Imágenes de páginas
PDF
EPUB
[merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors]

FIGS. 140, 141.-AUTOMATIC TUBE IGNITER - ROBEY

burner used is arranged so that the mixture of gas and air issues laterally towards the centre from an annular groove surrounding the tube, thus impinging directly upon it. Air is supplied in suitable proportions below and above the flame to ensure

FIG. 142.-AUTOMATIC TUBE IGNITER--FIELDING

perfect combustion; the usual chimney lined with non-conducting material is provided above the burner, which is arranged so that falling flakes or scale from the tube may drop clear without danger of clogging up the gas or air passages.

CHAPTER XVII

IGNITION TUBES

THE life of an ignition tube depends upon the material it is made of, but varies greatly according to the usage it receives. This life is shortened by intense heating, with sudden changes in pressure and temperature produced by the inrush of comparatively cool gases, whereas gradual changes tend to longevity. It is found that wrought-iron tubes deteriorate rapidly owing to high pressure within the cylinder at the moment of ignition. The thin tube, raised to white heat, is only exposed to the

atmospheric pressure outside, and the great pressure suddenly produced within has a tendency to burst it, and care should be taken that it is not over-heated or softened too much. It is found that the fresh mixture of air and gas when introduced cools the inner surface, and that it is necessary to use thin tubes, since thick ones are not so easily raised by a flame outside to such a temperature that even poor gases may be readily ignited by contact with their inner surfaces.

Ignition tubes made of 4-inch steam tubing will give very satisfactory results without a timing valve when using an angle chimney. Here part of the mixture is gradually compressed into the tube under the most favourable conditions. The tube is constantly open to the cylinder, and the length of the heated part of the tube is so adjusted that the small portion of the explosive mixture gradually forced into it becomes ignited, and has just time to fire the main charge in the cylinder at the proper instant. Ordinary wrought-iron gas tubing is used, though the life of such a tube is very uncertain. Numerous so-called alloys are used, and give fairly good results.

[ocr errors]

The Wellington' tube, when carefully handled, gives very good results. This tube consists of specially combined silica of earths exposed to certain treatment in drying and baking.

The heating of this tube is very quick, and with care the life is some years. At the same time the heat of the tube cannot equal in intensity that of the electric spark or voltaic arc, which is near the temperature of volatilisation of the insulated points between which the arc passes. Hence the continuous stream of electric sparks, which is practically unaffected by ordinary currents of cold gas and air, affords the surest and most powerful means of igniting a poor gaseous mixture under high pressure. For practical everyday use the disadvantages, however, more than counterbalance the advantages, and even when using producer gas from coke, a Bunsen burner can be arranged to use the same gas, and give very satisfactory results.

CHAPTER XVIII

TIMING VALVES

THE more control the timing valve has over ignition the less 'lead' is required, so that the tube should be heated as low down as possible; and to ensure the immediate contact of the charge with the heated part of the tube, a bulb is attached, into which the inert gases retained from the previous cycle are forced. If the clearance leading to the valve is long, a bulb is connected with the passage as near the tube as possible, so that the gases in the clearance space may be partly forced into it, as well as to give the fresh charge greater tube space. Should the valve require much lead' to fire the charge on the dead centre, its control over the ignition will be lessened. Some timing valves are not made air-tight-that is to say, a small groove is made in the valve face causing a slight escape at each stroke of the engine, just sufficient to draw by induction some of the products of combustion out of the ignition tube. For this purpose a separate shifting-valve is used.

Crossley's Timing Arrangement

Fig. 143 is a side elevation, fig, 144 front elevation, and fig. 145 a plan of timing arrangement used by Messrs. Crossley Brothers.

[ocr errors]

The lead in this type of timer is very small, the valve opening very rapidly. During the compression stroke the communication between the combustion chamber and hot tube is closed, and inert gases are allowed to escape to the atmosphere.

The end of spindle acts the correct time of firing. The stroke of the valve is

as a valve, and is responsible for This is a very important detail. inch, and when wear and tear

16

[blocks in formation]
« AnteriorContinuar »