This cannot be attributed entirely to the mechanical condition of the engines but to the load factor as well. The fuel consump tion of these old American Diesels is less than 10 per cent. over the fuel consumption of the modern Paris engines on the same load. From this the reader can rightly conclude that the effi ciency of the Diesel is fairly constant, regardless of the mechanical condition of the units, as long as the engine will function.
Gas Engine potential competitor of the Diesel engine where natural gas is available is the natural-gas engine. Table XIII is a published report on the gas engine plant of the Willard Storage Battery Co.; this report, as it appears herein, has been amended by the elimination of the depreciation and insurance charges. The production cost totals 9.14 mills per kw.-hr. at a load factor of 41.8 per cent.. This is in striking contrast with the Paris Diesel Plant in Table XII where the production cost was 4.98 mills per kw.-hr. and is only slightly better than the results secured with the Tyler second-hand units. The heat units per kw.-hr. in these gas engines do not differ materially from the B.t.u. per kw.-hr. at half load in the Diesels of Table XII. The advantage the Diesel possesses is princi pally in the lower cost per B.t.u. of its fuel over the natural gas. This can be placed in the form of an equation.
If c = cost of fuel oil per gallon (19,000 B.t.u. per lb.) a = cost of gas per 1000 cu. ft. y = B.t.u. per cu. ft. of gas then a y 133 if the cost per B.t.u. is to be the same in fuel oil and gas. As example, if fuel oil can be obtained for 3 cents per gallon and the natural gas contains 600 B.t.u. per cubic foot, the gas must be obtained at a cost of 13% cents per 1000 cubic feet to equal the fuel oil as a fuel.
At the same net cost per B.t.u. the natural-gas engine would possess a lower total cost per kw.-hr. because of its lower fixed charges. Each installation must necessarily depend on the vary ing considerations that enter into the problem.
Producer-gas Engines vs. Diesel Engines.—From the view point of fuel cost alone, the producer-gas engine is usually more economical than is the Diesel. Table XIV is a summary of a test on a 200 h.p. producer-gas engine conducted by the Lehigh University. The dry coal per brake horsepower was 1.04 lbs. Based on usual generator efficiency, the coal per kw.-hr. would
approximate 1M lbs. or 20,000 B.t.u. The coal must then be purchased at $4 per ton to allow the fuel cost to equal the Diesel fuel cost in Table XII. In normal times anthracite pea coal can be purchased at a decidedly lower price in the vicinity of the mines.
Comparative Estimates for Diesel, Steam Turbine, Natural gas and Producer-gas Engine Plants.—Table XV covers esti mates on various types of prime movers. It will be observed that the Diesel engine is the most economical unit under these conditions. The estimates are based on war prices and should be revised to prevailing quotations as should also be the fuel costs. The producer units approach the Diesels in total efficiency; in fact, in most cases the producer unit will prove more efficient. The serious drawback to the installation of a producer engine is the unreliability of the producer. This apparatus requires skilled attendants not easily procured; producer breakdowns are of common occurrence in such plants.
Steam vs. large units, above 5000 kw. the steam turbine will probably prove more economical than the Diesel. Plants of less capacity will find that the Diesel gives a lower total cost per kw.-hr. Under 1000 kw., the turbine is not as attract ive as the reciprocating steam engine and cannot compete with the oil engine. If a manufacturing establishment or office building creates a demand for exhaust steam, the Corliss or the Uniflow steam engine may be the type to install. For all other plants, where• no demand exists for exhaust steam, the Diesel is superior.
Plant power plant should have a daily log in which are entered the hourly load, fuel consumption and other details. Figure 192 is a log employed in a number of Diesel plants and is quite complete. Along with this daily log, a summary sheet, such as appears in Table XI, should be maintained. This sheet, being made up daily, gives the management a check on daily efficiencies.
Comparative Heat 193 to 195 show graphic ally the heat balances of various prime movers. It is apparent from these comparative charts that the Diesel engine is ap proached in heat-absorption efficiency only by the natural-gas engine. The actual efficiency of the producer engine is identical with that of the natural-gas engine, but the loss occasioned in the producer or gas generator must be charged against the engine.