The field for this oil engine is not limited to new installations. Indeed, its greatest opportunity lies in those steam power plants where the load is light during the major part of the day. Often it is possible to install an oil engine to care for the light loads, relying on the steam unit to meet the demands of the peak load which are beyond the capacity of the oil engine. An example of such a combined steam-oil engine plant is the Municipal Power Plant at Pala Alto, California.
The records of a somewhat similar plant, although much smaller, are given below. This plant originally contained a small Corliss and a high-speed engine, and two tubular boilers. The output of the plant was used in lighting the municipality and in pumping water.
Figure 356 shows the hourly load carried as well as the fuel costs. This excessive fuel expense was instrumental in the in stalling of a 60 h.p. low-pressure oil engine which, with the boiler alterations, cost $4200. In order to even up the load, the hours of pumping were slightly changed.
The plan of operation is to have the oil engine handle the load from midnight until 7 P. AL the next morning. At that hour the steam boilers, which have partially maintained their pressure all day, are fired up, using fuel oil. The corliss engine then carries
that part of the evening peak load which exceeds the capacity of the oil engine. Figure 357 gives the hourly load and hourly fuel costs while operating under this arrangement. The fuel expense during the peak load is high since fuel oil is burned under the boilers. The daily expenditure under the old and the new plan is as follows: Conclusion.—The low-compression oil engine does possess merit. It has a field of usefulness that is gradually increasing in extent. Its competitors are the Diesel and semi-Diesel en gines, although the fields of these three types do not overlap to any marked extent. The operating costs just discussed, when taken in conjunction with the Diesel costs in Chapter XVI, afford a means of comparing the values of the two types of oil engines in respect to their efficiency as heat-converters. It must be remembered that operating costs are by no means total costs; consequently in many plants where the Diesel oil engines of low powers are installed, low-compression engines would prove far more economical in respect to total costs per horsepower hour.