MOST people are aware that much heat is generated by an internal combustion engine while it is working and no known materials would stand continuous high working temperatures unless means were taken to dissipate the heat within practical limits. The usual running temperatures lie between 180 and 200 degrees Fahrenheit, or 80 and 90 degrees Centigrade. Sometimes control is achieved by direct cooling, and the heat is dissipated directly from the cylinders into the atmosphere. An engine cooled in this way is said to be air cooled. Or, alternatively, temperature control may be effected by indirect cooling, as when fluid is circulated through appropriate jackets surrounding the cylinders, and conveys the heat to a radiator which, in its turn, is copied by the atmosphere. Water is usually employed for this purpose on motor vehicles and will be the only fluid to concern us, but glycol, having a basis of glycerine, is extensively employed on aircraft engines.
Air-cooled cylinders are furnished with a large number of fins to increase the surface area for the radiation of heat. Such engines are confined chiefly to motor cycles, but when used on larger vehicles, a fan is sometimes era-ployed to deliver a forced draught through the system. Air-cooled engines installed in aircraft are fitted with special cowlings and baffles designed to concentrate the flow of cooling air over the cylinder cooling fins. Generally speaking, with air-cooled engines it is difficult to regulate the temperature, and consequently nearly all motor car engines use a water-cooling system in which either the water circulates by natural means, called thermo-siphon, or is forced round by a pump.
- 1 Thermo-Siphon System
- 2 Pump Circulation
- 3 Caution
- 4 Always refill the cooling system with soft water— ordinary tap water usually deposits substances detrimental to the system. If anti-freeze solutions are not used, or the garage is not heated, do not forget to drain the cooling system in frosty weather. Lubrication
- 5 Clutch
- 6 Gearbox
- 7 Propeller Shaft
- 8 Rear Axles
- 9 Differential Gears
In this system the heated water circulates, by convection current, to the highes t point in the system. For successful operation the bores. , of all pipes and jackets should be free of obstruction and as large as possible. The cylinders themselves are placed as low as possible, relative to the radiator, so as to accelerate the natural upward flow of heated water from the jackets to the radiator. The hot water cools as it passes into the hollow cellular structure of the radiator. It then flows down through the radiator into the cylinder jackets to take the place of the hot water flowing out. This circulation is maintained so long as the water level in the radiator is kept above the delivery pipe from the cylinders to the radiator. Otherwise the circulation will cease and the water in the jackets will boil away. So it is most important with this system to keep the water level well up in the radiator. It should be checked every day. This is particularly important where an aluminium cylinder head is embodied, for overheating will tend to produce cracks during the cooling of the metal. Another advantage of this system is that it prevents the engine from being run too cold, for the circulation will not commence until a certain heat is reached.
In this system the water is circulated under pressure by a pump or impeller, usually located between the radiator and the inlet into the cylinder jackets. The pump is driven by the engine and commences to circulate the water immediately the engine is started. A normal consequence would be for the engine to run too cold for some considerable time, to its detriment, and it is therefore the modern practice to instal a thermostatic valve in the outlet pipe from the cylinders to the radiator. This valve remains closed, preventing the water from circulating, but it automatically opens when a predetermined temperature is reached. In fact, the temperature of the engine is automatically regulated under all conditions. There is, however, a further consideration which must not be overlooked. Suppose the vehicle is held up in a traffic jam, with the engine running. It is true that the pump will circulate the water and the thermostatic valve will allow it to pass into the radiator, but, since the vehicle is not in motion, air-draught through the radiator will be absent and the engine will tend to over-heat. To counteract this disadvantage it has become a general’ practice to provide a fan to create a suitable flow of air through the radiator when the engine is running.
Always refill the cooling system with soft water— ordinary tap water usually deposits substances detrimental to the system. If anti-freeze solutions are not used, or the garage is not heated, do not forget to drain the cooling system in frosty weather. Lubrication
All the moving parts of a vehicle, whether they are components of the engine, gearbox, transmission or chassis, are subject to friction and wear. It is the all-important job of a lubricant to reduce this wear to a minimum. No doubt the reader is aware that many different systems of lubrication are used in different designs for this purpose, but though he may not be particularly interested in the system itself, he must acquaint himself with the makers’ instructions. It is, for instance, most important to use a suitable lubricant for a specific job. There are many different oils and greases whose properties and useful characteristics vary considerably. Nobody knows better what is suitable in the way of lubricants for a particular vehicle than the manufacturers, and one cannot do better than take their advice. For example, an oil which might be very suitable, and give good results in an engine employing a full force feed lubrication system would probably be entirely unsuitable for another engine employing splash lubrication. Also, the grade of grease specially suited to the lubrication of wheel hub bearings would be quite unsuitable for lubricating the water pump spindle. While not embarking on a detailed analysis of the various lubricants and their uses, we must point out that whereas the designer has equipped the vehicle with suitable means of lubrication at all places where it is required, it is the responsibility of the driver to see that regular maintenance is carried out with the lubricants as recommended. A lubrication chart is generally issued with the vehicle, or is available on application to any of the reputable oil companies.
For those who do their own maintenance, the points to watch in collaboration with the instruction book are: 1. Use the right lubricant for the right part. 2. Never mix oils or greases of different makes. 3. When checking engine oil level, always see that the vehicle is standing on level ground. The check should be made a few minutes after the engine is shut off. 4. When changing the lubricant in engine, gearbox or rear axle units, drain off the old lubricant immediately after a run—the lubricant, being hot, will then run most freely. 5. It is not good practice to use paraffin as a flushing medium. Use a special flushing oil. 6. It is usual to employ a lighter grade of oil during the winter period, than that used during the summer period. 7. Ascertain the position, etc., of all the filters provided, and see that they are cleaned or renewed as occasion requires. 8. Remove any blocked grease nipples and clear or replace them, making sure that the fresh lubricant gets to the part concerned. 9. Too light a grease in wheel hubs invariably gets on to the brake linings, with detrimental effect. 10. Never over-do the use of the carburettor strangler. Excessive use causes dilution of the engine oil.
TRANSMISSION AND MAINTENANCE
TRANSMISSION may be said to mean, the efficient conversion of engine crankshaft rotation into a motive force, to be transferred to the road wheels, by means of those mechanical components which link the engine crankshaft to the road wheels. Briefly, the transmission system includes the clutch, gearbox, propeller shaft and rear axles, which latter are driven through a set of differential gears by the main propeller shaft. The location of each of these – components, in relation to the engine and car chassis, and the functions of each are described in the following paragraphs.
Before proceeding further with a description of these components the reader’s attention is drawn to the fact that the type of transmission herein described relates only to the more general principles of transmission. There are, for instance, car transmission designs which incorporate front wheel drive; others are so designed as to locate the engine power unit at the rear of the car with a transmission materially affected in design by this measure. There may also be minor variations in design at any point throughout the whole of the transmission system for cars of different makes, which incorporate transmission of the more orthodox type and, occasionally, extensive differences in clutch design. These are the main characteristics which distinguish one make of car from another. Since these variations are numerous’ they cannot be dealt with in detail and it is proposed to describe only the fundamental requirements and functions of car transmission.
The problem of the suitable conversion of engine power into a motive force by means of the transmission system is not confined to the simple transference of a mere rotation of the driving road wheels. Several requirements in design must be satisfied before effective transmission is possible.
The first of these is the necessity of isolating the engine, at will, from the transmission. This requirement is fulfilled by the embodiment of a foot operated clutch and a neutral position of the gears in the gearbox.
The clutch, situated between the rear of the engine and the forward face of the gearbox remains in effective operation until it is operated by the foot lever, or, as it is generally known, the clutch pedal.
The gearbox, incorporating a selection of gears, remains inoperative to the transmission of crankshaft rotation, via the clutch, when the gear selector lever is in neutral position, I.e. out of gear. Thus it will be understood that until the gear selector is moved to a different position the propeller shaft (directly connected to the gearbox main driven shaft by a universal coupling joint) will not rotate although the engine is running. The clutch, therefore, is necessary for the temporary isolation of the transmission system for the changing of gears either from the stationary position or during actual motion of the car. A second requirement influences the design of the propeller shaft and its connec- tion to the gearbox. This is raised propeller shaft imparts its rotation, by the necessity of mounting the rear axle cover (or housing) on springs to absorb the shock norm;1.! To the movement of the car over uneven road surfaces. In consequence some point along the propeller shaft must be flexible to up and down movement of the rear axle without impairing the efficiency of transmission. This need is met by the introduction of a universal coupling at the point of attachment between the forward end of the propeller shaft and the rear end of the gearbox main driven shaft.
A further requirement, essential to the safe driving of a car, quite apart from the excessive wear which would be incurred by the tyres of the driving road wheels, is the necessity of ensuring that each of the rear driving road wheels shall turn at the correct speed required when negotiating a bend or corner on the road. Obviously the inner wheel should rotate more slowly than the outer wheel in relation to the bend in the road. This problem in design is solved by the incorporation of a differential gearbox located in the rear axle cover and it is at this point that the pinion wheel of the via the differential gears, to the axles of the driving road wheels. These three main requirements are, however, complicated by other considerations, the specialized reasons for which are described separately under the headings of the transmission components, the clutch, gearbox, propeller shaft, differential gears and rear axles.
This component is generally enclosed within the gearbox casing but is, in effect, a separate unit from the gearbox assembly. The clutch comprises three main members; the clutch pressure plate, flywheel and the clutch disk or centre plate, which covered with a material possessing a high coefficient of friction is interposed between clutch pressure plate, and the rear face of the flywheel, the clutch pressure plate only being operated by a foot lever located on the toe board of the interior of the car. A popular type of clutch is known as the Single Plate Clutch. In simple terms of operation, when the clutch pedal is fully depressed the clutch pressure plate is drawn away from the clutch disk, thus isolating the engine from the gearbox. The clutch pressure plate is heavily spring loaded and if the foot pressure is relaxed on the clutch lever the pressure plate will return to its initial position, firmly contacting the clutch disk and forcing the disk against the flywheel face. The flywheel, which is driven direct from the engine, then transmits its rotation without slip, to the clutch disk. As the clutch disk or centre plate is keyed to the gearbox shaft this shaft will be turned as soon as the pressure on the clutch pedal is relaxed. If a gear has been engaged by movement of the gear selector lever, rotation will be transferred to the propeller shaft and thence to the road wheels.
The leverage of the clutch pedal is so arranged as to permit a sensitive application of the clutch pressure plate to the clutch disk. This feature is an essential, otherwise the sudden load imposed on the engine by the inert weight of the whole car would cause excessive strain on the transmission system. Thus it will be seen that slow application of the clutch provides for a gradual transference of loading to the engine when the car is stationary, eliminating jerky starting from the stationary position and smoothing out any sharp differences in engine loading when the gears are changed during car motion.
The gearbox is closely inter-related, in function with the clutch, as it would be impracticable to change gears without the intermediate aid of the clutch assembly. But before describing the function of the gearbox it would be advisable to consider certain innate characteristics of the internal combustion engine.
It is obvious that an engine turning slowly will not develop so much power as when it is turning over at great speed. Thus at low engine speeds the power unit will, as previously mentioned, cut out if a sudden load, in excess of the power developed, is applied to the crankshaft. If, however, in the absence of a clutch, the engine is suddenly geared to the transmission the whole car will be jerked forward at a speed directly proportional to the power developed and transferred to the road wheels, before the engine cuts out. The effects of such a sudden loading on the various components can be imagined. Hence another reason for embodiment of the clutch.
Another significant factor in the employment of a gearbox is the variation in the load applied to the engine, such as would occur when the car negotiates hills, down slopes and level road surfaces. This will explain the range of gear ratios available in the gearbox. Since it is occasionally necessary to reverse the car a separate gear for this purpose is embodied in the gearbox. Each of the gear ratios is calculated to suit the power developed by the engine as against the range of loads’ likely to be encountered and speeds required during the normal running of the car. Thus the gearbox is there to ensure an appropriate transmission of adequate driving power to the road wheels in all conditions. To understand the operation of a gearbox it is necessary to realize that the average individual can, with the aid of a crane, lift a heavy weight which could not normally be lifted by hand. It is this principle which is applied in the design and incorporation of different gears in the gearbox. Briefly, if a large gear, attached to the drum of the crane, is turned by a small gear fixed to the spindle of the crane handle, a heavy weight may be easily lifted from the floor. The rate or speed at which the load is lifted will, however, be slower than would be the case if a somewhat larger gear was used with the crane handle (turned at a uniform speed) to operate the drum, but in the latter instance the speed and exertion of lifting the load would be proportionately greater.
For.example, if the large gear in the drum has 40 teeth and the small gear has 20 teeth the gear ratio will be 2 to 1. If the large gear has 80 teeth and the small gear has 20 teeth the gear ratio will be 4 to 1, and less exertion will be necessary to lift the load; but, if in both instances, the handle is turned the same number of revolutions per minute the speed of lifting will be faster in the former instance where the gear ratio is 2 to 1.
That in its most simple form is what happens in the gearbox.
Low ratio gearing being utilized for heavy loads, such as those imposed when the car is stationary and about to be put in motion, hill climbing, etc. The intermediate gear ratios are utilized for those transitional phases of car motion to ensure smooth engagement in top gear, one of high ratio, when the car has negotiated an incline or has gathered speed on a level road surface.
Another mechanical feature in gear running must be noted. If a gear having 60 teeth is driven direct by the engine crankshaft, rotating at 1,000 r.p.m., and a gear having 120 teeth is meshed with the former, the speed of the larger gear will be 500 r.p.m. If a lower gear ratio for the driven gear was used, say with a gear having 240 teeth, the speed of this gear would be 250 r.p.m. Thus by drawing a comparison it will be seen that speed of car motion is sacrificed in low gear ratios but the ability to apply higher engine r.p.m. Ensures that the engine will be capable of overcoming the load imposed without any danger of cutting out. The following table, which gives approximate figures and average ratios with resulting road speeds for an 8 h.p.car will serve to correlate the foregoing information, bearing in mind the fact that the higher the r.p.m. The higher is the engine power output.
It will be noted that the small gear on the main driven shaft is always in mesh with the larger gear secured on the layshaft (sometimes called the countershaft). These are known as the Constant Mesh Gears. Thus the layshaft will rotate when the engine is running, but, as has been previously mentioned, no transmission of rotation will be imparted to the propeller shaft when the gear change or selector lever is in the neutral position. Furthermore, the lay-shaft will rotate at a lower speed than the engine crankshaft which drives it. It will be noted that the layshaft will also carry other fixed gears which may in turn be meshed with one or other of the sliding gears each of which, being of different gear ratios, will transmit a different speed of rotation to the propeller shaft, assuming that the engine is turning at a uniform rate. First, second, top and third speed sliding gears are all free to move along the splined main or driven shaft but all must rotate with the driven shaft when one or other of the sliding gears has been meshed with its appropriate gear on the layshaft.
When the driver disengages the clutch and moves the gear selector lever to first gear speed, the sliding first gear is moved along the splined driven shaft and into mesh with its fixed mating gear on the layshaft, the other sliding gears remaining free.
When the pressure on the clutch pedal is released the clutch will transmit the crankshaft rotation to the constant mesh gears and the layshaft will transmit its turning moment, via the first gears, to the main driven gear shaft which will cause the propeller shaft to rotate.
The same operation will apply to the selection of any of the different gears and the conditions of use will be governed by the duties imposed on the engine during car running. In top gear, however, the main driven shaft in the gearbox is driven direct from the engine at crankshaft speed.
The propeller shaft is coupled direct to the rear end of the gearbox main driven shaft by a special fitting known as a universal joint. This fitting, necessary to the efficient operation of the propeller shaft, ensures that the up and clown movement of the axle cover or housing, to which the shaft housing is secured, will not interfere with the rotation of the shaft; for the axle cover is secured to the rear springs of the car and a certain degree of flexing movement at the springs, due to uneven road surface, will be encountered. The propeller shaft is contained in roller and thrust bearings situated in the shaft housing and at its rear end is secured a driving pinion which meshes with a large driven gear in the interior of the axle cover or differential gear box. The rotation of the propeller shaft driving pinion is transferred to the axles which operate in the following manner.
The rear axle cover or housing, it should be noted, is braced by two torque tubes which are firmly secured, in horizontal positions, to the propeller shaft and the outer extremities of the axle housing. Furthermore the weight of the rear of the car is imposed on the axle cover only, the axles being free to with the pin and disk, but they are also free to revolve on the pin. In mesh with these two differential pinion gears are the two planet gears fixed to the axle shafts. Consequently if the disk is turned by the handle, the differential driving pin and the two differential pinion gears will turn with it as one. As the differential pinion gears are in mesh with the two planet gears, they and the axle shafts will also have to revolve. Rotate within special type roller bearings located in the axle cover.
The large driven gear, previously mentioned as being in mesh with the propeller shaft driving pinion, carries the differential gear cage which is secured to its face.
It is at this point that a further reduction in the ratio of engine speed to road wheels is made, about 4 to 1 for private cars and approximately 10 to 1 for commercial vehicles.
First you have to imagine that the disk is fitted with a handle so that it can be turned in the direction indicated by the arrow. On the face of the disk are two bosses in which the differential driving pin is fixed so that when the disk is turned by the handle the pin turns with it. Mounted on the differential driving pin are the two differential pinion gears so that they must run round
But as the two pinion gears are free to revolve on the driving pin, unless the resistance to turning is the same on both axle shafts, the axle possessing the least resistance will be speeded up while any increase of resistance on the other axle will result in its reduction in speed. This will be clearer if the extreme case is taken, I.e. if one axle shaft is held so that it cannot rotate, the other shaft will rotate at a high speed, as the differential pinion gears will be turning on their driving pin at their highest speed. Next if the load on each shaft is the same, the differential pinions will not rotate on their driving pin, and therefore both axle shafts will rotate at the same speed. Any variation in speed between the two rear wheels is, therefore, automatically balanced. Now, if the handle is dispensed with and teeth are provided on the disk so that it can be turned by a driving pinion on the propeller shaft, the principle of operation in the differential gears will be exactly the same.
Some designs employ worm instead of bevel gear teeth, and recently double helical and hypoid gearing have become popular, but all function broadly on the lines already described in this section.