blasto9000

Not saying electric fans are unreliable, I’m saying that they are expensive for the air flow provided, and fore-thought needs to be applied before using them. A drag car runs 1320 feet at a go, and the 5,000 hp fuel cars don’t even have a liquid cooling system. A 4x4 may be slogging through mud and sand for hours, so for cooling it is flow, flow, flow.

As for the alternator comment. Agreed, the alternator RPM varies in proportion to engine RPM. But the more electrical load, the more power it takes to turn. You do not get power from an alternator for free.

If you have a 1 hp electric motor, at 100% efficiency it would take a generator with 1 hp to run at max output. You do not simply freewheel a generator and expect it to turn a 1 hp motor with no feedback resistance. Why do you think the electric company charges you by the kilowatt hour? The more power you use in the home, the more fuel they need to stoke their generators. Likewise, you cannot simply free-spin an alternator and load it to 100 amps and not expect it to increase drag.

My point here being, a lot of people are under the impression that an electric fan frees horsepower. This is a false assumption. Two fans moving the same mass of air in the same amount of time will require the same amount of power whether it is driven by an electric motor or a belt. However, a belt-driven fan has the advantage of its RPM being directly proportional to engine RPM, so its power draw is also variable. Most electric fans are 1 speed (MAX). I see you guys are looking to use a two-speed electric, which is an improvement, but it is still not continuously variable like a belt-driven fan.

Am I “against” electric fans? Certainly not. But for the OP’s purpose of maximizing engine output I think an electric fan will hurt him more than help him. Having an auxiliary electric fan in addition to the belt-driven fan is certainly a good idea to help with idle cooling. What I’m trying to dispel here is the notion that a common $40 electric fan will provide adequate cooling by itself. It won’t.

hercman

What you failed to tell us is what CFM fan did you try? Not any fan will do you need to match the CFM to the motor.

I know there are E-fans out there that will outperform a clutch driven mechanical fan at low rpms/low vehicle speed. And the minor electrical draw on the alternator is no where near the power drain on a mechanically driven fan.

UserID6095

No one said a $40 china made pos fan will work.

To do it right you need over 2800cfm to keep the XJ cool. The best setup is three ten inch fans with two running in place of the mech fan and the third wired like the stock e-fan.

Yes, you will gain power from swapping out the mech fan. Yes, it will increase the load on the ALT but no where near what the mech fan robs at full lock up. This will get you a minor HP boost and maybe an extra mpg.

This is a nice setup, http://www.ffdynamics.com/cherokeeB.html

1991Jeep_Man

The alternator is little more than the old electro-magnetic generators we played with elementary school. The concept is basic:

A coil around a magnet

If memory serves me right, the alts in our XJ's run a normal 12V when running standard, and ups to near 14V when the battery needs charging. All of this is done internally via a voltage regulator. The internals still spin at the same speed with the same resistance; the only difference is that the voltage regulator allows more of the current to be let out at once instead of regulating it down. So the alternator does not increase drag on the engine. And your idea with the electric motor does not follow with how our engines are using the alt. An electric motor is essentially a reverse electric generator, with a magnet driving a coil enwrapped rotor by alternating magnetic fields. It is a completely different concept than an alternator.

So, using more power doesn't increase draw on the engine. There is no increased magnetic resistance, only a change in voltage regulation. When it comes to an alternator, the energy is always the same cost unless you upgrade to a higher amp alt that will have a different resistance set-up.

blasto9000

Guys, guys. I’m not saying it’s impossible to use electric fans. Obviously there are electric fans available that work. Desert race trucks use them when mounting the radiators out of the engine bay. If it works for a Dakar racer, it should certainly work on a trail-running XJ.

What I’m saying is that you have to realize the shortcomings when using them; it is not a perfect device, nor is a mechanical fan. However, it is difficult to find an affordable electric that will provide adequate cooling, and this conversion can run a substantial cost. (And cost is always an issue, right?)

As for this comment:

I’m not trying to be a jerk here, but I want to make this short, so please understand that everything I’ve said is with all respect. Okay? But... this theory of how alternators work is incorrect.


You need to understand the difference between a volt, an amp and a watt. A watt is a unit of POWER conversion. POWER. You don’t freewheel an alternator and expect a continuous stream of electricity to come out of it. POWER in, POWER out.

Outside the U.S., engines are rated in kilowatts (kW) instead of horsepower. There is no escaping that 750 watts is about 1 horsepower. If you consume 750 watts, that is 1 hp additional drag on the engine at 100% efficiency. (In the real world it’s more like 65% efficiency so 750 watts of electrical output is actually closer to 1.4-1.5 hp.) Argue with me until the cows come home, but you don’t get something for nothing.