Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Energy implications of drafting - a question from todays show

A caller today asked about the effect of another car drafting behind him. You answered that drafting was a slight energy benefit to both cars (and also unsafe). The caller was not convinced - he felt that if the drafting car was using less energy that energy has to come from somewhere - probably his car.

The answer is the aerodynamics of the system (2 cars, 3 cars etc together) has been improved. The two cars drafting together are aerodynamically superior to the sum of the aerodynamics of the two cars separately. When ever you improve efficiency you are doing the same work with less energy - you are not creating new energy

I agree with your response about the statement on efficiency but that doesn’t answer the question if the fuel consumption of the lead car is better by drafting. Even if the sum of the two cars aerodynamics are better than individualy, that doesn’t assume any portion of the savings is from the lead car.

There is a high pressure zone in front of the car and a low pressure zone behind the car. When two cars get very close, the high pressure zone in front of the following car cancels out the low pressure zone behind the lead car and reduces the power needed for both cars. The very least power is used by the middle car which is both drafting and being drafted.

Watch NASCAR racing and you will frequently see a pair or trio of drafting cars slowly close the gap on the lone leader. If the lead car wasn’t getting any benefit, the pair would only be able to go as fast as the lead car would go solo.

Did I hear wrong, I thought the caller said he drove commercial truck. The laws change slightly when drafting a big rig. With two vehicles the same size, the closer the better, but with a large differential, then that is not the case.

There is some benefit to both vehicles if the smaller vehicle doing the drafting stays further behind. If you get right behind a commercial truck in a car, most of the car is exposed to the rush of air coming from underneath the truck. This air has been compressed and when it exits the rear, it blasts backward. But the box does have a low pressure area behind it. The low pressure area looks kinda like the sock and a small airfield when the wind is blowing, but not to hard. It sticks out, but droops. A car will get the best draft about 5-10 car lengths behind the truck, and it will is some small way help the truck by filling the void so to speak.

A car at the right distance will act like the long slopping tail seen in those old pictures of aerodynamic cars from the 40’s and 50’s Popular Mechanic’s magazines and such. 5-10 car lengths should be reasonably safe as well.

Does it really matter? It is unsafe and talking about the energy it conserves might convince some idiot to do it.

That being said, the law of conservation of energy states that for each action, there is an equal and opposite reaction. With no car behind the truck, the reaction affects the wind only. With a drafting car, some of the reactive wind transfers energy to the car. In order for this action-reaction sequence to steal energy form the truck, it would have to benefit from the absence of the drafting car. Without the drafting car, the reaction is spent on wind turbulence only. If there is a benefit to the truck, I don?t think it is enough to be measured.

I have noticed with my 98 Civic that I can follow a semi at a safe distance and bring my fuel economy from 36 MPG to more than 40 MPG. Drafting isn’t necessary.

The tv show Mythbusters addressed this question a couple of months ago and using semi-truck and the chase car was a Chrysler sedan proved that there is an energy savings in drafting a big rig. But, and this a big but the distance behind the truck was extemely dangerous to say the least. The speed involved was I seem to recall 70 mph and the dinstance behind the truck was only 4 feet!

There was some talk about “conservation of energy” on the talk show and in this thread. The actual reference should be to the first law of thermodynamics which states that there is conservation of energy in a CLOSED SYSTEM. A truck and a car are not in a closed system as there are many factors at work.

This is all moot since the discussion is academic. Nobody should be following closely enough to realize the energy savings. This reminds me of the discussion in my high school physics class where we debated whether or not you would survive if you jumped barefoot on the third rail of the NYC subway. The key to the debate was that you had to jump on with both feet at the same time. The theoretical answer is “nothing bad would happen” (e.g. bird on a wire). None of us wanted to test this theory in the real world.

Your post is wrong. First of all, conservation of energy is one thing; Newton’s third law (the action-reaction law) is about forces.

The original poster is correct. It is possible to improve efficiency, so the “new” energy comes from it not being wasted, and is not robbed from the truck in front.

And it is dangerous.

“This reminds me of the discussion in my high school physics class where we debated whether or not you would survive if you jumped barefoot on the third rail of the NYC subway. The key to the debate was that you had to jump on with both feet at the same time. The theoretical answer is “nothing bad would happen” (e.g. bird on a wire). None of us wanted to test this theory in the real world.”

Never say “nobody”.

The real problem here is of course aerodynamic, as others have mentioned there is a pocket of air created behind the big rig. The real question is if the pocket of suction created by the big rigs unaerodynamic rear end is smoothed out at all by another vehicle. If it worked it would be much like the dead zone of air created by a closed pickup bed compared to the suction zone created by an open pickup bed (there also was a test done on this by mythbusters). The drafting vehicle can’t take energy from the big rig it can only take advantage of and, depending on its size and distance, maybe affect the suction zone behind the rig. Nothing following the rig could rob as much energy from it as the pocket of air pulling back on it, in fact the suction zone created by flat surfaces is why fast back roofs were add to muscle cars, to remove the drag created by suction on the vehicle at high speed NASCAR tracks.

If you watch a Nascar of Indy car race, you can see the benefit of drafting, it happens all of the time in racing, it save fuel for the cars, and the wind is doing the work in this case, by creating a slipstream of air over the cars, and pushing the rear car forward. It would be dangerous to do this on the highways as you have to be too close to each other. The Car Talk Boys were right on the mark with their answer!

Much of the energy used to keep a car going is that of overcoming wind resistance. That’s why you can get much better mileage in a high wind if it’s a tailwind rather than a headwind. Airplanes use this to their advantage.

Two major elements of the wind resistance are the high pressure area in the front of the car and the low pressure area behind the car. That pressure difference uses energy from the engine to overcome.

Real drafting at high speeds, like you see NASCAR drivers do, has a significant advantage. It reduces the low pressure area behind the lead car (essentially moving it to behind the trailing car) which is part of the wind resistance equation, and substantially reduces the high pressure area in front of the trailing car, which is also a variable in the equation. Both cars benefit. The pressure differences between the front and rear of both cars become lower.

HOWEVER, drafting close enough to have an affect is extremely dangerous and should be left to race drivers. On the track they are all highly skilled, highly trained, highly experienced professionals in good physical condition. There are strict rules that they all must follow and they all know exactly what to expect. And their cars are all in excellent shape…there are no unmaintained vehicles out there. Their cars are designed to protect them in unbelievable crashes, and full-out emergency crews and equipment are seconds away.

Do not draft on the highway.

I agree with the others, it will benefit both cars (mostly the following car because it will see more drag reduction), and will not “steal” energy from the lead car (that energy has already been expended anyway. However, it is unsafe; don’t do it.