If the power rating is reported for a basic engine, sometimes called the gross power rating, it was tested with all accessories removed. These accessories include the air cleaner, muffler, generator, governor, and fan and radiator. If the power rating is
for a fully equipped engine, sometimes called a net power rating, these accessories were on the engine when it was tested.
If basic engine power is reported, the engine must be derated 10% for all accessories other than the fan. An additional 5% must be deducted if the engine will have a fan and a radiator instead of a heat exchanger.
Problem: How much usable power is available if the basic engine rating is 75.0 hp, and the engine will use a fan and a radiator for cooling?
Solution: Derating 10% for the accessories and 5% for the fan:
hp = 75.0 hp x (1.0 - (0.10 + 0.05)) = 75.0 x 0.85 = 63.75 or 64 hp
Note: the same answer can be determined by subtracting the lost power.
hp = 75.0 hp - Loss = 75.0 hp - (75.0 x (0.10 + 0.05)) = 75 - 11.25 = 63.75 or 64 hp
The effect of the accessories is to reduce the useable power from 75 to 64 horsepower.
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