Science Buddies: "Ask an Expert"

So, my proposition is this: (Note that this is a purely theoretical project, and no models/experiments are necessary.)
When a car moves forward, wind moves backward, right (relatively). So if we were to attach a small, retractable wind turbine to the side, connect it to the alternator, we could potentially power the headlights/alternator stuff-essentially taking load off the engine.
Ordinarily, the rotation of the wheel powers the alternator (which consumes petrol). So, instead, if we connect a wind turbine to the alternator, we use less petrol (I've calculated approx how much by using calorific value, energy reqd. for battery charging, energy reqd. for head lights etc.) I've also calculated turbine size/capacity that is required to save a decent amount of petrol.
Is this a valid theory? Is it too simplistic for an 11th grader? What are the points I need to include in my project report? Do I need essential additions that I left out?

Hi quietfury,

If I'm understanding you correctly, your analysis neglects a couple of key factors. First, the amount of power generated by a wind turbine depends on the rotation rate of the rotors. The rotation rate of the rotors, in turn, depends on the speed of the wind blowing past the rotors. In this case, the wind speed is controlled by how fast the car is traveling, which is set by the petrol-consuming engine of the car. So, you won't get improved fuel efficiency with this approach: the motion of the car creates the "wind", but that motion is powered by the petrol engine; therefore, any energy produced by the turbine is ultimately sourced from the petrol engine. In addition, the turbine would likely adversely affect the aerodynamics of the vehicle, further decreasing the efficiency of the vehicle. If you don't have time to refine the calculations, you could mention in your report that these additional factors would need to be taken into account for a more correct analysis.

I'm not sure what your teacher's requirements are, so I would ask him or her what should be included in your report. Science Buddies also has some great articles about each of the sections that are typically required in a science fair project report. You can find those articles here:

https://www.sciencebuddies.org/science- ... ndex.shtml

Thanks for your input!

I have factored in the wind speed, assuming the car to be moving at an average speed of 40 km/hr. This will provide impulse + reaction torque in the blades of the turbine. I calculated the torque, the total energy generated and everything else. The amount of petrol that's conserved came out to be 32%, which is a substantial amount.

For different speeds of the car, wouldn't the amount of petrol consumed increase proportionally with how much energy the turbine's generating? Thus meaning that the total amount saved wouldn't vary by much?

If the 'wind' powering your turbine is coming from the car's engine, then so is all of the energy coming from the fuel. There will be no net fuel savings unless *perhaps* you factor in a strong headwind. In that case, the overall system will be getting additional input from the wind. But in general, adding the increased drag of the turbine will cost you more fuel then you get back inn electrical power.

Here's another discussion of the same on the web: https://www.physicsforums.com/threads/w ... tly.422295

It's a nice idea but there are much better ways to save fuel. A better aerodynamic design, a smaller engine, etc.

Here, we're directly connecting it to the alternator, though. So, although it may affect the aerodynamics of the car, I don't see how it'll derive energy from engine fuel.
Also, the other thread was discussing a slightly different issue. I'm considering the drag force now, though. So thanks!

Hi quietfury,

I hope I can clarify a couple things further, let me know if this helps:

1. Aerodynamic drag is one of the major forces the engine of a car has to work to overcome. Going faster increases drag (drag is roughly proportional to velocity squared), but you have controlled for that in your calculation with a constant velocity. However, anything that adds to the exposed surface area of the car also increases drag - like adding a roof rack, passengers sticking their arms out the windows, or adding a wind turbine. Anything that increases drag means the engine has to work harder (and thus consume more fuel) to overcome that drag and maintain the same velocity. This is true whether or not you try to recover any of the energy (e.g. a roof rack vs. a wind turbine).

2. This is also true regardless of where your recovered energy goes. It doesn't matter if your turbine is connected to the alternator of a regular car, the battery of an electric car, or a charger for your iPod. Ultimately, the car's engine (whether gas or electric) will have to use more energy to overcome the drag introduced by the wind turbine than you can gain from the turbine acting as a generator. Otherwise, this setup would allow you to build a perpetual motion machine (a car powered entirely by energy from wind turbines attached to the car), which is impossible.

So, your original statement "So, instead, if we connect a wind turbine to the alternator, we use less petrol " isn't true - the engine still has to work harder (consuming more petrol) to overcome the additional drag from the wind turbine. I hope that makes sense.

-Ben