alternators on race cars

[Kev_205]

I am having a little debate on another forum
There is a product available that claims to give 5bhp + by turning off the alternator at high revs.
Now the product can be found here
http://www.gmcmotorsport.co.uk/freepowr.html

now the arguement has two sides
one saying an Alternator only put 'resistance' on the belt when they are actually compressing or making power. Otherwise there is no resistance as the units simply freewheel!

My theory is that the frictional losses from an alternator are pretty much constant and changes in electrical load are of little consequance
The resistor in the alternator regulates the current created by monitoring the difference between the alternator and the supply the battery is producing. So if no electrics are on the alternator is drawing less current to the electromagnets inside
So a fully charged battery requires no power from the alternator and using GMC's theory the car will gain power as the alternator is "off" in their mind
That would mean by plonking a new battery from halfords my car will gain 5bhp minimum

Discuss!! Ive reached the limit of my technical knowledge on this one lol

[zefarelly]

alternator 4-5kg
race battery 4-5 kg

no alternator 0kg
large battery 10-12kg

at least thats my theory for my car, and it wouldnt last an hour without charging
for most engines I think the power loss is so negligible as to be not worth worrying about

[Tainan]

The higher the load on the alternator (ie the more current there is coming out of it) the more power it takes. If you're not sure get a small electric motor, short the terminals together, and try turning it by hand. Rather than buying a unit that turns it off at hi revs you could just install a manual switch to turn it off while you're racing. Either option is probably against the rules for any series where a working alternator is required though. As above, once the battery's re-charged after starting the engine, the alternator isn't doing much anyway so its not really worth the bother.

[dtype38]

Hmmm... I may be wrong (has happened before ) but my understanding is that the battery gets drained a bit when you start the car (to power the fuel pump, ignition and starter motor). As soon as the engine is up to speed the alternator will start recharging the battery (time to fully recharge depends on revs and how much it took to start the car). Once the alternator is spinning, though, it supplies all the current for the car (as well as charging the battery) up to its maximum output. The latter usually depends on revs. At tickover it should cope with a fuel pump and the ignition, but may not be up to headlights and a rad fan running. As soon as you start driving, though, the alternator should supply all the current demands and still keep the battery fully charged. In this situation you can think of the battery as a sort of "back up" which will take over supplying current if the alternator fails or isn't big enough for the demand.

Anyway, the more things you have swithed on, and the bigger your equipment the more current the alternator has to put out, so the more power it takes from the engine. The figure of 5hp will only apply to one situation, but doesn't sound unreasonable for peak demand on an alternator. Beware, though, that this peak may only be what is actually being used on a dark rainy day with all your lights on, the windscreen wipers running, the demist fan on max, and your engine is overheating so the rad fan cuts in!! On a cool sunny day your alternator will be taking much less. Soooo.... what I'm getting at is the high revs cut-out theory for the alternator is sound, but might not give you nearly the power improvement you were expecting. It also will not give you any extra power at lower revs, which might be important to your acceleration out of corners rather than your top speed.

I don't know how well the battery would cope with the constant charge/discharge cycle, but I would have thought a proper race battery would handle it better than a wet battery. A possible improvement on the whole idea would be to look for a system which is linked to throttle position and cuts the alternator when you floor it, but lets it draw hp when you are feathering or off the throttle... then there should be no down side at all

[greenamex2]

Aha, this old chestnut.

Turning off the alternator at either high revs (via a decent ECU) or full throttle (via a microswitch) would probably help most cars a small but measurable amount.

On my car (9000+rpm) it will LOOSE my power because my ignition coil needs all the help it can get at those ridiculously short dwell times.

Running without one at all. There are two really incredibly stupid reasons for not finishing a race and often then not winning a championship -

- Running out of fuel
- Running out of battery

I'll swap probably less than a quarter of a second for a finish any day.

[R59]

(Stolen from a text book...)

If using a gasoline engine instead of a diesel, with a slightly lower 2 to 2.5:1 drive ratio, figure on a requirement of about 1 hp per 15 amps of output power + about 1-3 hp for engine operating/muffling loads -- i.e., 11-14 hp for a 150A alternator.

-----

So, you'r compact 45A alternator is going to take about a third of that power, especially these new super efficient ones. So, you're looking at 4hp at full output.

Choosing the right oil, and some clever oil additives, you can get that back.

What does make me laugh are people who get in front in a one close make series, and then turn on their headlights. Suddenly the alternator is a full load, and you've lost that couple of horsepower that keep you in front...

In something like mine, 4hp at full rate charge - who cares!

As Zef said, no alternator, big battery. Weight disadvantange. Then when it's heading for flat, your super sparks system starts to falter, misfire sets in, you come last / DNF.
Tried that, not good idea on fuel infected engine!

Rob.

[R59]

Denis, I've told you not to mention running out of fuel when I'm around.

[greenamex2]

I wasn't actually thinking of you, but the three people that have run out of fuel already this year and the one person who kept developing a voltage related misfire.

Don't worry, if anyone starts a thread on defective injection wiring looms you'll be the first person pointed at!!!!

[greenamex2]

Back to the original question.

Frictional losses will be AT LEAST proportional to revs, but more likely proportional to the square of the revs.

The load sensing characteristic detailed aren't totally correct. The alernator will ALWAYS be producing a higher voltage than even a fully charged battery (typical alternator = 14.4V typical new battery = 13.2V). And as a result of one of the fundamental electric equations (I=V/r) there will be a current draw and hence an increased load.

So, for a given revs the alternator (and belts and pulleys etc) will ALWAYS draw a certain frictional load but the additional 'supply' related load will depend on how much the battery is contributing versus the load presented by all the electrical gubbins being powered.

Hope that makes sense.

[Al Weyman]

Well I rigged up switch on the field coil on a chevy v8 and when I had it on the rolling road after it had been running a bit so it had charged itself up, I said to the operator, can I try flicking the switch to see if it makes any difference and he laughed. But not for long because as soon as I switched it off you could see a 5 to 7 hp rise and he was gobsmacked and could not believe it but it was 100% true.

From another angle, a friend of mine used to run in a short oval series where for some unknown reason you could not run an alternator. Well he reckoned this was a bad idea for the reasons posted above so rigged a secret alternator running off a belt on the main propshaft (it was an independant rear suspension seirra so no problems with the prop going up and down). So you see us racers if we can't do it we want to, :-)

[graham bahr]

when i only had around 190-200 bhp to shoot with i ran without an alternator, but to be honest i'm really not sure the hassle of constantly changing and charging batteries was worth it, now i have an alternator i use a much smaller battery, so the cars no heavier, and with 350bhp to shoot with now the losses from the alternator is now only consuming 1.5% of my engines power, which you will struggle to notice and i now know if i spin out or stall my engine WILL definately restart, i once had a DNF due to a flat battery, the sickening thing was i was leading the race and would of stayed there until a voltdrop missfire started and got gradually worse and worse

[Kev_205]

so are you beter off minimising the electricity usage of the car? If the car electrics are not needing much juice then you can fit a small alternator which will give less frictional loss at minimum and less when at full load.
So for a racecar theres no real point?

Al Weyman - did you ever experiment with this further to give an advantage on the track?

[zefarelly]

the belt Idea for the propshaft isnt a new one, the Diva GT did it in the mid 60's, it may relaesa a few horses at the flywheel, but you'll just loose it again in friction losses

I wonder if you could get one to run off your tyre like we used to have on our pushbnike dynamos in the 70's . . . . it'd bea good incentive to drive faster asit'd generate more charge.

the reality is almost definitely that the alternator is the most efficient way of generating power otherwise everyones cars wouldnt have one !

[Al Weyman]

I had it rigged up and used to turn it off sometimes but like Grahame I had a fair bit of power so I could not really notice the difference it is very easy to do though, just a switch break in the field coil side of the alternator, switch it off it does'nt charge, simple as that. I think on a low powered car it could be used to advantage during certain stages of a race if for example you were neck and neck with a similiar car 5 bhp for say the entire straight at snet may just put you an all important couple of feet in front at the end of the straight, but to be honest I really don't know. Got the tip out of an American Stockcar prep book by Steve Smith.

[graham bahr]

heres another angle,

i also run an electric waterpump which is proven to save the same amount of power that the alternator uses, so if i ditch the electric pump, i can also ditch the alternator and have the same usable power output, but then my engine wouldn't have such lovely stable water temp, and i'd probably need to fit a fan and use up more battery power and need to put the alternator back on!

[RED21]

I think this will solve your problems if placed correctly on the car.
http://www.maplin.co.uk/Module.aspx?...8845&doy=16m6D

[graham bahr]

minimise electrical losses, yes i can see sense, although my car started out on webbers and the only electrics were a coil,

but by the time i'd finished i was running full engine management, wasted spark coils, double injectors (8 on a 4 cyl engine), two fuel pumps and an electric waterpump, my electrical load had gone up sereral hundred % but i also had 30bhp and 30lbft torque more and a much wider powerband removing that lot to save 5bhp doesn't add up, although most cars dont have so much in the way of electrics as i was running

now its turbo'd i have even more electrical load as the ecu looks after the boost level via an electric solenoid and measures boost with a map sensor

[Al Weyman]

No but on say a Cortina GT or whatever, a Mini maybe with about 140bhp max that 5bhp may, in certain circumstances be useful and as it is switchable would not need leaving on (or off) all the time.

[graham bahr]

Originally Posted by Al Weyman

No but on say a Cortina GT or whatever, a Mini maybe with about 140bhp max that 5bhp may, in certain circumstances be useful and as it is switchable would not need leaving on (or off) all the time.

agreed, removing the alterbnator altogether will save a little more power not to mention weight, but on balence i dont think its worth removing it altogether

[zefarelly]

I have fitted a larger ally pulley to a std alternator to slow it down a little, my only electrical loads are points ignition and lights (brake and fog are used mainly, have yet to run in the dark) alternator weighs around 3kg

the 2 k sound system in the boot is solar powered

[Kev_205]

so really the best solution is to work out how much electrical power your car needs and then buy the smallest alternator you can get away with. This will loose less bhp but not leave you worrying if you will make it to the end of the race.
And maybe a switch to turn it off if you are struggling to gain on someone down the pit straight

[Mackmot]

Dont forget that in MSA rules you have to be able to start your car 5 times. Its something that has been done in scrutineering several times.

[Al Weyman]

I did not know that Mackmot good point, my truck battery looks more attractive all the time!

[zefarelly]

5 times, I didnt know that . . . .I can do it though . . .as long as the engines in tact !

[maddogf3]

Originally Posted by graham bahr

heres another angle,

i also run an electric waterpump which is proven to save the same amount of power that the alternator uses, so if i ditch the electric pump, i can also ditch the alternator and have the same usable power output, but then my engine wouldn't have such lovely stable water temp, and i'd probably need to fit a fan and use up more battery power and need to put the alternator back on!

I agree getting to the finish and reliability are most important . We dont have an alternator on F3 and we run for 25 mins and we hardly use a slave battery to start the engine. we monitor the standing voltage at the end of a race if it is anthing below 12.5 it gets charged off a car ( More noticeable when testing, when the car will have run for more than an hour ).In the past we had the dreaded low voltage misfire . but a watch full eye on voltage stops that . I set fuel pump to 90lbs any more and current goes up . giving a faster drain on the battery . I would of said 2-3 bhp is allways worth having against the opposition .having said that I wish we had alternator to save all that farting around . We were at Spa last year and the team next to us put the charger on and forgot to switch isloator on ( Battery not Charged) they did three laps . and i think they had qualified second or third . Mind you, we ran out of petrol with 2 laps to go . so we've no room to talk. It was a lot of money to get to Spa as well . None of us will do that again