59 replies to this topic

[EunUCh]

Interesting , the average standard 6 cyl. starter motor is 1HP , not sure how that all equates to a thermo. fan motor rating , they must be

pretty good little motors to pull around 60 amps ?, dunno.

[orangeLJ]

The power loss is from the air resistance acting in the fan blades. The alternators resistance would be much less.

[StephenSLR]

My understanding of power generation is rudimentary at best, but the alternator does not provide power to the accessories (fans, radio etc), they are fed from the battery. The alternators role is to keep the voltage and charge in the battery the same. Hence why you can run fans, radio etc while the engine is off.
This would mean by my reasoning that the alternator is never working more or less at any given time but at a constant rate.

 

That's what I thought too.

 

s

[StephenSLR]

I don't think the pulley on the alternator gets harder to rotate as draw demands increase hence the effort required by the engine to turn it remains constant

 

I've always thought they were like the generator you had on your bike tyres to run the headlight on the handlebars, the faster the engine cranks the more power is pushed out, not the harder the alternator works the more it drains from the engine.

 

s

 

[StephenSLR]

You guys are thinking Generators. GM phased them out in the late 60's.

 

I thought it was later than that or am I thinking of a primitive alternator? I recall an older style alternator that had a separate module whereas the Torana alternators have that inbuilt.

 

s

Edited by StephenSLR, 07 May 2017 - 05:13 PM.

[EunUCh]

Alternators generate 3 phase AC which is then rectified to produce DC and are regulated by excitation by the regulator.

Generators produce DC which was  regulated by a fairly "crude complex" system of relays.Three phase is far more better for anything

from generation of lectrickery to running motors.

 

Any load connected to either system will result in some sort of power being sucked out of the engine , thermo. fans . seem to be quite efficient other wise they would not be used.

As far as a couple of thermos. drawing 2HP goes ? , equates to 2 stock 6 cyl. starter motors driving a fan each , them little motors not much bigger than a wiper motor must be bloody good gear or something is not right and the drain is not what we are being told ?

Just try testing an alternator on the bench and see how much it takes to hold the thing in place with next to no load.

[RallyRed]

as above....

Heres my take on it, 

 

Talking Thermo fan load only,not all the others stuff

My LJ has one thermo fan, pulls around 30 amps.

30 x 13 = 390 watts.

If we assume some poor efficiency like 50%..then it would take appx 800 watts input to the alternator  to make the 390w output.

Allow for some alternator windage/alt fan  and bearing drag,or whatever, lets say 1000w input power is reqd. to make it happen.

 

1000w = 1kw  = 1.34Hp

Seems light on, but thats how it seems to be .  I guess back in the day they always said an engine fan was about 7hp..........7 versus 1.3...we have a winner?

Edited by RallyRed, 07 May 2017 - 07:14 PM.

[RallyRed]

yep, that's that old thing that Newton ( not Bert) figured out...."when one object exerts a force on a second object, the second object exerts an equal-and-opposite force on the first object.."... or in our language....""listen up.....if you push me to make 200 amps, well, I'll push back with the same amount of force""

..or something like that...lol

[koalasprint]

About time this thread dispelled some myths. Completely agree with all Bomber has said, particularity where alternators will vary the the load to suit (current/power)demand. Thermo fans WILL save HP when they are NOT on. They are not needed MOST of the time, so you save HP most of the time. When they are on, you normally don't need 100% of the HP available. EG Slow speed, stop start, idling.

 

Thermo fans do not switch off at high speed unless your car is ECM controlled and has that functionality built in. They are normally only coolant temperature controlled.

 

If you are building a burnout car then a big mechanical fan makes sense.

 

I am a bit surprised that the clutch fan taxed so much HP when cold. I wonder if the HP taxed would be less if the test was done with say 60km/h airflow to the radiator to simulate real world conditions. I suspect it would be reduced. because it's less work to spin a fan into a head wind than in still air.

[StephenSLR]

Thermo fans do not switch off at high speed unless your car is ECM controlled and has that functionality built in. They are normally only coolant temperature controlled.

 

In my car, I can see my temp. gauge needle dropping at high speed which I guess means the coolant temp is going down. For me high speed is limited to driving on the highways. Ymmv if you have a drag car I guess.

 

s

Edited by StephenSLR, 08 May 2017 - 10:02 AM.

[LC-GTR-1969]

Thermo fans WILL save HP when they are NOT on. They are not needed MOST of the time, so you save HP most of the time. 

 

As I earlier mentioned- my tests indicated any HP savings are negligible (when turning thermos off compared to on). However, that does not mean there are not HP savings, just that its like measuring a bees dick with a tape measure. 1hp will never be felt by seat of the pants, and its even not enough to really be picked up accurately by a dyno or drag strip.

 

Alternators will create extra resistance when the electrical load is increased- I've never disputed that... its a known fact, and something I have observed... However, over the years I have heard so many people tell me that I should be turning the thermos off as they will drain HP etc due to greater load on alternator and so forth- which was why I tested at the strip... again, the hp drain would be approx 1hp which is negligible. Needless to say, I just run them full time now (no mechanical fan). I could understand 1hp being more significant in a 30hp donk but in 250hp-400hp donks like typical torana engines, its just not enough loss to even think about. 

Edited by LC-GTR-1969, 08 May 2017 - 10:31 AM.

[TerrA LX]

, the strange thing is that most water pumps these days are still engine driven , not saying the

lectric ones don't work , just saying , engine speed /water pressure in sympathy with engine speed ?

 

Just picking up on this, I think it not only comes down to economics, but also, with the sophistication and demand on today's electronics in cars it may be a very powerful electric motor to drive for little gain for modern cars, in saying that, coolant speed needs to be linear with engine/coolant temp, not engine speed as such.
You will notice high end stuff comes with a speed controller which senses engine temp, doing away with the need for a thermostat.

[Bruiser]

Old thread, I know, can't keep my mouth shut here. I am an electrician, not automotive, but a volt is a volt and an amp is an amp

Alternators, and gens before them create voltage when spun. Disconnect the wires, there is no load, but still voltage present at the terminals. Regulated externally or internally to 13 - 15 volts roughly at all rpm.

Anything connected up and switched on will be a load, and draw current (amps)

The alt is basically an electric motor driven to make volts, rather than having volts applied and spinning.

A load will cause current to flow in the alt's windings, and have the effect of changing the magnetic fields between the turning part (rotor) and the stationary outer part (stator) causing a resistance to being turned. The bigger the electrical load, the harder it is to spin.

Peteruc's wattage to horsepower calculations are correct, but I don't think it directly can be compared to the horsepower needed to turn the alt pulley while under load. Only the engineers who designed the things would know how much torque is required to keep a loaded down alternator turning at whatever rpm. Plus, I reckon the pulley gearing between the balancer and alt pulley would halve(?) The required torque at the balancer.

The battery is connected in parallel with the alt, and will be seen as a load as well, probably not much. Proper auto sparky needed to answer this sort of thing...

2 x 100 watt spotlights will draw 16.6 amps.
Davies craig fans range between 8 and 19 amps

I've never felt any drag in my 2.4lt workvan with the spots on at all

Lc-gtr-1969's finding on the strip seem to suggest the same.

I reckon electric fans wouldn't use up much engine power.

Giddyup
B

[Ice]

Oh shit wait till Byron sees this thread 

[Ice]

Good to see you are getting around Bruce 

you will fit right in here 

[Bruiser]

Thanks gene, lurked around forums for years getting questions answered about speaker boxes, fishing, basses all sorts of things - good info out there. Never joined in on one though. Must have checked out all pages on this engine section at leats 3 times, there is some great stuff here and some pretty knowledgable cats too. Gotta laugh, I wont go near facebook but this is a bit of a novelty, I must say... I promise I will calm down soon hahah I crack me up

Is byron an actual auto spark who can show me the error of my ways?