Topic: Saving power with a "kit" alternator

[NickO]

Sorry to hijack the SOHC forum, but there are some top guys on here who have forgotten more than I will ever know about tuning engines, etc.

Has anyone got any definitive figures regarding how much horsepower a "kit" (i.e. single phase) alternator saves over a standard three-phase one on a modern 600 engine?

There are two good reasons to go down this route, one being the smaller rotor is less revolving mass on the crank and the second is the lower back EMF (electromotive force) caused by the smaller single phase unit.

Like everything else, I would like to know if the potential power gains are worth the effort, (i.e. the cash)!!!

All information appreciated.

[Retro Rocket]

There's a guy on this site somewhere that has used a VFR800 charging system with good results, 3 or 4 times more power than the stock item, he did this mod because his bike is running fuel injection and he needed more power to run the fuel pump. I am going to do the same mod to my 1000cc build....

[turboguzzi]

hei 62,

with the current crop of high tech batteries being so light, total loss ignitions can weight very, very little and of course, rob zero power from the motor

or is it a street bike / endurance racer?

[NickO]

It's for a modern supersport 600 fuel injected engine TG, (hence my apology for hijacking the SOHC forum), so we need the generator!

I've heard figures of +3 bhp, which would be great, but I'm not sure if someone is blowing smoke up my ars*?

[Leino]

My friend used to dragrace a CBR 600 with a kit you described.
I'll ask him tomorrow if he remembers any numbers and let you know.

[TwoTired]

1 HP = 745.69987 Watts.

We're going to need to know the electrical power budget of the bike; which is a list of anything that uses electrical power.
Computer,
Headlight(s),
Tailight (s),
Stoplight,
ignition.
alternator excitation voltage/power,
Hand warmers
Beer refrigerator,
Anti-gravitic drive,
tractive suction fans,
etc.

This will give us the watts consumed,what the alternator has to provide on a routine basis, and for a foundation on the electromotive force stolen from the crank during run time.
Using some estimate numbers, say 10 amps for bike devices and 1.5 amps for battery charging, we get 11.5A*13V=149.5 Watts consumption, which the alternator must provide to maintain the status quo.

We will have to asses a conversion efficiency of the alternator.  Meaning it will have to be more than what is used, due to losses in converting from rotation force to EMF (every time you convert energy, losses occur).  Rule of thumb for alternator is 60-80%.  But, even if we say it's a 50% conversion efficiency, this means our 150 watt consumption would rob 300 watts from the crankshaft.
If we divide 300 watts by 745 we get 0.4 HP consumed at the crank for driving the alternator rotation.

None of this accounts for power absorbed when fighting inertial effects.  It takes energy to both increase rotational speed and reduce rotational speed.  But, usually the energy needed to increase the rotational speed (acceleration) is more a factor in racing than reducing the rotational speed (braking effects).
Mass/energy/time calculations ARE possible.  But, just an exercise unless some numbers are available.  Weight of the rotational mass, the rpm variance, and rate of change.

Cheers,

[turboguzzi]

OK, i see

3bhp is around 2000 watts, sounds a bit much too save on the swap alone.

that said, fixed magnet type (rather than excited field) do over-produce power that has to be then dissipated through the regulator. most japanese bikes run their regs at close to boiling temperature, so that's lost power for you. still doubt its 3bhp2000watts, plain physics....

[NickO]

Wow......I'm impressed, thanks guys.

Alternator is fixed magnet type and this does explain why the regulator/rectifier has bigger fins that our beloved SOHC Honds's They really do get very hot.

I guess the only way to be sure is to run the bike on a dyno back-to-back with the original "street" 3 phase alternator and then the single-phase "kit" type?

I'll be interested on what your drag racer friend says Leino, thanks

[turboguzzi]

three phase thingies output goes up to 100 volts at high rpms, so reg has to more or less get rid of 82 volts... reason for this atrocity is that you still need to have decent voltage/power at 2000 rpm for lazy urbanites sitting at the stoplights.

our beloved fours though have excited field so the regulation happens turning on and off the field coil, so little wasted energy.   

[NickO]

................................................. at 2000 rpm for lazy urbanites sitting at the stoplights.

.................................. 

There's no way this one will drop below 14,000 rpm (for more than the blink of an eye at least), if we decide to use it TG! The man holding the twist-grip knows non other than "flat stick"

[raymond10078]

Generally, I wouldn't think the change in crank HP would amount to much, if any.

The actual power, in watts, consumed by the electrical system (and thus needed by the crank to spin the alternator) depends on your actual electrical load.  Single phase or three phase conversion to DC - doesn't make any appreciable power difference.  Anybody that has jumped a dead battery has probably noticed how the engine loads when you connect the two cars together.  The electrical demand on the "live" car jumps up, and the engine loads down.  Point is - more load - more engine HP needed to turn the alternator.

Sure, the permanent magnet (PM) alternators can produce much more voltage (and it will, since its output voltage is directly proportional to rotational speed), but this is not power, or watts, or HP.  1000 volts and zero amps output is zero power.  (For the purists, yes, there are magnetizing losses in the windings, but generally, these are quite low compared to the output of the alternator itself (think a few percent, tops)).

You've already identified the real "why" behind going to such a system - and that is the ability to produce power amounts greater than the stock alternator.  The motor benefit is not electrical, but is due to the reduced rotational mass of the PM system.  Think of it this way - we all know that reduced mass, whether it be in the motor or the bike itself, will help you get down the track faster.  Less mass will be reported as more HP (the engine, the bike, uses its HP to accelerate the bike - less is "wasted" accelerating the mass of the stock alternator).

[Leino]

CBR 600 PC25, difference between stock and kit system was 0,2 seconds @ 1/4 mile. I hope this helps.

 Juha