GAS ENGINES

Table II gives some instructive data relating to these engines.

Two cylinders, in tandem, act upon each crank. The normal full brake horse-power is that which the engines are capable of maintaining continuously; the 'overload' is 10 percent greater and should only be imposed during short emergency periods. The powers as stated are, moreover, for engines working at sea-level and in a temperate climate; when the plant is installed above sea-level a deduction of about 3 percent should be made from the power rating for every 1000 ft. of height. In Table II the initial temperature of the gaseous mixture before entering the engine is also assumed as 60 degrees F.; in hot countries some power loss is incurred by reason of the diminished density of the mixture when supplied to the engine at a higher temperature; the loss from this cause may be estimated as 1 percent of the sea-level rating for every increase of 5° F. above the standard temperature of 60° F.

The normal full revolution speeds given in Table II are also maxima values for continuous running; it is well, in order to minimize wear and tear, to arrange wherever possible to run ordinarily at from 5 percent to 10 percent below the 'normal full' values. The flywheels referred to in the table are as used with engines driving direct-current electrical generators, air-compressors, etc.; when alternating-current generators are being driven the necessary steadying rotational inertia is usually furnished by the generator rotor itself, and in such cases only a coupling at the crank-shaft end, or a light wheel for barring round the engine, is necessary. The total 'flywheel effect' required in parallel running is, however, in general settled by the suppliers of the alternating-current generators.