posted 03-08-2014 08:58 AM ET (US)        

In another discussion, jharrell points out that variations exist in typical test data. I cited two test reports with the following data of fuel burn at full throttle:

FOURSTROKE 115 = 10.5-GPH
VERADO 200 = 20.03-GPH

jharrell cited two test reports from the same pool of data

FOURSTROKE 115 = 11.1-GPH
VERADO 200 = 18.7

Because the engines are of different horsepower, we scaled the fuel flow of the 115-HP engine by 1.74:1 to be in proportion to the 200-HP engine. The data would then look like this:

FOURSTROKE (200) = 18.26-GPH
VERADO 200 = 20.03-GPH

and

FOURSTROKE (200) = 19.3-GPH
VERADO 200 = 18.7

By selecting one set of data or the other, various calculations using the fuel flow could be shown to illustrate one theory or another. Clearly there is some variation in the test data on the rate of fuel flow.

It was assumed that a way to know BSFC of an engine was to run it at full-throttle, where it must be producing its rated power, and then to use the fuel flow rate at that power level as a way to calculate BSFC. Yet, we see, just in the above two examples, that fuel flow at full throttle may be variable.

It seems to me there are at least four possible causes for variation in the fuel flow data:

--a random variation in the actual power being produced by the engine,

--a consistent variation in the actual power being produced relative to the rated power

--a random variation in the engine efficiency

--a random error in the measurement of fuel flow

If test data is obtained by using different engines, it is possible that variations in production of the engine cause variations in the actual power the engine can produce. There is a provision for a manufacturing tolerance of ten-percent in power output in the ICOMIA specifications. This suggests that on the assembly line of 200-HP engines, one particular engine might be capable of 210-HP while a second engine made by that same assembly line might only be able to produce 190-HP.

If test data is obtained using the same engine, it is possible that other factors could cause the engine to have variations in the actual power output. There could be environmental influences, such as air temperature, humidity, and air pressure. There could be load influences, such as the load of the propeller.

Because it is difficult to know with precision the exact power of an engine at any load or application, it is often seen in test report comparison that an engine is assumed to produce its rated power when at full throttle (and running in its rated engine speed range). If a particular engine is always producing some power output that is different from its rated power, any calculation based on power would be affected. For example, if an engine is rated at 115-HP but really can only produce 108-HP, calculations that assume it is making 115-HP will contain errors. Or, perhaps the 115-HP engine consistently produces 121-HP. Again, the assumption that the power output is 115-HP will produce errors in any calculation using power.

There is also the notion that a particular engine might have a variation in fuel flow while producing the same power due to some natural variation of its fuel efficiency at various engine speeds and loads. There may be particularly favorable or unfavorable combinations of load and engine speed for a particular engine which causes the efficiency of the engine to convert fuel to power to change. Without having access to carefully measured data for a particular engine, to know these characteristics seems impossible.

Finally, the accuracy of the fuel flow data may be inconsistent. In many modern engines the engine itself is relied on as the source of fuel flow data, as these engines report that parameter as part of the electronic engine instrument data. I believe this fuel flow data is imputed from the actual instructions contained in the engine controller that controls the combustion process. That is to say, the engine is trying to provide itself with the fuel flow rate it reports. The engine does not actually measure its own fuel flow. If there is some variation in the performance of the fuel system, the engine could deliver more or less actual fuel than is being reported. In my opinion, this realm has the least likelihood of being a source of inconsistency. If the engine fuel system delivers the wrong amount of fuel to the cylinders, there will probably be running problems.

Sources:
For my test data:
Verado 200 at http://www.boatpoint.com.au/engine-reviews/2004/mercury/ mercury-verado-8294
FourStoke 115 at http://www.mercurymarine.com/engines/engine-tests/boat-house-bulletin/ ?ID=1129&

posted 03-08-2014 10:04 AM ET (US)            

It is difficult for me to imagine much variability in modern engine manufacturing, but that is my assumption.

Measurement equipment is frequently calibrated so that SHOULD not be a variable.

I am ignorant of how loads are applied to outboard engines in laboratory tests. I've seen a special propeller arrangement used to subject engines to a load for tank testing at repair shops but would expect something more precise to be used in an engine laboratory. I suspect connection of the propeller shaft to a variable load device is used for consistency.

It's quite a puzzle.

posted 03-08-2014 10:19 AM ET (US)            

I choose the sample by choosing the first 16 test reports presented in the library of test reports for the Mercury FOURSTROKE 115-HP engine. The average GPH value reported in those tests is 10.3-GPH.

I now compare the average of 16-values to the values chosen by me and jharrel:

For my comparison, I used a value of 10.5-GPH, which is actually above the average value. In choosing this value I actually hurt my comparison. It would have better for me to choose a lower value. For example, if I had been cherry picking the data, I would have chosen a value of 7.3-GPH, which is contained in the data set and is the lowest value.

For jharrell's comparison, he chose a value of 11.1-GPH, which is also above average. Choosing a value above average helped jharrel's comparison. It would have been better for jharrell to use an even higher value. For example, he could have cherry picked the value of 11.3-GPH, which is contained in the data set and is the highest value.

However, if the average value of the data is used, my comparison is sustained and jharrell's comparison fails.

My conclusion from this: since there is a variation in data, to use a larger sample and take the average is a better method than selected individual data.

posted 03-08-2014 11:16 AM ET (US)         

I agree averaging is a better way of getting data. However your report of 7.3GPH shows even an average can be severely skewed by incorrect propeller selection. So the larger the sample size the better.

This should also be applied to the Verado did you gather multiple samples and average it as well?

posted 03-08-2014 12:54 PM ET (US)            

I have not yet studied a wide range of fuel burn data for the Verado. If I do study that data, I will post my findings here. You are also welcome to report on the same.

posted 03-08-2014 01:32 PM ET (US)         

I went ahead gathered data from 2 200 hp engines and produced an average for each:

Mercury Verado 200: 19.66 GPH over 39 engine reports
Evinrude E-TEC 200: [18.26] GPH over [22] engine reports

[Data corrected; see author's note below--jimh]

I would like to point out your choice of 20.03 was above the Verado 200's average as well. There are also over 20 reports of the 115 consuming 11 GPH or more. There is also a report of the Verado 200 only consuming 14.9 GPH, I guess I should have picked that one.

So what those this data tell us? Both the E-TEC and Verado are less efficient than a non-existent 200HP 1.7L naturally aspirated Mercury FOURSTROKE that only consumes 18.26 GPH.

posted 03-08-2014 01:37 PM ET (US)            

Re the "nonexistant" FourStroke: if Mercury had chosen to make a six-cylinder non-supercharged engine from the Verado six-cylinder block, we would have another candidate for comparison.

posted 03-08-2014 02:52 PM ET (US)         

You did prompt me to review the data and there is an error, Evinrude listed 4 225HP engines under the 200 hp performance reports. This brought the average down to 18.67 GPH over 22 reports, still above the 18.26 GPH 1.7L NA 200.

posted 03-18-2014 01:09 PM ET (US)            

jharrell sampled a number of reports about the 200-HP Verado version of this same engine block, and found the average GPH was 19.66-GPH.

I now re-calculate the fuel flow difference as in my first example. First, I scale the 115-HP engine fuel flow to 200-HP, using a factor of 200/115:

"200" FOURSTROKE = 18.26

When comparing to Verado, we see that 1.4-GPH more fuel is burned.

If we assume that the conversion of fuel to horsepower is done at the same basis in the Verado as in the FOURSTROKE (since they are identical engines other than the supercharger), we see the rate of horsepower per GPH is

115-HP/10.5-GPH = 10.95-HP/Gallon/hour

Thus suggests that when the Verado burns an extra 1.4-GPH, it ought to be turning that fuel into

10.95-HP/Gallon/Hour x 1.4-Gallon./1-hour = 15.3-HP

We attribute this lost power to the supercharger. This suggests the total power is about

200 + 15.3 = 215.3-HP

and the percentage of total power consumed by the supercharger is therefore

posted 03-18-2014 01:31 PM ET (US)            

There has been a notion put forth that production of a lot of horsepower from an engine of small displacement is somehow more efficient than production of that same horsepower from an engine of larger displacement. If one accepts this notion, then the implications are:

--when the Verado is making 200-HP from the 1.7-liter displacement, it must be taking advantage of more of this efficiency than when the FOURSTROKE is making 115-HP from the 1.7-liter displacement;

--due to the above increase in efficiency, it is likely that when we note the difference in fuel consumption, we should actually attribute more than just the noted difference in fuel to the load of the supercharger;

--the calculation of the load of the supercharger with the method I have used is likely to be lower than the actual load.

posted 03-19-2014 08:56 AM ET (US)            

--displacement is radically different (3.3-liter versus 1.7-liter)

--design is radically different (two-stroke-power-cycle versus four-stroke-power-cycle)