The ambient temperature must be considered because as the temperature of the air that the engine breathes increases, the density decreases, and there is less oxygen per cubic foot of air. The decrease in oxygen decreases the efficiency of the engine. The important air density is the density of the air as it enters the engine. For turbocharged and supercharged engines the density of the air has been increased above ambient removing the effect of temperature. Therefore no adjustment for temperature needs to be completed. For naturally aspirated spark ignition engines the power rating must be reduced 1% for each 10°F (5.6°C) ambient temperature rise above 85°F (29°C). For diesel engines the adjustment is 1% for each 5°F (2.7°C above 85°F (29°C).

Problem: How much usable power will a 165 Bhp spark ignition engine produce if it will be operating in 100°F air temperature?

Solution: Using the recommended derating for temperature:

162.525 or 160 hp

The effect of the 100°F temperature is to reduce the useable power from 165 to 160 horsepower.

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