660-mm. (26 in.) stroke; each engine has a scavenging cylinder of 1,050mm. (41.25 in.) diameter and 600-mm. (23.6 in.) stroke; also a threestage, four-cylinder high-pressure vertical compressor, both directly connected to the engine. This compressor delivers the air necessary for fuel injection and for starting the engine. The scavenging pump, which is larger than usual, delivers the air to blow off the products of combustion and fill the cylinders with fresh air at the beginning of the stroke This pump was increased in size to be able to fill the cylinders with air at 211⁄2 lb. gage pressure at the beginning of the stroke. This gives nearly the same initial absolute pressure and allows the engine to generate nearly the same indicated horsepower as it would at sea level. The work done in the scavenging pump is, however, increased and the horsepower available is approximately 95 per cent. of sea-level output. The fuel consumption per horsepower is increased over sea-level conditions because of the extra work of the scavenging pump and the resulting lower mechanical efficiency of the engine when operating at the elevation of this plant. FIG. 3.-SECTION OF CHART SHOWING VARIATION IN POWER LOAD. The plant has been in operation for 17 months, but as the mill was not ready until the middle of April, 1916, up to that time the load was carried on one engine. The fuel used is California asphaltum base oil 14° to 18° Bé. gravity, averaging 16° Bé. and 18,360 B.t.u., costing $1.85 to $1.98 per barrel. The number of heat units in the oil was not determined at Tyrone, but at another plant which gets the oil from the same shipping point and under the same contract. The pistons, cylinders, heads and exhaust pipes are water-jacketed; the jacket water of the exhaust pipes, which can be easily varied in temperature without danger, is used to heat the fuel oil to 120° F. as the heavy fuel oil cannot be used if cold. The engines are started with a lighter fuel oil of about 25° Bé. and run on this oil until the heavy oil is heated to the required temperature; when shutting off an engine, it is run for a few minutes on light oil to fill up the oil piping with light oil. The curve in Fig. 4 shows the oil consumption per kilowatt-hour delivered at the switchboard when engines were tested new; it is the aver age of several tests. It is interesting to compare the test figures with the actual consumption as shown on power reports of the company. Although individual months show variations from the curve, the yearly average for 1915 checks very closely with it, showing that the efficiency of the engines is well maintained and that the variable load has very little detrimental effect on the fuel efficiency. By the courtesy of Phelps, Dodge & Co., owners of the Burro Mountain Copper Co., I am allowed to publish in the accompanying table the operating costs from Jan. 1, 1915, to June 1, 1916. These costs are as they stand on the books and are higher than is to be expected. They cover the running cost of the complete plant, exclusive of the air compressor. These costs do not include taxes, overhead charges or deprecia Fig. 4.—AveraGE FUEL CONSUMPTION TESTS ON DIESEL ENGINE, BURRO MT. COPPER CO. Diesel Engine: 5 cylinder, 1250 hp. (sea-level rating), 2 cy. ch. Plant Elevation: 5,950 ft. Fuel: 16° Bé California crude oil, 18,360 B. t. u. per pound. tion. The cost per kilowatt-hour available is based on total power generated less that used for auxiliaries in power plant, such as lighting, circulating pumps for oil and water, etc. To simplify bookkeeping, the compressor is not charged with the cost of circulating its water or with its proportion of the lighting. The labor cost is high because we have operated from the first with the full labor needed for operation when the power house is delivering the power necessary to run the mines and mill at the rate of 1,500 tons per day, a condition that has not yet been reached. The repair cost is high, for soon after operations were started the construction account was closed and several items, such as labor for installing recording thermometers, measuring gages, some lubricating |