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Author Topic:   Fuel Flow Measurement Instrumentation
lhg posted 12-16-2005 05:33 PM ET (US)   Profile for lhg  
[Are fuel flow measurements performed in same way in Bombardier/Evinrude I-Command Gauges as they are in Mercury SmartCraft gauges?] On another thread, Evinrude fan Peter is saying this system is not accurate. What gives here, Peter? Good for E-TEC but not Smartcraft or Verado?
Peter posted 12-16-2005 06:48 PM ET (US)     Profile for Peter  Send Email to Peter     
Actually Larry, it was someone who has an Optimax with the SmartCraft gauges as well as a FloScan who said that the SmartCraft was reading low relative to the FloScan. Perhaps his SmartCraft system is not accurate. Perhaps is FloScan is not accurate but the poster seemed to put more faith in the FloScan readout than the SmartCraft readout.

Maybe one of the Optimax experts can tell us how the SmartCraft system actually measures fuel flow with the Optimax whether it too counts the injector pulses or simply performs a table lookup as the poster in the other forum suggested.

LHG posted 12-16-2005 07:45 PM ET (US)     Profile for LHG    
Peter - this is your post I was refering to:

"However, I'm curious to know on what basis the Mercury people think the SmartCraft fuel measurement system is more accurate than any other measurement system and even more curious why there appears to be in this particular case a much greater deviation in this particular test from the lows, highs and averages reported elsewhere than there is in the case of other manufacturers. Is there that much variability in the SmartCraft fuel flow measurement system? "

From what B& WB indicates about Smartcraft, and Seahorse here about E-tec, the fuel measurement systems appear similar, and one would expect, with same degree of accuracy.
Since Yamaha "Command Link" is also similar, I would think all three engines would similar, and comparable, fuel measurement. So I would think B&WB's comparative fuel economy statistics on the three engines would be valid.

Also, I noted in all these shootouts, 200, 225 and 250, the Optimax achieved the best economy, Yamaha in the middle, and E-tec at the bottom.

Peter posted 12-16-2005 08:57 PM ET (US)     Profile for Peter  Send Email to Peter     
Larry - Please explain how using the same or similar measurement system should lead one to expect the same degree of accuracy. I think I missed something along the way there. That's like saying that one should expect the same degree of accuracy from residential kitchen grade graduates and scales and laboratory grade graduates because they use the same measurement system. Try selling that one to the chemists of the world.
jimh posted 12-16-2005 09:16 PM ET (US)     Profile for jimh  Send Email to jimh     
I put more faith in a system which has a fuel flow sensor, like the paddle wheel fuel flow sensors in a FLO-SCAN or NAVMAN system. If fuel flow measurement is deduced from counting injector pulses, it is an indirect measurement. The actual fuel flow is deduced by observation of some other event. It is very possible for the indirect system to be quite consistent, that is, it will state the fuel flow with the same precision all the time, but it can also be inaccurate. In a direct system, the fuel flow is the cause of the sensor output. Gas flows, it spins the paddlewheel, and this is measured. A more direct measurement

I don't see any basis to make the giant leap of faith that because two systems measure fuel flow using similar indirect methods that they would be equally accurate. They might both be consistent in their measurement, but that could very likely be consistently inaccurate.

I don't find any reasonable basis to judge the SmartCraft or the I-Command to be absolutely accurate. Both need to be tested against direct measurement and confirmed.

jimh posted 12-16-2005 11:38 PM ET (US)     Profile for jimh  Send Email to jimh     
{Move this discussion from another thread.]
jimh posted 12-17-2005 12:26 AM ET (US)     Profile for jimh  Send Email to jimh     
Oops--here is a comment by seahorse to which an allusion is made above:

"The fuel flow system on the E-TEC gauges use the calculated flow from each injector pulse. Yes, the EMM "counts" every injector firing. It also re-calibrates each injector as it wears. It counts in multiples of 250,000 pulses.

I compared the EMM calculated fuel flow with a fuel flow meter and every time it was within 0.1 or 0.2 gallons per hour."

Seahorse has given us great information on the I-Command system from Bombardier. It sounds like it has the potential to be very accurate. Just to expand on this, the fuel injectors on an E-TEC engine are very precisely calibrated. Each injector has a measured coefficient which describes its characteristic fuel injection rate. This coefficient is stored in the engine management module, and it is used to calculate the precise electrical signal to send to each individual injector in order to control the fuel flow to each cylinder in a very precise manner. Because this fuel flow is related to injector pulses, total fuel flow rate can be deduced from total fuel injector pulse duration or counting.

I don't know where there is a source for definitive information about how the SmartCraft gauge fuel consumption measurements are made. It would be interesting to know how they do it. If (as apparently as been suggested elsewhere) the SmartCraft system simply deduces fuel flow from looking up stored values that correspond to some other parameter (perhaps engine RPM or throttle position sensor output), it may not be as accurate as the injector-pulse counting technique.

Also, an allusion is made above to some information in BASS & WALLEYE BOAT magazine about the SmartCraft fuel measurement techniques:

"From what B & WB indicates about Smartcraft.." (LHG)

In reading the article about the 200-HP outboard tests, I cannot find a single word on this topic. Are details about the measurement technique given elsewhere in that issue? If they explain how SmartCraft measures fuel and certify its accuracy, it would be good to get that information.

There is interest in how fuel consumption measurements are made because the price of fuel is becoming quite a factor in overall boating expense. In a comparative test it is vital that fuel consumption measurements be made in a uniform and accurate manner.

As I said before, there is no fun in a bake-off where Betty Crocker is allowed to judge her own cookies.

sosmerc posted 12-17-2005 12:45 AM ET (US)     Profile for sosmerc  Send Email to sosmerc     
The FloScan units that I am familiar with all have dip-switches on the back of the guage for adjusting the calibration of the unit. To me, this means that the system may not be that accurate right out of the box. You have to "calibrate" the instrument after having burned a specific amount of fuel. Once the calibration has been set for the specific engine and boat fuel should then be very accurate.
I do not know exactly how the SmartCraft guage determines gph or "fuel used", I can only guess that it is based on known flow rates of the injectors at various rpm and engine load factors. I would think it should be quite accurate and based upon many hours of test data. however, if one of the injectors is not performing as designed (partially plugged and not fully opening, or maybe not completely closing properly) the accuracy is going to go downhill. The engine ECM controls injector timing and duration based on a carefully designed "map". The ECM is constantly collecting data from sensors and then the ECM adjusts fuel flow and ignition timing based on the instructions engineers have programmed into the map. The ECM should know precisely how much fuel is being used at any time and under any conditions and thus fuel flow and fuel used is just another calculation that the ECM can output to the SmartCraft Guage. Manufacturing tolerances dictate that each injector probably flows within a certain percentage plus or minus of its target flow rate. So one would guess that there is going to be an expected percentage of error in actual flow rate and fuel usage.
I have "flow tested" my FloScan guages using an electric fuel pump to draw fuel 5 gallons of fuel through my fuel system. Using a stopwatch, I was able to test the GPH reading...and test the totalizer at the same time. AFTER calibration, the guage was very accurate in multiple re-tests.
I would be interested to hear how the FloScan compares to the SmartCraft readout on a boat that is equipped with both systems....especially after the FloScan has been calibrated.
erik selis posted 12-17-2005 08:06 AM ET (US)     Profile for erik selis  Send Email to erik selis     
The "opto-electronic" turbine flow meters that are used with the FLO-SCAN or NAVMAN systems have been around for decades. They are actually a low-cost flow measuring device mainly used for fluids. The accuracy of these flow meters are basically very low compared to other flow meters. Typically 10% of full scale. This means for a flow range of 3LPH to 150LPH you could have an error of 15LPH over the whole range(10% of 150). The calibration function of the FLO-SCAN or NAVMAN will compensate for this error, at least partially and if your boating habits are constant. Let's just presume we could bring down the error probability from the "opto-electronic" from 10% to 2%. This would still mean that the actual measurement of this device would have an error of 3LPH, dragging it along down to the minimum flow rate. Meaning you could be reading 6LPH instead of 3LPH.

Concerning the flow at the injector position: I have often wondered about the need for measuring the real-time flow of the fuel at the injector itself. Where does the EMM or ECM get its information from in order to know how much fuel it has to deliver in the first place? IMO, mainly from a very accurate, highly sensitive AMF meter (air mass flow meter). Together with the throttle position. If it knows exactly how much fuel the injectors have to deliver to the chamber can't it just deliver that same information to the fuel flow instrumentation? Just a thought.


swist posted 12-17-2005 08:49 AM ET (US)     Profile for swist  Send Email to swist     
Erik is right. Flo-Scan is not really high tech - has been around for years. I kinda doubt you would find the same system measuring fliud flow in a nuclear reactor or such.

And for a long time was the only game in town for recreational vessels so it's what all the testing people (Powerboat Reports etc) have always been using. SO there's a bit of an unsupported tendency to feel that because Flo-Scan said so, it must be right.

Chuck Tribolet posted 12-17-2005 09:02 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Eric, but remember that the automotive engine management
system incorporates feedback from the O2 sensors. It may
know that for X amount of air it needs Y amount of fuel, but
if the sensor say it's running rich, it will cut back Y a
a bit, compensating for any number of error sources.


Peter posted 12-17-2005 09:48 AM ET (US)     Profile for Peter  Send Email to Peter     
The O2 sensor just closes the loop and is another sensor input for the EMM. The EMM is the one that determines the rate of fuel to be injected at any given moment in time based on whatever sensor inputs it uses to make that determination from its stored fuel/timing map. Seems like it should be able to simply provide an estimated consumption read out, in parallel or nearly simultaneously, from wherever it currently is on its fuel/timing map which is the basis for injecting fuel.

What I think Seahorse is saying is that the injector's capacity to inject fuel each time it is cycled changes over time as it wears (these injectors are little fuel pumps) and the E-TEC's EMM takes that into account as part of its injector control function and recalibrates its instructions to compensate for the wear. For example, when the injector is new, let's say that the EMM issues instructions to cycle an injector 100 times to provide X volume of fuel at a given point in time to the cylinder. Some time later, after so many cumulative cycles of the injector, the injector wears a bit and now needs to be cycled 101 times to provide the same X volume of fuel. (On the contrary, perhaps it "breaks in" and only requires 99 cycles to provide X volume of fuel.) Based on knowledge of how the injectors wear and how many cycles they have cumulated, the EMM adjusts or recalibrates to provide instructions to cycle the injector 101 or 99 times as predetermined, for example, instead of 100 times based on the known wear to get back closer to the X volume of fuel it wants to deliver to the cylinder and that it used to be able to deliver by cycling the injector 100 times when it was new.

Adjusting for injector wear simply keeps the desired amount of fuel to be injected and the actual amount of fuel injected close. I don't think it really changes or impacts the ability to read off an estimated fuel consumption rate from the fuel/timing map.

jimh posted 12-17-2005 09:48 AM ET (US)     Profile for jimh  Send Email to jimh     
To get off the topic for a moment to answer the question:

" Where does the EMM or ECM get its information from in order to know how much fuel it has to deliver in the first place?"

There are two ways that the engine can operate: open loop and closed loop. Initially the engine supplies fuel according to a stored map or look-up table. This is the open loop mode. If the engine has been designed with an exhaust sensor, it can measure the exhaust gas mixture and get feedback on the stoichiometry of the fuel burn equation, usually deduced from an oxygen sensor. The engine can use this information to make adjustments to the fuel mix. Closed loop systems have been used in vehicle four-stroke engines since at least 1993. It is not clear to me if any of the current four-stroke outboard motors or the E-TEC use a closed loop system. That is really another topic in itself.

The turbine transducer in a FLO-SCAN or other fuel measurement gauge is a direct measurement of fuel flow. By this I mean that it is the flow of fuel itself which causes the sensor to react. The sensor is directly operated by the flow of fuel.

In contrast I described the EMM based fuel flow systems as indirect, because they impute the amount of fuel flow based on something else happening. Basically they measure how long the valve was open that controls fuel flow (the injector). If the fuel pressure and flow rate is constant and well known, then you can calculate the fuel consumption by adding up all of these times that the valve was open. It is just that this technique is an indirect measurement.

ASIDE: The increase in computing power in an engine or vehicle has allowed for many new features to be added based on analysis of existing data. The fuel flow computation is a good example. Another example in vehicle applications is the deduction of air tire pressure differences from analysis of wheel rotation speeds. There is a federal regulation that requires vehicle manufacturers to be able to monitor tire inflation pressure. The direct method would need special sensors installed in each wheel and coupled via RF to the monitoring system. The indirect method compares the rotation speed of the wheels on a long term basis. It deduces a tire is under inflated by the difference in the rotation speed. This method saves money because separate sensors were not needed. (The wheel rotation speed sensors were already in place for use in the anti-lock brake system.) Another example: a misfire can be deduced by careful analysis of instantaneous crankcase velocity, which is deduced from the crankcase position sensor data.

The question still to be answered: how does a SmartCraft gauge compute fuel consumption? Is there a flow sensor in the engine? Or is the fuel consumption computed by an indirect method.

seahorse posted 12-17-2005 09:52 AM ET (US)     Profile for seahorse  Send Email to seahorse     
>>>Concerning the flow at the injector position: I have often wondered about the need for measuring the real-time flow of the fuel at the injector itself. Where does the EMM or ECM get its information from in order to know how much fuel it has to deliver in the first place?<<<<<

Knowing exactly how much fuel is delivered to each cylinder is the E-TEC specialty and the heart of the system. Where other motors (Opti and HPDI) can only "guess" at the fuel delivered per the time the injector is open and depending on fuel rail pressure, the E-TEC injector and EMM are
"matched" with the calibration number assigned to each E-TEC injector. That is why each injector is "assigned" to a particular cylinder and can't be interchanged.

The EMM even compensates for wear during the millions of pulses each injector endures. The EMM knows precisely how much fuel is injected for each cylinder and the calculations on flow are very accurate.

That precision in fuel delivery is why the E-TEC works so well and is the lowest emission outboard at this time.

Peter posted 12-17-2005 09:58 AM ET (US)     Profile for Peter  Send Email to Peter     
Seahorse, was that not also the case for the Ficht system?
seahorse posted 12-17-2005 10:28 AM ET (US)     Profile for seahorse  Send Email to seahorse     
>>>>>Seahorse, was that not also the case for the Ficht system?<<<<<<

The FICHT system worked a lot the same way, but never had the accuracy and consistancy that the E-TEC system does. That's why they never acheived 3-star emission certification and, just like 4-strokes, the emission outputs increased as the hours accumulated.

Peter posted 12-17-2005 10:44 AM ET (US)     Profile for Peter  Send Email to Peter     
I realize that the solenoid like Ficht injectors are not as precise as the voice-coil like E-TEC injectors but I had thought that there might have been some injector wear compensation built into the Ficht control scheme.
sosmerc posted 12-17-2005 12:17 PM ET (US)     Profile for sosmerc  Send Email to sosmerc     
"The question still to be answered: how does a SmartCraft gauge compute fuel consumption? Is there a flow sensor in the engine? Or is the fuel consumption computed by an indirect method."

There is no fuel flow sensor, so it looks like SmartCraft lets the ECM perform a "calculation" I guess this is what you refer to as an "indirect method".

LHG posted 12-17-2005 01:04 PM ET (US)     Profile for LHG    
Since Smartcraft was introduced back in 2002 or 2003, and just now "I-Command" and "Command Link" are available for 2006 models, I wonder if it ever occurred to anybody that perhaps the "Command" family of digital instrumentation is copied and reverse engineered from Mercury? Sure looks like it to me. Even the gauges look similar. In that case, the process of measuring fuel flow could be presumed to have similar accuracy.

I have no idea whether any of this is more accurate than the Navman paddle wheel method, but I am now convinced that the Mercury Smartcraft system, being the innovator in this field and now obsolete compared to the newer "Command" technology used by all other brands, is being calibrated so that Mercury engines consistently record less fuel than actual consumption, allowing Optimax to win the "shootout" fuel economy categories. I am also convinced that the Evinrude system as defined by Seahorse is the least accurate measuring system, which is why the Evinrude, which is actually getting superior fuel economy to Mercury and Yamaha, recorded the highest fuel use.

Psst: that ENERTIA prop is likely to be no good either.

Peter posted 12-17-2005 01:44 PM ET (US)     Profile for Peter  Send Email to Peter     
Larry, your assumption that there is similar accuracy in fuel consumption "measurement" between the E-TEC and the Optimax would require the assumption that the fuel injection systems comprising the E-TEC system and the Orbital system that Mercury uses in the Optimax to be the same. I don't think they could be any more different.

That's a bit like saying that your Cadillac and a Porsche that both use fuel injected reciprocating piston engines driving rear wheels with a transmisson are similar and therefore it should be safe to assume that they would get similar performance. LOL.

By the way, no one on CW at any time during the discussion of the B&WB results ever suggested that the E-TEC was the economy winner or the lowest fuel consumer. The only question raised was about the gap between the E-TEC and Optimax and the Yamaha in the B&WB results, the gap between the Optimax results in the B&WB results and the several Optimax results published by Whaler and the gap between the B&WB results and the several Yamaha results published by Yamaha. The gap between published Evinrude test reports and the B&WB results were the smallest which sure does make it look like the Evinrude's fuel consumption montioring system, if that is what B&WB used in the test, to be the most repeatable.

sosmerc posted 12-17-2005 01:46 PM ET (US)     Profile for sosmerc  Send Email to sosmerc     
I have about 4 customers with new 3 cylinder Optimax engines and they all have been amazed at how fuel efficient these engines are. One of them uses a portable 6 gallon tank and he specifically commented on how accurate he felt the fuel totalizer was. I am sure all of the digital system currently employed will turn out to be more than satisfactory for the vast majority of boaters.
I recently installed a FloScan on my Ventura. My engine is an old '98 135 Optimax that is not compatible with SmartCraft, but I am assuming should have a flow rate very similar to what Merc and Whaler have published in their tests. It's been so cold out here in the Northwest that I haven't had a chance to take the boat out to calibrate my FloScan yet, but when I do I will be reporting my results. (I've got alot of new stuff on the boat to test...Bennett Sport M80 Tabs, Garmin 178C GPS/Map, dual head temp guages, water pressure guage, DDT dash hookup, new custom made kicker motor bracket and 2004 15hp ML 2 stroke Merc kicker). All I need now is some warmer weather!!
jimh posted 12-17-2005 02:50 PM ET (US)     Profile for jimh  Send Email to jimh     
Larry--To answer your question, it has never occurred to me that NMEA-2000 compliant systems like I-Command from Bombardier or Command Link from Yamaha (which I assume is NMEA-2000 or will soon be certified) were reversed engineered from SmartCraft. I-Command systems were engineered to be compliant with an open standard, the NMEA-2000 protocol. There would be no point in trying to reverse engineer them to work with SmartCraft because SmartCraft is a closed system.

If you have any evidence that computation of fuel consumption using the injector pulse counting and accrual method is something that was an invention of Mercury, could you please follow up with the patent number for us to read.

NMEA-2000 has been in development since 1994. Let me say that again. NMEA-2000 has been in development since 1994. Much of the underlying technology in NMEA-200) is based on CAN-BUS, and this is a much older standard which is very mature and widely used in real-time automotive data systems. It was really a very intelligent decision to piggy-back on CAN-BUS because there are already hundreds of devices available at very low cost to implement CAN-BUS. There is no way to know much about what SmartCraft is or how it works, because it is a closed system. I would bet they also used CAN-BUS technology.

As for the shape of the gauges, rectangular digital gauges have been very common in marine instrumentation for decades. We used to have some on our sailboat back in the 1980's. I don't think there is anyone who really thinks that Mercury invented the rectangular digital marine instrument in 2002.

We have quite good information provided about how the E-TEC engine computes fuel consumption, but so far no information at all about how the SmartCraft system works. It is hard for me to make the inference about one being a copy of the other when we don't really have any information about the SmartCraft system.

Once we finally discover how SmartCraft computes fuel flow, we can compare it to other techniques that are known.

To relate back to the initial discussion about fuel measurement techniques used by the boating magazine, the bottom line question is whether all the measurements were made using the same instruments. If a magazine presented speed results and used three different systems of measurement, there would be the same interest. For example, if one boat's speed was measured by a stop-watch, one by GPS, and one by a paddlewheel hooked to its own gauge system, I think people would be asking the same type of questions.

The real concern is did the fuel consumption data all come from the same measurement technique? I have not seen the details of the test technique used. I am curious if it was the same for all motors.

Whether SmartCraft fuel consumption is accurate is a separate question.

There is no doubt in my mind, or probably in anyone else's, that if the magazine used the I-Command data to measure the fuel consumption of the E-TEC and declared it the winner, there would be a similar inquiry.

I also think that there will be little reason for Mercury to modify their SmartCraft instruments to become compliant with NMEA-2000. It goes against their way of thinking. Mercury, through Brunswick, likes to think in vertical integration. They like to own everything in the boating business, from the boat builder, the engine maker, the dealer, the finance company, the bilge pump maker, and even perhaps the marina and slip where you keep the boat. They don't want to let anyone else in. So they probably will never make their system open. They probably looked at NMEA-2000 and intentionally decided not to participate in it so as to exclude other vendors from being able to compete with them for connecting to Mercury engines.

On the other hand, the following companies are producing and shipping NMEA-200 compliant devices:


--Offshore Systems

LHG posted 12-17-2005 03:20 PM ET (US)     Profile for LHG    
Jim - These are the Smartcraft gauges I meant. BW is using on them on most of their boats. It appears that everyone has now copied this idea and display mode configuration. Actually, Mercury does not make the square digital gauges like Yamaha is now offering under "Command Link". They only have a little square "System Montior" to supplement an analog gauge display, and the large GPS type display unit. Mercury brought Smartcraft and these round gauges out in 2000, not 2002 as I had previously indicated.

All Mercury's Smartcraft instruments are made by VDO, a highly respected instrument maker.

jimh posted 12-17-2005 04:03 PM ET (US)     Profile for jimh  Send Email to jimh     
Ah, the combination pointer and LCD gauge. Yes, those are similar. I have always liked VDO gauges, too. They had a particularly attractive style to them, a sort of "German engineering" look.
LHG posted 12-17-2005 04:11 PM ET (US)     Profile for LHG    
Yes, and the Verado is also "German engineered" to some degree, I believe!
Perry posted 12-17-2005 04:13 PM ET (US)     Profile for Perry  Send Email to Perry     
A Boating Industry Canada article states that the digital fuel mamagement system for 2005 and newer Honda outboards (135 HP and up) reads "More accurate than those of paddle-wheel fuel-line sensors used with other gauges."

I assume the system used by Honda is similar to the ones used by Yamaha and others outboard makers. Like jimh said earlier, a test using the traditional paddlewheel type with the [direct] method is probably needed to make any accuracy claims valid.

where2 posted 12-17-2005 10:33 PM ET (US)     Profile for where2  Send Email to where2     
I'll check with the former Pratt & Whitney technician down the street and find out what sort of cheezy little flow meters they used to use on aircraft engines. If it's cheezy enough for a fighter plane, maybe it's cheezy enough for your Whaler...

I seem to recall said technician had very little difficulty dialing in his Yamaha fuel management gauge (1997 2-Stroke engine), or his Floscan. With the floscan, he did have to re-orient the flow meter to face UP since trapping bubbles in the turbine path tends to mess up your readings. (the previous owner or installer apparently disregarded the note about pointing the flow sensor vertically!)

As to measuring injector pulse time of a fuel injector for fuel management purposes, the system works pretty well if your fuel pressure is consistent.

rtk posted 12-19-2005 10:59 PM ET (US)     Profile for rtk  Send Email to rtk     
I just received my service manual for my Smartcraft ready 2003 Mercury 250 EFI. It does not explain how the instantaneous fuel economy number is calculated. The Smartcraft operation manual does not either.

But the service manual does explain how the EFI system delivers fuel through the injectors. The ECM (engine control module) monitors the air temperature, manifold absolute pressure (MAP sensor), engine head temperature, and throttle position (TPS- throttle positioning sensor).

In the MAP sensor description it says "The MAP sensor is functioning through the full RPM range and is continually signaling induction manifold readings to the ECM. The ECM in turn determines fuel flow as signals are received."

I would think it is not a stretch to assume if the ECM makes the decision on how much fuel is to be delivered by the injectors, it can easily calculate the sum total of fuel sent, convert that to gallons per hour and send an instantaneous fuel consumption figure to a simple display.

As far as accuracy, I have not tested it against another metering device. I don't think you could really test instananeous consumption accurately by known gallons of fuel burned- you would have to run the boat at an absolute fixed speed uninterupted over a period of time, then accurately determine the exact amount of fuel consumed to be precise. You could certainly calculate an average using that method, but I think all test results we are talking about display instantaneous burn at a snapshot moment-rpm and speed at that instant.

A well calibrated paddle wheel system, which produces accurate and consistant readings I think is easier for us to accept as accurate because we actually saw the fuel level drop a certain amount after running for a while during the calibration process. Fuel level actually droped "x" amount, and the meter said it consumed that amount. A little wheel spinning is easier for us to accept than a computer matrix calculating very minute amounts of fuel dispersed from multiple injectors. It appears that you calibrate the paddle wheel system by getting the system to accurately display a fixed amount of fuel gallons consumed over a period of time. If it will accurately display fuel consumption over a period of time, it is safe to assume that the instantaneous reading is also accurate. That is a bit of a leap of faith too I think.

My vehicle displays miles per gallon. It is wrong. It is not instantaneous, but it does seem to give a progressive miles per gallon over a tank full or even longer period. If you divide the displayed miles per gallon by 1.12 after it calculates the figure over a tank of fuel, the result is consistant with the tried and true fill the tank, reset the trip odometer to zero, run 300 miles, fill the tank, calculate miles per gallon. 320 miles, 20 gallons used, week after week tells me I get 16 miles per gallon. Computer says 18 miles per gallon. This is a recent Toyota 4Runner, design is by a Lexus Engineer!

I think it is logical to build some error into all methods used to calculate fuel consumption, be it instantaneous or averaged. Five to ten percent, in my opinion would not be unreasonable when using the data. Any study of data always has some degree of variation built in to take into consideration variations of all sorts in test conditions. Especially when testing boats, there is just too much variation built in from weather, sea conditions, salinity, horsepower variations of motors, hull differences.


Perry posted 12-20-2005 12:18 AM ET (US)     Profile for Perry  Send Email to Perry     
Something else to consider when figuring out the accuracy of a fuel flow meter's MPG reading is how the unit determines boat speed. Is a unit that gets its speed reading from water pressure (Smartcraft etc.) less accurate than a paddlewheel design (Navman etc.) that gets its speed reading from a GPS?
oysterman posted 12-20-2005 01:24 AM ET (US)     Profile for oysterman  Send Email to oysterman     
Would you happen to work at Sande's? I see you are from Belfair. Just wondering- I purchased our Optimax from them this last summer.
Bthom posted 12-20-2005 11:03 PM ET (US)     Profile for Bthom  Send Email to Bthom     
In the field of process control, positive displacement type flowmeters(which a fuel injector could be considered to be)have much better accuracy specifications than a turbine/paddlewheel type of flowmeter.

I would bet that any of the digitally calculated flowrate measurements would be inherently more accurate than any of the paddlewheel types,and any calibration you do is only going to be as good as your measuring equipment.

The only accurate way is to have a graduated volume of fuel that you can switch into and out of the fuel system at various speeds and accurately measure distance travelled.
The speed side is relatively easy with today's GPS units but
you'll need a pretty specialized fuel system installation to get better accuracy results than any given manufacturers engineered system.

Just my 2cents,

sosmerc posted 12-21-2005 12:19 AM ET (US)     Profile for sosmerc  Send Email to sosmerc     
Oysterman.....I worked there for about 10 years, but have been operating my own outboard service shop on the South Shore since 1990. I am a Mercury outboard specialist and have been at it since 1979. I do mostly service, but I do sell new engines from time to time through a relationship I have with a large Oregon dealership. I have a number of Whaler customers.....and finally, just this year, I too became a proud Whaler owner.....and I still have my "go fast" boat as well. Stop by and visit some ahead though, as I'm not always here. 360-275-4690
jimh posted 12-21-2005 01:00 AM ET (US)     Profile for jimh  Send Email to jimh     
rtk--I am surprised that your automotive fuel measurement system exaggerates the vehicle's range. That could leave you stranded. I would have assumed the engineers would have tried to err on the conservative side, so you'd be pleased to arrive with more fuel in the tank than you thought.

Perry--that is a very good point you made. The accuracy of the speed data of course would directly affect the accuracy of the computed miles-per-gallon data!

I would assume that there must be some setting in the SmartCraft instruments which allows one to pick the source of the speed input. A GPS would probably be more accurate than a paddlewheel. In my NAVMAN 3100 Fuel instrument, you can select the data source for speed information to be either a paddlewheel or GPS.

Even among GPS data, however, there may be some difference in accuracy depending on the Horizontal Dilution of Position (HDOP) of the GPS measurement. Units with Differential GPS or WAAS capability would be more accurate. And I would not be surprised to learn that there might be some variation among units in the accuracy at which they deduced the speed from comparison of successive position plots.

One minor point: GPS speed data includes the effect of current, as it measures speed over bottom. A paddlewheel will measure speed through the water. When operating in water with a current, there will be a big difference.

Bthom--good information on the comparative accuracy of the displacement pump versus the turbine blade type of measurement. I would tend to accept that as true. I don't think the issue has ever been that the injector pulse counting and accumulation method of measuring the fuel is not accurate.

The fundamental issue that really began this discussion was the lack of information on what sort of technique was being used in the SmartCraft system on Mercury motors. I think we are still cloudy on that.

It seems a bit ironic, but perhaps in this day and age of high technology, the most acceptable measurement of speed and fuel consumption (from which we could compute MPG) might be to use a stop watch to time the boat over a measured course, and then to actually measure the amount of fuel used from a calibrated auxiliary tank.

Chuck Tribolet posted 12-21-2005 02:39 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     

First, we don't care about GPS speed in calculating MPG,
just distance, measured as successive positions. And unless
we are doing some really convoluted course, the total
distance will be VERY precise. Assume GPS position is
accurate to 45. The distance will be accurate to a MAX of
90' over many miles. Any errors cancel each other out,
and don't add up. If one segment is short, the next will be
long, zero net error.

Second, even if we did care about speed, the same thing would
be true: any errors would cancel each other out. If one
segment is fast, the next will be slow, zero net error.

Finally: My '87 Vette has an MPG meter. I don't know whether
it has a sensor in the fuel line or counts injector pulses,
but it's spot on vs. odometer, gas pump, and pocket calculator.


Perry posted 12-21-2005 03:07 AM ET (US)     Profile for Perry  Send Email to Perry     
When the magazines compare several outboard's fuel flow in MPG then declare a winner, I think it does make a difference how speed is measured because any error in speed measurement will throw off any MPG figures. Is it not true that a GPS is more accurate in measuring speed than a paddle wheel or pitot tube? Wouldn't speed measured from a GPS give a more accurate MPG figure?

I wonder how B&WB measures MPG when doing their tests; with the factory equipped fuel flow meters from Mercury, Yamaha, Evinrude etc?

tombro posted 12-21-2005 07:49 AM ET (US)     Profile for tombro  Send Email to tombro     
I did a very informal comparison last October with my Verado 225/210 Ventura. It was a 60 or 70 mile trip mostly in the ocean.

Old school method--Started with a full tank, and filled up at the conclusion of the trip. Checked my GPS trip log (all the trip was under power, either cruise or trolling-no drifting)and divided by gallons burned==2.7 nmg.

Smartcraft==2.6 nmg.

Isn't there some velocity made good reading on the GPS, showing your actual speed over the surface? That would be a better gauge of miles. Maybe I should just pick a windless day and make a measured run on flat water at slack tide to better compare?

seahorse posted 12-21-2005 08:02 AM ET (US)     Profile for seahorse  Send Email to seahorse     
For fuel comsumption and milage tests the magazines should compare motors at the same speeds, not rpms, due to prop designs and gear ratios. For example, use common speeds depending on the type of boat, such as 30 mph, 40mph, etc. That is a lot better than one motor running 4000 rpm and getting 33.2 mph and X gph, and another motor running 4000 rpm and getting 30.8 mph and X gpn.

Another item that should be published is the ICOMIA fuel milage in average gallons per hour. A motor is run at the same duty cycle as the emission tests and the result would be the fuel comsumption for an average boater. I don't have all the "test points" handy at the moment, but it is 6% at full throttle, 14% at 80% throttle, on down to 40% at idle. This is the same standard that all outboard manufacturers use to compute fuel usage.

The auto makers use a standard procedure also to compute the fuel milage that you see on the window sticker of a new car. Every car company uses that standard.

The only problem with outboards is that many "flo-scan" type units cannot accurately read flows less than 1 gallon per hour. Some E-TECs idle at 0.2 gph and many 4 strokes at 0,5 gph. You would need almost lab grade instruments to be accurate.

Peter posted 12-21-2005 08:41 AM ET (US)     Profile for Peter  Send Email to Peter     
I agree. Fuel economy at certain common boat speeds is relevant. Fuel economy at certain common engine speeds is not.

With respect to the low flow rate accuracy of paddle wheel sensors, my Fichts idle around 0.3 GPH as I understand from Evinrude test reports and the Navman 3100 I have shows 0.3 GPH at idle for each motor.

In two of my cars, I have trip computers that display averge MPG, near realtime MPG and range. Both read optimistically by 1 to 2 MPG.

jimh posted 12-21-2005 09:45 AM ET (US)     Profile for jimh  Send Email to jimh     
Chuck--I beg to differ. I think the fuel devices compute a rate of fuel flow and compare it with a boat speed. Check on the NMEA sentence being supplied. I'll have to go look this up in the documentation for my GPS and my fuel flow computer.

As far as know, a GPS does not transmit "distance" data. It can transmit position and speed data. However, if the GPS just supplied the fuel computer with the position, the fuel computer would have to retain the position data in memory and calculate the distance. I think it works on a much simpler basis. It gets the current speed from the GPS (miles-per-hour), and it also gets the current fuel consumption rate (gallons-per-hour) from its own measurement of flow. From these two it computers the miles-per-gallon.

Your hypothesis would require that the fuel computer perform the flowing:

--get a position fix (1) from GPS and remember it;
--note the time of this fix
--start measuring the amount of fuel consumed
--get a new position fix (2) from the GPS and remember it
--stop measuring the amount of fuel consumed
--calculate the distance between positions 1 and 2 (which would require the fuel computer to have a great deal of knowledge of spherical trigonometry it otherwise would never need to know)
--divide this distance by the amount of fuel used in the same interval

I really do not think it works like that. I think it works like this:

--measure fuel flow over predefined intervals and extrapolate this to a flow rate in gallons per hour (A)
--accept data from the GPS about vessel speed in miles per hour (B)
--divide A into B to compute miles-per-gallon

Chuck Tribolet posted 12-21-2005 10:20 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Jim, even if it is your second hypothesis, slight speed
errors would cancel out, one slightly high, one slightly

And WAAS GPS (essentially all of them these days) are reputed
to be accurate to .1 MPH. I'd believe that. I've run a
a measured mile on cruise control in my truck, and a stopwatch
and the GPS agreed.


oysterman posted 12-21-2005 05:57 PM ET (US)     Profile for oysterman  Send Email to oysterman     
Thanks for the info. I'll try to look you up next time I need some service.
Jerry Townsend posted 12-21-2005 08:40 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Chuck - hopefully, I have not misunderstood you - but, instrument error does not work that way - you can't count on a high reading and then another high reading with a net error of zero. The error can be accumulative - one high and the other low, or in-btween so that a 5% error instrument will give a combined error of within 10% error when two readings are used in a calculation.

Now, I have not read this entire thread - but Jim's concept of the mpg calculation is right on - with the possible exception that the conversion is probably based on a shorter period of time - seconds or minutes rather an hour. All GPS systems receive a very!! accurate (probably micro or nano seconds) time from the satellites and it is this time that is used in the speed and triangulation position calculations. Speed calculations/display could be a short term moving average - or something else - I just don't know.

Another comment; I have noticed where some have mentioned about being provided little or lack of information regarding how a device works. That is one of my major pet-peeves and will turn me off faster than anything else - someone not telling me how his/their device works. And I will accept "...I don't know, but I will find out and let you know..." I have been known to refuse to buy sophisticated instrumentation systems from a vendor who refused to tell me how their systems perform certain operations. And saying that the entire system is proprietary doesn't cut it. ---- Jerry/Idaho

where2 posted 12-22-2005 12:11 PM ET (US)     Profile for where2  Send Email to where2     
As your fuel injector gets deposits growing on it from that 50 gallons of fuel you left in the boat over winter, how does that effect your accuracy of the flow rate? What level of deposits puts the injector accuracy inline with the expected accuracy of a turbine flow meter?

Just curious... As for automobiles and instantaneous MPG, my 1990 VW can be set to a diagnostic mode where it will display instantaneous MPG based on manifold pressure and instantaneous velocity. While coasting it reads 99.9mpg. In normal mode, it reads "average MPG" based on the current trip, or the overall trip.

Chuck Tribolet posted 12-22-2005 12:16 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
The way GPS speed measurement works, you will get one high,
one low. It works by taking successive positions, computing
the distance, and dividing by time. The successive positions
will have canceling errors (if due to noise, and no error
in the distance if due to offset) and GPS time measurement
is so much more accurate than required here as to not be a
source of error.


Perry posted 12-22-2005 10:21 PM ET (US)     Profile for Perry  Send Email to Perry     
Chuck when you say you don't care about GPS speed in calculating MPG, what exactly are you saying?

Are you saying that when I plug my GPS into my Navman 3100, it doesn't use the speed input to calculate MPG?

According to the manual of my Navman, speed input is required to display MPG. It also says that speed input can be either from a GPS ( speed over land) or via paddlewheel ( speed over water). I read this to mean that speed is needed to calculate MPG.

Chuck Tribolet posted 12-22-2005 10:30 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Speed is not needed. Distance is needed. It appears that
that the Navman works backwards from speed (and time) to get


jimh posted 12-23-2005 12:07 AM ET (US)     Profile for jimh  Send Email to jimh     
The NAVMAN 3100 FUEL instrument can accumulate mileage. I do not know the details of how it does it. I would assume it computes the mileage itself from the speed data. My reason for making this assumption is the instrument will also do this with a paddlewheel sensor. Therefore I do not think it is relying on the GPS to compute a distance, but just to compute a speed.

If you think about it, if the NAVMAN were relying on the GPS to compute a distance, then the GPS and the NAVMAN would somehow have to agree on the interval of time that the distance information applied to. As far as I can tell, the GPS computes its measurement of distance using an interval that it finds convenient, and the NAVMAN similarly computes its measurement of distance at an interval that it finds convenient. The two do not have to agree on the interval. If the Navman accepts speed data from the GPS, it can use it to compute the distance without regard to the interval that the GPS is using to compute the distance.

Revenge 25 posted 12-24-2005 01:01 AM ET (US)     Profile for Revenge 25  Send Email to Revenge 25     
I pulled this from the NAVMAN website:

NAVMAN flow transducers are of an optical type. Within the transducer are a transmitter and receiver that output an infrared beam through the body of the transducer. Inside the transducer is a very precisely dimensioned turbine running on jewelled bearings. As the fuel flows it spins the turbine and the blades cut the infrared beam, which transmits a pulsed signal to the fuel computer.

Chuck Tribolet posted 12-24-2005 04:59 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
The GPS sends both time (of day) and position information.
It may even be in the same NMEA sentence. I'm out of town
for the holidays, and my NMEA doc is on the computer at home.
I'll check when I get back next year.


Perry posted 12-24-2005 10:40 PM ET (US)     Profile for Perry  Send Email to Perry     
According to my Navman the GPS sends speed (in MPH or knots) to the fuel flow sensor.
Peter posted 12-28-2005 07:44 AM ET (US)     Profile for Peter  Send Email to Peter     
In the NAVMAN 3100 Fuel setup, you can select between a paddle wheel speed input and a GPS speed input. If the NAVMAN were connected to a paddle wheel input, it would still compute total distance traveled based on the speed input on its own computation cycle, whatever that is. Whatever information the GPS sends to the NAVMAN in the NMEA sentence, all the NAVMAN cares about is the speed information, as it performs its own distance calculation based on the speed input from whatever source.

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