[C38] Another idea on alternators

[les]

A lot of things are done in racing that yield small performance gains,
and sometimes not by the way that one might think.

An alternator won't charge at its rated current, unless there is a very
low load on it.  In your example of the 90A alternator, the alternator
is about 35% efficient.  That means 90*12=1080w is the output.  To look
at the power to drive that, we get 1080/.35=3085Watts.  There is a small
fixed loss of the alternator due to bearing wear, and pulley
inefficiency and other mechanical things.  But looking at the 3085 Watt
number to get horsepower, you divide that by 745.7, and you arrive at
4.1 horsepower. A car battery is typically about 120AH rated, and given
normal charge states, you can recharge that with a current of 12 A.  To
get to 90A, something would be radically wrong with the battery or cars
wiring system.  If the battery were shorted, then you could easily get
to 90A or more.  But in that case, you would probably notice a LOT of
heat on the cables.  If you attempted to start the car, depending on the
engine, you might need up to 400A to run the starter.  A typical
alternator will not start a car.

As to the horsepower savings through the efforts used in the race car,
minimizing the batter would save more in weight than anything else.  The
alternator although 35% efficient, if the battery is charged, the
alternator is essentially just running the car.  If you lowered the
alternator below about 6 or 7A, you would be running the car on the
battery (typically a car battery just starts the car, and the alternator
carries the load).  I would think that you ran the risk of the car
battery going flat and ending the race.  If not, you were probably
running a sprint car on a short track and short races.

One of the dragsters I worked on in the 60s had no starter, or
alternator or generator.  I used a good sized battery to run the
electrics and we push started it.  But it only needed to run for about 3
or 4 minutes at a time.

A boat is a different character.  If you are racing, using a slip, you
can just run the battery and not even have an alternator, assuming you
have enough battery power to run the electric fuel pump, if you have
one.  Use a walkie talkie with penlight batteries and a handheld GPS.
I don't know how you could push start a large boat, maybe just use a
small kicker to get in/out of the slip?

But if you are going off shore for cruising, you need better radio
capabilities, you need lighting, you need refrigeration (well, I do),
and so on.  This takes power and that means big batteries (you don't
want to full cycle them do you?)  and it means a big and probably
inefficient alternator and probably other means as well to recharge
them.  There are means to reduce the alternator load, such as lifting
the field to reduce the current production.

Regards,
Les H
On Sun, 2011-09-18 at 18:54 -0400, tdtron at earthlink.net wrote:
> I was doing some thinking today and it's something I don't have a lot
> of practice with so bear with me.
>  
> I was going through some old stuff I had and I came across a picture
> of a race car the company I worked for had.  In the '70's I worked as
> an automotive technician (mechanic in those days) and I worked for a
> multi franchise import store near Atlanta.
>  
> We did a lot of work on sports cars that were used in the Sports
> Car Club of America racing series, the SCCA.
>  
> The rules said every car competing had to have a battery and
> alternator in working condition.  At the start of every race, each car
> had to crank by starter and battery without any outside assist.  Once
> the race started, a starter wasn't needed any longer and the battery
> needed only to supply a very small amount of current to the ignition
> and brake lights.
>  
> One performance trick we did was exactly opposite to what we sailors
> do, we made our alternators smaller and ran the smallest and lightest
> battery that would start the cars.  We took the stock factory
> alternators and had them rewound to produce only a few amps of charge.
> This relieved the engine from having to pull the effort of the
> alternator charging the battery which robbed horsepower.
>  
> Where am I going with this?  If we put larger output alternators on
> our vessels, we can charge batteries faster but at what price?  We
> know high output marine alternators aren't cheap, and we know they put
> more mechanical strain on our engines and alternator brackets, water
> pump, etc.  One thing we have not discussed is the extra cost in fuel
> by the increased load on the engine.
>  
> At any given RPM, a 150 amp alternator will obviously cause more drag
> on the engine than a smaller alternator.  If you have to add more
> throttle to power the larger alternator, you are burning more fuel but
> at a shorter period of time vs. the smaller alternator.
>  
> I made a 12v power generator once so we could fast charge cars at the
> dealership without having to drag an extension cord and charger to the
> back storage lot.  I put a 90 amp alternator on an old lawnmower deck
> with a Briggs and Stratton 3hp engine running the alternator.
>  
> It didn't work!  We had to put a series resistor in the field line to
> reduce the charge output to keep the alternator from stalling
> the engine.  We thought 3hp would be sufficient but evidently not.  It
> was an eye opener for me to see how much horsepower it takes to run an
> alternator connected to a discharged battery.
>  
> If you are running a Universal 5424, 3 hp is about 12.5% of the 24hp
> redline throttle, or in another way, it's about 1/8 of your total
> engine power.  Since my alternator stalled the 3hp engine, the actual
> load had to be more than 3hp.
>  
> Since our original alternators only put out about half of the amperage
> of my experiment alternator, that 3hp would still be over 6% loss of
> engine power on a 5424.
>  
> My point is, the faster charging large alternators will charge faster
> but it remains to be seen how much they save on fuel by their shorter
> run times.  Of course, if that 150 amp alternator destroys your water
> pump, mounting bracket, belt, etc., you will save a lot of fuel
> because you will be sailing home without an operable engine.
>  
> Any comments, thoughts, etc?
>  
> Tom Troncalli
>