Minnesota. What are the biggest factors in seasonal variation of fuel consumption? I think exclude driving style. What is left? My son and I were at a freshman (college) orientation last summer and a physics professor said as an aside, "who would think I could use my Pontiac Grand Am as a calendar. The graph of fuel consumption (mpg) was a perfect sine wave with a period of exactly one year." On my query, he stood on atmospheric resistance, 3% greater in cold and 3% less in warm weather. His observed fuel consumption varied roughly +/- 3%, lowest mpg in January, highest mpg in July. I asked about significance of fuel vapor pressure which varies by design on roughly the same calendar schedule. What say bitogers?
seasonal variations in fuel consumption
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The only reference I could find quickly (and from a somewhat sketchy webpage) suggested that winter blend gasoline has 2% less energy per unit volume than what is used in the summer.
I would also add the effect of winter tires for those who use them, would imagine they tend to result in lower fuel economy compared to most all season or summer tires.
I drove back home from the mountains yesterday and tried hard to keep the trip above 30mpg by the car's own calculation, was going just a bit above the speed limit (which made me stick out like a sore thumb on New England highways) and avoiding sudden braking/acceleration and unnecessary stops. It became clear as I got close to home I wasn't going to make it and drove more like my usual self at the end, ended up with 29mpg. I believe I have been able to do 32mpg under similar conditions in the summer. Started with what I think was an ice cold engine, it had been sitting outside for a couple of hours in ~15F weather and it took quite a while to hit the minimum 90F or so for the oil temp to start displaying.
I would also add the effect of winter tires for those who use them, would imagine they tend to result in lower fuel economy compared to most all season or summer tires.
I drove back home from the mountains yesterday and tried hard to keep the trip above 30mpg by the car's own calculation, was going just a bit above the speed limit (which made me stick out like a sore thumb on New England highways) and avoiding sudden braking/acceleration and unnecessary stops. It became clear as I got close to home I wasn't going to make it and drove more like my usual self at the end, ended up with 29mpg. I believe I have been able to do 32mpg under similar conditions in the summer. Started with what I think was an ice cold engine, it had been sitting outside for a couple of hours in ~15F weather and it took quite a while to hit the minimum 90F or so for the oil temp to start displaying.
Ethanol has a lower energy content than gasoline.
Blending ethanol into gasoline lowers the resulting blended fuels energy content.
Butane has an energy content much lower than pure gasoline similar to the energy content of ethanol.
Winter gasoline (whether blended with ethanol or not) contains a much higher percentage of butane than summer gasoline (4% to 6% higher in winter). Butane raises the RVP (Reid Vapor Pressure) of the blended fuel allowing it to more easily vaporize in cold weather. Easy vaporization of fuel allows a vehicle to start and run smoother in cold weather.
The EPA dictates the maximum RVP of blended fuel by State and County across the US. Higher RVP is allowed in winter months and lower RVP in summer months.
Oil refiners blend more Butane into gasoline in the winter months causing the resulting fuel to vaporize more easily in colder weather yet contain less energy than the fuel sold in summer months.
The higher the energy content of the fuel used, the farther the same vehicle will go on a gallon of fuel. That's why the same vehicle will generally get better mileage with pure gasoline vs. an ethanol-blended gasoline. It's also the main reason why higher energy content "summer gas" generally returns better mileage than lower energy "winter gas".
Blending ethanol into gasoline lowers the resulting blended fuels energy content.
Butane has an energy content much lower than pure gasoline similar to the energy content of ethanol.
Winter gasoline (whether blended with ethanol or not) contains a much higher percentage of butane than summer gasoline (4% to 6% higher in winter). Butane raises the RVP (Reid Vapor Pressure) of the blended fuel allowing it to more easily vaporize in cold weather. Easy vaporization of fuel allows a vehicle to start and run smoother in cold weather.
The EPA dictates the maximum RVP of blended fuel by State and County across the US. Higher RVP is allowed in winter months and lower RVP in summer months.
Oil refiners blend more Butane into gasoline in the winter months causing the resulting fuel to vaporize more easily in colder weather yet contain less energy than the fuel sold in summer months.
The higher the energy content of the fuel used, the farther the same vehicle will go on a gallon of fuel. That's why the same vehicle will generally get better mileage with pure gasoline vs. an ethanol-blended gasoline. It's also the main reason why higher energy content "summer gas" generally returns better mileage than lower energy "winter gas".
Is atmospheric resistance the biggest factor or is fuel vapor pressure (summer gas w/ more "BTUs/gal" vs. winter gas w/ less "BTUs/gal") the bigger factor? Any other factors more significant than either of the two mentioned? Again, discount driving style. The professor's weekly drive was from Eau Claire, WI to Sioux City, IA and back.
What is "atmospheric resistance"?
For all of the various reasons, my 2018 F150 2.7L ecoboost gets 18.3MPG in wintertime and not one bit more, regardless of how slow I drive. In the summer, it's possible to achieve 22MPG.
air drag? air resistance? whatever the proper terminology is, in for instance, bullet flight. In a vacuum there is essentially none. We have an atmosphere on earth. Hence atmospheric resistance. Not wind resistance for purposes of this discussion as it is not reliability constant.
So mean of 20.15 mpg +/- 1.85 mpg - is that really a 9.2% variation? A lot (relative/significant?) more than the professor's +/- 3% or Greatlake's +/- 2.5% (a mean of 5% and 2.5% each way).
So mean of 20.15 mpg +/- 1.85 mpg - is that really a 9.2% variation? A lot (relative/significant?) more than the professor's +/- 3% or Greatlake's +/- 2.5% (a mean of 5% and 2.5% each way).
I can demonstrate the same type of fuel mileage variation as the professor based on my fuel records going back to around 2000. Every one of my vehicles demonstrates the same patterns. It is more dramatic on some than others.
Air density plays a role. Both as resistance and as increased fuel burned to match the denser air available to maintain optimal emissions and operations. Energy content of gasoline plays a role, as noted that there is less energy in the fuel since higher vapor pressure is allowed. Cold fluids are all more viscous on startup and play a role (more dramatic with short tripping, but still a factor). Did he keep his tires inflated exactly the same? Odds are he did not, meaning the tires are often at a lower pressure in cold weather which increases rolling resistance. Engines run rich on cold starts to get the cats going, etc... And all that takes more time and fuel when cold. Running the heater on high takes energy (though so does running the A/C).
Which one is the biggest? Air density is a good one, but not fully explained by "resistance".
Air density plays a role. Both as resistance and as increased fuel burned to match the denser air available to maintain optimal emissions and operations. Energy content of gasoline plays a role, as noted that there is less energy in the fuel since higher vapor pressure is allowed. Cold fluids are all more viscous on startup and play a role (more dramatic with short tripping, but still a factor). Did he keep his tires inflated exactly the same? Odds are he did not, meaning the tires are often at a lower pressure in cold weather which increases rolling resistance. Engines run rich on cold starts to get the cats going, etc... And all that takes more time and fuel when cold. Running the heater on high takes energy (though so does running the A/C).
Which one is the biggest? Air density is a good one, but not fully explained by "resistance".
OK thanks.
Yeah, 3% is not something that can be attributed to much at all. A study I posted a while back showed that the energy density of gasoline varied up to 4% at the same station, regardless of the season.
Measuring it is one thing but attributing it to a specific variable is another.
Yeah, 3% is not something that can be attributed to much at all. A study I posted a while back showed that the energy density of gasoline varied up to 4% at the same station, regardless of the season.
Measuring it is one thing but attributing it to a specific variable is another.
Air density will also affect fuel mileage of a vehicle. Air density changes with altitude, temperature and humidity. At one atmosphere of pressure at sea level and identical humidity, air density will increase about 17% as the air temperature falls from 80F to zero F.
About 50% of the horsepower made by an engine is used to overcome aerodynamic drag (depending on vehicle speed, frontal area and overall vehicle shape). About 50% of the fuel energy is actually used to move a vehicle down the road (the rest is wasted as heat out of the tailpipe). About 25% of the fuel burned in a vehicle actually pushes it through the air (the % greatly varies with vehicle speed).
Therefore, depending on the air density, vehicle speed and the vehicle's engine efficiency, who knows which has more effect; fuel energy content or outdoor temperature without precise measurements?
Generally, reduced fuel energy and increased aerodynamic drag in cold weather (higher density air) have about the same effect on vehicle fuel mileage - both negative.
About 50% of the horsepower made by an engine is used to overcome aerodynamic drag (depending on vehicle speed, frontal area and overall vehicle shape). About 50% of the fuel energy is actually used to move a vehicle down the road (the rest is wasted as heat out of the tailpipe). About 25% of the fuel burned in a vehicle actually pushes it through the air (the % greatly varies with vehicle speed).
Therefore, depending on the air density, vehicle speed and the vehicle's engine efficiency, who knows which has more effect; fuel energy content or outdoor temperature without precise measurements?
Generally, reduced fuel energy and increased aerodynamic drag in cold weather (higher density air) have about the same effect on vehicle fuel mileage - both negative.
What could make the graph look like a nearly perfect sine wave with a period, of for all intents and purposes one year, except something I would say manipulated with intent/purpose such as fuel vapor pressure? His records were for several years as well. I keep the same kind of records (although I have not graphed the data) and I'll say my gasoline-fueled vehicles get the best mileage in September and the first part of October.
Not a perfect sinewave but it recognisable and the variation between summer and winter is fairly consistent. This is my bike and although I have similar records and graphs for my car which does show some variation between summer and winter it's not recognisable as a sine wave at all.
Dense cold dry air provides more oxygen than hot humid air per piston stroke, so self-compensating modern engines with ECU's and O2 sensors provide more gasoline to stay at the correct stoichiometric ratios for clean burning.
Winter blend gasoline (already mentioned).
Dense air (already mentioned) is harder to push aside at high highway speeds, but this is less of a factor than stoichiometry and winter blend. But put all 3 together and you have a measurable difference in fuel economy (for many vehicles, anyway).
Winter blend gasoline (already mentioned).
Dense air (already mentioned) is harder to push aside at high highway speeds, but this is less of a factor than stoichiometry and winter blend. But put all 3 together and you have a measurable difference in fuel economy (for many vehicles, anyway).
I don't have the nice graphs handy to post, but I'd agree on two things:
My graphs are not perfect sinewaves - but there are distinct trends between summer and winter.
On top of that, going back, I've got a lot more than 3% variation. The truck nets out at nearly 6% higher than average in the summer (best month is August), and 15% lower in the winter (worst month January) than average (Turbocharged DI engine - loves the cold air for power!). Note my miles are not balanced year around either.
The Explorer with a V8 nets 12% higher than average in its best month (July) but 8% lower in its worst month (February, though its within a tenth of December and January as well).
A LOT more variation than 3%.
My graphs are not perfect sinewaves - but there are distinct trends between summer and winter.
On top of that, going back, I've got a lot more than 3% variation. The truck nets out at nearly 6% higher than average in the summer (best month is August), and 15% lower in the winter (worst month January) than average (Turbocharged DI engine - loves the cold air for power!). Note my miles are not balanced year around either.
The Explorer with a V8 nets 12% higher than average in its best month (July) but 8% lower in its worst month (February, though its within a tenth of December and January as well).
A LOT more variation than 3%.
My winter fuel economy is usually pretty bad compared to summer. Summer id get close to 500 a tank and winter about 400. I plug my car in too and rarely idle it.. it seems to vary depending on the tank though. But Id think its about 20% worse for me in winter.
3% falls within the error bars all on its own.
Your prof is a smart man. Air density does increase, and the drag effect is not linear.
I suspect most of you know that drag increases as the square of speed. Drag at 60 is four times that at 30. So a 3% increase in air density results in far more than 3% drag increase.
Vapor pressure shouldn't have any effect while driving. Compression of the air charge will vaporize the fuel that isn't in a boundary zone.
The increase in oxygen content due to higher air pressure has no effect. You run a smaller throttle under those conditions. If you didn't, you'd fun at higher speeds.
I suspect most of you know that drag increases as the square of speed. Drag at 60 is four times that at 30. So a 3% increase in air density results in far more than 3% drag increase.
Vapor pressure shouldn't have any effect while driving. Compression of the air charge will vaporize the fuel that isn't in a boundary zone.
The increase in oxygen content due to higher air pressure has no effect. You run a smaller throttle under those conditions. If you didn't, you'd fun at higher speeds.
Problem is the professor didn't say it was more than a 3% variation (as a few of us are noting is common). He said it was a 3% change in air resistance, not density, and a 3% +/- on fuel mileage.
Vapor pressure itself is not the issue - the issue is what can go in the fuel that results in a higher vapor pressure. Those contents typically have a lower energy content.
Vapor pressure itself is not the issue - the issue is what can go in the fuel that results in a higher vapor pressure. Those contents typically have a lower energy content.
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