Pop-up headlights, CD, and gas mileage
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From: Saint Louis, MO, USA
Car: 91 Formula
Engine: 305 TBI
Transmission: 700R4
Pop-up headlights, CD, and gas mileage
Anyone know to what extent having the Firebird headlights up affects gas mileage (by increasing drag)? Or maybe just some general estimates of the relationship between Coefficient of Drag (by what % it changes) and gas mileage?
I was debating this issue with a friend whose H***a A****d's (please, no flames) has its lights permanently up. I said that besides the silly look of driving around with pop-ups up all day, her gas mileage was suffering (she wasn't buying that it made any noticeable diff...).
Just curious.:lala: :lala:
I was debating this issue with a friend whose H***a A****d's (please, no flames) has its lights permanently up. I said that besides the silly look of driving around with pop-ups up all day, her gas mileage was suffering (she wasn't buying that it made any noticeable diff...).
Just curious.:lala: :lala:
Last edited by Joel Geerling; Mar 20, 2002 at 12:41 AM.
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You're increasing the frontal area by about 1½ ft², so figure it out from there. The cD on a typical Firebird with the headlights stowed is about .31-.32. I don't know what it is with the headlights deployed, but it can't make much of a difference under 45 MPH.
On the other side of the coin, one of the major reasons highway patrol cars don't often have externally mounted light bars any longer is the increase in fuel mileage from eliminating the drag. If you operated a fleet of 600 cars state-wide and had them running 12 hours per day, you would want the extra ¼ mile per gallon, too. That's a lot of fuel in the course of a year (and a lot of tax dollars).
For your car (or your friend's Accord), either of you would be far better off using synthetic engine oil and installing a fresh oxygen sensor, air filter, plugs, etc. THAT will get you some better fuel mileage.
On the other side of the coin, one of the major reasons highway patrol cars don't often have externally mounted light bars any longer is the increase in fuel mileage from eliminating the drag. If you operated a fleet of 600 cars state-wide and had them running 12 hours per day, you would want the extra ¼ mile per gallon, too. That's a lot of fuel in the course of a year (and a lot of tax dollars).
For your car (or your friend's Accord), either of you would be far better off using synthetic engine oil and installing a fresh oxygen sensor, air filter, plugs, etc. THAT will get you some better fuel mileage.
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...Oh, and wash your car, too. The extra mass and turbulence from all that dirt is not helping the mileage or drag coefficient, either.
Incidentally, I wonder what kind of wax is best for mileage? I'm sure a race team somewhere has studied the effects of finish on drag - I know the Air Force has. One of the reasons the old F-15 still spanks the Mig-29 is that the skin rivets on the Mig are not very flush, and at 1,800 MPH they create a lot of drag. The F-15 skin is as smooth as a baby's behind, all the way to 2,150... (Grumman used to make a pretty sleek canoe, too.)
Now who has too much time on their hands?
Incidentally, I wonder what kind of wax is best for mileage? I'm sure a race team somewhere has studied the effects of finish on drag - I know the Air Force has. One of the reasons the old F-15 still spanks the Mig-29 is that the skin rivets on the Mig are not very flush, and at 1,800 MPH they create a lot of drag. The F-15 skin is as smooth as a baby's behind, all the way to 2,150... (Grumman used to make a pretty sleek canoe, too.)
Now who has too much time on their hands?
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From: Saint Louis, MO, USA
Car: 91 Formula
Engine: 305 TBI
Transmission: 700R4
Thanks for the info, Vader. At least someone took the q. seriously! :hail: :lala:
I didn't think it was a huge factor, but it has to contribute something - like you wrote re. the highway patrol lights. I'm not worried about the difference on my car in a practical sense (it's running like a top), just curious. And I do keep it clean!
I didn't think it was a huge factor, but it has to contribute something - like you wrote re. the highway patrol lights. I'm not worried about the difference on my car in a practical sense (it's running like a top), just curious. And I do keep it clean!
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Vader,
Ok, I know that having the lights up increases CD, that is obvious. Is it due to increased frontal area, or because it creates an obstruction to airflow. It doesn't seem to me that frontal area would change, since the sillouette (looking at the front of the car) does not change with the position of the lights. Now, if the lights went up far enough to stick up past the roof, or popped out sideways past the fenders (just to illustrate my point), then frontal area would change. I'm just thinking that the frontal area actually remains the same, with the difference in CD coming from the more turbulent airflow with the lights sticking up. Anyways, fill me in if I'm wrong, I've been wondering about the frontal area thing for a while now.
MikeS
Ok, I know that having the lights up increases CD, that is obvious. Is it due to increased frontal area, or because it creates an obstruction to airflow. It doesn't seem to me that frontal area would change, since the sillouette (looking at the front of the car) does not change with the position of the lights. Now, if the lights went up far enough to stick up past the roof, or popped out sideways past the fenders (just to illustrate my point), then frontal area would change. I'm just thinking that the frontal area actually remains the same, with the difference in CD coming from the more turbulent airflow with the lights sticking up. Anyways, fill me in if I'm wrong, I've been wondering about the frontal area thing for a while now.
MikeS
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From: Birmingham Mi, USA
ok, I found the number, it is 406 million gallons of fuel every year.
here is the formula:
"For several years now Canada has required all new cars sold to have daytime running lights. Any time the car is running the headlights are on, but the taillights and other lights are off. You have to turn on these other lights from the dashboard at night. Studies seem to indicate that having the headlights on during daylight hours reduces the number of multiple vehicle accidents (although there has been some controversy about people forgetting to turn on their other lights at night -- a mistake that causes extra accidents, and a good example of the "law of unintended consequences"!).
The US has not adopted this law, but if it did they would definitely consume gasoline. Headlights require power, and a car's engine produces power using gasoline. If you make a few assumptions, it is possible to estimate how much gas the law would consume.
A typical headlight bulb uses about 55 watts; sometimes the daytime running lights run at a lower wattage so they use a little less power. Let's say the daytime running lights use 100 watts since there are two bulbs.
To calculate the energy used, we need to figure out how much time people will spend with their lights on. According the to NHTSA, vehicles in the US drove 2,560 billion miles in 1997. We need to make a guess at the average speed people drive including stops in order to figure out how much time people spent driving their cars. Let's guess 30 mph, which means each mile takes two minutes. That makes 5,120 billion minutes or 85.3 billion hours. Now if each car normally drives at night about half the time, that means that the daytime running lights would be on 42.6 billion hours a year. Multiplying by the 100 watts we get 4,260 billion watt-hours or 4.26 billion kilowatt-hours. The U.S. uses about that much electricity nationwide in 12 hours.
Now we need to figure out how much electrical energy we can get out of a gallon of gas. A gallon of gas contains about 60 kilowatt-hours of chemical energy, but this energy has to go through two conversion processes before we can use it in a light bulb. First the chemical energy must be turned into mechanical power by the engine of the car. Car engines don't do this very efficiently -- only about 25% of the chemical energy can be turned into mechanical power, and the rest is wasted as heat. After the engine gets done with our gallon of gas we have 15 kilowatt-hours left.
Now the alternator on the car has to turn the mechanical power from the engine into electrical power. The alternator does this a lot better than the engine, but it is still only about 70% efficient. In the end we get about 10.5 kilowatt-hours of electrical energy out of a gallon of gas.
To calculate how many gallons of gas this is, you can divide the 4.26 billion kilowatt hours of energy that the daytime running lights consume each year by the 10.5 kilowatt-hours of energy each gallon of gas yields. If daytime running lights were on all the vehicles in the U.S., we would burn an extra 406 million gallons of gas each year. That's only a couple gallons for each vehicle, but in total it is more than all of the vehicles in the country burn in a day. At $1.50 a gallon, that's $600 million per year. Looking at it another way, an extra 8 billion pounds of Carbon Dioxide would be added to the atmosphere by this law."
---thanx to www.howstuffworks.com
here is the formula:
"For several years now Canada has required all new cars sold to have daytime running lights. Any time the car is running the headlights are on, but the taillights and other lights are off. You have to turn on these other lights from the dashboard at night. Studies seem to indicate that having the headlights on during daylight hours reduces the number of multiple vehicle accidents (although there has been some controversy about people forgetting to turn on their other lights at night -- a mistake that causes extra accidents, and a good example of the "law of unintended consequences"!).
The US has not adopted this law, but if it did they would definitely consume gasoline. Headlights require power, and a car's engine produces power using gasoline. If you make a few assumptions, it is possible to estimate how much gas the law would consume.
A typical headlight bulb uses about 55 watts; sometimes the daytime running lights run at a lower wattage so they use a little less power. Let's say the daytime running lights use 100 watts since there are two bulbs.
To calculate the energy used, we need to figure out how much time people will spend with their lights on. According the to NHTSA, vehicles in the US drove 2,560 billion miles in 1997. We need to make a guess at the average speed people drive including stops in order to figure out how much time people spent driving their cars. Let's guess 30 mph, which means each mile takes two minutes. That makes 5,120 billion minutes or 85.3 billion hours. Now if each car normally drives at night about half the time, that means that the daytime running lights would be on 42.6 billion hours a year. Multiplying by the 100 watts we get 4,260 billion watt-hours or 4.26 billion kilowatt-hours. The U.S. uses about that much electricity nationwide in 12 hours.
Now we need to figure out how much electrical energy we can get out of a gallon of gas. A gallon of gas contains about 60 kilowatt-hours of chemical energy, but this energy has to go through two conversion processes before we can use it in a light bulb. First the chemical energy must be turned into mechanical power by the engine of the car. Car engines don't do this very efficiently -- only about 25% of the chemical energy can be turned into mechanical power, and the rest is wasted as heat. After the engine gets done with our gallon of gas we have 15 kilowatt-hours left.
Now the alternator on the car has to turn the mechanical power from the engine into electrical power. The alternator does this a lot better than the engine, but it is still only about 70% efficient. In the end we get about 10.5 kilowatt-hours of electrical energy out of a gallon of gas.
To calculate how many gallons of gas this is, you can divide the 4.26 billion kilowatt hours of energy that the daytime running lights consume each year by the 10.5 kilowatt-hours of energy each gallon of gas yields. If daytime running lights were on all the vehicles in the U.S., we would burn an extra 406 million gallons of gas each year. That's only a couple gallons for each vehicle, but in total it is more than all of the vehicles in the country burn in a day. At $1.50 a gallon, that's $600 million per year. Looking at it another way, an extra 8 billion pounds of Carbon Dioxide would be added to the atmosphere by this law."
---thanx to www.howstuffworks.com
Last edited by Joshua Leslie; Mar 21, 2002 at 06:44 PM.
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