It would be more efficient to use the vehicle's alternator to charge the battery directly, and to cut out the propeller step. Which is sort-of-why the propeller idea isn't done.

However, the propellor-generator was a useful device in the saving of many lives when an aircraft ran out of fuel in mid-Atlantic and had to glide to the Azores.

It would more than counter any power generated... else you'd have the possibiliy perpetual motion..
We've had this idea posted several times.
Del

How about always parking on top of a hill so that you always only drive down hill? That will surely save gas...

I think we had this discussion.

Chris

"I think we had this discussion."

Yes, we did. And this one: another discussion

Would the increased air friction (gas consumption), counter any increase in power generation?

Of course.

Efficiency may be attained if you mounted a small generator and a propeller on the outside of a static vehicle in moving air...

I realize this borders on perpetual motion which I know is a no-no. I was thinking more along the lines of a very small propeller driving a small generator to provide a trickle charge similar to that you would get from a solar panel. Placed on a big rig truck, very little drag would be added.

I believe similar devices are used on boats and airplanes. On a boat it is located underwater to indicate speed and on a plane, to do the same.

One really interesting version of this is called an Autogyro. Basically you can do what you are asking. The rotor is a net drag on the system, which is tilted to the rear. All forward motion is provided by the propeller. I believe modern versions can feather the rotor blades, but you can maybe change the rotor tilt to alter lift profile too.

Chris

Many vawt types placed at locations large trucks pass would be a net gain.

I've never seen a propeller driven speed measuring device on an airplane, though I won't say it hasn't been done. Normally, a pitot tube and static port pressure are compared and the difference displayed as airspeed. Time lag, and erroneous readings if the airplane is not flying in a straight line without pitch or yaw, will eliminate use of a propeller as a viable means of obtaining speed information. Add the need for moving parts, and it is dead.

In any case, consider: if a propeller / wind turbine is attached to a moving vehicle, there are losses in coupling the relative wind to the prop; there would be less loss by coupling to a moving wheel in case of a car or truck, or better still, to the engine itself. Power transmission between prime mover and electrical output will involve more loss through airstream and propeller than it would to use direct drive, gearing, chain, or belt drive - and perhaps even hydraulic or pneumatic drives. And the load imposed upon the engine by the added drag will further skew results in favor of more direct means. Period.

Placed on a big rig truck, very little drag would be added.

Very little drag added, and even less power produced. As soon as you claim to generate more power than the loss from the drag created, you are talking about "over unity," "free energy" and perpetual motion. If there is some need for electrical generation, it is most efficiently handled by direct drive from the prime mover.

Planes do not use propellers for speed indication. Some boats do, and the tiny propeller reduces the boat's speed potential ever so slightly.

A case in which a propeller would "make sense" (in a theoretical sense, but not in a practical one) is if you used it as an air brake. When you wanted to slow down you'd deploy the retractable propeller/generator unit to slow down, and charge batteries. When acceleration was subsequently required, you'd power the prop from the batteries. It would be a crude, complicated, and inefficient form or regenerative braking.

Ron, TANSTAAFL and the third law of thermodynamics (they are essentially the same.) always applies, you never get back as much energy as you put in.

Also, after the alternator has recharged the energy used to start the vehicle, for all intents and purposes the car is running on the alternator. The battery is essentially a parasitic load at that point.

I think you've stumbled onto something most have missed so far.

You're description is not of replacing or generating extra-power for the vehicle. It is a given the vehicle in motion will use energy.

What hasn't been observed is the energy which maybe reaped without interfering in the normal use of said vehicle. A freight hauler for instance could provide a means to collect, store then discharge energy to a storage unit periodically.

Is this the thought keeping you awake?