Steam Locomotive Development

With the "King George V." was sent the "North Star," one of the first engines on the Great Western Railway of England, de signed by Sir Daniel Gooch, of that company, and built by Robert Stephenson and Company. The "North Star" as well as others of the "Star" class of locomotives, of the 2-2-2 type, as designed and built by Sir Daniel Gooch, at the Swindon works of the Great Western about 1846, for fast passenger service, was adapted to a 7 ft. gauge of track; cylinders, 18 by 24 in.; one pair of driving wheels, 96 in. ; heating surface, 1,952 sq.ft.; boiler tubes, 300; and was equipped with the Gooch fixed link valve motion. This locomotive had a maximum speed of 78 m. per hour, and when evaporating about 2,500 gal. of water per hour, the fuel consumption averaged about 2.5 lb. of coal per horse-power hour, which compares most favourably with present performance. The Delaware and Hudson Company high-pressure freight locomotive, No. 1,401, the "John B. Jervis," was also exhibited. The out standing feature of this locomotive is the water tube—fire flue type of boiler carrying 400 lb. boiler pressure ; one high-pressure cylinder, 221 in. and one low-pressure cylinder, 381 in. diameter, by 3o in. stroke; the weight on four pairs of drivers, 295,000 lb.; driving wheels, 57 in. diameter; and the tractive power of 85,000 lb. in simple gear, 70,80o lb. in compound gear and i8,000 lb. additional tractive power in the tender truck booster for starting and accelerating trains.

Locomotive Classification.

Prior to the locomotive classifi cation system developed by F. M. Whyte, mechanical engineer of the New York Central and Hudson River Railroad Company, the designation was by name only, as Ten-Wheel, Mogul, Con solidation and the like. Whyte's classification, which has been generally adopted, is by numerals. It consists of three divisions, the first to designate the total number of leading truck wheels; the second, the total number of coupled driving wheels and the last, the total number of trailing wheels. In some cases, the number of leading and trailing truck wheels in the tender is also added.

Capacity and Efficiency.

The capacity of a locomotive is rated in terms of i—Its starting tractive force and 2-The horsepower developed at speed. The latter determines the available tractive force at various speeds. The starting trac tive force on the other hand is limited by the available adhesion at the driving wheels. For this reason, the starting or rated trac tive force should not exceed approximately one-quarter the weight on drivers.

The tractive force is effected by the action of the steam against the pistons in the cylinders and the connecting rod mechanism, resulting in a torque applied to the driving wheels. This is resisted by the moment of the tractive force at the rim of the wheels. Friction in the mechanism, however, reduces the effective torque applied to the driving wheels, and friction in the main journal bearings further reduces the moment of the tractive force at the rail. This total friction loss reduces the actual tractive force

developed at the rail and which propels the train, to a certain percentage of the cylinder tractive force. The ratio of the actual tractive force to the cylinder tractive force (i.e., the tractive force developed by the cylinders without friction) is termed the ma chine efficiency. At low speeds, where the torque and traction force is large, the machine efficiency is relatively high, whereas at the higher speeds, it is considerably lowered and the correspond ing losses materially limit the output of the locomotive. If it were not for the resistance of the locomotive and tender idle axles, the drawbar pull and tractive force at the drivers would be identical in value. Actually, the train resistance of the locomotive as a ve hicle, reduces the drawbar pull below the tractive force developed at the drivers. Obviously, during acceleration, the difference may be considerable, due to inertia resistance of the locomotive.

The performance of a locomotive at speed, in fact from 50 R.P.M. or 25o ft. piston speed, and above, is dependent upon the horsepower characteristics of the locomotive as a power plant. The horsepower rating of a locomotive is essentially dependent upon the boiler evaporation rate per hour and the cylinder steam consumption rate per horsepower hour. At the lower speeds, the steam consumption of the cylinders increases over that at the higher speeds due to the inherently longer cut-offs, together with a somewhat reduced evaporation rate of the boiler. For this rea son, the horsepower developed varies with the speed, increasing to a maximum value at the operating speed wherein economical cut offs are possible.

The horsepower rating of road locomotives is, therefore, usually stated at an arbitrary piston speed at from i000 to 1200 ft. per min. or a corresponding rotational speed of the drivers at from 200 to 750 R.P.M. In this speed range maximum horsepower is reached. It is then dependent upon the normal maximum boiler evaporation rates and the normal minimum cylinder steam con sumption rate.

In heavy freight drag service, it is desirable to improve the operating efficiency in the low speed range. In this case, more economical cylinder steam consumption is obtained by early cut offs in the low speed range. Such locomotives are frequently termed "limited cut-off" locomotives.

To increase the Rankine efficiency, high pressure locomotives have been introduced, using compound cylinders. Aside from the increased economy, the compound cylinder, properly propor tioned, gives a much wider speed range for economical fuel con sumption and low water rates.