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.

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.

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

of cam, roller, &c., reduces the stroke to inch the worn part should be renewed. If this precaution is neglected the engine may reverse at starting. The ignition valve and guide are made of phosphor bronze, and are easily removed for grinding the valve in.

The method of taking the ignition tube out for renewal is simple, and when the ignition valve is placed vertically it acts well.

'Stockport' Timing Valve

Fig. 146 is a sectional elevation of the timing arrangement combined with a self-starter used by Messrs. Andrews on their engines, F being the timing valve and A the valve used for the self-starter, G ignition tube, D lever for operating timing valve, H mantle for enveloping flame round tube. The ignition tube employed is a small one, and is arranged so that the gas and air mixture can at each ignition blow out some of the products of combustion remaining from the last ignition. This is effected through the small annular passage for the air and gas mixture between the inner and outer tube, as shown in the section. The valve F is not air-tight, a small groove being made in its face so that there is a slight escape at each

suction stroke of the piston, just sufficient to cause some of the products of combustion to be drawn out of the ignition tube. For engines of large powers two complete timing valves are arranged in one box, having one common passage leading

FIG. 146. TIMING VALVE (STOCKPORT)

to the combustion chamber. Ignition is arranged in duplicate. so that in the event of accident to ignition tube-say, by bursting—one timing valve may be thrown out of gear of cam, and the other valve brought into action. This can be done automatically, the resultant force from the broken tube being made to act on a vane placed at the top of the chimney. This firm's ordinary timing valve works well, and the 'lead' can be regulated at will.

Tangyes' Timing Valve

Fig. 147 is an external elevation, and fig. 148 a sectional plan of the timing arrangement fitted to Messrs. Tangyes' engines. A is the combustion chamber, and B the timing valve held

L