Motors and Motoring
The Modern Motor Some Special Features
In a previous issue I haveerieferred to one or two features which are apt to puzzle those selecting a car or motorists who take an interest in their machines but have not much experience. The next item for mention is the- thermostat. A number of makers now fit such a device as standard, and I have received several inquiries as to the object and advantages, if any, of a thermostat. The primary aim of this arrange- ment is to keep the cooling water at a more or less constant temperature, and so generally help the efficiency of the engine, for the unit which is run at too low a temperature cannot develop full power and is wasteful in fuel consumption, while in operating an engine which has recently been started up from cold it is of the utmost importance to the life of the machine that it be not run under load or quickly before the oil is sufficiently warm to get properly to the respective bearing surfaces. If, therefore, a device reduces the amount of patience required from an owner-driver or a chauffeur by accelerating the process of warming up it is clearly helpful. The working of a thermostat may sound somewhat like a conjuring trick, but it is really very simple, and consequently reliable. The aim of it is to short-circuit the water circula- tion by cutting out the radiator, and so, until a pre-deter- Alined temperature is reached, allow the water to circulate only round the cylinders themselves. Conversely it enables an engine which is being shut off to keep its warmth longer, by slowing down the cooling-off process. A thermostat usually consists of a copper coil or series of small, hollow, thin copper discs, filled with some volatile liquid to which is attached a valve or diaphragm. As the water rises in temperature, the coil expands like a bellows and opens the valve, so allowing the water to pass to the radiator, from which it returns cool to the engine in the ordinary way. The device is set to begin its opening movement at a fixed number of degrees, and as the water rises in temperature so does the aperture enlarge, restricting the circulation less. As the water cools off, so the valve begins to close. In some cases thermostatic control is arranged to govern the move- ments of a shutter in front of the radiator, and the principle is now being applied to carburation.
* * * * I am often asked whether it is advisable to have a gear-box with three forward speeds or four. There are advantages and disadvantages to both, but in the main I have no hesita- tion in favouring the four-speed box, for clearly higher efficiency can be obtained on give-and-take roads with a machine on which one can meet the requirements of the engine with a greater range of gear ratios. It may often happen in a day's tour that one meets a slope which is just too severe for climbing on top with a three-speed car and yet is not steep enough to allow any but a small throttle opening on second, unless the engine be raced to its detriment. In such a case driving is tedious and the average journey speed is lowered, while of course, since for a given road distance the engine makes more revolutions, a larger quantity of petrol is consumed. It is in such cases, and in being able to change down early and so tackle a hill at really good vehicle speed, that the third gear of a four-speed box comes in. The obvious drawback to the four-ratio box is that it calls for more changing, because the toti ratio is generally rather higher, and therefore for more skill on the part of the driver. In America motor-car construction has been developed on non-gear-changing lines. This suits the type of chassis production. Petrol is comparatively cheap and there is no tax on horse-power based on the diameter of the cylinders. Thus we get comparatively large engines built on much less efficient lines than the British, and which are consequently cheaper to manufacture, together with a three-speed box with a relatively low top ratio. This plan makes for simple driving, but not for efficiency and economy. A word here must, however, be said in favour of the three-speed box, from a general practical standpoint, for the smallest and lightest types of car. In these it is the manufacturer's object to give the greatest possible value for money and so to popularize his productions. For this reason, although British designers, in fact one may say European designers, have almost universally adopted the four-speed box on cars capable of carrying five persons, those of from seven to fourteen nominal horse-power still often have but three speeds. This should not prove a deterrent to those pros- pective new car owners who wish to get a machine which comes in the light class. The American argues that with a six-cylinder engine the four-speed gear-box is not necessary. It is not essential with his type of car, but with the average European one, which is built to give the highest efficiency with the most economy, it is, in my opinion, an asset.
Servo assistance in braking is the next feature for comment. Brakes workable wholly mechanically and dependent solely upon the physical effort of the driver were found to call for a good deal of force on a big car, with the result that servo mechanisms in various guises were introduced to obviate this drawback. The principle of servo action is that it allows the momentum, which is the weight of the car multiplied by its speed, to be used in assisting brake application and so reduce the force required by an operator. There are various kinds of arrangement in use on modern cars. In some cases the brake-shoes themselves are devised to supply servo action by allowing a shoe in each brake to be half-floating. In some a mechanical servo motor is utilized, while in others tydraulic means are employed. Here it is proposed to refer :briefly to the latest and most widely adopted form which is known as the vacuum servo. The advantage of this Isystem is that it entails but few working parts, calls for no .adj‘strnent, is light, is automatically lubricated, and should anything go wrong with it it allows the braking to be used in the ordinary way. Moreover, it is available with the car travelling in either direction. The depression in the inlet manifold is utilized to assist the physical effort of braking, and since the driver releases his accelerator before applying his brakes, the required depression in the induction pipe is ttirailable. The device consists of three main parts, a dis- tributor and two valves, one communicating with the induction for braking and one with the atmosphere for release or de- braking, a cylinder with a piston, and the requisite connecting -levers. A pipe couples the inlet manifold of the engine with the brake cylinder, which is placed between the brake pedal and the control rods of the brake mechanism. When the pedal is applied, the valve between the vacuum pipe and the cylinder is opened and a depression is created behind the piston, so that the greater pressure on the other side moves it in the cylinder. The piston, in its movement, pulls the rod connected to the brake mechanism, and so broduces the required mechanical effort. So long as the rake pedal is kept down the valve stays open. The power is generated according to the pedal travel, and so progressive- ness, in the ordinary way, is obtained. In between the extremes the evenness in graduation is obtained by a ".hunting " movement. To provide a natural feel on the l and so prevent a heavy-footed driver from applying is brakes too harshly, the pull applied to the brakes is partially returned to the pedal itself to act in opposition to the foot pressure.- When the pedal is released, one of the distributor valves opens as the other closes, and because the one now in use is connected with the air in the crankcase, the servo brake is let off, since the pressure is raised and the vacuum replaced. The air coming from the crank chamber carries with it oil mist and so effects lubrication automatically. One or two other items will be dealt with at a later date. YOUR MOTORING CORRESPONDENT.
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