59 replies to this topic

[Ice]

Yeah we do have some gurus here prob more than any other forum on Holdens 

[claysummers]

He’s taking unusually long to do the math.

Obvious advantage of anything electrical is it can be switched at demand. Only robs power when necessary. But it ads complexity and capital.


Sent from my iPad using Tapatalk

[RallyRed]

Bruier's comment above, I generally agree.

However would think that any pulley, gearing  between the crank and the alt. etc would not be in the calc.? You cant get something for nothing?

Some pages ago, someone referred to 30amp fan, is equal to appx. 360w ( 12 x 30 =360).

This is the Output of the alternator, assuming 70% efficiency , means around 470w to make it turn.

 

Likewise, the dragstrip fan on, fan off tests, in my little mind , are not conclusive, as depending on the battery charge at that exact moment, the current for the fan could be coming from the battery and not the alternator at all.  That test needs to be redone after you have parked the car and disconnected the battery, then jump started it, to prove the fan current is 100% being supplied from the alt. ( no responsibility if you torch the alt).

 

I may be wrong with the above, but I think it is a little more complicated than just turning the elec. fan on and seeing the difference. 

I guess , even if we triple the 470w and call it 1.5kw ( 2hp) , it's still not much.

Edited by RallyRed, 08 May 2021 - 08:56 AM.

[Shiney005]

 Plus, I reckon the pulley gearing between the balancer and alt pulley would halve(?) The required torque at the balancer.
 

I reckon with the bigger crank pulley driving the small alternator pulley the required torque would at least double. This would be why the XU1's and L34's came out with a 3 inch alternator pulley. If the factory is going to go to the trouble of making this change to it's performance cars, then there must be a good reason. 

I bet you couldn't hold an alternator with your hands that was having 15 amps sucked out of it.

[axistr]

Yes you don't get something for nothing, but the ratio is way in favor of the electrical powered fans. I can remember Holden WB's & early commodores had electric over vacuum switch which increased the distributor advance and increase the idle speed. The very inefficient at low speed mechanical fans drew a bit more air through the radiator when stopped at the lights increasing the engine speed by a further 300 rpm.

 

You wouldn't have a hope in hell these days trying to keep a 500hp engine cool idling on a hot day for very long with mechanical fans. The theory is most fans mechanical or electrical really don't have any effect over 60kph. Although my fans cut in by temperature, override or when the A/C compressor cuts in. At 85kph on a hot day with the A/C on and fans cut in my temp drops down a bit further. If you have a scan tool and can see the idle air control valve position and injector pulse width you can quickly see how much the engine adjusts for load when the fans cut in. I am going to have a stab in the dark and reckon is about a half the load to a six blade mechanical fan, but the higher the RPM the more efficient the electric fan become and the ratio increases in favor of the electric fans. 

 

Trivia.

 

Cummins engines stated in their operators manual (NTC 400 engine) when the mechanical clutch fan cut in the total deduction was 60hp. When the fan did cut in it was back at least one gear on a hill, but generally two.         

[Bruiser]

Shiney, you got me there, well spotted mate, you made me think of a pushbike. Smaller rear cog makes it harder to turn. Xu1 pulleys were bigger to slow the alt down at high revs. The electrical theory part is one thing, then there's the physics involved is another. Pity the electrical output and mechanical input are both measured in kw - it just makes it harder to get your head around.
Rallyred, I agree with your maths, the efficiency bit is the key to the whole thing. Maybe an auto sparky can fill us in on the behaviour of the alternator/battery load sharing idea, I think you're onto something there too. Just how much does the mechanical resistance increase for every amp/watt demanded of our alternator?

[Shiney005]

Just a note on the importance of shrouds here too. My 14 litre N14 Cummins had a crack in one of the fan blades so I cut off 25mm from every one. (It was 815mm diameter overall)

Now when it cuts in, it is much quieter and it isn't "felt" as much from inside the cab. I guess it is the outside part of the blade that is doing the most work. But with the extra gap between the blade tip and the shroud, it now takes nearly twice the amount of time to bring the engine temp down to where the fan cuts out again.

I guess I should explain that the big diesels have a clutch fan, but the clutch is air actuated and controlled by the ECM.

 

As far as electric fans go, I bought one a while ago that had to go into a tight space, so I bought the shallowest one I could find. It had a very flat shaped motor and the whole thing was only 60mm from mounting face to back of motor. Because of the motor's small size, it didn't spin the blades anything like the old Davis Craig fans, and it didn't shift anywhere near as much air. So some will draw more power than others.  I had twin Davis Craig fans on a Torana years ago, and the headlights would dim when they cranked up. 

[Bruiser]

Hoho, when in doubt - I just googled "mechanical load on car alternator". I am a computer dumbo, and can't do that clever post a link in blue print thing yet. The first one to show up "electronics.stackexchange.com" looks like a pretty good answer to this. There is even a graph there. See what ou guys think.

[Shiney005]

https://electronics....f-an-alternator

It looks like they are saying 6-8 horsepower for the alternator.

[rodomo]

Just a note on the importance of shrouds here too. My 14 litre N14 Cummins had a crack in one of the fan blades so I cut off 25mm from every one. (It was 815mm diameter overall)

Now when it cuts in, it is much quieter and it isn't "felt" as much from inside the cab.

Bushy