This type of engine, shown by Figs. 98 and 100, is open to the disadvantage that it requires the largest and most expensive foundation of any steam-blowing engine, and that it requires also a great area of building and ground, leading to high cost on this account, but the building cost as a whole is not so excessive as might be expected because of the very moderate height required with this engine, so that only a relatively low building is needed for its accommodation.

Fig. 98. Large horizontal cross-cornpound blowing engine. The Wm. Tod Co., Youngstown, Ohio.

Its great foundation also acts as a huge anvil to absorb inertia stresses and unbalanced centrifugal effects of counterbalance. This tends to cut down to a minimum the possible vibration of the unit as a whole.

Owing to these considerations the engine is capable of being run at the speed best adapted to produce the highest economy, and this higher speed of unit permissible as compared with most other types of engines goes far to offset any disadvantages in total first cost under which a unit of this type of a given size labors, as compared with that of some other types. The whole of the engine is down on or near the ground level, and I believe that operators who have had experience will bear me out in saying that one attendant will look after two engines of this type more effectively than two attendants can after one of the steeple type of the same size.

This engine has, of course, the desirable feature of transmitting the stress from one cylinder to another through rigid members and not through moving joints, and in consequence it is capable of realizing the benefits of this system to the maximum.

Fig. 99. Inside-crank blowing engine. Slick air cylinders 36 in. and 72 in. x 54 in. steam, 72 in. and 72 in. x 54 in. air.

Allis-Chalmers Company.

Through the kindness of Mr. E. T. Child, of the Southwark Machine Company, I am able to reproduce in Fig. 101 a set of steam and air cards, one of a large number of sets taken by him personally from an engine of this type, running at a proper speed, in which the efficiency for the average of the whole set was about 91 or 92 per cent. This was in actual operation, no particular attempt being made to obtain a record, and I have no doubt that other well-designed engines of this type would do as well, if run at the proper speed.

The Tail Slide

One of the essential features in the design of the modern horizontal cross-compounds is the means for relieving the bottom of the cylinders of the drag of the heavy pistons necessary in large units. This is accomplished by putting a cross-head and slide at each end of each of these large cylinders and making the piston rods strong enough to carry the pistons clear of the cylinder, so that nothing but the packing ring touches the wall of the cylinder.

In some cases the piston rods are made of high-grade steel and cambered upward before use to the amount by which the weight of the piston will deflect them so that they are perfectly straight in service.

Mr. Nordberg has gone still further and uses a hollow cast-iron trunk of large diameter whose only function is to carry the weight of the piston and take compression stresses, the tension stresses all being taken by a much smaller steel rod through the center.

This design not only frees the engine from a very heavy friction load, but also relieves the cylinders from wear and puts them on a parity in that respect with vertical cylinders.

The Inside-Crank Engine

The inside-crank type of engine is shown in Fig. 99. The crank-shaft lies between the two cylinders and the piston rod instead of passing directly from one cylinder to the other; is divided into two parallel rods symmetrically placed with reference to the axis of the cylinders, and in a plane about 45 deg. from the horizontal.

These two parallel rods take hold of suitable projections which are provided on the cross-heads for each cylinder.

This arrangement makes an uninterrupted open space in the axis of the engine and between the two cylinders; the connecting rod and crank work in this space.

The type shown is built as a cross-compound steam engine and this type of construction is also much used in gas engines. It has many of the advantages of the horizontal tandem engines, but on account of the stretcher rods from one cross-head to the other the simplicity and accessibility of the running gear are not so great as in the straight tandem type, and I have never known what advantages this engine possessed to offset these disadvantages unless it be the accessibility of the cylinders due to the elimination of the tandem arrangement. It is a type of engine approved by engine builders very generally, and is recommended by them for high speeds.

Fig. 100. "Mesta" cross-compound steam blowing engines.

Fig. 101. Southwark steam and air cards showing a mechanical efficiency 92 per cent.