Sources of Error in Fuel Flow and Fuel Volume Measurements

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jimh
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Sources of Error in Fuel Flow and Fuel Volume Measurements

Postby jimh » Thu Jan 17, 2019 9:01 am

Some users of Evinrude E-TEC engines that are attempting to monitor the engine fuel flow rate or to calculate a volume of fuel used have configured their multi-function display device so that its FUEL MANAGER function should use the FUEL FLOW RATE data in the NMEA-2000 data (being sent by the engine the device is configured to monitor) as the method to deduce or calculate the volume of fuel consumed by the engine, and there are reports that in some instances these values of fuel volume are in error. We explore the possible sources for error in these measurements.

On an EVINRUDE E-TEC engine there is nothing at the engine to configure regarding the sending of the FUEL FLOW RATE data. The E-TEC engine is hard-coded to send the FUEL FLOW RATE data in the standard NMEA-2000 parameter called parameter group number or PGN 128489. The engine just sends this data to the NMEA-2000 network. There is no method available to the user to alter or adjust or tinker with this data; the engine just sends it. Also note that there is nothing in the E-TEC engine that tracks or measures a fuel volume used over time. The only data about fuel available from the engine is the instantaneous fuel flow rate.

We presume a multi-function device that is displaying some data about fuel volumes has some sort of function built into it which is generally called a FUEL MANAGER. Exactly what a particular multi-function device will do with the data it gets from an E-TEC engine via PGN 128489 is completely up to that device and how that device has been configured to work. There is no guarantee that a device will come out-of-the-box configure in the precise manner that a particular user desires. For this reason, the user should become familiar with the device and its configuration. Generally the best method to become familiar with a device is to read the user guide carefully and for comprehension.

A well-designed FUEL MANAGER function using engine FUEL FLOW RATE data will be able to accurately integrate the instantaneous FUEL FLOW RATE data over time in order to develop a volume of fuel used, and this volume calculation is usually called FUEL USED. This process is accomplished by periodically getting (from the NMEA data stream) the value for FUEL FLOW RATE at a certain interval of time. For example, if the actual engine fuel flow rate is steady at a rate of 0.05-gallons-per-hour, and if the FUEL MANAGER is programmed by its designer to sample the fuel flow rate every 6-seconds, then every 6-seconds (i.e. ten times per minute) the FUEL MANAGER will compute the volume of fuel consumed, as follows:

0.05-gallons/1-hour x 6-seconds x 1-hour/3600-seconds = 0.000083333333333-gallons.

The FUEL MANAGER then adds that amount to the stored value of FUEL USED, and the process repeats every 6-seconds. In one hour, assuming the flow rate remained steady, the FUEL MANAGER would add 0.000083333333333-gallons to the FUEL USED value a total of 600-times, and the value of FUEL USED would be increased by 0.000083333333333 x 600 = 0.0499999999998, which is exactly what the engine was reporting, 0.05-gallons-per-hour.

This method can be subject to some variation. For example, we know the fuel flow rate is not going to be completely steady over time, so the frequency at which the FUEL MANAGER samples the instantaneous flow rate and then computes the FUEL USED can affect the accuracy of the calculated volume over time. The more frequently the FUEL MANAGER samples the FUEL FLOW RATE and computes an incremental FUEL USED volume, the more accurate the calculation will be. Also, the volume of fuel being reported as the instantaneous fuel flow rate is often very small, and if the FUEL MANAGER function is not able to keep an accurate track of these very small volume numbers, there could be some rounding or arithmetic errors.

Another variation is due to bad data from the data's source. It is generally assumed that in the case of an E-TEC engine there is no actual physical measurement of the fuel flow rate, that is, there is not a sensor in the fuel line trying to detect and measure fuel flow. The fuel flow rate data comes from the stored algorithms of the E-TEC engine's fuel map that tell the engine how much fuel to use at various combinations of throttle, load, engine speed, temperatures, and barometric pressure. If there is a malfunction in the engine fuel system, for example, a fuel injector that is working improperly and uses either too much fuel or too little fuel, this variation in fuel flow rate won't be reported.

Another source of error can be from how the engine reports fuel flow rate when the engine is not running but the engine has power. Some owners of the E-TEC engine have wired their boats so that the power for accessories (like a multi-function display that has a FUEL MANAGER) comes from the engine ignition switch ACCY circuit, and they tend to leave their E-TEC engine with the ignition key in the RUN position while the engine is not actually in operation. Exactly what the engine will send as the FUEL FLOW RATE data in its PGN 127489 data could cause some FUEL MANAGERS to become confused. If the engine sends a FUEL FLOW RATE of zero, and if the FUEL MANAGER has an algorithm for performing the integration of rate to volume over time that has trouble handling the calculations when the fuel flow rate value is zero, the result can be an error in the amount of fuel used that is calculated. Problems of this type were reported to have occurred with the infamous DATA STORAGE MODULE aka MEMORY MODULE aka FUEL DATA MANAGER MODULE from Lowrance, and several firmware revisions were offered to remedy the problem.

With regard to computation of tank levels with the indirect method, the greatest source of error is probably lack of diligence by the operator to correctly inform the FUEL MANAGER when fuel is added to the tank. Inputting the wrong volume or failing to input any volume data will cause the tank level calculation to be in error. If the device is configured so its FUEL MANAGER is to track a fuel tank level using the INDIRECT METHOD, you will have to be very diligent about providing input data to the FUEL MANAGER so that it knows when you have added fuel to the fuel tank you have configured the device to track.

In most implementations, a FUEL MANAGER allows the user to input a volume of fuel added to the fuel tank. If the user makes an error and inputs the wrong value, there are two possible situations: the input value was more than the actual volume of fuel added; or the input value was less than the actual volume of fuel added. If the user inputs a value that is more than actual, there is no remedy for this because as a general rule the FUEL MANAGER only allows the user to ADD fuel to the tank, not subtract it. The workaround is to make a note of the error, and to correct the error the next time fuel is added to the tank by inputting an intentionally wrong value, that is, a value that is LESS than actual by the amount of the prior error. For the other case, when the user input a value that is less than actual, there is a simple remedy: just immediately repeat the procedure and enter a correcting value that compensates for the understated value entered earlier.

For more about the INDIRECT method of monitoring the level of fuel in a fuel tank, see this article:

http://continuouswave.com/whaler/refere ... Level.html