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Engine Drive Systems: Considerations with Engine Applications - Sizing

In sizing an engine for a specific equipment application, a proper evaluation of the power requirements, rotational speed (rpm), capacity control or unloading of the equipment to be driven, along with its operating characteristics need to be performed. The power rating of the engine is based on the projected use. For primary power applications, the prime power rating applies; for standby or intermittent use, the continuous rating for standby power would apply.

Higher Heating Value (HHV) and Lower Heating Value (LHV)
There are two ways to define the energy content of natural gas in common use -- Higher Heating Value (HHV) and Lower Heating Value (LHV). The difference can be especially important when reviewing the performance of engine-driven systems.

Higher Heating Values for a fuel include the full energy content as defined by bringing all products of combustion to 77°F (25° C). Natural gas typically is delivered by the local gas company with values of 1,000 - 1,050 Btu per cubic foot on this HHV basis. Since the actual value may vary from month to month, some gas companies convert to therms. A therm is precisely 100,000 Btu. These measures all represent higher heating values.

Some engine manufacturers rate their engines using Lower Heating Values (LHV) which can be both confusing and potentially misleading to the casual user of their product literature. Lower heating values neglect the energy in the water vapor formed by the combustion of hydrogen in the fuel. This water vapor typically represents about 10% of the energy content. Therefore, the lower heating values for natural gas are typically 900 - 950 Btu per cubic foot.