Generator units are “work horses” that fulfill the need for emergency and standby power. They arem available from small 1 kVA units to those of several thousand kVA. When properly maintained and kept warm, they dependably come on line within 8–15 s.

In addition to providing emergency power, engine-driven generators are also used for handling peak loads and are sometimes used as the preferred source of power.

They fill the need of back-up power for uninterruptible power systems. Where well-regulated systems, free from voltage, frequency, or harmonic disturbances, are required, such as for computer operations, a buffer may be needed between the critical load and the engine-driven generators.

Diesel Engine Generators
Diesel Engine Generators
Individual models by various companies may be different. Lower speed units are heavier and more costly, but are more suitable for continuous duty.

Diesel engines are somewhat more costly and heavier in smaller sizes, but are rugged and dependable. The fire and explosion hazard is considerably lower than for gasoline engines. Sizes vary from about 2.5 kW to several MW.

Gasoline Engine Generators
Gasoline Engine Generators
Gasoline engines may be furnished for installations up to about 100 kW output. They start rapidly and are low in initial cost as compared to diesel engines.

Disadvantages include a higher operating cost, a greater hazard due to the storing and handling of gasoline, short storage life of the fuel, and generally a lower mean-time between overhaul. The short fuel storage life restricts gasoline engines’ use for emergency standby.

Gas Engine Generators
Gas Engine Generators
Natural gas and liquid petroleum (LP) gas engines rank with gasoline engines in cost and are available up to about 600 kW and higher. They provide quick starting after long shutdown periods because of the fresh fuel supply.

Engine life is longer with reduced maintenance because of the clean burning of natural gas. However, consideration should be given to the possibility of both the electric utility and the natural gas supply being concurrently unavailable.

Unless engine compression ratios are increased, engines lose approximately 15% in power when operated on natural gas, compared to gasoline. Considerations in selecting natural or LP gas-fueled engines are the availability and dependability of the fuel supply, especially in an emergency situation.

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