2009 RTI spin hoist
 

Diesel Engine Fuel Consumption - Quick calculation

  • Diesel engines use approximately 1 gal per hour per 20 HP developed

 

  • Fuel consumed is proportional to the power developed

 

  • Power developed is approximately  proportional to the cube of the engine revs

 

  • Hence fuel consumed is proportional to the cube of the engine revs

 

If you ½  the revs you reduce the power & hence fuel consumption by 1/8

 

 

Assume a sailing boat has 40 hp engine

Max revs 3800 rpm

At 3800rpm developes 40 hp ( consumes 2 gal per hour )

 

If normal cruising revs are 3000 rpm ( 80% of max )

Power developed 80% X 80% X 80% = 51.2% of the max

ie 0.51 x 40 = 20.4 hp

Fuel consumed 1 gal per hour

 

Therefore by reducing the engine revs from 3800 to 3000 you have halved the fuel consumption. & most probably only reduced the speed from say 7kn to 6kn.

 

Now reduce the engine speed to 2500 rpm ( 65% of max )

Power developed 65% x 65% x65% = 27% max

ie 0.27 x 40 = 10hp

Fuel consumed 0.5 gal per hour

 

The same rule apply to much larger diesel engines.

A 400hp engine will consume 20 gal per hour at full revs and only 2.5 galls per hour at ½ max revs. 

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