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How a Motorvan Works.

30th May 1907, Page 5
30th May 1907
Page 5
Page 6
Page 5, 30th May 1907 — How a Motorvan Works.
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The stereotyped trade descriptions of self-propelled vans mud Wagons, whilst they convey a perfect understanding of their construction to an engineer or mechanic, are all more or less unintelligible to the prospective buyer, who has not the advantage of any mechanical or engineering training. The novice asks : How does it work? This question, as regards the less-understood internal-combustion system, we hope to answer in this article by outlining the general principles which govern the construction of all vehicles in which the se-called " petrol " engine is the source of power. The different types of frames, and the methods of spring suspension adopted, will not be discussed in this article, which will deal exclusively with the development and transmission of the power from the engine to the road wheels. In such an article, it will be necessary to commence with a description of what takes place within the cylinder, or cylinders, as it is primarily intended for those new readers who are almost, if not entirely, unacquainted with any detail of motor construction, and of the manner in which cars are driven.

The type of internal-combustion engine most commonly used on commercial vehicles is known as the four-stroke motor, and may have one or more precisely similar cylinders. Starting with the piston at the top of the cylinder, the first down stroke causes a charge of fuel, mixed with the requisite quantity of air, to be drawn into the cylinder, by aspiration, through the inlet valve : this is generally spoken of as the suction stroke. On the following upward stroke, the piston compresses the mixture already in the cylinder : this is the compression stroke. Just as the piston begins the second downward stroke, or slightly in advance of that point, the mixture of fuel vapour and air is caused to explode by the application of an electric spark, and the force of this explosion, transmitted by the piston and connecting rod to the crankshaft, supplies the driving power : this is, therefore, called the explosion or working Woke. Throughout the two latter strokes, both the inlet and exhaust valves have been tightly closed, but, just before the piston commences the second upward stroke, the exhaust valve opens and permits the exploded charge to be pushed out as the piston rises : this is known as the exhaust stroke. Briefly, then, the four-stroke principle may be summed up as : First stroke (down)—Gas drawn into the cylinder. Second stroke (up)---Gas compressed into the combustion chamber (top portion of cylinder). Third stroke (down)—Gas fired, causing the piston to be driven outwards, Fourth stroke (up)—Spent gas driven out of the cylinders.

The whole cycle of operation takes four complete strokes, or two complete revolutions, and it is repeated so long as fuel and ignition are furnished.

In the diagrams illustrating the four cycles of the engine, the automatic type of inlet valve is shown : this simply consists of a mushroom valve, which is held up to its seating by means of a „spiral spring only just strong enough to support the weight of the valve and ensure that it is returned to its seat. So long as there is any pressure within the cylinder, the inlet valve is kept closed, but, as soon as the piston moves dawn the cylinder on the suction stroke, a partial vacuum is created in the cylinder, and, intto this, the air passes, by reason of the atmospheric pressure without. The air which thus enters is conducted to the cylinder through an in:et or induction pipe, from the carburetter, and, during its passage through the latter, it !takes up a quantity of petrol, which, becoming atomised, mixes with the air and forms the explosive mixture. The valve may be operated by means of a cam as is the case with the exhaust valve.

What Takes Place in the Carburetter.

As the air is sucked into the cylinder, a corresponding partial vacuum is created in the jet chamber of the cat-buretter : the level of the petrol is thus disturbed and causes a jet of petrol to be squirted into the passage towards the cylinder.; This jet of petrol is taken up by, and mixes with, the air which is rushing in to fill up the vacuous space in the cylinder. The speed of the engine may be regulated by intrOducing a throttle-valve in the inlet pipe, and, when a governor is fitted, it generally controls the engine by opening or closing such a valve. Explosion of the Combustible Charge.

There are many ways in which the charge may be fired, but the general practice is to employ some font of electric spark for the purpose. An attempt to give 'anything approaching to a clear explanation of how the spark is produced would occupy more than the space at our, disposal. here, and we therefore purpose to make it the subject of a subsequent article, in which the action of coil and accumulator, high-tension magneto, and low-tension magneto will be fully explained. For the present, it must suffice tostate that there is a contact-breaker mounted on the camshaft, and, as this shaft only makes one revolution for 'two revolutions of the crankshaft, the contact-breaker is so arranged that it completes an electric circuit for any cylinder once in each two revolutions, the spark 'thus passing at, or nearly at, the end of the compression stroke. The insulated portion of the contact-breaker is capable of being moved round the cainshaft, which has the effect of making: the spark earlier or later in the cycle.

The Escape of the Spent Gas.

It has already been stated that just before the piston, commences on its second upward stroke the exhaust valve islifted, and the piston drives the spent gases out of the cylinder. The exhaust valve is lifted by the cam, as wilL be seen on reference to the diagrammatic arrangement of the engine. The reason why this valve should open just before the upward stroke begins, is, to relieve the pressure. in the cylinder ; the motion, otherwise, would be retarded, and the speed and power of the engine would be considerably diminished. When the exhaust gas leaves 'the cylinder it is conveyed to a " silencer," in which it is cooled, down, and consequently has its volume and pressure reduced, before being finally discharged into the atmosphere.

The above is only a bare outline of the principle of the four-stroke, internal-combustion motor, but it will serve• to convey to the uninitiated a clear idea of exactly what. takes place during each cycle : it must be remembered that the same process goes on continually so long as the throttlevalve is left open, and the ignition remains uninterrupted. Various modifications have been made, in the matter of arranging the valves and ignition-timing gear, as well as. in the cooling system, which, generally, is effected by water which circulates in a jacket round the cylinders.

How the Engine is Started.

Before attempting 'to start the engine, it is necessary 10. make quite sure that the following points have had attention. Firs..., the throttle-valve should be open wide enough. to admit a full charge of gas to the cylinder ; second, see that the switch is in the " on " position, and that the sparkadvance lever is in the "retard " position, otherwise a back-. fire.might be the result; 'third, see that the petrol cock is turned on so as to admit the passage of petrol from thetank to the carburetter, which should show signs of flooding before starting ; and, fourth, see that the change-speed. lever is in the neutral position. Having ascertained that these points are in order, the starting lever spindle should be pushed inwards to enable the ratchet coupling (A) to. mesh with the corresponding coupling (B) which is fixed on the forward end of the crankshaft (C). The lever is now given a few sharp turns so as to draw in a charge of gas, compress it, and then fire it. Having got one good explosion, the engine will go on running automatically, and, as the connection between the starting lever and thecrankshaft lever is of the free-wheel order, it will be readily understood that the lever is not carried round with the crankshaft.

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