks per revolution, and the magneto armature must run at crankshaft speed. A six-cylinder engine working on the four-stroke cycle would require three sparks per revolution, but the armature of the magneto only supplies two, therefore it must be driven at one-and-a-half times the crankshaft speed.

Fig. 49.—End View of a High Tension Magneto,showing the Earthing Terminal (P).

The high tension distributor consists of the carbon brushH driven by gearing from the magneto armature and the metal segments M1, M2, M3, M4, which are mounted in a block of insulating fibre. There must be as many segments on the distributor as there are cylinders on the engine, one segment for each sparking plug; but the armature cannot supply more thantwosparks per revolution, and therefore if the distributor has four segments it must be driven at half the armature speed, and if it has six segments it must be driven at one-third of the armature speed. Each metal segment is electrically connected to a sparking plug lead such as L1, L2, L3, L4. The high tension electrode D is attached to a light carbon brush which presses on a gunmetal collector ring at the high tension end of the armature winding. A special terminal is provided at P, so that when a wire is attached to it and connected to the frame of the engine (usually through a switch) the low tension windings areshort-circuitedor closed on themselves, and the make-and-break has no effect, because there is always the path through the switch until it is opened again. Under these circumstances the voltage of the high tension circuit is not sufficient to cause the spark discharge, and the ignition is then said to beswitched off. The instant at which the spark occurs may beadvancedor made earlier by moving the rocker arm K, which carries the stationary cams T1, T2backwards, whereas if it is movedforwardthe ignition isretardedand occurs later in the stroke.Normalignition occurs when the lever is midway in its range of movement and corresponds to the position of the piston when the crank is on the top dead-centre, whereas advanced ignition occurs just before the piston has completed the compression stroke, and retarded ignition will take place after the crank has passed the dead-centre and when the piston has moved down a little on the power (or explosion) stroke. Advancing the ignition increases the speed, and retarding the ignition reduces the speed, except when the engine is overloaded, and then it may pick up speed a little or runbetter if the ignition is slightly retarded—but the exact behaviour will depend on the temperature of the metal walls and piston within the cylinder.

Fig. 50.—An Ignition Coil, showing the Trembler Mechanism.

We have mentioned that normal ignition occurs when the crank is exactly on the dead-centre and the piston at the top of its stroke. If we set the magneto when the engine is at rest so that ignitionoughtto occur on dead-centre when the arm K is in its mid position the actual sparking will belateon account of thetime lagof the electric current. The current takes time to flow and in that brief element of time the crank has moved a few degrees off the dead-centre, at high speeds. Hence the ignitionmust be advancedif the charge is to be correctly fired when the engine is running fast. If the ignition is too far advanced it will cause the engineto “knock,” especially under heavy loads. If the ignition isretardedthe charge is not fired at the commencement of the stroke so that a portion of the power theoretically available in the fuel is lost to exhaust at the end of the stroke. Retarded ignition always causes overheating of the exhaust system.

If the arm K is fixed mechanically in its mid position so that the ignition can neither be advanced nor retarded, we have what is known asfixedignition.

Fig. 51.—Ignition Coil Case.

Fig. 52.—Low Tension ContactBreaker for Single Cylinder CoilIgnition System (Wipe Contact).

An Ignition Coilsuitable for a single cylinder engine is shown in Figs.50and51, in which A and B are the low tension terminals and C is the high tension terminal. The trembler blade is shown at D, with the adjusting screw Fand the platinum-tipped contacts G1, G2. The iron core of the coil projects a little above the case, as shown at E in Fig.50. The strength and character of the spark may be varied considerably by slightly screwing F up or down. When current is supplied to the low tension terminals of the coil it flows through the primary winding and magnetizes the iron core, completing its circuit by passing across the platinum contacts. When the trembler blade is attracted to the iron core the primary circuit is broken by the temporary separation of the platinum contacts, and therefore the magnetism ceases, the trembler is released, and the circuit is completed again. Thus the trembler blade is set rapidly vibrating and making and breaking the primary circuit as long as the roller attached to the rotating arm H of the low tension contact breaker shown in Fig.52is in contact with the metal segment K, and this results in the production of asuccessionof sparks at the sparking plug which is connected to the terminal C of the high tension winding. This is very useful especially when starting an engine, but with modern high-speed engines the trembler has only time to give one spark at high engine speeds, and therefore the magneto has the advantage except for easy starting. This has led to the introduction ofdualignition systems, and in particular to that system in which the main ignition is by magneto, but there is a supplementary coil fitted to supply high tension current to the ordinary high tension magneto distributor when the engine is at rest,the coil being cut out after the engine has got up speed. But this has been largely superseded by the use of electric motors for starting the engine, although themagnetois still relied upon for the ignition of the charge in the cylinders. The contact breaker and coil just described would be very suitable for a single cylinder petrol engine, or a non-trembler coil might be used in conjunction with a contact breaker of the quick break type used on magnetos and illustrated in Fig.48. In the case of a multi-cylinder engine having coil ignition we may use separate coils without a high tension distributor, or a single coilanda high tension distributor having as many segments as there are engine cylinders and arranged similarly to the magneto distributor of Fig. 48. When no high tension distributor is fitted there must be a separate coil for each cylinder, and the high tension wire runs direct from the coil to the sparking plug, so that the character of the spark as well as the exact instant at which it occurs may not be the same in each of the cylinders. If there is a high tension distributor it should be mounted on the same driving spindle as the low tension contact breaker, in order that the ignition may besynchronized, i.e., the spark will occur at thesamepoint in the piston’s stroke for all the cylinders. The ignition may be advanced or retarded by moving the casing of the low tension contact breaker relative to the roller arm, thus causing it to make contact either earlier or later in the revolution.

At one time it was thought thattwo-point ignitiongave increased power and efficiency. Two-point ignition meanssimultaneousfiring of the charge from more than one plug. Sometimes two high tension leads were led from each distributor segment and connected to the two plugs in the corresponding cylinder—this constituted theparallelsystem. Another system employed a special plug withbothelectrodes insulated from the engine frame; this was coupled inserieswith an ordinary plug so that the spark jumped the gaps in succession. It is quite evident, however, that ifthe gas is thoroughly mixed up and in a state of violent agitation as the result of rapid compression, a single well-placed spark will fire it successfully and so no gain results from simultaneous ignition at another and less favoured point.

Fig. 53.—Wiring Diagram for Four CylinderEngine with High Tension Magneto Ignition.

Wiring Diagram for Magneto Ignition System.—The electrical connexions are extremely simple in the case of a high tension magneto ignition system. In Fig.53we show a four-cylinder engine fitted with high tension magneto. The only wires required are the four high tension cables from the high tension distributor to the sparking plugs and theearthingwire leading from the short circuiting terminal to the frame of the engine through a switch as indicated. The firing order of the cylinders may beeither1, 3, 4, 2 or 1, 2, 4, 3, as desired (provided the cranks are arranged in the usual manner, that is, in the order shown in Fig.21). In determining the order of firing of the respective cylinders the engine should be turned round very slowly by hand and careful note made of the order in which the firing strokes occur. To determine thefiringstroke the piston should be moving downwards and the position of the valves noted; ifbothvalves areshutthen this is the firing stroke, but if the inlet valve is opening it is the suction stroke.

Wiring Diagram for a Coil Ignition System.—Theelectrical connexions for a coil ignition system are slightly more difficult to follow out; they are shown in Fig.54for the same engine illustrated in Fig.53. In the diagram we show four separate trembler pattern coils, each of which can give a succession of sparks as long as contact is being made on any one segment of the low tension contact breaker connected to it. All the low tension terminals of the coils are connected together to a common busbar, which is supplied with current from the accumulator direct. The current flows from the busbar through the low tension windings of each of the coils in turn, as it comes into operation through the engine-driven contact breaker, and returns to the battery through the frame of the engine. High tension cables lead from the high tension terminal of each coil direct to the sparking plugs, and therefore the ignition is not necessarily synchronized.

Fig. 54.—Wiring Diagram for Four Cylinder Engine with Trembler Coil Ignition.

When the switch in the low tension circuit is opened the ignition isoff, because the current is then permanently interrupted; when the switch is closed the ignition ison.To economize current a quick make-and-break device should be used instead of thewipeform of contact breaker illustrated, and a non-trembler coil used. It is very important to fullyretardthe 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, F2are 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.

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 evenseizurewill 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 aninflammablevapour, i.e., one which will burn. This temperature is called theflash pointof 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 ofcarbon depositon the metal walls of the cylinder and the valves; others again are gummy or tooviscouseven at high temperatures. Such oils must be avoided equally with those which lose theirviscositytoo 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 ofit will fall into these holes and lubricate the bearings. This is a very cheap method of lubrication infirst 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.

Improved System of Splash Lubrication.—This is a combination of the splash system and theforcedsystem, and is shown in Figs.55and56. In these figures A2and A3represent two of the main engine bearings which supportthe crankshaft; C1, C2, C3are three of the crankpins; F1, F2, F3are oil troughs placed under the crankpins; D2, D3are oil feed pipes to the main bearings. Generally speaking, the oil is drawn from the bottom of the crankcase by means of a pump, and this pump delivers the oil to some form of indicator mounted on the dashboard of the car. After passing through the indicator the oil flows by two main pipes, one of which feeds the main bearings by means of branches D2, D3, etc., and the other feeds oil troughs by means of branches such as G2. When the troughs arefullthe oil overflows into the bottom of the crankchamber, and so there is always a constant depth of oil for the scoops attached to the connecting rod ends to dip into, and one great drawback to the splash system is overcome; also the main bearings are always sure of being amply supplied. The oil pump may be an ordinary plunger type pump or a rotary pump.

Fig. 56.—Sectional View of Endof Connecting Rod, showingArrangement of Scoop andOil Trough.

Forced Lubrication.—One system of forced lubrication is shown in Fig.57. The general arrangement of the system is very similar to the preceding one, except that there are no troughs in the crankchamber andall the bearingsreceive an ample supply of oil underpressureso that the journals are supported in their bearings on afilm of oiland the metals never come in direct contact with each other. After entering the main bearings the oil passes through holes drilled in the crankshaft and thus positively lubricates the crankpin bearing, passing up the connecting rod either internally as shown or by an external pipe it lubricates the gudgeon pin and then falls down into the crankchamber. On its way down it gets splashed about and thus lubricates the cylinderwalls and piston; sometimes these are positively lubricated by leading the oil through the centre of the gudgeon pin direct to the surface of the cylinder walls—but this often gives an excess of oil and causes a smoky exhaust. In Figs.58and59we show two views of a very popular form of oil pump for forced lubrication systems. It consists of two gear wheels, one of which is driven by a spindle from the engine crankshaft, and it drives the second wheel by means of the projecting teeth. The oil is picked up by the teeth and passed round from the suction to the delivery side of the pump on theouteredge of the wheels; no liquid can pass direct across between the teeth which are in mesh, and hence the direction of rotation is as shown by the arrows.

Fig. 57.—Forced Lubrication System.

Fig. 58. and Fig. 59. Two Viewsof a Rotary Oil Pump forForced Lubrication.

The difficulty of securing a really good lubricant for petrol engines must be apparent from a study of the prices of the various oils. It will be observed that they are all considerably more expensive than petrol, and therefore we must economize in their use. The old splash system was very wasteful and consumed oil at the rate of one gallon every hundred miles at least, but a modern system of forced lubrication will not require more than one gallon of oil every thousand miles. Perhaps an average everyday figure for ordinary motor-car engines would be one gallon every 250 miles. The pressure of the oil in a forced feed system varies in different makes of engines from 5 up to 40 pounds per square inch—a very common figure, however, is 10 pounds per square inch. The speed of the oil pump also varies considerably, and ranges from 500 up to 2,000 revolutions per minute at normal engine speed. Generally a small relief valve is fitted in the pump casing, which returns oil to the crankchamber if the pressure tends to rise above the desired limit due to the engine speed increasing. We have mentioned already that the flash point is generally over 400° Fahrenheit when the oil is new, but after it has been in the crankcase some time and got used over and over again it is found that the petrolvapour leaking past the piston rings of the engine condenses when the engine cools down after a run and drops into the oil in the sump, thus lowering its viscosity and its flash point. According to Mr. Morcom it may come down as low as 200° Fahrenheit (about), but if the oil is heated and the petrol driven off the flash point goes up again. Therefore it is a good plan with forced lubrication systems to empty the old oil out periodically and fill up entirely with fresh oil.

We have already explained the necessity for cooling the cylinders of a petrol engine by means of a water-jacket, and we now proceed to show how the circulation system may be arranged. There are two forms of circulation in use: (1) Natural; (2) Forced.

Fig. 60.—Thermo-syphon Water Cooling System.

Natural or Thermo-Syphon Circulation.—This system is shown in Fig. 60, and may be explained as follows:—The heat generated by the successive explosions within the cylinder causes the water at the top of the cylinder jacket A to get hot. As a column of hot water is lighter than one of cold water of equal height, the heated water rises up the pipe B and flows into the top of the radiator D, while colder water from the bottom of the radiator flowsup the pipe C and into the cylinder jacket A. It is important that the height of the water in the radiator D should be at such a level that the outlet from the pipe B is submerged.

Fig. 61.—Forced Water Circulation by means of a Pump (P).

In the radiator the water falls through a series of tubes E, havinggillsor fins on the outside for the purpose of dissipating the heat. The cooling of the water is also assisted by the fan F, which is driven from the fan pulley G and draws air past the radiator tubes at high speed. Sometimes the water in the radiator is made to fall through a series of cells which are formed of cast aluminium; such a radiator is called ahoneycombradiator. It is important that the pipe C should not have any sharp bends and it should not rise very much in height, but the outlet pipe B may have a considerable rise with advantage. Both the inlet and outlet pipes should be of large diameter with this system of circulation, and the radiator should be soarranged that there is a goodheadof water above the cylinders. In the drawings H is the front cross-member of the chassis, K is the starting-handle clutch, and L is the starting handle.

Forced or Pump Circulation.—With this system the water is positively circulated through the jackets; it is drawn from the bottom of the radiator by the pump P (Fig.61), which is mechanically driven from the valve shaft of the engine, and delivered under pressure to the jacket A. The outlet of the pipe B need not be drowned, and the pipe C may be arranged in any way most convenient to the chassis. Sometimes when a pump is fitted the pipes are arranged so that the system may be operated as a thermo-syphon in the event of a breakdown of the pump. It is not uncommon to experience trouble due to leakage at the pump gland, which results in gradual loss of water from the system, and therefore the thermo-syphon or natural circulation has much to recommend it. Also it may be said that the pump represents an additionalcomplicationto the engine and means increased first cost. Every moving part we add to the engine is of course an additional potential source of trouble, but the addition of a really first-class water circulating pump of the type shown in Fig.58cannot be said to be anything but a reasonable precaution. The weight and size of every part of a motor-car engine and chassis have been so much reduced recently, owing to competition with American firms, that many manufacturers who adopted the thermo-syphon principle experienced great trouble with it owing to the small size of radiator fitted, as well as faulty arrangement of the connexions. Considering any one engine, it follows that if a certain size of radiator and a given quantity of water in the circulating system will keep the engine cool when a pump is used to give a positive circulation, then a larger radiator and greater quantity of water will be required for natural circulation. Thermo-syphon circulation also means ahighradiator andbonnet, which many people object to on the score of appearance, without considering its utility. With natural circulation greater care must be exercised to keep the radiator well filled, but this often leads to other difficulties on bad roads owing to the water splashing from the overflow pipe and finding its way on to ignition appliances. Before starting an engine it is always advisable to remove the radiator filling cap and examine the water level; if it should happen that at any time while the engine is running the circulating system runs quite dry, owing to a breakdown or leakage, do not attempt to pour water into the radiator, but simply raise both sides of the bonnet and leave the engine to cool down first. Again, when filling the radiator for a forced circulating system, it is desirable to give the engine a turn or two with the starting handle occasionally to operate the pump and prevent air locks; very often the radiator appears to be full, but as soon as the engine commences to run the water disappears owing to the system not being full, due to the above-mentioned cause. In cold or frosty weather all the water should be drained off from the circulating system when the car is in the garage, unless the garage is heated or some anti-freezing solution is used. Glycerine or alcohol added to the water will prevent it freezing, but as an additional precaution in cold countries one often sees travelling rugs strapped over the radiator and bonnet.

Occasionally one gets trouble due to the water boiling in the jackets, and on this account reasonable care should always be exercised in unscrewing the radiator filling cap if the presence of steam is suspected. An engine may have been running well for a long time without trouble and then develop symptoms of overheating in the circulation system. This overheating may be eitherlocalorgeneral. Local overheating may result from some partial seizure of the piston in the cylinder due to dirt on the walls, or from the presence of grease on theoutsideof the cylinderwalls, in the jacket space. If grease is suspected or there isfurringup in the passages of the jacket due to bad water supply, the trouble may be cured by adding some common washing soda to the water in the radiator and running the engine with the car at standstill for half-an-hour or so. After this drain off all the water and sludge, allow the engine to cool down, and then fill up again with clean water.

General overheating may result from leaky pistons and pistons rings, or from the use oftoo weaka mixture in the carburettor, or from overloading the engine. If the mixture supplied to the engine is very weak, the overheating will be very marked on theexhaust sideof the engine. Local overheating causes the engine to “knock” badly.

In arranging the jackets and the pipes care must be taken to arrange that a cock is placed at thelowestpoint in the system, so that the whole may be completely emptied, and the inlet pipe to the jacket should enter at the very bottom of the jacket chamber for the same reason. It may be thought that all that is necessary is to provide plenty of space in the jackets round the cylinders and plenty of water in the whole system, but experience shows that it is very important not to make the jacket spacetoo large, so as to ensure positive circulation and avoidlocal circulationin any one portion of the jacket. When cylinders are cast in pairs the back pair have a tendency to discharge their hot water into the front pair and so back to the inlet pipe again, hence this should be guarded against in arranging the outlet pipes.

Pipes suitable for use with multi-cylinder engines are shown in Fig.62, in which (a) is an outlet pipe for a monobloc casting, and (b) and (c) are inlet and outlet pipes respectively for engines having separate cylinders. It is advisable to modify the diameter of the branches by the insertion of metal orifice plates at the flanges to ensure an equitable distribution of the water among the several cylinders.

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