Check my math: car electronics.

[skids]

Modern gas IC engines are 30% efficient.

The alternator in a 12V auto system is typically 55% efficient.

Let's say gas averages $2.70 a gallon -- a very generously hopeful rose-colored-glasses estimate, given likely future trends.

That would mean that the electricity used to run electronics inside the car costs 50 cents per kwh.

(It may be more than that. I don't know whether this is the case in more modern vehicles, but I saw it mentioned that the alternator actually has it's field coils turned off a good portion of the time, running the car on battery until a low hysteresis threshold is reached, then charging the battery up in spurts. That means most, or at least a good portion, of the electric power enters and leaves a lead-acid battery, a process that is only 75% to 85% efficient.)

Why worry, right? We don't use that much power in cars, right? Not so anymore. Now, the living-room-on-wheels models are moving in the coming years to a 42V system with a more efficient (75% to 80%) alternator perhaps even with regenerative braking even in non-hybrids.

But even in your normal car, more and more electronics are in use -- HUDs, fuel injectors, computers. And don't forget the headlamps at night. I can't nail down a definite number for normal cars -- the living-room-on-wheels are starting to draw 2kw plus. My guess is that averaging out a normal headlamp usage pattern we are talking between 200W and 1kW. Likely 200W is a good floor to work with.

So at least a dime per hour of commute, possibly as high as a buck.

People have been using little solar panels to top off their car battery for years now. Depending on the car they can be plugged into the cigarette lighter or may have to be directly wired. The point with these is more to preserve battery life by float charging than it is to generate usable power, so they are low wattage panels. They pay for themselves in lower battery replacement rates and less jump-starts.

Obviously solar doesn't work at night. But thermopiles do (even moreso as outdoor temps are lower.) Right now those are at $8/W if you or I were to buy raw units off the web today. They aren't heavy, and neither are the electronics to professionally tie them into the system. If you shaved the bottom off the alternator load with one of these attached to the exhaust or block and installed, say, 200W of capacity, say at a cost of $2k (adding $400 for fitting/heatsink/etc) and gas didn't get cheap again, payback would occur at 4000kwh, or 20000 hours driven. Whether this is within the lifetime of the car itself depends on how much you drive. If the cost of a complete thermoelectric system were brought down, or gas prices go up, payback could be within a typical car loan period (interest subtracted) for average drivers and a stellar deal for professional drivers (pizza, taxi, whatnot.)

So post-factory mod systems like the one Hi-Z has fitted to a semi trailer could enter market parity now even for passenger cars, not necessarily just for rigs. If my numbers are right. Are my numbers right?

http://www.hi-z.com/websit07.htm

(Not that it would help me as much as others, as my Prius has a more efficient generator and and smaller 12v system.)

[Turbineguy]

1. Let's

not forget those welding cables to power amplifiers so that the rest of us can enjoy having our cars shaken at stop lights...

(don't get me started!)

[NNadir]

2. First of all, you are missing one important variable, the current.

In AC systems, the RMS power is a function of the RMS voltage and the RMS current, but even if the RMS power were 2000 W, the actual power requirement, in comparison to the engine drive, would be very small.

You are neglecting that most of the power generated by an internal combustion engine is devoted to moving the car, and not running the electronics.

One horsepower is equal to 746 watts. Thus the electronics of a car, even with all kinds of electronic crap in it, will seldom ever require more than three horsepower (electric) to operate, ignoring generating efficiency.

A Honda Insight has a 67 horsepower motor. A Toyota Prius has a 77 horsepower motor.

It may be that a solar battery can be of some use on a car like a Prius or an Insight which can actually use the electrical power produced by solar cells but the amount of energy would be relatively small. Even if the solar flux is 1000 W/m^2, and the efficiency of the solar cell is 100%, this is just barely over one horsepower, assuming that the car remains perfectly aligned with the sun, which of course, it won't.

Indeed a large back-up power generator from Kohler, capable of powering a piggish McMansion requires only a 13 hp engine.

http://www.continentalengines.com/pdf_files/kohlergen__resgen8511rmy.pdf

This generator has a power output of about 10 kW.

[skids]

9. Don't quite get your point about AC.

When I say most current alternators are 55% efficient, that is their energy efficiency in converting mechanical power to watts DC, post rectifier. All car electronics run off post-rectifier DC these days.

(I'm actually a degreed EE, so I do understand AC RMS quite well.)

I'm not neglecting that most power goes into propulsion. Depending on your driving speed and overall mileage, at current gas prices, the cost per hour of drive time is much higher (several dollars) than the amount I calculated for the cost of electrical load.

Your estimate of horsepower required is a bit low (not much) mainly because generator efficiency cannot be ignored. Check the automotive discussion boards and you'll find that alternators drain 4 or 5 hp, sometimes as much as 7 or 8. Probably due to the inefficiency of both the alternator itself and the mechanical coupling. Further, the vast majority of the time that you drive a car, you are not using the full hp of the engine, only a small fraction of it to maintain speed. When this is the case, again, most of the time, the proportion of hp used for electronics is signifigantly higher.

I'm not really concerned about solar/hybrid (that was covered by an earlier article in this group, where a working model bodged together with consumer electronics parts produced a modest but respectable gain in mileage in-flight, suggesting an optimized design with off-time-charging capability might produce a signifigant gain) but rather thermoelectrics. Those work on engine waste heat, which is available whenever the engine is up to operating temperature.

Like I said, without the ability to utilize the energy for propulsion, solar isn't around when you are running headlamps. It's good for extending battery life, but can only be considered for fuel economy on a hybrid with a battery pack available for propulsion.

I'd be surprised if the kohler generator had a mere 55% efficient alternator.

As far as whether it would be cheaper to simply replace the alternator with a more efficient basic design -- yes there are a few available, but it would also be easier to void manufacturer warranties that way. A post-production "accessory" for current automotive stock could be developed that would not do so as it need only interface with the battery terminals and a surface from which heat can be drawn. The only real challenge is getting the two charge controllers to pay nicely together.

[NNadir]

12. I'm sorry, I didn't get you're point. My bad.

You were discussing thermoelectric generation. This is an interesting idea.

Thanks.

[bemildred]

3. I dunno.

I recently drove 4 hours in stop and go with the sound system
on the whole way, and guess what? My battery went dead.
Maybe a solar panel on the top would help, but I would bet
that a better alternator (or fewer speakers, or a cooler day)
would be more to the point.

[NNadir]

4. If your battery went dead, and you noticed it, you need a new alternator.

[bemildred]

5. I know. It's "underperforming" right now.

I got it jumped, and it got us home (no sound system),
at night, at speed, and had enough charge to start it
several times along the way, so it's not like the alternator
is dead, yet.

I'm faking it with a trickle charger now, which also allows
me to keep an eye on it, until I feel like springing for the
rebuilt alternator.

[NNadir]

6. On the other hand, maybe you're getting into the blame game with the

alternator. It's doing the best it can under the circumstances. No one could anticipate that you would play the stereo. I think the battery has the primary responsibility any how. And where, by the way was the voltage regulator? I'm sure that communication with the wires hasn't been the best.

And let's look at your own behavior. You could have evacuated the car but still you chose to stay with it.

[AZCat]

7. wow...

I had to read your post several times.

Awesome work, NNadir, awesome work. I think I'll print this one out.

[bemildred]

8. True, I should have just left when the opportunity offered.

Who am I to blame the alternator? The battery was already dead.
I could have taken the hint in a timely manner and saved myself.
Perhaps I even endangered other drivers by my selfish obstinacy.

[skids]

16. We've had enough blown fuses.

Please stop the blown fuses. We need to declare a moratorium on blown fuses. It's too damn late. Get off your ass and jam a penny in there.

You need to find your alternator and replace it. Get a caring alternator. Get a sympathetic alternator. Just don't get the same alternator.

[Massacure]

14. Haha. Taking a page out of the Shrub's playbook. I love it!

:rofl:

[rfkrfk]

10. 46 cents

a kilowat-hour of electricity is 3412 BTUs.
gasoline has about 120,000 BTUs per gallon, I assume that to
be the lower heating value.

because of two inefficiencies, 0.3, and 0.55 ,
you need 20,679 BTUs of 'gasoline input'.

which is 0.17232 of a gallon

multiply by 2.70

result, .46527

[NNadir]

11. This is a very good calculation so far as it goes.

However, unless one disconnects the alternator, one will recognize very little savings with a solar panel. If the alternator is 55% efficient then 45% of the energy produced is simply losses from friction and heat in the alternator.

But who am I to say? One can put a solar cell on one's car roof if one wants to do so. It certainly would be of value for hybrids, as some hobbyists are showing. It doesn't do any harm and it may do some good.

[skids]

15. It's mostly resistance and flux loss.

That's why I didn't include the pully loss in the calcs.

When the alternator is off, the feild coils are off. You do still get friction and pulley loss but most of the losses are not in the bearings, rather in the source resistance in the primary coils and the heating of the core laminates from flux hysteresis, and to a lesser extent airgap flux leakage and smaller eddy currents -- those latter two are minimized by the laminates and general design of the alternator -- but there really is no cheap way to control the former two.

[muriel_volestrangler]

13. I'd say 20000 hours is a long, long time for a car to last

2 hours a day, every day, in your car, for 15 years is 10950 hours. If you managed to average 25mph then that's 270,000 miles. So 20000 hours is huge. Unless you spend the whole time in traffic jams.

[skids]

17. Yeah, just on the virge of economic viability.

If you're a pizza delivery driver, it might work out. They need to bring the cost down on these things a bit more. If it were built in to the radiator that would bring down the cost signifigantly, but as far as a non-intrusive post-factory mod it is teetering on the edge of being practical.

I hope the guys at Hi-Z have success with their big rig version, though they seem to charge an awful lot for their modules compared to tellurex.

[Isntapundit]

18. Anyone hear of this?

This page is saying that adding very low volumes of acetone (1 part per 3000 to 5000) can increase your fuel efficiency by up to 30%

http://pesn.com/2005/03/17/6900069_Acetone/

http://peswiki.com/index.php/Directory:Acetone_as_a_Fuel_Additive