What happens when highway drivers are forced to go 55 mph (it's not pretty)
Everyone's probably familiar with the idea that engines in cars produced today are tuned to burn gasoline most efficiently when the car is moving at about 55 mph, right? I thought I'd heard that some newer cars were tuned for a slightly higher speed (60 or so), but I haven't been able to confirm that. Anyway, the point is that cars do not run their best at 70 or 75 miles per hour.
Nonetheless, most of us don't drive 55, even in areas where that's the speed limit. Some young people in Atlanta decided to commit an "act of civil obedience" and enforce the 55-mph limit on a busy Atlanta-area highway. Let's just say the people behind the 55-mph drivers were not happy, as the video above shows (I wish the kids put as much time into editing the clip as they did with the stunt itself).
Anyway, this video came to our attention thanks to a reader who uses the event to think about whether or not cities should be sending out fleets of cars to do what the young people in the video did. Ticket-enforced speed limits obviously don't make people drive 55, and knowing that you're emitting more than you would at a lower speed doesn't do the trick, either. How important is it to go 55? If you're interested, you can read the full argument in favor of enforcing the limit. What do AutoblogGreen readers think; should we consider the community at large and work harder to make 55 mph the speed people actually drive? Or is 70-75 the right highway speed, and we should push automakers to develop engines that are more efficient at that level? Will EVs make this entire debate moot in two decades' time?
Finally, I think it's only fair to put Sammy Hagar's preemptive response to this idea after the break.
[Source: YouTube h/t to ]
Reader Comments (Page 1 of 2)
tim mcleod 2:44PM (8/25/2007)
We need more facts here - is there an engineer in the house? Somebody who knows please weigh in here.
Talk about cars being "tuned" to a certain speed intuitively seems to me like a minor factor in the pursuit of economically optimum speed. I thought that drag (in my oversimplification that's essentially air friction, not wheel friction) was a minor part of the fuel economy equations at lower, say "neighborhood", speeds and then went up roughly by the square (or cube?) of airspeed to become the dominant portion of the fuel economy influences at (or above?) typical "highway" speeds.
If that is even remotely accurate, then all the "tuning" in the world can't beat slowing down as the greater influence on fuel economy.
Seriously, if you know some engineering please put some more facts on the table for all of us to chew on.
thanks,
tim
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Sos10 5:15PM (8/25/2007)
In some countries (like the Netherlands) police "pace-cars are used (tested) to slow down traffic or to keep the traffic at a sustained speed... this prevents many traffic jams... and by preventing jams, you save gas.
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Tim 5:28PM (8/25/2007)
The point here is ENFORCE or ABOLISH. To selectivly enforce is unfair to everyone.
I'm thinking immigration...
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tim mcleod 7:13PM (8/25/2007)
Couldn't agree more with comment #3. How many times have you seen this useless attempt at parenting: daddy or mommy repeatedly screams "stop it johnny" while the brat's behavior only worsens. Unenforced rules only serve to weaken authority.
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mlsommer 10:54PM (8/25/2007)
This happened last year didn't it?
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MikeW 10:18PM (8/25/2007)
Engines aren't tuned for a specific road speed.
There is a BSFC curve which has a plenty broad minima because of variable intake and exhaust valve timing.
http://www.worldcarfans.com/3060316.002/photo1027/lexus-rx-350
Isn't that about 2000-3300.
So you get a modern transmission to go with that. 6/7/8 speed automatic.
The mileage doesn't drop out that much
Take a stick shift honda insight: 100mpg@50mph-too slow for the highway, 50mpg@100mph-too much work to keep the car going straight with such narrow tires, 165/65 14, and zero toe-in, so 75mpg@75mph-best speed for time for mileage for highway travel.
Its what people automatically do already. Pigment on metal doesn't overrule common sense.
Those kids are clueless, a 4 lane highway like that should be 80mph.
We define the speed limit, how we drive, set at the 85-90th percentile.
But corrupt politicians stick their dirty hands in the way, and lower the speed limits (how they are able to do that? it doesn't seem legal to override a legitimate traffic survey)
and lower speed limits on non-highways cause yellow/red light duration/timing to be off. So they then propose a solution to the problem they created. (problem/reaction/solution)
http://www.thenewspaper.com/
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Bob 1:17PM (8/26/2007)
Aerodynamic drag goes with with the square of the speed, while the power required for aerodynamic drag goes up by the cube of the speed. This second function is due to the fact that the work is done in half the time. So for just aerodynamics, dropping the speed by half cuts the energy needed to just 12.5%. As the speed drops, rolling losses start to dominate over aerodynamic.
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Joe 2:33AM (8/26/2007)
Air drag does a good bit. Horsepower is dependent on the cube of the velocity. Drag = .5*density*velocity^2*Frontal area*Drag Coefficient. Horsepower is roughly equal to Force*Velocity/550. So, using some rough numbers, and assuming A) I didnt mess up, and B) This is air drag only, it takes about .075HP to move along at about 10mph, about 12HP to move at 55mph, about 20HP to go 65 and about 73HP to go 100mph.
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Joe 2:37AM (8/26/2007)
I should note, I based my calculations off the dimensions of a Chevrolet Cobalt, and assumed conditions at sea level and guessed a drag coefficient of .4. The EV1 had a Cd of .19, a Flat Sheet of metal has a Cd of 1.0.
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tim mcleod 4:59AM (8/26/2007)
Thanks Joe.
How much fuel could I save then, if I were willing to use an ordinary car only around the neighborhood, and this allowed me to swap to a lower hp engine?
To keep the math simple, say that this Cobalt tops out at 100 mph with a 100 hp engine. Does this mean that roughly 3/4 of my hp (73/100), and therefore fuel, are spent on drag and 1/4 on all other resistance (mostly drive train frictions and tires' rolling resistance)? Then, assuming linearity between your example points, I could top out at a speed of 32.5 mph ((10+55)/2) while using an engine with only about 33hp ((.075+12)/2) for drag + (100-73) for other resistance)?
And, continuing to take liberty of oversimplifying that engine hp and fuel consumption are directly proportional, use only 1/3 as much fuel, thereby increasing, for example, from 25 mpg to 75 mpg by foregoing the highway and using this "lawnmower" engine? So 2/3rds of the gasoline burned in America is used to preseve the potential of using highway speeds (assuming I have the world's greatest torque converter and can get away from a stop sign fast enough not to be shot by those with more realistic time constraints)?
Interesting. Someone, please steer me back to the real world before I get too far out on this limb.
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tim mcleod 6:44AM (8/26/2007)
My insomnia seems to have helped me to conveniently neglect the not small issue of neighborhood/city driving's frequent requirement for fuel-consuming speed changes.
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rgseidl 7:51AM (8/26/2007)
People want to drive faster than 55 because their lives are busier than those of their parents. Time is money and, fuel is still comparatively cheap.
Besides, the purpose of the 55 limit had essentially nothing to do with fuel economy. Researchers simply discovered in the 1950s that 55 was the speed at which roads tended to deliver the greatest vehicle throughput per lane.
It's also nonsense to claim that an engine is optimized for a particular vehicle speed. If anything, you would need to tune the transmission gear ratios to keep the engine running at lower RPM when cruising at higher speed (a.k.a. overdrive).
Otherwise, to save fuel at high speeds, you need to focus primarily on the aerodynamics in the direction of travel. This doesn't just mean reducing the drag coefficient of a single vehicle (cp. Loremo). It also means adding automated control systems (e.g. based on short-range radar or lidar) that let cars travel in tightly spaced yet still safe slipstream convoys. Such adaptive cruise control systems already exist, but they are expensive and currently only available on high-end models. You also wouldn't want anyone whose vehicle did not have this equipment join such a convoy (except as the lead vehicle).
Pickup trucks and SUVs should be required to drive in the right lane only, where traffic is slower, except when passing commercial vehicles. It's virtually impossible to get their aerodynamic resistance down to reasonable levels. Besides, it would put a severe damper on their outsize popularity. Fewer of them in the fleet would save massive amounts of fuel.
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Paul 1:07PM (8/26/2007)
"Pickup trucks and SUVs should be required to drive in the right lane only, where traffic is slower, except when passing commercial vehicles. It's virtually impossible to get their aerodynamic resistance down to reasonable levels. Besides, it would put a severe damper on their outsize popularity. Fewer of them in the fleet would save massive amounts of fuel."
Not a bad idea, however, I'd modify this to the right two lanes (currently enforced in Atlanta for commercial trucks), remove the exclusion for passing, and put in traffic cameras EVERYWHERE to enforce the heck out of it ($50 ticket sent automatically to the registered car owner for each violation). Also prohibit them form carpool lanes regardless of the number of passengers. It won't make a difference on two or four lane roads, but who cares. It'll discourage what I consider to be the #1 consumer-based evil of cities today--that is suburbanites commuting 50 miles single in 12-15MPG vehicles. AND it'll trickle down over time to smaller locales.
Hard to draw the line at pickups and suv's because of the rise of crossovers and such (really nearly as bad). You'd need either an MPG cutoff or something similar to what London is doing. Then combine this with true London'esque congestion charges (okay perhaps a pipe-dream) and you'd likely cut large vehicle sales in half almost immediately.
On the negative side, short-term it'd make exiting a nightmare and likely worsen traffic problems (and emissions), but that would diminish in a couple of years.
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Nils 3:19PM (8/26/2007)
Don't forget that a lot of energy is spent accelerating/decelerating the car linearly as well as each rotating part. Cars have way too much power to drive at legal speeds but they need it to get there. People want zippy cars you know...
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gnoble 8:54AM (8/27/2007)
Yah, the math behind this is sort of complicated, so the program I work on had to get a doctor of physics to do the math for us. If you want see the formula (it doesn't cut and paste in) it's here - http://drivewiser.ca/default.asp?mn=1.181.184.207 .
The basic principle behind it is that with a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as quickly. Since power is the rate of doing work, four times a work in half the time requires eight times the power.
I dunno about 'merican miles, but in km/h this works out to 150km/h uses 125% more fuel than 100km/h. So you travel 100km, you're only going to be shaving 15 minutes off of your time but you'll be using more fuel that it would take you to get there an back again.
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Don 4:37PM (8/27/2007)
What a bunch of drama queens.
Just get in the right lane and do 65.
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Mike Ogburn 12:46PM (8/28/2007)
Read about a similar incident (involving fines!) in Canada.
http://freerepublic.com/focus/f-news/678348/posts
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Brian Hague 1:22PM (8/28/2007)
They should come to Dallas and try that... I can see a different outcome from camera the bridge.
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Richard 11:29AM (8/29/2007)
"Don't forget that a lot of energy is spent accelerating/decelerating the car linearly as well as each rotating part. Cars have way too much power to drive at legal speeds but they need it to get there. People want zippy cars you know..."
Sounds like a perfect argument for hybrids, actually, which was their original business case. A small efficient ICE that's optimized to produce just a little bit more power than is necessary for highway cruising, combined with a potent electric motor that's optimized for brief bursts of torque and is recharged over time from regen braking and that slight available overpower on the highway.
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Jon 10:54PM (8/30/2007)
While I am a 4th year mechanical engineering student, that does not qualify me to know in detail all the answers in all fields, so I will not claim to. But I believe that I can offer some insight and tips to help you save a lot of gas.
First, I think that it is important for people to realize that speed is not the varible to look at for economy. It's RPMs. Most vehicles get the best mpg at 2500 rpm. This is because work conversion mostly occurs at the gears (trans), much like a bike, but resistance due to friction is more apparent in the case of an engine. The real trick in optimizing your mpg on the highway (besides how much you use your brakes ) is minimizing drag while keeping your car at about 2500 rpm. Notice that required power at high speed goes up much more per unit (mph) than at lower speeds, with this we can draw the conclusion (assuming we are in the same gear) that 2500, or actually a bit below just because of the increases in drag at high speeds, is the optimum rpm. Once you go below 2000 you start drawing a line. My truck actually runs at 75 mph at 2500-2400 rpm or so. When I get to 80 mph the drag is noticably larger. I get the best mpg at about 75 mph (this is in overdrive). You must also take into account hills. If you are trying to keep a certain speed and your drop 5 mph below that speed going up a hill, it will take more work (gas. again assuming same gear) to get back that 5 mph at 1500 rpm than at 2500 rpm (this also depends on drag, but with my care is the case).
Unless your car is at 2500 rpm at 65 mph, I can not agree that 65 should be a standard, nor do I believe in a standard, the range is too broad from car to car. And if something like that were to come about, it would only be a matter of time before people were pushing for 60. I believe that people should be educated in efficient driving (as stupidity wastes more gas than anything) and the most economic highways speed should be published in owners manuals.
I'm going on a limb, but if you do not take these things into account already, there are two things that I bet will save you from 5-10% on gas. 1. what we talked about above. As a human to best guage drag and rpm, start at 2500 rpm on the highway and find that speed. At that speed with your foot on the pedal (until you shift), go to neutral and time how long it takes to drop 500 rpm. Repeat that for speeds 5 mph above and below and even 10 if your not satisfied. The speed at which it took the longest time to drop 500 rpm and closest to 2500 rpm. This works because at 2500 rpm and the longest drag interval you are finding the lowest wind/rolling resistance and comparing to the optimum mechanical speed and resistance. WHEN IN PLACES OF HIGH AIR DENSITY, HUMIDIY, OR WHEN IT'S RAINING, DRAG BECOMES EVEN MORE IMPORTANT, so you will want to slow down in these conditions a bit. Of course be careful and do this when no cars are around. 2. Easy on the gas and brakes. Not much need at all for brakes at the end of a turn or going up a hill. And not much need for gas going down a hill or entering a turn. Once again safety is most important, but with a little judgement and practice, you will get good at this and will save a noticeable ammount of gas. Of course the usual (oil changes, tire pressure, filters, etc.) are important too. No matter what speed you are at (55, 25, 70), mechanical efficiency is usually optimized at 2500 rpm. So find a comfortable, legal, speed as close as you can get to 2500 for cruising on these roads (like 55 mph roads) and it will also help you and the environment. E-mail me if you would like, jcortel@uark.edu, and happy savings.
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