Unfortunately, such ideal loads seldom are encountered. The usual Diesel engine carries a load from one quarter to full load, and the fuel consumption is vastly different from full-load test results.
Table VIII is a compilation of the hourly loads in a plant containing three old 225 h.p. Diesels and a 500 h.p. Busch-Sulzer Type B Diesel. The 500 h.p. engine has an individual fuel tank while all three of the older engines obtain their supply from a common tank. Hourly readings of the fuel consumed, actual kilowatts delivered to the switchboard and the indicated kilowatt load are entered in a station log. It will be noted that the 500 h.p. unit operated sixteen hours daily and carried a load ranging from one-third to one-half load. One hour's fuel con sumption (from 9 A.M. to 10 A.M.) is evidently erroneous, but this was rectified in the subsequent hourly readings. The aver age consumption of .711b. per kw.-hr. at half load is exception ally good and is evidence of the high-class attendance employed in the plant. The interesting figures in this log are the fuel con sumptions of the obsolete 225 h.p. Diesels. These engines are from ten to fifteen years old and have had hard service. Never theless, the fuel consumption of these engines at less than half load averages .909 lb. per kw.-hr. Although the design and workmanship of these early American Diesels are often ridi culed, it is questionable if the more modern engines will show any better results after the same length of service.
Table IX is the summary of a daily log of a plant having two 225 h.p. and one 170 h.p. old Diesels and one 500 h.p. McIntosh & Seymour Diesel. The average fuel consumption was .972 lb. per kw.-hr. This high value is due to the condition of the old engines and to the engineer's unwillingness to have the 500 h.p. engine carry the night load even though it was less than the engine rating. Frequently the engineer is blameable for a high fuel consumption since he is unwilling to work the engines at their economical load factors.
Production Costs.—While the Diesel is superior to any other form of prime mover in thermal efficiency, the actual commercial efficiency of the engine may be even less than a steam turbine at low loads. It is obvious that the Diesel plant investment is high with consequent high interest and depreciation (fixed) charges. If the plant is operated at a low load factor, the fixed
charges and the labor charge will cause the total cost per kw.-hr. to be excessive. Figure 191 is a chart which shows the total cost per kw.-hr. in a graphic manner. This plant had a total installed capacity of 500 kw. The entire plant cost $81,600. Table X gives the various charges used in developing the curves in Fig. 191. It is apparent that the two factors that determine the total cost per kw.-hr, are the labor and fixed charges. These two items, divided into hourly charges, are assessed against the hourly load and cause the total cost per kw.-hr. to mount very rapidly on low loads. The solution of economical Diesel opera tion is a high load factor which would tend to reduce the overhead and labor charges. This particular plant employs seven men, and the capacity could be doubled without any increase in the labor cost, although the fixed charge would increase in ratio with the engine capacity. These curves prove that Diesel plants must be fairly well loaded to be economical. For this reason it frequently pays a plant to give a very low power rate in order to increase the load factor.
Table XI is the daily report of a Diesel plant having a 350 kw., two 125 kw. and one 100 kw. Diesel units. The smaller units are very old and have a high fuel consumption. The load factor is very low which results in an excessive fuel charge even on the modern 500 h.p. unit. The old units required much over hauling,—this shows up in the maintenance charge. The total production cost was 18 mills per kw.-hr. while the fixed charges are about 10 mills per kw.-hr., based on the daily output. How ever, this total cost of 28 mills per kw.-hr. compares very favor ably with steam plants having similar load conditions.
Table XII covers the results of three months' operation of two Diesel plants of the Texas Light and Power Co. The Paris plant possesses three modern 500 h.p. Diesels, while the Tyler plant has two double-engine units. These latter were second hand units that were originally installed in an Eastern industrial plant. The condition of these units is revealed in the main tenance charge of 4.48 mills per kw.-hr. The fuel cost of the Tyler plant is 30 per cent. greater than in the Paris plant.