ELEMENTS OF THE CAB

Southland Times, Issue 19291, 9 July 1924, Page 9

ELEMENTS OF THE CAB

THE CYCLE OF OPERATIONS The first stroke in the “cycle of operations” was once described as the inlet stroke, but it can equally well be described as the suction stroke. Assuming that a simplified carburettor is connected to the cylinder by a pipe, it wall follow that when the piston moves downward —the linet valve being open—it creates suction and forcibly draws petrol from the jet and air through the apertures, consequently the petrol is sprayed and infermixed with the inrushing air and drawn into the cylinder, and, providing the jet and air apertures are of suitable sizes, the explosive mixture will be “correct” in proportions of air and gas. By the time the piston has reached the end of its first down stroke, the cylinder will be practically full of an explosive mixture of petrol and air. It is necessary now for the inlet valve through which the mixture entered to close, so that when the piston makes its second stroke upwards the mixture is retained in the cylinder. THE COMPRESSION STROKE. As the piston rises it compresses the cylinderful of mixture into a comparatively small space at the top of the cylinder, this space being approximately one-fifth of the total capacity of the cylinders. This act of compression enables the mixture to explode, or burn, with much greater force and rapidity than if it were not compressed, but it is important not to compress it too much; otherwise it would explode spontaneously, as the compression creates great heat —this heating being very advantageous strictly up to a certain point. THE FIRING OR “EXPLOSION” STROKE. Assume now that the second stroke has just been completed, and that the piston is on the point of moving downwards again for the third stroke of the “cycle.” This is the most favourable instant for exploding the charge so that the pressure created will act maximum effect on the piston and force it outwards. The ignition spark is, therefore, timed to occur at the moment of full compression, or it may occur advantageously even just prior to this condition, as the operations of ignition and combustion occupy an interval of time; although in a non-techuical sense they are instantaneous. The allowance for this interval is known as “Advancing the ignition.” THE EXHAUST STROKE. The piston will now have reached the end of its second outward stroke, the third stroke of the cycle, namely, the power stroke, being complete. The position of affairs now is that the cylinder is full of burnt gases at considerable pressure, and the piston is on the point of beginning its upward stroke, therefore the exhaust valve is opened, preferably a little before the actual end of the firing stroke, so that the gases partly escape by their own pressure and relieve the load on the rising piston, which then sweeps out the main quantity of burnt gas remaining in the cylinder. Instead of the gases being swept straight into the air, and thereby creating noise, they are first passed into a “silencing” device. A TWO-STROKE CYCLE PRACTICABLE. The four-stroke engine is universally adopted for motor cars, but it may be pointed out that it is quite practicable to have an engine working on a two-stroke cycle of operations, so that, instead of obtaining only one power impulse for two complete revolutions, an impulse is obtained every revolution; thus the power developed is less intermittent than in the standard engine. There are no valves in the two-cycle engine, the gases being admitted and exhausted through apertures or

ports in the cylinder wall, the piston in the course of its movement covering and uncovering these ports; thus it is a much simpler engine than the four-stroke, but rather less efficient. TYRES. THEIR CARE AND REPAIR. Considerate driving is of the first importance where a big mileage is to b" obtained from outer covers, as nothing destroys the rubber and fabric structure more than violent braking or accelor fast driving over rough, flinty roads. There are other points also which require attention. One of *-he most important is the air pr 7 ~.e tube, as if the tyre be too soft the walls are apt to be flexed unduly, resulting in time in their destruction, while if they are too hard there is some danger of the tyre bursting through expansion due to the heat generated in runing. The front tyre’, in view of the fact that they generally, have less weight to carry, and take no driving strains, can be inflated to a pressure of 51b or 101 b less than those at the rear. Non-skid covers also require a slightly higher pressure than those of the ordinary pattern. The correct pressure naturally depends to some extent upon the weight of the vehicle. Cross-section of the tyres has also some bearing on the question. It is, for instance, possible, where tryes very much larger than necessary are fitted, to run with them somewhat soft, thereby obtaining increased comfort. The makers of most tyres furnish on request a table showing the pressure to which their tyres should be inflated. This may very from about 601 b per square inch for a 3jin tyre fitted say, to a Ford, to a pressure of about 951 b in a heavy car having tyres of 150 mm. cross-section. Every week or so a profitable halfhour can be spent jacking up each wheel in turn and examining carefully the treads for cuts and sharp stones, the latter should be extracted and the cuts filled with a reliable make of tyre filling. This prevents rot-inducing moisture from attacking the fabric which forms the base of the tyre’s construction. Severe cuts are best repaired by vulcanising. Quite a number of small vulcanisers for the owner-driver’s use are now on the market, and very little practice is required to obtain satisfactory results. At the same time, fairly satisfactory results can be obtained by filling the cut with vulcanising compound and pressing upon it for a considerable time the blade of a large screwdriver which has been previously heated placing a scrap of paper between the tool and the rubber, so as to prevent sticking. The difference between using a vulcaniser and an ordinary tyre cement or filling is that in the former case the edges of the cut are bevelled off so that the opening is larger at the top than near the cord foundation, while in the latter case the edges of the incision are, so to speak, undercut, so as to hold the filling more rigidly in place. If the car has to be left for any length of time in the hot sun, it is always advisable to hang a large cloth round, or even on old coat over each wing, so as to keep the entire wheel in the shade. The question of repairing tyres nowadays seldom troubles the motorist on the road, as detachable wheels or rims are standard on practically every modern automobile; at the same time, the method of repairing an inner tube, whether on the road or in the garage, is the same. The method of removing the outer cover depends on whether it is of the beaded edge pattern, which is common practice in Europe, or the straight-edged kind popular in America. With beaded edge tyres it is invariably necessary to use stout steel levers which are generally fairly sharp, fairly thin at one end and hooked at the other. To remove the cover, its wall is first pushed with the palm of the hand so as to bring the beading away from the rim, permitting the thin end of the lever to be inserted, after which, by simply levering and with some assistance from the other hand, it is possible to get one side of the cover off all the way round. It is a simple matter after that to remove the other side of the cover. In replacing, put the hooked end of the rever over the inward curled edge of the rim, when the beading can gently be levered into position. It is advisable to have a certain amount of air in the tube before the cover is replaced, to avoid the risk of nipping the tube and causing a burst,