Getting to the Trailhead; My MPG experiment !!

[SkierSteve]

With the prospect of $5 per gallon gas looming, I'm looking for any edge I can get get to reduce my transportation costs to my hiking destinations. So I decided to try a very un-scientific analysis of how highway speed affects fuel economy. Now I know this is pretty basic and probubly well known to all but the differences in MPG between speeds was a bit of a surprize.
I tried a series of 20 mile tests on Route 93 in NH, both North and South bound. I drive an Acura MDX and used the trip computer for distance traveled and to measure exact fuel consumption. I also used cruise control to maintain a constant speed. Fortunately, I was traveling in both directions at periods of low traffic and was able maintain fairly constant speeds. The results are:

80 MPH- 20.1 MPG
75 MPH- 21.5 MPG
70 MPH- 24.7 MPG
65 MPH- 25.8 MPG
60 MPH- 26.2 MPG

I found the results to be a little startling. Obviously all vehicles will react differently but for me 65 -70 is now the sweet spot. Hope this information is useful. Happy Trails!!

 

[bikehikeskifish]

What is the bigger cause of higher fuel consumption?

Higher RPMs?
Increased friction due to wind resistance?

If the former, we need to get cars with a 6th gear!

FWIW, a recent tank of gas included the 285 mile trip to do the Carter traverse. I broke the 32MPG barrier on that tank, including using the remaining 150 gallons or so for around town/commuting to work... I kept the cruise control about 5-7MPH lower than usual.

Tim

 

[Viewseeker]

gas

What about this additive of 10% ethenol??? Around here we see a difference in mpg just by which station is pumping it into there tanks...less mpg for the stations with it. Is that a factor where your from? Summer blend/ winter blend..

 

[DougPaul]

What is the bigger cause of higher fuel consumption?

Higher RPMs?
Increased friction due to wind resistance?

Gasoline engines (as set up in "normal" cars) tend to become more efficient at higher power levels (up to a fairly high rpm). This is why smaller engines tend to get higher mileage than a bigger engine in an otherwise identical vehicle--they have to operate at a higher percentage of their maximum power levels.

Wind resistance at driving speeds tends to increase with the square of the speed. And, of course, there is tire rolling resistance and losses in the moving parts of the power train. Simple friction (ie between two rubbing solids) is often constant (independent of speed) and fluid resistance tends to be a combination of linear (proportional to speed) and proportional to the square of the speed.

In some of my previous cars, I found my mileage to be fairly constant over a range of highway speeds--presumably the higher engine efficiency at higher speeds offset the increased drag. Don't know about my current car...

Other things that one can do to improve mileage:
* Don't carry unnecessary weight
* Removed unused carriers, etc.

SkierSteve:
A nice little experiment, but as you note results are likely to vary with different models.

Doug

 

[DougPaul]

What about this additive of 10% ethenol??? Around here we see a difference in mpg just by which station is pumping it into there tanks...less mpg for the stations with it. Is that a factor where your from? Summer blend/ winter blend..

Ethanol has less energy per gallon than does gasoline. I have read reports of very significant decreases in range per tank of high ethanol fuels. (Ethanol is a fairly nasty solvent and will damage the seals in a engine not designed for high percentages.)

Summer blends tend to have fewer highly volatile components to reduce pollution and winter blends have more for easier starting in the cold.

Doug

 

[peakbagger]

Aerodynamic drag is the big culprit at those speeds. I dont have my hands on the formulas but the contribution of drag goes up by a cube at highway speeds (thus the observation that hybrids dont do as well at highway speeds). The only way to do much on drag is reduce the drag coefficent or start drafting big trailer trucks . You can reduce the drag coeffcient slightly by removing your roofracks and other addons.

Ethanol has a much lower Btu content than gasoline but it does have octane benefits in small amounts. So a little bit of Ethanol is good but a lot makes the mileage go down. The "evil" MBTE is far more effective as an octane booster but its environmental effects and drive to keep the midwest farmers busy drove the switch to ethanol. There is another more refined version of grain alcohol called biobutanol that is arguably better than ethanol with a higher Btu content and it doesnt absorb water like ethanol, unfortunately a byproduct is buteric acid which has the delightfull odor of vomit when poured

All in all, slowing down is the best option assuming your vehicle is in good shape.

 

[MichaelJ]

Also, get rid of that old carburetor-based car if you actually still have one. There is a serious fall-off in performance when the engine gets too fast for the electrical phenomena in the points (which I won't go into) that results in substandard sparking by the plugs and a corresponding falloff in fuel efficiency.

Nowadays you really want fuel injection, electronic ignition, and a good overdrive in the transmission can make a world of difference, too.

 

[DougPaul]

Aerodynamic drag is the big culprit at those speeds. I dont have my hands on the formulas but the contribution of drag goes up by a cube at highway speeds (thus the observation that hybrids dont do as well at highway speeds).

My aerodynamics books say drag (a force) goes up at the square of the speed at highway speeds. (The required power goes up at the cube (power=speed * drag), but since moving faster reduces the time to go a given distance, the total energy required to go the given distance only goes up as a square.

From an alternative viewpoint, energy=drag * distance. The distance is a constant, so the energy required to travel the distance is proportional to the square of the speed.

Ethanol has a much lower Btu content than gasoline but it does have octane benefits in small amounts. So a little bit of Ethanol is good but a lot makes the mileage go down.

Octane is a measure of how much one can compress the fuel-air mixture before it self-ignites. (This produces knocking or pinging in the engine.) It does not measure the amount of energy in the fuel. However, a higher octane fuel can be used in a higher compression engine and higher compression engines are more efficient--but one would have to replace the engine or car to get a higher compression engine. (And of course, the higher octane fuel is more expensive...)

The "evil" MBTE is far more effective as an octane booster but its environmental effects and drive to keep the midwest farmers busy drove the switch to ethanol.

IIRC, lead was another "good" octane booster. Unfortunately, the amount of energy required to produce ethanol from corn is almost as much as one gets from the ethanol. Much todo resulting in very little overall benefit. (On the other hand, ethanol produced from some other sources (eg sugarcane as practiced in Brazil) does result in a sizable energy gain.

All in all, slowing down is the best option assuming your vehicle is in good shape.

Getting the engine tuned and replacing the air filter can help too.

Doug

 

[DougPaul]

Also, get rid of that old carburetor-based car if you actually still have one. There is a serious fall-off in performance when the engine gets too fast for the electrical phenomena in the points (which I won't go into) that results in substandard sparking by the plugs and a corresponding falloff in fuel efficiency.

I think you may be confusing two different things that were changed about the same time:
* Fuel injection gives more accurate control over the fuel-air ratio and more even distribution of the fuel to the cylinders.
* The change from point-based ignition to electronic ignition gave a stronger spark at high rpm. The points (mechanical contacts) were also subject to wear which would change the timing (and dwell) while the magnetic pickups used in electronic ignition are much less subject to wear.

Nowadays you really want fuel injection, electronic ignition, and a good overdrive in the transmission can make a world of difference, too.

All good stuff. It would also help if manufacturers used their improvements in engine design to increase the efficiency rather than the maximum horsepower...

One of the advantages of the hybrid engine systems is that the electric motor can provide the short-term power for acceleration and the gasoline engine can be tuned for efficiency rather than power.

 

[Quietman]

My kids were watching Mythbusters the other night and they had a segment about drafting trucks.

The daredevils on Discovery's Mythbusters demonstrated that if you followed a big rig by 100 feet, you could decrease your fuel consumption by 11 percent. The intrepid duo pushed the envelope further by following a truck at only 10 feet--a dangerous maneuver we don't recommend viewers try at home!--and managed to decreased their gas consumption by 40 percent.

They did say that the safe distance for following trucks is in the range of 125'.

 

[DrewKnight]

Getting the engine tuned and replacing the air filter can help too.

And don't forget to check your tire pressure -- even minor under-inflation can result in a 5% or greater drop in fuel mileage at highway speeds (not to mention reduced tire life and increased odds of failure). Proper inflation reduces rolling resistance fairly dramatically -- if you don't believe me, try airing down your bike tires and riding... you will feel the difference.

If your car nominally would get 30mpg on the highway, you might drop to 28mpg or so with under-inflated tires. Put another way, if you have a 15-gallon tank, you're paying about $4 extra per tank if you're running around on squishy tires.

You can also - safely - exceed the "recommended" tire pressure (typically found on the door post of the driver's door). Most manufacturers bias recommended pressure for improved ride. Don't, however, exceed the maximum pressure on the sidewall of the tire. I run my 2005 Volvo V50's tires in the high 30psi range (approximate 38psi) all around (about 8psi more than the factory recommendation). The ride is a bit firmer -- but I also consistently get fuel mileage equal to or better than the EPA estimated highway mileage.

W/R/T drafting larger vehicles... well-known but somewhat dangerous trick. I do it regularly, but never get much closer than a 2-second buffer, which at 70mph is about 200 feet. Even at that distance, you do gain an advantage. This AM I had to run to Plymouth for an appointment and was playing around with my instant-MPG gauge... as I closed in on a trailer truck on the flat, the MPG rose from the low 30s to the low 40s under constant load... then dropped off as I pulled out to pass, and fell back to the high 20s when I was out in the turbulence of the trailer. It works, but saving a few MPG isn't worth the risk of becoming a bumper sticker... remember the old trick of marking a spot as the truck passes it, then counting 'one-thousand-one, one-thousand-two".

 

[DrewKnight]

...and managed to decreased their gas consumption by 40 percent.

Loved that episode. They also tried to run at 2 feet behind the trailer -- but discovered that their mileage was actually much worse -- they figured it was because the driver was justifiably nervous and constantly varying the throttle position... which brings us to another gas-saver: use your cruise control. The little electronic brain is much more efficient at maintaining a relatively constant speed and engine load than all but the most attentive drivers.

 

[Kevin Rooney]

My kids were watching Mythbusters the other night and they had a segment about drafting trucks...

Same principle works if you and your buddy are cycling together. You can alternately draft each other. Reduces the effort considerably.

 

[TCD]

Carrying the above discussion a little further.

Gasoline engines as tuned in normal cars are inefficient when undergoing hard acceleration. Also, it's obvious that using the brakes wastes energy, much of which is dissipated as heat.

So the least fuel efficient way to drive is to frequently accelerate and frequently brake (that's why the tailgater gets poorer mileage). But that's how about half the drivers out there still drive. You would think gas was $1.00 per gallon, watching these drivers.

Do you think such drivers are unaware of these effects? Or do they know that they are costing themselves money, and they are just overcome by emotion behind the wheel?

TCD

 

[Neil]

If you had continued with your experiment down to 0 mph. you would have discovered great economical advantages.

 

[dentonfabrics]

80 MPH- 20.1 MPG
75 MPH- 21.5 MPG
70 MPH- 24.7 MPG
65 MPH- 25.8 MPG
60 MPH- 26.2 MPG

Thanks for the information. You were really rolling the dice driving 80mph for 20 miles on Rt 93. With many people driving slower now, the H'way Patrol has to work harder to achieve it's quota for speeding tickets!


bob

 

[chomp]

Some info from the folks at Edmunds:

http://www.edmunds.com/advice/fueleconomy/articles/106842/article.html

 

[peakbagger]

To add to the thread drift, the former Georgia Pacific Mill in Old town Maine (Red Shield) is converting the pulp mill into a combination pulp and hemicellulosic ethanol plant. The government is kicking in 30 million. This process bypasses the problem that is bedeviling most of the cellulosic conversion processes in that the lignin (the "glue" that holds the cellulose fibers together) is taken out by the pulping process. The lignin is burned in the recovery process to power the plant and recycle the pulping chemicals. The cellulose from the process is used to make pulp for paper and the hemicellulose that is useless for paper and usually would go along with the lignin will be converted to ethanol. So rather than growing corn specifically to make ethanol, the process takes a waste product from an existing industrial process and turns into a higher value product. The process can be tuned to produce a varying split of pulp to ethanol (within limits) so the facility can be less dependent on the paper market only.

Whats the down side?, the "fuel" to the recovery boiler will have a lower btu content, so the plant may have to burn more external fuel (usually low grade wood chips) to support the process and those who wish the pulp and paper industry to go away in new england may not be too happy.

BTW, its still a caustic based process so the smell shall return.

 

[RoySwkr]

65 MPH- 25.8 MPG
60 MPH- 26.2 MPG

Next time you have a few minutes to spare, I'd be interested in the results at 55 and 50 mph which are often proposed as speed limits to save gas.

Years ago, I was surprised to discover that my mpg were like 20% less driving 65 mph on flat Interstates in South Dakota than 45 mph on twisty roads in West Virginia.

Ethanol has less energy per gallon than does gasoline.

You're the guy to ask! Awhile ago I read that since auto engines aren't 100% efficient there is more energy in the gas than you actually use, and that since water had a greater coefficient of thermal expansion than gasoline you could actually get better mileage if there was a way to reliably mix a small proportion of water with the fuel at the time of injection.

 

[DougPaul]

You're the guy to ask!

Maybe... I know a little bit, but am far from an expert.

Awhile ago I read that since auto engines aren't 100% efficient there is more energy in the gas than you actually use, and that since water had a greater coefficient of thermal expansion than gasoline you could actually get better mileage if there was a way to reliably mix a small proportion of water with the fuel at the time of injection.

The modern automobile gasoline engine has an average efficiency of 25-30%, peak ~75%. (Ref http://en.wikipedia.org/wiki/Engine_efficiency.) So there is a bit of room for increased engine efficiency as well as more efficient use of the available mechanical energy. (One might also do better by running the engine at max efficiency to charge batteries and then running an electric motor off the batteries--a form of series hybrid.)

The gasoline engine is a thermal engine: (it sucks in cool gaseous mixture, compresses it, heats it to increase the pressure (Pressure * Volume is proportional to Temp), extracts mechanical energy by letting it expand (which also reduces the temp), and finally dumps the still warm gasses. All thermal engines have theoretical limits on their efficiency (which depend on the peak temps in the chamber and the external temps). (See Carnot cycle: http://en.wikipedia.org/wiki/Carnot_cycle http://en.wikipedia.org/wiki/Carnot_heat_engine. )

IIRC, good quality fuel cells (using hydrogen) can have efficiencies of 90+%. If there were fuel cells using gasoline (or some equally available fuel), we could do much better. (Or if using a hydrogen fuel, use a fuel cell rather than burning it.)

I don't know if water injection could help the internal combustion engine. There have been a number of such claims, many of which belong in the snake oil category. I do know that the original Boeing 707 jet engines used water injection during takeoff. Evaporating water absorbed some of the heat and produced a massive expansion to help power the engine--without the water, the engine would have overheated at that power level. (Jet engines are thermal engines and have compression, heating, and expansion phases just like car engines.)


Methinks this is getting farther and farther from outdoor recreation, but someday some of the above techniques may allow us to get to the trailheads on a little less primary fuel.

Doug