IMPROVEMENTS IN RECIPROCATING ENGINES.

For battleship work, in order to meet the turbines on more even ground, the principal changes made in the design of reciprocating engines are as follows:

1. Increase in ratio of L.P. to H.P. cylinder volumes from about 7 to 1 to about 10 to 1.

2. Lengthening the main steam valves in order to give short and straight steam and exhaust ports, with consequent reduction in clearances and in steam frictional losses.

3. Increase in vacuum carried in main condensers.

4. Use of superheated steam.

5. Slight decrease in bearing pressures of crank-pins and cross-heads. 6. The fitting of forced lubrication to crank-shaft, crank-pin, and cross-head journals and to eccentrics and to cross-head slides.

All these changes have not as yet been made, but 1, 2, and 4 were utilized in the engines of the South Carolina and Michigan, 1, 2, 4, 5, and 6 in the engines of the Delaware, and 1, 2, 3, 5, and 6 in the engines of battleships Nos. 36 and 37, the designs of which have just been completed by the Navy Department.

RESULTS OBTAINED BY THE ABOVE CHANGES.

Unfortunately, in the cases of the South Carolina and Michigan, no measurements of actual water consumptions of the machinery were made on the trials of these vessels. An idea of the economy realized can be obtained, however, by comparing them with their sister ships of the Connecticut class, the Delaware also being included in the tabulation:

Let us compare the above results still further, using a series of vessels having the same type boilers as the above vessels, but all being without superheat and having the old type of naval engines.

SUPERALES AND IMPROVED ENGINE

EPPER G.S.

UNO SUPERHEAT

STANDARD CONDITIONS

A

HS (GENERATING) = 48.1
GS

IMPROVED ENGINES

GEN H.S.='9 TOTAL HS. SUPERHEAT AT ENG-ABOUT 60

B STANDARD CONDITIONS AS ABOVE WITH EARLY TYPE
ENGINES NO SUPERHEAT

SCALE OF AIR PRESS IN FIRE ROOMS IN INCHES OF WATER

In the above table and in the following one the boilers are all reduced to a standard ratio of generating heating surface to grate surface of 48.1, and the air-pressures calculated as varying inversely as the ratio of the equivalent grates to the actual ones.

VESSELS WITH OLD-TYPE ENGINES AND NO SUPERHEAT.

Plotting the results obtained from the above tables on Plate I, we see that for an air-pressure of 2 inches of water, which is the naval limiting pressure for coal-burning Babcock and Wilcox boilers, which are fitted in all the above vessels, the use of superheat and the improved engines gives 25.6 I.H.P. per sq. ft. of grate against 20.6 I.H.P. per sq. ft. of grate with the old equipment. This is a total increase in economy of 24.3 per cent. The commonly accepted saving due to superheat is estimated at 1 per cent. for every ten degrees, which, as the average superheat used in the above vessels at full power amounted to about 60 degrees, gives a total saving due to superheat only of 6 per cent., leaving a balance of 18.3 per cent. due to the improvements in the engines themselves.

COMPARISON OF WATER RATES OF OLD AND NEW TYPES. The only vessels having reciprocating engines for which accurate engine water consumption measurements have been taken, up to the present date, are the scout Birmingham and the battleship Delaware. The necessary engine data for comparison are as follows:

FCURVES OF POUNDS BACK PRESSURE PER □ OF L.P PISTON AND REVOLUTIONS PER MINUTE OF VARIOUS NAVAL ENGINES

DESIGNED REVOLUTIONS FOR DESIGNED FULL POWER MARKED O.

REVOLUTIONS PER MINUTE

75

85 90 95 100 105 110 115 120 125 130 135 140 145

150

The Birmingham engine, of course, is handicapped at the start by its smaller size, higher piston speed, and lower steam pressure, but these handicaps cannot possibly explain away the great differences existing between the steam consumptions per I.H.P. of the two types of engines at equal fractions of power. These consumptions are as follows:

The above amounts include drainage from the jackets. All leakage from stuffing boxes is unaccounted for, but if this were taken into account, the difference would be still more favorable to the Delaware, as not the slightest evidence of leakage through her valve stem or piston-rod stuffing boxes existed during the trials.

The collier Cyclops has engines built along the same lines as the modern naval reciprocating engines, and the results obtained by her bear out fully the results obtained by the Delaware. Although no superheat exists in the case of the Cyclops, the water per I.H.P., measured from her indicator cards, shows at full power less than 12 pounds, neglecting leakage and cylinder condensation.

To show the effect of the short, straight steam ports on exhaust pressures, Plate 2, showing the amount of back pressure exerted per square inch of L.P. piston area for several engines of the old type and of the Delaware, is herewith submitted.

The very low pressure shown by the Delaware is not entirely due to the ports alone, but is partially accounted for by the fact that with the Delaware's engines no live steam was admitted to the I.P. and L.P. receivers during trials, full benefit being taken of the expansion of the steam from cut-off in the H.P. cylinder to the exhaust in the L.P. cylinder.

The Delaware's engines were still further aided in the search for economy by the forced lubrication system, which, by insuring that all bearings were oil borne, quite materially decreased the friction of the load, and thus allowed the engines to turn up more rapidly than would have been the case if this system had not been installed.

The following table, giving steam and exhaust velocities, will be of interest as showing the benefit of the straight ports in reducing resistance to the flow of the steam, and thus increasing the efficiency of the engines: