mileage computer for a carbureted engine

[JohnnyGrey]

If you want to go the cheap route, forget about all the flow measuring junk.

Get a decent sized 16v capacitor, maybe 100,000uf.

Take a line from the fuel pump's + terminal.

Run it through a variable current regulator, through a diode to the + of the capacitor. This is called an integrator.

Use a very high resistor (1M+) between the capacitor and a DMM to measure your fuel usage at the end of your trip. Do some trial and error and find the right current value so that 1v corresponds to 1gal. That way, the meter will be good up to 12 gallons. For a reset button, all you need is a button wired through a 10 ohm resistor between the terminals of the capacitor.

Keep a little four function calculator to get MPG numbers at the end of your trip.

[dcb]

With a better pump that would work. These are cheap constant speed, continuous running, variable output, pumps. It is a spring load piston setup where the spring provides the limited output pressure.

A fixed displacement, pressure shutoff pump would simplify things, but again at a cost.

Adding a pressure switch to the output of the cheap pump might just be close enough though...

[LostCause]

Carbureted fuel economy gauges existed during the 1970's. One unit, called the SpaceKom, occasionally shows up on Ebay. It used a flow meter and a VSS, I believe.

Good luck on your project.

- LostCause

[dcb]

My problem is the low flow rate at idle, but I never idle anyway so I don't know why I'm being so picky about it

[LostCause]

Originally Posted by dcb

My problem is the low flow rate at idle...

That's easy, increase your idle...

What do you estimate the flow rate to be? In any case, I bet the SpaceKom device was pretty inaccurate. Coming from vacuum gauges, I bet anything that resembled instant MPG feedback was sellable.

- LostCause

[dcb]

I'm guessing less than .1gpm at idle.

Anyway, I like the pressure switch idea now I got a Low Pressure Switch off of ebay. This setup is showing some promise, but my bike computer is freaking out with all the noise from the pump (resetting and not in a useful state). It is actually on the bike and mostly hooked up though, so that is extra promising

I can blow into the tube and make the computer count, so it is electrical mayhem. When I use my thumb to controll the pressure, the pump pumps up and shuts off fine, but the computer freaks.

Theory of operation:
The pressure drops as fuel is consumed,
the N/O part of the switch closes and the pump turns on,
the N/C part of the switch opens, the computer does nothing

the pressure builds back up,
the N/O part of the switch opens and the pump turns off,
the N/C part of the switch closes and the computer registers a tick

(just guessing) the pump flow rate is probably 100 times the max consumption rate of the engine, so any variance in fill volumes between low consumption times and high consumption times will be minimal.

The number of pump cycles will be porportional to the fuel consumed (fingers crossed).

Note, there are no electronics at all currently, excepting the bike computer.




more reading:
a foot of 3/8 tubing helped installation. Store only had 3/8 tees and I added a 3/8 check valve just downstream of the pump because it wasn't sealing reliably (I don't think it liked the water experiments , what with the black stuff coming out). The rest is 5/16, occasionally jammed onto a 3/8 plug, plus a few 5/16 nylon tube joiners. Attaching the pressure switch cheaply was accomodated by grinding off the threads and shoving it into some 3/8 tubing.

Maybe I just go to sciplus and get a few mechanical counters w/resets(one for current and one for tank tracking), and put this computer back on a bicycle

[dcb]

Messed around with it a little more this morning, I think I'm close. Here is a diagram of the setup(check valve omitted, add in if pump wont hold pressure):


On the bike it is giving just a real quick pulse, like maybe 2 strokes, and when idling it is another 1&1/2 minutes till the next pulse. It's looking good, but still have to get a working counter. So, guessing here, if 90 seconds between pulses = .1gph, then 3 gph would be 3 seconds between pulses?

But since this is a pressure system, it also holds promise for automobiles.

[diesel_john]

the VW burns 1 GPH, just a relative number.

Still working on the flow meter, I am using a glass bead.

What happens when the trapped air is is eliminated.

So the bike meter just needs the two pickup wires connected together to record a cycle.

If the pump tries to get two thumps in one cycle, would limiting the voltage to the pump slow it down. That's what I couldn't get it to do, singulate the thumps.

[dcb]

Cool! You got a reading! So did you go with an aluminum ball? Is it already calibrated?

re trapped air, don't know. I left it there as an accumulator so the pump and switch don't have to cycle so much. I will probably try adjusting the switch pressure down to get more pump cycles, and thus more data samples, but not so much that i'm clacking the pressure switch at 60HZ I'm not convinced that it matters how many strokes there are in a pump cycle though. Except if you are trying to get close to real-time data.

Bike meter, yup, touching the leads together works. Leave a pigtail on the reed switch and hang on to the magnet, they could be useful yet.

Oh, I picked these up for a couple bucks a piece. But unfortunately they are 120 volt (6 watts, 1.6kohm coil) and I have to rewind the coils for 12 volt operation, still figuring that out. But I like 'em and think they would make an interesting addition to the bike, one for tank trip and one for current trip fuel consumption They would just hook up in parallel with the pump and count the pump cycles.

[dcb]

Sadly, the extra pressure, low though it may be, is causing my old carbs to leak. I had to yank it all off ( excepting the sight gauge ). Cei la vie