The valve is specially designed with cavities so arranged as to co-operate in increasing the area of opening to double the amount obtainable with an ordinary valve having the same travel.
Without some special provision, the full power of the compound engine cannot be exerted in starting, as the steam operates primarily on the high pressure piston only, which has led to the condemnation of the compound as a traction engine. This objection has been thoroughly overcome in the Woolf by means of a “converting valve,” rendering it possible to admit steam directly to the large or low pressure cylinder, thus largely increasing the power obtainable from the engine, even when exerting its maximum power as a compound. This arrangement can be used not only in starting, but also in cases of emergency, such as climbing steep hills, getting out of bad places on the road, or disposing of an especially tough cut in sawing, etc.
Without increasing the boiler pressure beyond that ordinarily used, the compound engine gets fully one-third more force out of the steam used than is at present obtainable with the simple engine as commonly worked.
In other words, the compound will show results compared favorably with a good condensing simple stationary engine doing the same work.
[Illustration: Ball Tandem Compound Engine.]
The heavy case iron base is cast in two sections, the rear part being securely bolted to the front section. To the front section is also bolted the main engine frame. This frame contains the bar guides for the cross-head, and pillow block bearing for the double disc crank shaft, and also forms the front head for the low pressure cylinder, which is securely bolted to it.
The high pressure cylinder is attached to the low pressure cylinder by two brackets securely bolted, and is supported by a pedestal, bolted to the rear part of base. By this arrangement, both the high and low pressure pistons are upon the same piston rod, which necessitates of but one cross-head, connecting rod and crank.
The valve of the high pressure cylinder is operated and completely under the control of the automatic shaft governor attached to one of the band wheels, while the valve of the low pressure cylinder receives its motion from a single eccentric on crank shaft at opposite side of engine.