Topic: Alternator is killing my gas mileage...

[Pinhead]

When I first got my bike, there was a few things wrong with it. The first thing I had to do was clean the carbs. That wasn't a big deal, took a couple of hours or so. I put them back on the bike and I realized that I needed a new battery. Again, I figured that would be a given, having bought a non-running bike. I got a battery and it started right up. I drove it around for a while and soon realized that my alternator wasn't charging. I went through the tests to find my rotor was shorted and my R/R was bad. No biggie, I thought. I disconnected all of my fuses (less the main fuse, lets the ignition run, but I have no lights) and drove it. After a couple of months and more than a few tanks, I averged around 57mpg or so. Power was good, though didn't have much over 5000rpm. I figured that was due to low voltage getting to my spark plugs. Ok, I'll just drive nice for a while. Now here's where the weird part comes in. I finally got my alternator to work and it runs great! Good power all the way up to redline, no missing, idles good at 1000 rpm. The only "problem" is my gas mileage has dropped to borderline crap! It went from my normal 60mpg back down to factory 40mpg. Not being one to assume anything, I disconnected the alt and fuses again and my mileage has gone back up. I figured with the hotter spark and smoother running my mileage would've went up instead of down! My question is, what would cause such a drastic change? I know it takes power to make power (simple law of physics) but I didn't think it'd be that drastic... Could there be a fault somewhere that's causing my alternator to pull so hard? Where should I start looking?

[TwoTired]

Start looking with a voltmeter.

A) Battery voltage vs RPM with a fully charged battery.

B) Then measure the voltage across the Blk and White wires going to the Alt rotor.  Readings over the RPM range.

C) Since you know how to remove the system loads from the battery. Lets see a repeat of tests A and B without the normal driving loads.

D) Then measure the ohms across those Blk and White Field coil wires disconnected from the Reg/Rect.  (Note any measurement error variables so accurate calculations can be made).

Thinking out loud...
Depending on what is found above, I might suggest adding a low ohms resistor in series with the field coil.  This will reduce the magnetic field strength placing drag on the rotor/ crankshaft.  It will also reduce your alternator output strength.
A further, option might be to add a voltmeter to the bike in operator view and adding a switch to the bike to insert or remove the resistor for the field.

Then again, maybe the tests above with show a malfunction in installed components. 

Post data, then analyze, then make a plan.

Cheers,

[Ibsen]

I don't think you can expect the same gas milage with a non working charging system compared with a working one. I have got a slight difference in idle rpm,  with or without the head light switched on (The Europen models have got a main switch for the head light). And the reason for this will be that under load, the egenerator will be working harder, and it will use some of the engines hp since it is heavier to turn because of the increased magnetic field. And I believe that it will influence on the gas milage as well. Not as much as on your bike though, but still.

[bryanj]

Not sure of the math but i seem to remember being told years ago that a generator can take 10-20% of an engines power output.

[Pinhead]

So I guess even if everything tests out good (in my charging system) than the best course of action would be to add a switch somewhere to turn my lights off during the day to lessen the load on the alternator. Or replace all of the bulbs with LED's. The switch would definately be cheaper. LED's would be easier. Is there anywhere LED bulb replacements can be bought to simply screw into the stock lighting?

I know that with the alternator on, it idled at 1100rpm and when I disconnected it, the idle jumped right up to 2100rpm or so. That's quite a difference, but the engine only has a few horsepower at those speeds, so it probably wasn't pulling that hard.

I've got to go get a new multimeter (mine got rained on) before I can continue tests on my charging system. For now I'll just run with everthing but the turn signals and brake lights disconnected.

[Ibsen]

I have never tried to disconnect the alternator completely on my 650, so I really can't tell if the rpm would jump that much, but switching the head light on and off, especially when the engine is cold, does make the rpm change approx 200 rpm.  I have checked the charging system, and it puts out 13,4 volts at idle (1150rpm), and 14,5 at 2500rpm and above.

[super pasty white guy]

For LEDs, try here.

http://www.superbrightleds.com/

[Tim2005]

I don't think that the alternator will have that large an effect on gas mileage, maybe someone on here knows the calculation for converting watts to petrol used, I've seen it ages back in a study of how much extra fuel would be used nationally if everyone drove with their lights on all the time & vaguely remember it was in the under 10% region.  However, maybe using the battery only you are babying it along at under 5000 rpm, while with the alternator connected you're hitting the revs much harder... that'll make a big difference in mileage.

[Pinhead]

I don't think that the alternator will have that large an effect on gas mileage, maybe someone on here knows the calculation for converting watts to petrol used, I've seen it ages back in a study of how much extra fuel would be used nationally if everyone drove with their lights on all the time & vaguely remember it was in the under 10% region.  However, maybe using the battery only you are babying it along at under 5000 rpm, while with the alternator connected you're hitting the revs much harder... that'll make a big difference in mileage.

I thought that'd be an issue, also, but I ran quite a few controlled runs around over a set distance through the same ammount of fuel and it all came out about the same.

When I first fixed the alternator, I definately was revving it a bit tighter, simply because I couldn't do that before! But after the first tank of getting 35mpg I slowed way down. I ran the controlled tests, ran 3 tanks with the alt connected and 3 with it disconnected. Driving style the same through all 6 tanks. The only difference was the alternator. With the alt connected, 40 mpg average and with it disconnected, 57mpg average. This was a pretty controlled test (I've been testing MPG mods for years).

I'll have to add the watts consumed by all of the lights to convert it to horsepower.

1HP = 746w.

The entire charging system is only about 250 watts, I believe, so it should pull about 3 horsepower. Given any innefficiencies in the system, I'll be generous and say 4 hp.

If my bike produces 50hp, than 4/50=8%
At 3500rpm (where I normally drive) it doesn't have anywhere near 50hp, probably more like 10 or 12. If it's 12hp, than 4/12=33%.

If at 3500 rpm and 5th gear I get 57mpg, if I lose 33% of that I should be getting 37.62 mpg, or about 40!

Wow, That should prove it. I guess all I can do to try to combat this effect is to minimize any electrical load I have on the system.

[techy5025]

250 watts is one third horsepower....not three.  The headlight is probably about
120-150 watts which would be one fifth horsepower or so.  You might be able to
see that at an idle but not at road speed....I would think.

Jim

[Pinhead]

Oops, you're right!  Dang it, and I thought I had it all worked out. Wonder why the mileage just happened to work out in the math above lol. So after doing the above math incorrectly, and then realizing it's actually 1/3 instead of 3, I assume I have to look for something having to do with the higher voltage instead of having to do with the load on the crank shaft. Thanks!

[Steve F]

You said"....After a couple of months and more than a few tanks, I averged around 57mpg or so. Power was good, though didn't have much over 5000rpm. I figured that was due to low voltage getting to my spark plugs. Ok, I'll just drive nice for a while."  "..........I finally got my alternator to work and it runs great! Good power all the way up to redline,"
I can't help but think you're not takin' so easy anymore. 

[Terry in Australia]

Well I'd reckon that 57 Mpg is the exception rather than the rule, and 40 Mpg (or thereabouts) is about normal? In Oz, our old bikes all had switches so we could turn our lights off, but the fuel consumption is still around 40 Mpg, on average. (US Gallons, not Imperial)

You could always run an ARD Magneto for spark and ditch your alternator completely, (I will on my racer project) and just run a gel-cell battery just for brake lights and indicators etc, and minimal (read "bugger-all") headlight use. The Ard will still suck a little power out of the engine, but less I'd imagine, than your alternator? Cheers, Terry.