T&T: More on diesel fuel consumption

[LRZeitlin@aol.com]

Rob asks:

Larry, as you noted my HP and fuel consumption spec curves are flat enough to 
be considered linear in the mid range.
 
My specs show   rated SHP is consistently a bit less than BHP, maybe 15 HP or 
less, not surprising.
 
But "power - propeller load" only gets to rated SHP at WOT, 2,300 RPM. 
Otherwise, it drops off fast: At 1,600 RPM for example, while rated SHP is about 
333, power at the prop is way down at about 200 SHP. Why is "prop load power" so 
much lower at lower RPM??
 


Why is the propeller curve lower than the engine output curve?

The propeller hp required curve shows the actual power required to rotate the 
propeller at the indicated rpm. The required power to rotate the prop rises 
exponentially as the rpm increased, generally somewhat faster than the square 
of the increase in rotational speed. Doubling the rpm means that the prop 
requires more than four times the power. The actual amount of power consumed 
depends primarily on the pitch of the prop, the blade area, and the prop diameter. 

The engine output curve shows the maximum amount of power that the engine can 
deliver at a given rpm. These curves are usually determined by attaching the 
engine to a power absorbing dynamometer. The engine is set to rotate at a 
given rpm and the dynamometer load is gradually increased. When the engine begins 
to labor and slow down, the amount of power absorbed by the dynamometer at the 
given rpm is noted. The process is repeated over and over at different rpms. 
The readings are marked on a chart and the maximum power curve is drawn.

Thus there are two different curves. The engine power output curve and the 
propeller law curve. The difference between the curves shows the engine power 
left in hand at each point. If we are talking about car engines, it is the 
additional power available for acceleration. The boat engine never delivers its 
full power to the prop at low prop rpms. It is only when the prop power required 
at a given rpm reaches the the engine maximum power curve that the boat engine 
is fully loaded. That the point where the two curves intersect. The prop char
acteristics should be chosen so that the intersection point is at about 90% of 
maximum engine output.

This is, of course, true only for fixed pitch props. A CPP can increase the 
blade pitch at lower rpms to use the full engine power available at that rpm 
point. Since both gasoline and diesel engines are more efficient when loaded to 
about 80% of their maximum power output, the CCP can gain a few percentage 
points in efficiency.. The downside is that it is possible to overload the engine 
if you are not careful. It is difficult to damage an engine with a correctly 
specified fixed pitch prop since the prop serves as an rpm limiter.

Larry Z