LeSving wrote:
A fuel totalizer is pretty redundant and useless on modern engines with EFI and/or with a return line. Fuel is “measured” or meetered directly by the injectors, very precisely, and logged as part of all other engine parameters.
Fuel metering on an EFI engine is done by controlling the time the injectors are open, not fuel flow directly. There is no positive displacement pump in the system. Fuel pressure, which is not particularly accurately controlled, and any number of other variables affect fuel flow with a given injector pulse time. That is one reason why exhaust oxygen is measured on some engines, to reset the maps used for injector pulse time over a relatively long period of sampling and correct the fuel flow when it is not being directly metered.
On any injection system where the fuel circulates, flow can be measured on either side of the engine and fuel consumption can be calculated from the difference. This derived measurement is less accurate than measuring 100% of the flow into the engine on non-circulating system, and is not typically used for engine fuel control – it is for information, like any totalizer.
Exactly; none of these systems measure fuel flow accurately.
IIRC, the “mpg” or “l/km” readout on modern cars is implemented by indirect measurement of fuel flow, by reading the pulse rate of a solenoid operated fuel pump, or something like that. Certainly that is how some retrofit meters used to work.
Only a turbine sensor in the fuel pipe will give you an accurate enough reading to be able to fly safely to say 10-20% of the fuel tank capacity. I’d want a totaliser to be accurate to around 1%.
There is no positive displacement pump in the system.
Aren’t all electric FI fuel pumps positive displacement nowadays?
No
For Night or IFR the LAA has specifications about instruments, and also other items. This includes GPS models.
Silvaire wrote:
That is one reason why exhaust oxygen is measured on some engines
Yes, but not on any factory produced EFI equipped aircraft engine. Maybe perhaps the EFI Lycoming? not sure. ULPower and Rotax (iS), which is about 99+ % of all EFI equipped engines today, the map is fixed, but adjusted for air temperature and air pressure. AVGAS (lead) will destroy O2 censors I have heard.
When I had an engine course at the ULPower factory last year in Belgium, this was one of the things I learned; a fuel flow meter is redundant and inaccurate compared with the injectors. If one injector is broken, that’s another thing, but then you will have much bigger worries than measuring the fuel flow. Also, the bypass ratio is pretty high, I think more than 10:1 or more (much more at idle than full power), you don’t want to be too reliant on the pressure reduction valve.
From a measurement point of view, even if you have 1% accuracy on your turbine meters, the overall accuracy when you need two of them and subtract two large flows to get the difference which is less than 1/10 of the scale of the meters, the overall accuracy will only be in the order of 10-20%. It’s a dead end solution. The injectors are better than 1%.
T28 wrote:
Aren’t all electric FI fuel pumps positive displacement nowadays?
More like axial compressors actually. The idea is to get a constant pressure at the injectors independent of flow. A pressure regulating valve keeps the pressure. But who knows how it is on modern car engines with direct fuel injection in the cylinders. I would imagine a positive displacement pump necessary for that, like diesel engines.
alioth wrote:
Many of us are still puttering around with an O-320-B3B though
Yes, some live in the past 
My impression is that this “red cube” flow transmitter is pretty standard.
Also, the bypass ratio is pretty high, I think more than 10:1 or more
In that case a flowmeter is not going to work. Are these engines really returning 90% to the fuel tank?
The red cube flow transmitter is just a Floscan 201 turbine flowmeter, boxed so people don’t recognise a sub-$100 part made for the boating business 
There is probably a “wide spread of appreciation” of fuel totalisers within GA. If one does just 1-2hr flights then it is just a toy, and indeed there is no evidence that any non-N-reg TB20 GTs which would all have suffered from this error ever noticed, let alone got it fixed. In that short-flight sector of GA, you just work on “a range of 4hrs in the POH, so if I fly for 3hrs I have 1hr left in the tanks” and mostly it is good enough. But on longer flights, the entire strategy re diversions etc is driven by the forecast LFOB (landing fuel on board) so an accurate fuel meter is a must.
It is just a thought. Other potential DIY standalone installations might be things like engine monitors, which most manufacturers don’t seem to do well. For example most of them require a lot of man-hours to wire up the probes.
Peter wrote:
Are these engines really returning 90% to the fuel tank?
Not sure about other engines, but on a LOM Praha M337, I’d say it’s roughly 60-70% into the cylinders and 30-40% back to the tank. This is easy to estimate: on Zlin 42/43 series, fuel source is managed by the selector valve, but the excess fuel after the pump is always returned to the left tank. So when you fly on the right tank, the left one is refilled, but noticeably slower.
On the other hand, it’s always possible to install another flow sensor on the return line and subtract its readings from the main one.
Yes, and that is done, but if one was returning so much to the tank, the accuracy would be poor because the net flow would be the difference between two large numbers. Flow totalisers are only just accurate enough; a loss of a factor of 10 on accuracy would render them useless. One would have to develop much more accurate sensors, and probably make a correction for temperature.
For EFI this is a design choice. Or at least that what I gather. The injectors needs a constant pressure to inject the right amount of fuel. The pump(s) are constant rpm vane pumps with a certain characteristic (pressure decreases as flow increases, in a non linear manner). Then there is a pressure control valve keeping the pressure constant. The higher the bypass ratio, the less you need to rely on the pressure control valve. The downside is larger pumps. I guess with an O2 sensor, the injectors could be adjusted continuously independent of fluctuations in pressure, it would be a “better” design also by other measures, but then you rely 100% on an O2 sensor.
High by pass ratio is therefore a simple, but more robust way of doing it.
