Turbo diesel engine sizing?

jfritz_drfritz wrote:

naturist wrote:


at 10,000 feet, non turbo engines will be down 20% or more in power, and that 20% comes off the top. That is, it takes the same toll on your power to run the alternator, pump the water, overcome engine friction; the power loss comes entirely off the part that moves you up the road, so is a great deal more noticeable than a MERE 20% might otherwise be

.

I guess I don't understand.
the alternator, water pump, internal engine friction are all parasitic losses and are the same at 0 ft elevation as they are at 10,000 ft elevation. As such they are constant in absolute terms i.e if they draw 40 hp at one elevation they do also draw 40 hp at another elevation. They do make up a larger percentage of the power the engine actually produces as the power drops off due to elevation rise, but since their draw is constant, they can be discounted in any discussion of engine power production related to elevation change. Manufacturer hp ratings are net ratings i.e. taken at the flywheel with a functioning water pump, power steering pump and alternator. So there is no "top" there is just your net hp and as you correctly state it drops off less with a turbocharged engine because the boost level stays constant at altitude (to a point).

You got part of it;
The parasitic HP requirements remain constant so if the parasitic draw on a 100 hp engine is 20 hp., you have 80 hp to move the load, or 100% of the remaining HP.
now at 10,000 ft. you loose 20% of HP (100 hp), so now you have 80 hp total for the parasitic draw and moving the load. 80-20=60, now you have 80/60=.75, so you have really lost 25 % of the power to move the load.