does alternator cycle off?

9

92xjsp

NAXJA Forum User
How does an alternator work? If the battery is fully charged, does the alternator cycle off somehow and stop taking power from the belt, maybe going into a "free spool" type mode? Or does it keep generating power and just discharge it somehow?

I'm thinking about people who go to electric fans to gain HP from the engine. Seems like it's more efficient in the overall scheme of things to run the fan directly from the engine, than to use the engine to drive a belt to turn the alternator to generate electricity and then run the electricity through the wires to an electric fan. Of course, that wouldn't be true if the alternator is always running anyway and just wasting the extra power.
 
The alternator's mechanical load on the engine is proportional to the electrical load on the alternator; i.e., alternator turns freely when there is no electrical load.
 
As I recall, the alternator is always generating power - the difference being how much power at any given time.

When you start your truck, the battery is drained (not critically, but significantly) when you pull the 150A load for the fuew seconds it takes to crank the engine fast enough to light, as well as whatever it takes to fire up the ECM/PCM/FI setup.

Once the engine is running, the regulator senses the battery/system voltage, and energises the field coils to generate a current in the spinning armature (I may have that reversed.) It is spinning the armature within the field coils, and the opposing magnetic fields involved, that creates the resistance to turning that puts the load on the engine.

When the battery is fully charged, the regulator only energises the field/armature enough to generate enough current to power whatever is running at the time - the battery is now effectively out of the running, unless there is alternator trouble.

Is there a power savings involved in going to an electric fan? Probably - but I'd like to see how much HP is involved in spinning the "string driven thing" vice the electric, current consumption on the electric, and power input/current output curves on the alternator before I commit to an opinion - I don't have much data in that line. Anything I could give one way or the other is strictly a guess.

Personal preference? I've not gotten round to it yet, but I plan to make a solid spacer and delete the fan clutch, and keep the secondary fan on a manual switch. I think that's the best "compromise" setup for me (especially in Bay Area traffic!) and I don't mind losing a couple HP to running the fan overall, as it doesn't seem to effect torque output per "Butt Dyno" measurements and I like having a fan always running on the 4.0 (which is designed to run a bit too hot, IMNSHO.)

5-90
 
Good explanations of how the alternator works. Based on that, and the conservation-of-energy principle in physics, I’d have to say you can’t gain HP by converting from a clutch fan to an electric fan, unless the fan simply is pulling less air through (assuming that both fans are equally efficient mechanically). Conservation-of-energy says you can’t generate more mechanical power-equivalent worth of electricity than what the fuel holds that you’re using to generate the electricity with. Now, it may be that the clutch fan generates more air flow than you really need, so you could gain HP by reducing the air flow down to just what you need, but you could get that same result by swapping for a lower-capacity clutch fan. If the electric fan equals the clutch fan in CFM, then the process of converting the engine power to electricity and then back to rotary motion on the fan is bound to lose a little, maybe not much but at least a few percent, so you’ll actually be losing HP.

There may be other good reasons for converting, but it doesn’t look like you can gain HP.
 
The reason people say it will increase HP to go to an electric fan setup is that HP is often measured as Peak HP. With a belt fan resistant will go up exponentially with RPM but with an electric, the RPM and therefore power draw is constant. Fans are most efficient at a specific RPM so you get a diminishing return as you spin the belt driven fan increasingly faster over its design point (presumably it is designed for cruising RPM not peak HP RPM). The advantage of the electric is that it can always function at it design point, giving the most efficient work regardless of engine RPM and on top of that its power demand dose not sky rocket just when you need the power at the wheel the most like a belt fan. Electrics also let you run a power debit to the battery that you can repay after peak demand has ended.
The only problem is that nobody I have heard from who has tried an all-electric setup in their XJ seems to feel it can keep the thing cool enough. In other words the available E-fans do not have sufficient flow.
 
Several good points there, bronco. Especially your point about the clutch fan being intended for cruising RPM and possibly over-producing at high RPMs. Of course the engine produces more heat at high RPMS so needs cooling most then, and I would think that engine engineers would look closely at that relationship when designing the fan, but it still may be overproducing. Now that would be a real argument for HP gain at peak.

I haven't heard of an electric setup that can handle extreme hot weather, either, but we may soon have to change that opinion. What got me thinking is that Marcus in Australia just did this conversion ("gojeep" web site) and he usually seems to know what he's doing. In another couple months it will be high summer down there so we can see how it holds up for him. He's using a 16-inch, 2500 CFM fan, shrouded to direct all of the flow through the 12-inch high radiator. (Anybody know what CFM the clutch fan produces at various RPMs?)
 
5-90 said:
Told you I probably had it bassackwards - perhaps I'm thinking of a generator or a starter? I always think of the field as sitting still (kinda like any other field) and the armature going berzerk in the middle. I've been wrong before, tho...

5-90
Click to expand...

Technically, I think the field is defined as the part that provides the magnetism, and the armature as the part which puts out the current. On a DC motor or generator, the field surrounds the moving armature, but of course that requires that the current produced (or consumed by a motor) must all pass through the commutator, which acts both as a rotating contact and a rotary switch to keep the current going in the right direction. which leads to sparking and limits speed. There is always a compromise between the pressure required to reduce sparking, and the need to limit wear. The reason alternators took over from DC generators is not because alternators are somehow more powerful, but because generators are speed-limited by that hot commutator, and that means that if you gear a generator to provide adequate voltage at idle it will burn up at higher speeds on a modern engine.

There's a little more to it than that as I recall, because a DC generator also needs to be both voltage and current-regulated, while an alternator needs only voltage regulation, but that's the gist of it anyway.
 
When your figuring in the relative power draw of the mechanical fan versus the electiric fan. You have to figure in inertia (accelerating the wieght) (the mechanical fan wieghs a lot) and drag, again with the wieght. The mechanical fan acts just like a flywheel during acceleration, but doesn´t store much/any of the energy. Takes a lot of energy to keep it moving, and a bunch more for it to pull air.
There is also a drag on the alternator, required for the extra power to turn the electric fan. But I find it hard to believe, it´s as much, as the mechanical (at least during acceleration). The electric is most times spun up, before accelaration. And the blades wiegh a lot less.
On a purly theoretical level, the mechanical is probably more efficient than an electrical. The wieght difference, probably makes up for the energy lost, when changing from mechanical energy to electrical energy and back again.
Personnaly, I get tired of the mechanical fan, throwing water and mud all over my motor.
 
You also might want to consider that fact that you hear of racers using an electric fan and even an electric driven water pump to save ponies. Well, drag racers don't run an alternator so there is no additional load due to the alternator generating current. So in that application they WILL gain a bunch of HP by going to electric.

In our application I think it will save some HP but not alot. Sure when you punch it the motor must give up some horsepower to spin the mechanical fan faster due to inertia and additional air load and this was mentioned above. Well, I think this is where the electric will save HP since it is spinning at a constant RPM and doesn't have to speed up when you gun it. Also in a sudden load situation when the fan turns on it doesn't have to draw the surge from the alternator. It gets it from the Battery.

I think you need to look at ALL the pros and cons of each type of fan not just the HP issue.

I think for an off road vehicle there are several advantages to an electric fan. As mentioned above you can turn it off for water crossings. An mechanical fan isn't designed for water.. so not only will it spray water all over the engine. It can also get pulled into the radiator in deep water. Not good. Also for rock crawling the engine usually is turning very slow RPMS... Not enough for proper cooling so electric wins here too.

But there are som down sides to them too... More things that can break for one. Electrics are noisy as heck sometimes too.

Just a few points for you to consider.
 
Draw racer engines have to disperse so much energy so fast that they cannot count on using the water-cooling of the engine as they make there run. The system simply cannot more that much heat that fast, so the engine temp is going to go up dramatically regardless of weather they run the fans or even the water pump. The thermal mass of the engine is all that keeps it cool. My understanding is that in the really extreme dragsters that they run the water pump and fans off a battery at the starting line and when circling back to the pit to get plugged back in. Cooling essentially takes place after the race has ended.
For the sake of the current Jeep discussion, using an electric fan lets you do kinda the same thing. When you need peak HP you do not have to pay the price of cooling till later (after the race) because the engine block has to saturate with heat and then pass it on the water (this takes time). However with my jeep my goal is far from maximum power. What I want is robust, durable and reliable system. I’ve never been in an off road situation where HP was holding me back, and I try not to embarrass myself racing the ricers on the street, so...
The other thing I have heard of for slow rock crawling is running the fans in reveres. The theory goes something like, there is essentially no ram effect when going 1.6 MPH and buy pulling air in the hot air tends to get trapped under the hood, but by running the fans backwards you are bringing in cool air from under the jeep and the hot air coming from the grill is more likely to blow away in the breeze because it is higher. I think this would be alota tweakin’ to get working right and clearly you would need a way run them the right way so you could drive home from the trail.
 
I´ve probably had the theory in school at sometime or another (been awhile). Seem to recall, a fan pushes better than it pulls. I can´t recall, a centrifigal type fan or pump, installed by choice, in a pull configuration. They are much more efficeint pushing. Propeller type fans, seem to also push better (mounted before the restriction, heat exchanger, duct or cowel).
Worked with air handlers for many years, noticed early on, they seem to push better than pull, with most things being equal. Maybe it´s just, that a blade gets a better bite, into clean (unturbulent) air.
 
Stand six feet in front of a house fan and see if you can feel it then try that behind it. However the house fan has no shroud and the XJ dose wherefore any air that comes through the fan had to come out of the radiator.
I don't know just some more thoughts. I have a fever and I may not be in my right head so you don't have to listen if you think I’m full of it.
 
Never on this forum but came across this topic and it was asked how the twin electrics went. Well I can say that it performed as well as the machincal fan in keeping the engine cooling under control but stand by that it is not significantly better than before the conversion but importantly it is no worse. I did not expect that either as my viscous fan was working properly at the time of removal.
As for the discussion about power differences, the electric will always better off. One thing that no one at all mentioned is the fact that most of the time the electric fan is not even on at all so does not draw any extra power from the alternator! Even in 100*f temperatures it only comes on at a stand still while waiting for the lights to change and only then right before I move off. Off road in those temps it still cycles off and on even while towing up long steep hills or over sand dunes when they stay on, they switch off as soon as it flattens out and go back to cycling off and on. Now that we are back in winter I have not had the fans turn on now for months. Even when it was still in the 90's it was not even coming on very often while stting at the lights since I fitted my cowl hood as the hot air escapes so much quicker and does not build up in the engine bay.

Hope these coments help some. ;)
 
I also has replaced both my fans with after market electrical fans and love it. At 40+MPH, the fans never come on. I am in South Florida. The temp yesterday was 102* heat index 110*. Driving in Miami between 5.00 and 6.00 PM with the AC on and 5 adults in the XJ through miles of stop lights the electric fans performed flawless. Comes on and off with the AC compressor. Even the engine runs cooler now that it did with the mechanical fan. What I like best about it is moving off from a stop light. The butt dino smiles all the way to the next stop light. This coming weekend I will be making a 1300+ miles trip from Miami to North PA somewhere between NY and CT. Those hills on I-84 would be a good test for the new fans crusing at 75+MPH.
 
Electric fan conversion on Cherokee is a horrible idea. I did it, it caused vibrations, cooled less efficiently than the standard clutch fan, and stole more hp. The factory mopar clutch fan disengages all of the time, and puts negligible drag on the engine regardless of rpm. Whether at idle or 5,000 rpm, it isn't spinning that fast, one reason Cherokee's cooling system isn't too good, but electric fan doesn't improve it. Buy a fan clutch from Auto Zone for $35, it will speed up the fan and improve cooling, but give you significant hp losses as well as a loud roar.
 
5-90 said:
Told you I probably had it bassackwards - perhaps I'm thinking of a generator or a starter? I always think of the field as sitting still (kinda like any other field) and the armature going berzerk in the middle. I've been wrong before, tho...

5-90
Click to expand...

A generator generates DC power and alternator generates AC power. The alternators on vehicles generate AC as it is more easily controlled, and converts it to DC by the use of an invertor/converter. I/Cs use bridged diodes to essentially "cut-off" one part of the AC to have a chopped sine wave. A capacitor and zenor diode are used to then stabilize the voltage.

AC is way easier to generate than DC as with DC, the more RPMs, the higher the voltage and therefore, more difficult to control. Voltage control for DC generators is very complex.

With an alternator, the voltage is the same, only the cycles per second (CPS) changes. So, an inverter has only one thing to worry about: wave transformation. change the alternating wave to a steady (DC) wave.

In answer to your query: the alternator does go into a "sleep" mode when no load is needed. But that's extremly rare because there is alwasy some load, even slight, at all times. Everything uses power. The ignition, radio, guages, etc.
 
Last edited:
92xjsp said:
How does an alternator work? If the battery is fully charged, does the alternator cycle off somehow and stop taking power from the belt, maybe going into a "free spool" type mode? Or does it keep generating power and just discharge it somehow?

I'm thinking about people who go to electric fans to gain HP from the engine. Seems like it's more efficient in the overall scheme of things to run the fan directly from the engine, than to use the engine to drive a belt to turn the alternator to generate electricity and then run the electricity through the wires to an electric fan. Of course, that wouldn't be true if the alternator is always running anyway and just wasting the extra power.
Click to expand...


in the case of the 92 the alternator is controlled by the pcm, which has a alternator driver circuit internally, which grounds and ungrounds the charging system circuit rapidly to keep optimum charging voltage. this happens too fats to even count. similiar to injector pulses
 
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