Digital fuel metering

I'm looking for some kind of accurate (probably digital) meter of fuel usage that I can put onto a small outboard with either gravity or suction fed petrol supply.

I'm experimenting with various semi foiling hull appendages on a little inflatable with various outboards between 2.5 and 8hp and would like to create some tables of distance per litre (or similar) at different speeds and trim states.

I'd rather not get into weighing fuel before and after etc if there is a quicker alternative. I'd also like to do some videos of my experiences, and being able to show a number on a screen for real time fuel usage will have its benefits for that too.

I'm aware there are expensive bespoke fuel monitoring systems from reygar etc. And presumably the likes of Garmin too. I'm looking for a cheap workaround, say $50 max. So maybe something automotive I can adapt?

 

Nothing would meter that small amount that I know of..you'd need to graduate the tank, than run for a long enough time to make the graduations.

 

Are the engines carbureted or injected?

Flow meters for very small flow rates are available but not for $50.

If the engines are carbed with the fuel flow rate controlled by a float, the flow rate will vary at most speeds.
Say you start with a full float bowl at idle, the float drops a bit and some flow comes into the bowl, the bowl fills up, the flow rate as far as the gauge is concerned diminishes, the engine pulls fuel, the float drops, flow rate increases and the cycle tends to move on.
We used a flow meter on non -return V-8 engines and found at idle the gauge would cycle from almost zero up to a significant number. (it has been 20 odd years so cannot remember the upper end) We initially thought that
we had a faulty sender (impeller in line) and contacted the factory and they confirmed that this cycle is normal at low fuel usages due to the float/fuel flow issue and said that when fuel consumptions goes up the gauge gets more precise.

I am not sure if our meter was dampened, ie to average out the varying flow rates but meters can be purchased to average out the flow reading. As in your car, when you hit the brakes with a 1/4 tank, the gauge does not change.

I think that you would be better to plumb in a parallel fuel line with a burette and be able to switch to it when underway and record volume change with respect to time. The top of the burette would have to be equalized to atmosphere to inhibit it drawing a vacuum.

If the motors are injected it may be a different flow system. In the automotive injected end, they had a return line to return engine warmed fuel back to the fuel tank. Which then requires two fuel meters, one to meter flow to the engine and then subtract the flow back to the fuel tank.

In the marine end, when we began to use automotive engines that would normally have a return line, we could purchase EFI engines that had the physical return to tank line omitted. Another method was used to relieve the pressure that the positive displacement fuel injection pump could create. I suspect that the pressure relief system worked before the inlet into the high pressure injection pump. Perhaps someone can explain the process

 

Yes, they are all carburettor engines.
Maybe I'll need to set up a small graduated tank and run for some time at each throttle setting/ trim position to get the numbers.

 

you could perhaps use a clear fuel tube and close off the tube somehow..this way you could operate rather efficiently

I don't know fluid mechanics well enough to tell you how to rig it, but the float bowl issue could be gone; you'd fill the bowl, tube, close the flow off and allow only venting and measure graduations of the tubing and fairly non-invasive..and quick measuring only fuel tube amounts..

 

I bought this to start with 1pc Acrylic Semicircle 2 Way Flow Meter Indicator Port for PC Water Cooling | eBay https://www.ebay.co.uk/itm/324930606060?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=Uhs-9jY6TxW&sssrc=2349624&ssuid=IIXxNyPdRuW&var=&widget_ver=artemis&media=COPY

It's acrylic so will eventually melt in petrol, but I can wash it out after each use. I'm interested to see if it will spin slowly enough to let me count revolutions per minute.
If I can count it spinning, I might then calibrate it to a known volume.
I've also ordered a digital flow meter off Ali express for $20. The sender/ impeller is plastic on that too, but replacement ones are only $3 so if it works I'll buy a bunch of senders. I think it's for measuring flow through water filters, but who really knows with Ali express?

I'll have to use small amounts of fuel with it at any time, just in case it does all end in a fireball.

 

If you run a long tube from the tank and are able to snake it outside; add a shutoff valve to the top of the tube...and another vent valve under it; once the fuel drops below the vent valve, open it; use the tube distance for the gauge, a 6mm tube would be like .00282cc for each centimeter of tube change; it would be far more accurate than the meters...

0.00282cc/x seconds

so, say 1.5 seconds is x2400 cc/hr

6.768cc/h or some such...don't use my math please, I did it in like 3 seconds.

or just make a small tank with a tube...that you can hook on top...

No smoking please..

 

You can make a fill line on the tank. After a run, re-fill to line from a graduated beaker.

 

I like that idea, I could set it up in the boat as a vertical transparent graduated tube say a meter long. I could build it on a vertical wooden or aluminium frame clamped to the transom next to the motor. A metre long vertical run would mean I'd smooth out the effects of the float bowl accepting fuel at a different rate from that at which the engine is actually burning it. My AliExpress digital meter might end up as a paperweight.
The engines I'll be using have very small float bowls, as I run the bowls empty whenever I store them and it takes 2 minutes max for them to run dry with the fuel tap closed at a fast tickover and no load in neutral in a bucket.

I think a 1m long 10 mm internal diameter tube would give a pretty accurate idea of consumption within 2 or 3 minutes running at a set trim/ throttle setting etc.

Having to stay at the same throttle setting for 2 or 3 minutes to get each set of data wouldn't be too onerous.

I don't want to risk a glass tube. Anyone know of any common transparent plastic tubing material I could use that petrol won't immediately eat?

 

Tygon Tubing

 

Some things that dissolve in petrol will ruin engines, like sugar. Not sure if Plastics would have the same effect? Lots of complexity and Tiny orifices in the old style carburetors, as I recall.

 

You should be able to tape the Tygon to a measuring stick, like a yardstick and view a height either parallel or possibly even through it to get a length of column consumed

 

While calibrating fill lines on the tank is obviously the cheapest, lowest tech solution, it might not give you the most precise measurements.

I'm not clear what was all that wrong with using a simple scale, preferably one of the ones with fine gradations. Some can even be tared (set to zero) when the tank is empty, so the weight will be equal to that of the fuel alone - otherwise measure the tank weight once, and subtract.

However, both level measurements and weight measurements are affected by waves, as well as the current tilt of the boat. So, unless you can find flatwater to test in, and level the tank, I don't see how to use either to get precise and accurate results, unless you spent a lot of time, money and effort at it.

For the most part, a good flowmeter probably won't give particularly inaccurate results in waves or if the tank is off-level - though of course wind and waves will also affect the final mileage (i.e., miles/gallon or km/liter) a lot, no matter how you measure, because energy is wasted pushing through wind and waves - unless you are riding a following sea and wind, in which case you may gain energy (and get better mileage), at least at low speeds. Likewise, rates of current (tidal currents and river currents - which by the way often vary with depth - e.g., wind and bottom sheer can cause the top and bottom layers to have different speeds and directions) have a big effect on mileage too. In shallow water, so does water depth. Perhaps salinity, and other water composition elements, do too. (E.g., salinity, sediment and bubble inclusion can affect density and viscosity.) And so, of course, will the cargo weight.

So, if you are trying to figure out at what speed your boat uses fuel most efficiently, there are a lot of other variables to be considered and controlled.

For the greatest consistency, you could perhaps do all your measurements on a flatwater reservoir, over the same path, on windless days - but the results would only apply to those conditions.

Back in the days when I was writing image compression software for satellite weather images, I was told that the entire cost of the U.S. Navy's weather program (which included a lot of satellites, data links, weather buoys, people, computers, facilities, research money, and perhaps other things), was justified by fuel savings, because Naval ships were able to pick their course to minimize fuel usage. (Large vessels can consume A LOT of fuel.) There are other factors too - even the largest vessels, like aircraft carriers, are damaged by bad weather. But even the fuel savings alone are significant.

All of which goes to show you may need to take your measurements, under particular conditions, with a grain of salt, and allow for a margin of safety.

 

One flowrate meter technology I've used in the past for non-combustible fluids is based on a heating element upstream of a thermal sensor. As flow increases, the heated fluid has less time to dissipate the heat; so a high temperature is measured.

Like I said, this is for NON-COMBUSTIBLE fluids, so stay clear of this type of sensor for measuring fuel.