Fuel Burn - Net or Gross?

[Bensdad]

One of the things I love about my Cruisers boat is that so many things are labeled.

One of them, the fuel return lines on the diesel engines, got me thinking about my fuel flow gauges. I couldn't find anything in previous threads about this so I'll put it out there hoping someone knows.

Is the number I see on my flow gauge (example 10 gph) the amount of fuel going into the engine or is it the net amount burned after a small portion is returned to the tank?

If it's the net burned, any idea on what percentage is typically returned to the fuel tank?

Dick Grimes

[chrisr]

couple of questions

- how does your flow gauge actually work ? does it really measure 'flow' or does it look at tank level & time, and calculate flow ?

- where (in which line) is the sensor for the flow gauge located ?

cheers,

[rslifkin]

A proper setup should measure input flow and return flow and then display the actual fuel burned.

Return flow varies widely between different engine designs and may also vary based on power setting. Some engines may only return 5% of their total flow, others may return 50% or more under some conditions.

[Bensdad]

Good question about where in the fuel line is it located? I honestly don't know.

I have Volvo diesels and the multi-function Volvo digital gauges that came with the boat. One of the several displays is gph but I don't know where it gets its data.

It's beginning to sound like a $$-based decision whether I want to install meters for input and return flows or just cruise along using the existing display. For the immediate future, there are plenty of fuel docks in my typical cruising area, so they will have to remain as an interesting source of conversation and not part of a trip planning calculation.

Thanks for your replies!

[rslifkin]

If your engines are electronically controlled, the data is likely coming from the engine ECUs and should be a pretty accurate net flow.

[Niall leslie]

- It is very likely that the indications are for fuel actually consumed.
- That should be very easy to check by filling the tank(s) and then following a long run comparing the fuel required to refill with the indicated fuel used.
- An even more simple way to check is by asking your Volvo dealer or the vessel manufacturer.
Cheers,
Niall.

[Sailing4Jesus]

I don't think that Joshua Slocum had this issue to deal with. We have become too dependent of Doodads and such.

Senior Chief

[Michael Pope]

Ahmen Use a stick!

[vanderd]

I have Flo Scan GPH meters on my power cat. They do take into consideration the fuel return. In fact the calibration process requires filling the tank running the boat and then topping up the tank. I find the Flo meter most useful for slight rpm changes. Sometimes a slight changes yields a noticeable drop in fuel consumption.

[a64pilot]

Originally Posted by rslifkin

If your engines are electronically controlled, the data is likely coming from the engine ECUs and should be a pretty accurate net flow.

This, if an ECU, it’s calculated, not measured, however my experience with them is they are astonishingly accurate.
I know of no automobile that reports MPG that actually measures fuel flow, they are all calculated, and again real accurate.

Flow scans etc if a diesel all have two flow measures, one is subtracted from the other of course, and are as accurate as you make them. Nearly all have to have a “K” factor entered, often times this is done by burning a tank of fuel and refilling, if your really into it, burn a few tanks and keep track of how much you put in every time, then enter the total. The bigger the amount the more accurate as it averages any errors in filling out.

[HBrew]

On a slightly tangential issue, I have Volvo engines (D2-40) and I have been trying to get fuel consumption figures at various revs but all I can find in the Volvo docs is a chart that is indicative but not precise. Does anyone know a good source for numerical fuel consumption information?

[NorthCoastJoe]

Can you look at the lines and see if there is a meter on inlet and return lines?
My Yanmar 3gm30 uses so little I wonder if it eats ever. I have a stick as a fuel gauge on that boat. The other boat I am setting up a day tank. A usage meter might be something I put on that one.

[StuM]

Originally Posted by HBrew

On a slightly tangential issue, I have Volvo engines (D2-40) and I have been trying to get fuel consumption figures at various revs but all I can find in the Volvo docs is a chart that is indicative but not precise. Does anyone know a good source for numerical fuel consumption information?

The factory power and fuel burn curves v RPM are about as good as it gets.


To get anything more accurate, you would have to do a series of experiments. It is very dependent on your individual drive chain and prop set up.

[Boatwright]

KISS:

IMO most of the fancy stuff: Fuel Flow Gauges, Battery Meters, Garmin Maps, TV's, all the comforts of a 4,000 sg ft suburban give us a sailing experience filled with stuff that costs too much, needs constant maintenance, and yields marginal results.

With a voltmeter and a simple log of readings over time, with notes about engine run time and power loads you will have an excellent idea of your system's state of charge. You can buy a very accurate and reliable digital voltmeter for about $10.

Consult your engine manual for fuel flow at a given RPM. Since we usually motor at a constant speed that's all you really need. If you have a tank fuel level sender you can calibrate it by noting the readings before and after a fill-up. Pilots never rely on fuel gauges but always calculate fuel burn. Cost: $0

A GPS dongle for PC based navigation costs about $30 and will give you a position accurate to a few yards. Raster based chart files ditto. The complete set of Canadian Great Lakes charts (our summer cruising ground) costs less than $100 and includes [U]all[U] the published charts, including harbor and other special charts.

Entertain yourself with a good book. Get your head up, sail the boat with the compass and quit worrying about expensive crap you don't really need, and remember the wise man who said: "Heaven has no moving parts."

[GWilly]

HBrew, check on the site called boatdiesel.com. They have a ton of data sheets and info on every brand/make/model of Diesel engine. You may have to join (nominal fee, <$25US) but well worth it, IMHO.