spool and central spring attachment. Owing to the small amount of lifting lever surface contact, very little strain is thrown on the valve spindle. As the opposite end of the lever is always heavier, the end on the spool-lifting lever need not bear on the bottom of the spool. The adjustment for wear is made by a fine thread and four flats on the spindle, so that very fine adjustments can be made.

In fig. 122 a distinct departure is made both in the construction of the valve and the lifting arrangement, and was introduced by the author in 1892.

It is well known that where a large engine is working at full power great difficulty is experienced from the exhaust valve fusing. To overcome this the head is made of 'cylinder liner' metal and attached to a steel spindle, the valve seat being also made of the same metal as the head. It is not good practice to arrange the exhaust valve in a loose box, especially on large engines. There is not only the difficulty of taking the box out, but there is a great tendency for the box to become distorted, owing to the comparatively large hole which must be in the box for the exhaust gases to escape in proportion to the sectional area of the box. To mitigate this a rib is placed in the centre of the opening.

The lower seat (as shown in fig. 122) admits of the water space being brought well up to the casing, and is at once a good mechanical job. The method of lifting is arranged so that in all positions of the lift the lifting die is at right-angles to the valve spindle. The valve spindle guide is loose, the joint on the under-side of the combustion chamber being a metallic one. Renewal of the guide or bushing is by this method considerably cheapened. This arrangement has been used with marked success by Messrs. Robey & Co. on their larger engines.

The pressure on the head of an exhaust valve in large engines amounts to about two tons, and the strain thrown on the worm gear and lifting mechanism is very great. To overcome this two exhaust valves have been used, one valve having twice the area of the other, the smaller valve being arranged with a lead' of the larger one.

An equilibrium exhaust valve arranged with one-half the

usual lift, and which would have the pressure equal to the difference between the two areas, is practical, and will, no doubt, be used on very large engines in the future.

Although valves working in a vertical position give good results, great difficulty is found in lubricating the spindle; and owing to the high temperature, and the amount of moisture contained in them, a leakage past the spindle into the engineroom is not unusual after twelve months' hard work. To overcome this one maker has arranged a hole in a spindle guide, leading to the air pipe, to allow the escaping exhaust to mingle with the incoming air.

Another means of overcoming this unpleasant leakage is to arrange a bowl on the valve spindle, surrounding the top part of the guide, which very effectually prevents the exhaust rushing down the spindle guide.

The diameter of the air and exhaust valves vary considerably with most makers; some on small and medium sized engines use the same diameter for air as the exhaust, whilst others prefer the air valve larger than the exhaust. A common and good practice is to arrange the air valve for 100 and the exhaust for 80 feet per second.

CHAPTER XIII

GAS VALVE AND COCK

THE size of the gas inlet varies from 450 to 650 feet per second through the valve. There being no fixed rule for the diameter of the gas supply, the difficulty usually experienced is to get sufficient in; the same size is often used for three or four sizes of engines, the difference being made in the lift given to the valve and the size and number of holes in the air valve box.

A very neat form of gas cock and valve is shown at fig. 123. The plug is held in its place by three screws, two of them being used for determining the position of handle to 'open' and 'shut.' The gas valve can be readily taken out.

A good rule for the taper of the plug is to allow a taper of

1 inch in 8.

CHAPTER XIV

STARTERS

SINCE Brayton used compressed air stored in a reservoir for starting purposes, which then, as now, was found unreliable owing to the difficulty of keeping up the pressure, the possibility of leakage, and also the reservoir being emptied before the engine was started, great strides have been made in the method of starting, and the development has gone on lines which may be classified as follows:

1. Pressure starter with a hand pump.

2. Low-pressure multiple impulse self-starter. 3. Low-pressure single impulse self-starter.

4. Low-pressure double acting.

5. High-pressure single impulse self-starter.

6. High-pressure single impulse with previous compression of air.

7. Steam from producer plant boiler.

Wells Brothers' Starter

The first application of the modern method of starter was introduced by Messrs Hamilton and Rollason, of the firm of Wells Brothers, in 1889. Fig. 124 is a part sectional elevation, fig. 125 end view, and fig. 126 a starting diagram.

At the extreme end of the combustion chamber is fixed a hand pump A fitted with a suction and delivery valve, the inlet of gas and air being through a three-way cock.

The action is as follows: The crank is placed on the firing stroke, and the timing valve kept closed by means of a catch. The combustion chamber being already filled with air, a proper charge of gas is pumped in, the threeway cock is then turned so as to admit air to the pump, which is then forced into the cylinder. The catch is released, and the charge ignites by means of the ordinary ignition tube.