Engine specific fuel consumption vs. prop load

A few questions regarding the fuel-propellor curve. Few manufacturers provide this curve so is it calculated using the same formula for the prop curve:

HP = C X RPM^2.7 but we would simply substitute the data from the fuel burn curve?

The strange thing, for example, is at an operational speed requiring 140 hp, based on fuel burn the engine would be operating at 90% power output. However, if you simply look at 140hp/174hp, then it would be at 80% output. So which would be considered a more accurate representation?

 

You are misundinterpreting the diagram. The upper branch shows the max throttle values; ie maximum Power asf of rpm. The lower shows Power and fuel consumption at reduced throttle.

So, if your propeller is matching the max Engine Power (174 hp @ 1800 rpm), then the prop is requiring 140 hp @ 1670 rpm, with a fuel consumption of ~7.5 gph.

At the other limit, you could have a propeller designed for 140 hp @ 1400 rpm (on the max Power curve), where the corresponding fuel consuption would be ~8 gph. Between 1400 rpm and 1800 rpm your 140 hp's are available at different throttle settings (and different fuel consumptions, see below).

The reason for this is that the "Brake Specific Fuel Consumption" in "Gallons per horsepowerhour" (which is a measure of the Engine efficiency) is not constant over the entire operating map. Normally, the lowest BSFC will be found slightly above the max torque rpms, and at slightly reduced throttle.

This info will be seen in a torque/rpm diagram, where the BSFC curves form s.c. musslecurves. It must be understood that the exponent 2,7 in the cited equation for recalculation of propeller Power at varying rpms, is a curve fit, taking in consideration the variation in propeller inflow due to the varying hull resistance at different speeds. The "physically correct" exponent, according to the turbomachine affinity laws, is 3. But that requires constant inflow and outflow (=loading) angles, and this is violated by the hull influence. If you look at a waterjet (ie a "confined propeller"), the ducting losses are overriding the hull inflow influence. So recalculating Power asf of rpm is here done with the exponent 3, practically regardless of vessel speed.

 

Ok I understand all this. So it would seem that I need to generate a propeller fuel burn curve based on the standard 2.7 exponent in order to accurately predict fuel use at various speeds.

 

"Accurately" is inappropriate here, since there are two unknowns; the exact hull Power requirement (thus the exponent) and the Engine BSFC character. The exponent 2.7 does not cover all hulls over all speedranges. Using whatever exponent will give you the "Power burn" curve (not fuel burn). To get the "Fuel burn curve" with some accuracy, you must have access to the Engine BSFC at all the Power/rpm combinations along the "Power burn curve".

 

Gotcha. Glad it's a weekend today so I can sit down with Excel and create curves. Thanks.

 

OT: Is friday really a weekend in Qatar?

 

Yes the country is ruled by Islamic law and follows their calendar. The weekend is Friday (the holy day) and Saturday. Christmas day is a normal workday, as is our New Years Day.

Forget about watching the Super Bowl, Oscars, Emmy's, election cover live, etc.

Takes getting used to.

 

The graph you need is a fuel map, or BMEP map.

This looks like a series of bulls eyes with the most efficient band in the center , with lower efficiency in each outer ring.

You match the prop choice to the hp in the cruise or toeing regime you want to travel.

The hp\ "prop" crap mfg. give out is perfectly useless to match the engine and prop for efficiency.

Sadly the BMEP for most engines and power ranges is a more closely held item than the CEO's bookies home phone number.

Good luck in shaking one loose for your engine choices.

 

The graph you included seems to use 3.0 exponent, not 2.7. (1000/1800)^3*174=30 while (1000/1800)^2.7*174 would be 36.

Exponent 3.0 is valid for agitators (and bollard pull). It works well also for many true displacement boats (e.g. sailboats). Planing and semi-planing boats can have a totally different exponent and a constant exponent assumption is valid only for a subregion. E.g. a planing boat can have less than two in the planing region, but about 3 at displacement speed.

So you really need to know the thurst needed and propulsion efficiency in order to know how much power is used at different rpm.

Here is some interesting measured data and fitting to engine fuel map for a sailboat: http://www.hymar.org/upload/LIBS_130110.pdf

 

Well you guys are right about trying to "shake one loose" from the distributors. But I'm keepin-a-tryin

 

The best approach is use a company name and BS them that you are contemplating making some type of pump.

As pumps run at a variety of speeds and loads perhaps they will cough one up for the engine of your choice.

IF the mfg is no joy ,try importers in your country or anyone you can find on the net.

Good hunting.