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Author Topic:   Metrics of fuel usage: GPH vs MPG vs GPM

tjxtreme posted 03-10-2011 10:28 PM ET (US)
I am curious about the relative merits of the various methods for measuring fuel consumption and efficiency. I read a recent editorial arguing for converting car MPG to GPM as a more accurate measure of the amount of fuel used. Of course, in boating GPH is more common, I assume because it is easiest to compute, not being biased by currents and wind.
jimh posted 03-11-2011 12:17 AM ET (US)
Fuel economy measured by miles-per-gallon (MPG) is a good way to establish the overall efficiency of a boat, motor, and propeller combination, allowing for influences from the loading of the boat, the sea state, winds, tide, the helmsman's skill, the weather, and who knows what else.
Measurement of engine fuel flow rate in gallons-per-hour is a good way to isolate the engine from the other variables. Measurement of the rate in gallons-per-minutes is just a scalar change.

jimh posted 03-11-2011 08:22 AM ET (US)
To really compare the fuel efficiency of engines, use the engine's brake specific fuel consumption (BSFC) Brake specific fuel consumption measures the rate of fuel use per unit of power per hour. This index gives an excellent measure of the efficiency of the engine. In an earlier article I compared the BSFC of several popular engines of the same horsepower rating:
http://continuouswave.com/ubb/Forum4/HTML/007420.html
An engine's BSFC is not a constant figure, and BSFC will vary as the engine load and power changes. It is important to compare engines are the same rated horsepower. The easiest way to do this is to compare them at their maximum rated horsepower. When this is done the result is often surprising, particularly to those who have preconceived notions about what engine technology will produce the best efficiency.
tjxtreme posted 03-11-2011 05:04 PM ET (US)
The article shows that the relationship between MPG and GPM is nonlinear. Therefore, an increase in fuel efficiency at the lower end of the MPG spectrum uses much less fuel than that same increase at higher MPGs (see graph in link). Consumers, according to the referenced study, are less likely to pick the most fuel saving car when presented as MPG versus GPM.
This is one article of many online
http://dearscience.org/2008/06/20/ you-dont-understand-fuel-economy-blame-mpg/
It seems that GPH and BSFC are not good measures for real-world usage, since engine load is rarely constant in small boats. I think that GPM is the best metric of fuel consumption, but nearly impossible to accurately quantify for a boat.
tjxtreme posted 03-11-2011 05:20 PM ET (US)
Of course GPM and MPG are also dependent on averaging or assuming a certain engine load, but I think more useful for comparing motors/boats.
Jefecinco posted 03-13-2011 10:20 AM ET (US)
For most buyers of recreational craft and/or engines the only value of fuel burn numbers is to be able to make comparisons. Unless the engine and boat builders can agree upon a standard load and other important conditions affecting fuel consumption various tests are only marginally useful.
On the other hand BSFC provides a useful comparison of engine efficiency regardless of application. So, MPG or GPM numbers are only useful for the application from which they were measured. Neither are useful to most buyers though both can be helpful to the boat owner for planning purposes.
Thankfully, current fuel flow measurement technology for small boats does an excellent job of providing the data needed to plan an outing and to know about when it will be necessary to purchase fuel.
Butch
boatdryver posted 03-14-2011 09:02 AM ET (US)
Its interesting that the range of BSFC in the 3 engines article in Jimh's link is only a little under 10%.
A buyer would have to put lots of hours on a motor for a 10% difference in fuel costs to outweigh other things such as the availability of expert local service, installed cost, noise, exhaust smell, weight and appearance in the final purchase decision.
JimL
Jefecinco posted 03-14-2011 10:00 AM ET (US)
JimL,
Agreed. That was a major reason why I was not overly annoyed at being unable to specify a nice white ETec when buying my last boat.
So far I'm extremely pleased with the ugly black Verado 135 forced upon me by Boston Whaler. ;=)
Butch

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Author	Topic: Metrics of fuel usage: GPH vs MPG vs GPM
tjxtreme	posted 03-10-2011 10:28 PM ET (US) I am curious about the relative merits of the various methods for measuring fuel consumption and efficiency. I read a recent editorial arguing for converting car MPG to GPM as a more accurate measure of the amount of fuel used. Of course, in boating GPH is more common, I assume because it is easiest to compute, not being biased by currents and wind.
jimh	posted 03-11-2011 12:17 AM ET (US) Fuel economy measured by miles-per-gallon (MPG) is a good way to establish the overall efficiency of a boat, motor, and propeller combination, allowing for influences from the loading of the boat, the sea state, winds, tide, the helmsman's skill, the weather, and who knows what else. Measurement of engine fuel flow rate in gallons-per-hour is a good way to isolate the engine from the other variables. Measurement of the rate in gallons-per-minutes is just a scalar change.
jimh	posted 03-11-2011 08:22 AM ET (US) To really compare the fuel efficiency of engines, use the engine's brake specific fuel consumption (BSFC) Brake specific fuel consumption measures the rate of fuel use per unit of power per hour. This index gives an excellent measure of the efficiency of the engine. In an earlier article I compared the BSFC of several popular engines of the same horsepower rating: http://continuouswave.com/ubb/Forum4/HTML/007420.html An engine's BSFC is not a constant figure, and BSFC will vary as the engine load and power changes. It is important to compare engines are the same rated horsepower. The easiest way to do this is to compare them at their maximum rated horsepower. When this is done the result is often surprising, particularly to those who have preconceived notions about what engine technology will produce the best efficiency.
tjxtreme	posted 03-11-2011 05:04 PM ET (US) The article shows that the relationship between MPG and GPM is nonlinear. Therefore, an increase in fuel efficiency at the lower end of the MPG spectrum uses much less fuel than that same increase at higher MPGs (see graph in link). Consumers, according to the referenced study, are less likely to pick the most fuel saving car when presented as MPG versus GPM. This is one article of many online http://dearscience.org/2008/06/20/ you-dont-understand-fuel-economy-blame-mpg/ It seems that GPH and BSFC are not good measures for real-world usage, since engine load is rarely constant in small boats. I think that GPM is the best metric of fuel consumption, but nearly impossible to accurately quantify for a boat.
tjxtreme	posted 03-11-2011 05:20 PM ET (US) Of course GPM and MPG are also dependent on averaging or assuming a certain engine load, but I think more useful for comparing motors/boats.
Jefecinco	posted 03-13-2011 10:20 AM ET (US) For most buyers of recreational craft and/or engines the only value of fuel burn numbers is to be able to make comparisons. Unless the engine and boat builders can agree upon a standard load and other important conditions affecting fuel consumption various tests are only marginally useful. On the other hand BSFC provides a useful comparison of engine efficiency regardless of application. So, MPG or GPM numbers are only useful for the application from which they were measured. Neither are useful to most buyers though both can be helpful to the boat owner for planning purposes. Thankfully, current fuel flow measurement technology for small boats does an excellent job of providing the data needed to plan an outing and to know about when it will be necessary to purchase fuel. Butch
boatdryver	posted 03-14-2011 09:02 AM ET (US) Its interesting that the range of BSFC in the 3 engines article in Jimh's link is only a little under 10%. A buyer would have to put lots of hours on a motor for a 10% difference in fuel costs to outweigh other things such as the availability of expert local service, installed cost, noise, exhaust smell, weight and appearance in the final purchase decision. JimL
Jefecinco	posted 03-14-2011 10:00 AM ET (US) JimL, Agreed. That was a major reason why I was not overly annoyed at being unable to specify a nice white ETec when buying my last boat. So far I'm extremely pleased with the ugly black Verado 135 forced upon me by Boston Whaler. ;=) Butch