Naked Science Forum
General Science => General Science => Topic started by: Karsten on 05/10/2009 20:29:56
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Does a truck experience a decrease in fuel mileage if they are followed closely by a car that in turn experiences an increase in fuel mileage? Or, at least does the truck slow down a bit? In car racing they say two cars drafting can go faster than just one car. But what about the fuel consumption for leader?
I would say the truck uses more fuel, since someone is obviously helping the car to save gas and that costs the truck some fuel. True?
If yes, are the savings/losses equal?
If not, where does the energy to "pull" the following car come from?
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When slipstreaming in this way the pursuing vehicle is taking advantage of lower air pressure behind the lead vehicle and thereby experiencing reduced air resistance; this translates into a requirement to do less work and therefore lower fuel costs. This drop in air pressure is a consequence of the lead vehicle pushing the air ahead of it "out of the way"; this is work it would need to do regardless of who's following, so I don't think the lead vehicle suffers any significant additional cost due to the presence of a tailgater; it's just that the tailgater benefits.
What does everyone else think?
Chris
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That is what I initially thought too, until I was persuaded otherwise by sophiecentaur in this thread http://www.thenakedscientists.com/forum/index.php?topic=19075.0
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I think Karsten is right that tailgating will improve the fuel consumption (and speed) of both vehicles. The drag a vehicle experiences is due to displacing the air ahead (increased pressure ahead) and also due, to a lesser extent, the low pressure region behind. Manufacturers use spoilers, which are not always just for show - they change the airflow at the back of the vehicle and increase the pressure at the back slightly. Tailgating (if perfect) diverts the airflow over both vehicles so removing low pressure region from behind the lead vehicle. The guy at the back gains the most though as he does not have the dominant high pressure ahead of him.
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I think Karsten is right that tailgating will improve the fuel consumption (and speed) of both vehicles.
I am afraid that I am NOT right if tailgating improves the fuel consumption of BOTH vehicles. I thought it would improve the fuel consumption of the follower and reduce it for the leader. I still wonder if it is in equal proportions.
I liked the comparison in the above link to the other thread (which I vaguely remembered) by Sophiecentaur: Make the follower a really big truck and see what happens to the leader. It will have noticeable effects right when it happens. The leader will slow down and, if with a small engine, may not be able to pull away.
I would love to see this tried.
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Does a truck experience a decrease in fuel mileage if they are followed closely by a car that in turn experiences an increase in fuel mileage?
Yes. The car is saving fuel, but the truck is paying for it.
Or, at least does the truck slow down a bit?
It will either slow down, or increase fuel consumption to maintain the same speed.
In car racing they say two cars drafting can go faster than just one car. But what about the fuel consumption for leader?
(OK - I'm guessing) I suspect that if the trailing car gets close enough to the leader the two vehicles begin to act like a single vehicle. The combination has double the mass and double the power, but because it has reduced drag it can go faster for a given amount of power.
I would say the truck uses more fuel, since someone is obviously helping the car to save gas and that costs the truck some fuel. True?
Agreed.
If yes, are the savings/losses equal?
I don't think so. The combination is more efficient. In the case of a car following a truck, the cost to the truck is much smaller than the saving to the car.
If not, where does the energy to "pull" the following car come from?
Right. Somebody had to pay for it. Had to be the truck.
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When two vehicles are travelling independently both have to push air away in front and overcome the extra drag of the low pressure region behind. When tailgating, the front vehicle gains a little from the reduction in the drag by the lessening of the low prssure behind, and the rear vehicle does not have the high pressure in front. The rear vehicle gains more but the front vehicle also gains a small amount. There's no magic here and nobody is getting something for nothing. An exaggerated example - eight serially coupled railway trains would use a lot less energy than eight running parallel of separate lines.
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Fascinating stuff. Thanks!
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So, I am getting conflicting views.
One says the leader saves - the other the opposite.
I still have a few questions:
I read somewhere and many years ago that leading race cars experience acceleration when the car following in the slipstream leaves the slipstream to pass. Nonsense?
What if the leading vehicle is significantly smaller than the follower? Will the lead car experience a disadvantage in this case? (e.g. Smart car 2 meters in front of large truck)
How does this look aerodynamically if a vehicle drives through a tunnel? Do the walls of the tunnel cause extra resistance since they inhibit air flow? I would guess yes. May I conclude that objects in front of and next to a vehicle result in a disadvantage while objects behind the vehicle create an advantage? When (at what approximate angle) do the advantages begin? At what proximity?
Is it possible to stay in the slipstream of a vehicle with a light vehicle and just aerodynamic means? (For example a bicycle with a special sail following a large truck). If that is possible, does the truck slow down a bit in this case? Why would it get faster or run more efficiently? Is there an area in the slipstream where air would "suck" a vehicle along? I remember riding my bike close to tractors and doing very little.
I wonder if race cars go faster in tandem because they want to go faster. What if the follower tries to use as little power as possible instead and stay in this (imagined?) area of "suction"? This would be the case when someone drives behind a truck on a highway - they try to save on gas.
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My practical experience is somewhat limited. In my teens, I used to "slipstream" double-decker buses on my bicycle. I know for a fact that I was expending a lot less energy than I would have been without the bus in front! I also doubt that I was helping the bus, but I have no means to prove that. Perhaps I was. (BTW, while some might consider this a dangerous practice, the brakes on the particular double-deckers were so pathetic that the risk of my rear-ending one of these buses was extremely low.)
I also have some sense of a retardation effect produced by a vehicle approaching from the rear. We have a travel trailer (caravan in UKspeak) that we pull behind our truck. When a large semi (articulated lorry in UKspeak) is overtaking us, I am aware that my vehicle has reduced speed, or is consuming more fuel to maintain the same speed. This seems to contradict the view that the net consumption is reduced. However, it is a very complicated subject.