Fuel consumption overestimated by Garmin

[max from alaska]

I have a 2014 R27 with a since replaced chart plotter. This year it seems to overestimate fuel consumption by around 20%. For example last trip it said Fuel Remaining = 41.6 gal. When I filled up the actual fuel remaining was 51.2 gal. Thus the Garmin overestimated fuel consumption by about 11 gal on a half tank or 21%. Need help on how to calibrate or fix the problem. I did do a lot of idling while fishing. Didn’t use the Webasto much. Otherwise I cruised at 3050 RPM with 5 minutes at WOT

 

[Cutwater28GG]

on my boat, which has an inboard Volvo, I find the Garmin is more inaccurate when idling.

as I understand it, it is actually taking fuel flow from the engine and not estimating. the challenge is at low RPM's there is a larger margin for error when 0.3gal is flowing vs at cruise when 10+ gal is flowing.

 

[Chimo]

on my boat, which has an inboard Volvo, I find the Garmin is more inaccurate when idling.

as I understand it, it is actually taking fuel flow from the engine and not estimating. the challenge is at low RPM's there is a larger margin for error when 0.3gal is flowing vs at cruise when 10+ gal is flowing.

I may be totally wrong here and I'm sure someone more knowledgeable will correct me if that's the case.

I believe you are correct that the Garmin is taking fuel flow information from the engine ECM and not measuring it directly. In the case of a gasoline engine the ECM is actually able to measure the amount of fuel supplied for combustion with a high level of accuracy. For a diesel engine the fuel pump oversupplies fuel. The ECM decides how much is needed based on a number of parameters including throttle position, temperature, etc. Unused fuel is returned to the tank. The ECM does not measure directly how much fuel is used but rather incorporates an algorithm to estimate this from a standard set of curves generated on a factory dynamometer. The result is surprisingly accurate [in my opinion]. To get a more accurate number perhaps one could install two flowmeters, one on supply and one on return, and take the difference. Since a flowmeter accuracy could be +/- 2% depending on quality [they use really good ones with space vehicles 😉 ] the result may not be much better than the estimate method.

Running at a cruising speed the parameters used by the ECM are much more predictable than they are at idle speed. Once it stabilizes, operating temperature is reasonably constant for example, as is load. When idling both of these are more variable. The hysteresis effect of the thermostat could well be more significant in the PNW than in warm Florida waters. The ECM will take all its inputs into account and do the task that has been assigned to it, maintain engine speed at 700 rpm. It will deliver the fuel necessary to achieve that. If you spend a lot of time idling while fishing then you spend that time in, for the ECM, a more unpredictable operating range. It may well then be providing less accurate fuel flow data since it knows only what was measured at that RPM on a dynamometer.

As I said at the beginning, I may be totally wrong about this.

Chimo 🙂

 

[Todd Tugnut]

Is there a way to get a manual fuel level reading on a R31?

 

[Nwdiver]

A new update for the garmin MFD came out today, hopefully it has the fix for the gas issues

 

[Submariner]

A new update for the garmin MFD came out today, hopefully it has the fix for the gas issues

I’ll know tomorrow. Upgrading the software tonight, headed to Canada tomorrow.


Sent from my iPhone using Tapatalk Pro

 

[fidalgoisland]

A new update for the garmin MFD came out today, hopefully it has the fix for the gas issues

I’ll know tomorrow. Upgrading the software tonight, headed to Canada tomorrow.


Sent from my iPhone using Tapatalk Pro

Hi Martin, Did you have a chance to test this to see if it fixed the problem? Thanks and I appreciate all your efforts!!

Dennis

 

[Submariner]

A new update for the garmin MFD came out today, hopefully it has the fix for the gas issues

I’ll know tomorrow. Upgrading the software tonight, headed to Canada tomorrow.


Sent from my iPhone using Tapatalk Pro

Hi Martin, Did you have a chance to test this to see if it fixed the problem? Thanks and I appreciate all your efforts!!

Dennis

Not fixed. I came back from Canada today with 0.0 gallons of fuel left. <sigh>
(I really have 42 gallons left.)


Sent from my iPhone using Tapatalk Pro

 

[Nwdiver]

ARRG

 

[gregc]

Finally, I can point out to my wife where my procrastination has actually saved us from something.

(Still need to do MFD updates. 🙂 )

 

[NorthernFocus]

on my boat, which has an inboard Volvo, I find the Garmin is more inaccurate when idling.

as I understand it, it is actually taking fuel flow from the engine and not estimating. the challenge is at low RPM's there is a larger margin for error when 0.3gal is flowing vs at cruise when 10+ gal is flowing.

I may be totally wrong here and I'm sure someone more knowledgeable will correct me if that's the case.

I believe you are correct that the Garmin is taking fuel flow information from the engine ECM and not measuring it directly. In the case of a gasoline engine the ECM is actually able to measure the amount of fuel supplied for combustion with a high level of accuracy. For a diesel engine the fuel pump oversupplies fuel. The ECM decides how much is needed based on a number of parameters including throttle position, temperature, etc. Unused fuel is returned to the tank. The ECM does not measure directly how much fuel is used but rather incorporates an algorithm to estimate this from a standard set of curves generated on a factory dynamometer. The result is surprisingly accurate [in my opinion]. To get a more accurate number perhaps one could install two flowmeters, one on supply and one on return, and take the difference. Since a flowmeter accuracy could be +/- 2% depending on quality [they use really good ones with space vehicles 😉 ] the result may not be much better than the estimate method.

Running at a cruising speed the parameters used by the ECM are much more predictable than they are at idle speed. Once it stabilizes, operating temperature is reasonably constant for example, as is load. When idling both of these are more variable. The hysteresis effect of the thermostat could well be more significant in the PNW than in warm Florida waters. The ECM will take all its inputs into account and do the task that has been assigned to it, maintain engine speed at 700 rpm. It will deliver the fuel necessary to achieve that. If you spend a lot of time idling while fishing then you spend that time in, for the ECM, a more unpredictable operating range. It may well then be providing less accurate fuel flow data since it knows only what was measured at that RPM on a dynamometer.

As I said at the beginning, I may be totally wrong about this.

Chimo 🙂

As speculated above the fuel consumption in common rail diesels is calculated rather than directly measured. The calculated number comes directly from the injection rate that the ECM is commanding. The ECM calculates an injection rate(mass flow) based on demand(i.e. throttle position and rpm). The ECM then takes data for fuel pressure/temperature and manifold pressure/temperature to calculate the volumetric flow needed to satisfy the desired mass flow. It then uses that volumetric flow rate and fuel rail pressure to calculate how long to hold the injector open in order to inject the appropriate amount of fuel. The volumetric flow/calculated injection rate is the number that is exported and totalized by Garmin or other display. Small errors are introduced by each of the sensors involved in the calculations. The lower the flow rate the more significant those errors become and the overall error is larger. Which is why the difference in indicated fuel consumed vs actual tends to be greater when a lot of time is spent at low rpm/idle.

BTW if the automobile that you drive displays instantaneous mpg it's using the same injection rate described above and speed/miles traveled to calculate mpg.