I have basic maths programming skills, but little on the arduino. So here goes. Basically i fly radio controlled modelled aircraft, but next aircraft is a jet turbine model. One thing that would be useful is how to measure the fuel left in the tank. The engine is powered by an electric fuel pump, so could you integrate the voltage drop over the fuel pump, (or its current draw) with respect to time, which would give you a total fuel used, then as long as you know how much you had in the first place, hey presto you have a valid reading. Sound plausible??
Does the pump actually work harder at some times than at others? Or is the fuel used more simply related to how long the pump has been running?
Or, more likely, excess fuel simply gets routed back to the tank, and there is no relationship between pump on time, current draw, etc. and fuel consumed.
id guess the relationship is directly proportional, the speed and thrust of the engine is controlled solely by the amount of fuel pumped from the tank to the turbine. so when a higher voltage is applied to the fuel pump, (just a simple electric motor) more fuel is delivered to the engine, and it creates more thrust.
I'm not sure if this applies, but I know when people try to measure fuel consumption of a car with a gasoline engine, they calculate this by putting a known, low amount of fuel in the tank and counting the number of times the injectors fire until its empty. That gives them the amount of fuel used per injector pulse. A jet turbine engine must have some form of injectors, right? It can't just rely on pressure from the fuel pump can it?
yeah the engine is entirely controlled by the rate at which fuel is pumped into the engine. if the fuel pump shuts down, no fuel is pumped in, the engine doesnt 'suck' its own fuel, so it would flame out and stop running. there are no injectors, the fuel is merely pumped to the centre of the engine where the fuel is squirted directly into the hottest part, burnt and exits as exhaust gasses.
hall sensor is for magnetic fields right? sorry stupid question, but how would that measure fuel flow rate?
tuxduino:
Here comes the stupid question... a hall sensor on the fuel path ?
Nothing stupid about it, there are flow meters that work on that principle. Though I have no idea if the specific flow meter I linked to is the right size, or will tolerate whatever fuel is being used. Still, there are probably commerically available flow meters that are perfect for this application, the real question will be if they are affordable on a low volume basis.
hall sensor is for magnetic fields right? sorry stupid question, but how would that measure fuel flow rate?
There are flow sensors based on a rotating element and a hall sensor. The uP counts the pulses. The sensor datasheet states the volume of liquid per pulse, so the total measured volume is easily calculated.
im a bit hesitant about putting things in the fuel system itself. but wouldnt the voltage across the fuel pump be directly proportional to the fuel flow rate. higher voltage means faster fuel pump means pumps more fuel? just calibrate it first, that would work wouldn't it? or maybe current draw. then integrate the values against time to get total fuel been pumped since it started.
jaddion82052:
im a bit hesitant about putting things in the fuel system itself. but wouldnt the voltage across the fuel pump be directly proportional to the fuel flow rate.
Not if it's like the one I linked to, that one has a digital output for use by a microcontroller or other logic circuitry (so there's a little more than just a hall sensor). Basically every time the magnet on the pinwheel goes past the hall sensor on the outside of the tube there is a short pulse on the digital output, which can be counted and used to determine the flow rate.
jaddion82052:
One thing that would be useful is how to measure the fuel left in the tank. The engine is powered by an electric fuel pump, so could you integrate the voltage drop over the fuel pump, (or its current draw) with respect to time, which would give you a total fuel used, then as long as you know how much you had in the first place, hey presto you have a valid reading. Sound plausible??
I guess, although it's very indirect and may not give you a very accurate indication of actual fuel consumption given that the voltage, current, pressure and flow rate will all be varying and have non-linear relationships.
Might it be easier to measure the fuel pressure at the injector? I guess you could map out the relationship between pressure and flow rate easily enough and it ought to be fairly consistent as long as the fuel temperature is steady. If you're heating the fuel up all bets are off and you'd need to come up with a better way to measure actual fuel flow.
Ok so say if i use a micro flow meter, something that can measure between 50-300ml/min. Whats the next step in programming. Setting up a vector of time and value, then using trapezium rule or rectangle rule to integrate?
There are several small fuel flow sensors around, although most inexpensive ones I've seen are for water use and have standard 1/2" (12.7mm) fittings. I am sure these will work though, but you would have to check that it is suitable for Jet fuel.
As for the other issue about measuring the fuel quantity, I have been doing several experiments with capacitive sensors, and the real simple way to see how much fuel you have is to place a series of parallel conductive strips down the sides of the tank, (front, back and sides to get an average when not in level flight) and measure the capacitance of the strips. I used self-adhesive aluminium tape. This changes quite a lot from full to empty, but they are still in the region of only a few nanofarrads. A lot of processors have CapTouch interfaces these days and this makes it easy get readings. I haven't yet seen anything on CapTouch on the Arduino processors, but you could use a 3rd party micro to do this.
My first experiments involved using a LM555N timer IC and the capacitive fuel sensor as the timing capacitor in the circuit. I had to use the right resistor and capacitor values to get a reasonable range in frequency. Then you simply use one of the frequency counter libraries available for the Arduino and calibrate the readings. You will have to use the normal fuel you use to calibrate, as using water or some other fuel gives different readings. The dielectric constant of Jet-A1 is different to other liquids, like methanol (for piston powered models) or water.