Motors and Motoring
The Modern Motor Car.—I. Some Special Features
TICE motor car is fast becoming an object of interest to all users. The owner-driver, be it a case of a man with his first car or a woman who buys a seven or a nine as an auxiliary for her personal use, is naturally keen, even if knowledge of the mechanism is strictly limited. The sign of the times is rather to be seen in the fact that the big car owner, who seldom if ever drives himself, is beginning to want to know the why and the wherefore, and the day on which a new model is delivered is now quite a red-letter one. No-longer is it just a motor car—a convenient and comfortable means of loco- motion from place to place. On arrival it is carefully inspected, and not only externally but under the bonnet and the floor- boards as well. Among a large number of inquiries from many parts of the world in connexion with the selection of a suitable car I am asked fairly often about certain features : what is the object of having a six or eight-cylinder engine in com- parison with a four ? what is a vacuum servo motor ? are three or four speeds advisable ? and so forth. I therefore propose here and subsequently to refer briefly and in simple language to a few of these features which appear in the specifications of so many cars of to-day. First as to the number of cylinders. Speed in motor cars has really been great enough for some time now, and what is needed, and in consequence what designers are chiefly aiming at, is more 'acceleration power and a • reduction of piston impulse in conformity with power output. As roads, steering, suspension, and braking improve, cars ' can be driven faster than previously with a given factor of safety, and this emphasizes the desire for greater smoothness in engines running towards the top of their , speed curve together with cleaner and better balanced acceleration under load. Herein lies the reason for the wide adoption to-day of the scheme of multiplicity of cylinders. It will be clear, even to the noviae, that if fdr a given power output a smaller engine but one having more power strokes or impulses per revolution of the crankshaft can be used, the operation gene- rally will be more flexible and altogether more pleasant.
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Thus it is, with the increasing desire for a higher degree of luxury in road travel, that the six-cylinder engine has made such great headway in the last eighteen months. The British manufacturer realized, to some extent under pressure- of American influence and competition, that he must progress in the matter of engine flexibility and smoothness, and because of this he tackled the problem wholeheartedly, making the six-cylinder chassis a real commercial proposition even in the lower and less expensive grades of car. Already the eight and even the twelve-cylindered engines are accom- plished facts in standard productions, and, although the step is in some ways a bigger one from the six to the eight and the eight to the twelve than from the four to the six, I have long thought that the six-cylindered engine will become the common unit for cars up to medium size and that the eight will be generally- adopted for larger models. This, however, is still somewhat in futuro. The four-cylinder engine is by no means dead yet, and certainly for two or three years has its important work to do, for without it there are thousands of persons who would_ not be able to enjoy the benefits and delights of car ownership at the present time. For the ordinary size of car, however, the six-cylinder engine is to be advised, while those possessing more cylinders have their advantages and charms for the person who wishes to have the most up-to-date and luxurious mode of road travel. The increase in number of bearings carrying the main or crank- shaft is also a definite sign of progress, for it affords more refined running and so conduces to the desired end. The same may be said of the provision of counterweights on the shaft and the fitting of a vibration damper or harmonic balancer. The chief object is to reduce the fluctuations or- up-and-down movements in the 'curve of the power output and so obtain a steady stream or flow of power.
• * * * * A modern development aimed at economy in operation is crankcase ventilation. At present this is to be found on only it few makes. It is no exaggeration to say that the efficiency and durability of an engine is dependent not only upon its having a sufficient quantity of oil but upon such lubricant being of the right quality. If the oil becomes diluted it will lose much of its effectiveness. Unburnt portions of fuel in gaseous form, with a certain amount of generated water vapour, are known to leak past the piston-rings and so find their way through to the crankcase. It will be seen that in the relatively cooler atmosphere these vapours or gases are apt to condense, when they will dilute the lubricant. Crankcase ventilation is a scheme to obviate this by the extraction of the vapours before condensation can take place, and this is usually done by inducing a partial vacuum• and so setting up a constant draught of air through the crankcase.
* * * * The autovac or vacuum petrol tank has been so widely adopted on cars of all nationalities that it may be of interest briefly to describe what it consists of, especially in view of the fact that although it is really quite a simple device it is little understood in its working by the average motorist. The placing of the main petrol tank at the back of the chassis has many advantages in comparison with other positions and this arrangenient has therefore been widely favoured. It is clear, however, that with such a low level in comparison with the carburettor and the relatively large distance between the two components means must be provided for the delivery of the petrol from the tank to the carburettor. In old days exhaust pressure was generally used, and air pressure supplied by a snail pump driven from the engine has also been employed. Development then led designers to utilize the partial vacuum or depression in the induction pipe of the engine which is present when the pistons are descending in the cylinders on the inlet stroke. In this way the autovac auxiliary tank draws the fuel from the back of the chassis to the dash by partial vacuum, and from - here the fuel. descends to the carburettor by gravity. Briefly the autovac tank is as follows. Inside the circular tank, which is usually secured to the dash under the bonnet, is an inner tank. The latter is the vacuum one and fills from the main reservoir at the back of the chassis, while the outer circular tank is fed from the inner one. The outer one, being open to the atmosphere, is thus at all times a gravity tank, and with the tap turned on allows the petrol to flow naturally down to the carburettor, which is at a considerably lower level. I have said that a partial vacuum is created in the induction pipe when the engine is running, and a pipe connects the manifold by a non-return valve with the inner tank. Thus with the piston descending on the inlet stroke a pressure below that of the ordinary atmosphere is communicated to the inner tank. Since a pipe on the top of it connects with the main petrol tank at the back, in which the petrol is open to atmospheric pressure, it follows that the fuel is drawn along the pipe and so into the autovac inner tank. At the bottom of this tank, which does • not reach to the bottom of the outer one, is a drop valve. As the petrol flows into the inner chamber it raises an ordinary valve float, and when this has risen to a definite height it works a valve which cuts off any further vacuum from the engine and at the same time opens an air release. With the inlet of the atmospheric pressure the partial vacuum naturally disappears, when the drop valve is automatically freed. Petrol thus flows from the inner tank to the outer, which always has petrol in it.
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There is a small conical gauze in the entrance channel of the autovac. This should be cleaned fairly frequently and the drain tap in the bottom of the autovac should also be used. It is a simple matter to remove the filter. On the top of the autovac is a clamp which holds in place three parts—namely, the induction pipe connexion, the air vent to the outer tank, and the main petrol supply connexion from the rear tank to the inner or vacuum one of the autovac. If the single nut holding the clamp be slackened the_latter can be turned, and with the main petrol supply pipe unit unscrewed the elbow piece; which is a taper fit, can be removed and the gauze lifted out. Other features must be deferred to the next article
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