As the altitude increases, the barometric pressure decreases. This reduction in pressure reduces the efficiency of naturally aspirated engines because as the elevation of the engine increases, the pressure difference is less between the air pressure inside the cylinder, on the intake stroke, and the atmospheric pressure. Decreasing the pressure difference decreases the amount of air that will flow into the engine on the intake stroke.

Altitude is not a problem for engines with turbochargers or superchargers because they increase the intake system pressure. For naturally aspirated spark ignition and diesel engines the power rating must be reduced by 3% for each 1,000 ft (305 m) of elevation above 500 ft (152 m) above sea level.

Problem: What is the usable power rating for a 225 hp diesel engine operating at an elevation of 4,500 ft?

Solution: Using the derating recommendation for altitude (Note: the method used to solve this example is different from the previous one. Either method can be used.)

This example was worked using percentages (%).

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