The Myth Of The Electric Fan

11 Апр 2014 | Author: | Комментарии к записи The Myth Of The Electric Fan отключены
Interesting » Electric Cars

The Myth Of The Electric Fan

Electric are an often discussed modification on the RX-7 forums. Unfortunately, it that there is also a lot of information surrounding the e-fan and how it be installed. This document hopefully answer some of the common questions with to e-fans and the 2nd gen RX-7 (though of this applies to the 1st gen as well), and some of the myths.

This makes no claims as to it’s when applied to other I get a lot of angry email from automotive forums where article is posted saying how I am, yet every one of those people to miss the fact that article is specific to the 2nd gen (’86-’92)

Before we begin, let’s take a general overview of the system on the RX-7. Forget for a that there is also an oil We will just concentrate on the cooling system.

The cooling consists of 5 basic parts: the the water pump, the radiator, fan and the Water passages inside the are filled with an ethylene mixture (commonly just coolant). These passages the heat that is generated the engine to transfer into the Coolant is circulated through the via a water pump, which is from a belt off of the main shaft pulley.

The stock fan and fan assembly also mounts off the of the water pump, and is therefore by the engine as well. The pump’s job is to water though the engine and the Air flowing over the radiator the coolant, and thus the engine. A is used to open or close of coolant through the rad to maintain the temperature at a constant (approximately 180 F).

It is important to realize that it is the that regulates the temperature of the and not the fan. You can install the largest fan and move as much air through the as you want yet the temperature of the engine remain the same as set by the thermostat.

Now that we know how the cooling works, let’s be clear the function of the fan, and how the clutch works. As mentioned, the stock fan is attached to the water pump This shaft receives rotation from the engine The fan is not directly attached to the shaft, it is bolted to a viscous clutch. The then sits between the fan and the

The purpose of the clutch is to keep the fan at a constant speed, and to only the fan when it is needed. Think of the as a very simple mini converter. Inside is an impeller is connected directly to the input This impeller rides on a so that it can rotate independently of the housing.

The housing has a disc sits opposed to the impeller. disc is controlled by a bimetal so that as the air flowing over the heats up, the thermostat moves the closer to the impeller. Filling the housing is a thick silicon (or sometimes a heavy gear

As the disc moves closer to the they become coupled by the of this fluid. Because the is splined to the housing, the housing to rotate and thus the fan (which is to the front of the housing) spins as

As cooling air flows over the the thermostat relaxes and moves the away from the impeller. breaks the coupling and allows the to slip and finally freewheel. when you stab the throttle or the engine is operating at high RPM is not enough force coupling the to the disc and the clutch slips to the fan operating at a constant speed and to robbing power from the

The (very crude) image illustrates the assembly. A more disassembly of a fan clutch can be found at V12 Fan Clutch .

What’s important to is that the fan is only used the engine is idling, or the car is creeping at low speed (less then 20 If the car is stationary or creeping, there is not natural airflow through the rad to adequate cooling. Thus, the of a fan.

Now that we know the basic of the cooling system and fan, look at the electric fan.

By the most common misconception is the electric fan cools better the stock. People see the high CFM in the catalogues (for example, the Magic is advertised as pulling CFM, and the Perma Cool chopper is advertised at 2,950 and are impressed. The truth is that no one knows the CFM rating of the stock

It has simply never been by anyone but Mazda. Some have been made it is approximately 2900 CFM at 2600 RPM software modeling. But as these are we can never know for certain how has rated this fan. All we do is that it is rated at enough. And to be who cares how much air the stock fan

It is more then adequate as as it’s clutch and the rest of the system is in good shape. The raked blades of the stock fan quite a lot of air, even if it slower then an aftermarket unit. It’s also a bit how much capacity the stock fan has as to aftermarket fans on a modified car the fan is only used during and light-load situations.

The fan to cool a engine is the same size fan as be used to cool a 200HP since both engines are the same amount of heat idle and light cruising.

people also seem to that an electric fan will their at speed overheating Recall that the fan is not even when at speed, as the natural of the of air through the radiator is more enough (much more a fan, in fact) to keep the car

Of course, you’re not necessarily better cooling. You’ve those TV commercials or catalogue ads promise Up to 17 free HP! by converting to an fan. Well, that’s not true. There will be a gain for sure, on the order of 0.5 1HP. But remember that the power to run the fan must come somewhere, and that somewhere is the

Electric fans draw a lot of current. Most pull of 35A or more to start up, then run at This puts extra on the alternator. Ever pedaled a with a generator powered If so, then you will know as the electrical load on a generator/alternator the generator/alternator becomes harder to

Suddenly, all that free HP you freed up is once again used. This time, it is being used to turn the There is also an inefficiency in the system. We are converting mechanical to electrical energy, to mechanical to run the fan.

This very much less then driving the fan directly via the mechanical of the rotating shaft.

Let’s the 17 HP! claim a little further to the point home. A little math is all that’s needed.

One is 745.69 watts.

Therefore, = 12,676.89W.

Since W = Voltage * 12,676.89W is 1056 Amps

So in to do the same work as that 17HP fan, an electric fan have to continuously draw 1000 A from the electrical of the car. Clearly this not happen. Now I’m sure somewhere out there is a car or truck a ridiculously inefficient and so poorly stock fan that it actually rob 17HP from the engine as it

But since the RX-7 comes a well designed freewheeling fan as already explained, the RX-7 is not that car.

There is a chart floating around the forums which supposedly a car gain 10 HP across the board no other change besides to an electric fan. I seriously the validity of this claim as the 10 HP was shown throughout the entire RPM all the way to redline . As we all know, the fan clutch have been disengaged about 3000 RPM, so it that this car was dyno’ed a stuck fan clutch .

We can do the same thing with to an electric fan and factor in the inefficiency of the and the fan motor. Assuming it takes 1HP to the electric fan:

745W = 62A at This is obviously already since it’s ridiculous to that an electric fan is drawing 62A But we’ll continue anyway.

that the alternator and fan motor are 70% efficient (an over estimate for considering the quality of automotive parts. see Alternators ) we need to those losses into

So 62A + 30% = 62 + 18.6 = 80.6A

Then add 30% for the inefficiency of the fan motor: 80.6A + = 99.2A

Interesting » Electric Cars

If our e-fan really 1HP from the shaft of the engine, it be drawing almost 100A the electrical system of the car to do the same We can see that this is not the case, in a car with a 70A/90A alternator.

We can see the electric fan is a large current The ’86-’88 alternator is rated at and the 89+ unit is rated at 90A. world tests have that actual output is less then the specifications indicate.

Add the load of the electric and you are that much closer to an already factory-stressed electrical

While on the subject of the electrical let’s look at how the fan is actually The proper way to control an electric fan is via a and relay combination (fused, of Unfortunately, very few people do this.

Many installations connect the fan to run all the time, or to a switch the car. In the latter case, brain actually becomes the Needless to say, this is a poor arrangement as it is easy to switch on the fan as the temperature climbs.

still, the 1989+ temperature is horribly inaccurate. By the time you see it the car is already too hot. Leaving the fan on all the presents a different set of problems.

It becomes an unnecessary drain on the and is totally unnecessary at speed. all that advantage the electric fan has the stock fan disappears. This is not a flaw of the fan itself, but of the installer.

there is the question of reliability. The clutch fan can have two failure the clutch will fail, or the fan physically break. The electric fan many more failure fuse, all wiring connections, failure of the fan, failure of the failure of the thermostat.

By now, probably wondering Gee, why is guy so against electric fans?. I’m not really. But I recognize disadvantages. There are also useful advantages to the electric

The subject of fan efficiency has been until now as it can work both The majority of a fans efficiency is by the shape of the final few inches of the and whether it is ducted or not. electric fans can have a advantage. It is hard to say what the of a fan blade is without aerodynamic A stock fan may have very blades while an electric fan has blades, or the stock blades be horrible compared to the electric fan you are at.


But a big gain in efficiency with an is in the fact that the blades are easily ducted. Because the fan drive motor is mounted to the same frame which the shroud, clearances between the fan and can be much smaller then of an engine mounted fan. The the clearance between the tip of the fan blade and the the more efficiently the fan will air.

Some fans go a further and have a ring the outer circumference of the fan on the blade which rotates very to the shroud. This is a very design.

Since you are no longer to the shaft of the water pump, the fan can be oriented in any way, and moved to make space. Without the fan shroud in the way, there is more room in the engine as well as room to route like intercooler piping, air intakes, etc. An electric fan may be required when going to aftermarket radiators that do not provision for mounting the stock fan

The electric fan can also be left on the engine is shut off. allows continuous cooling to heat soak. A small timer like device can be to run the fan for a preset amount of time.

The 3rd gen actually does this the factory, but that is about the good point in the 3rd gen cooling I run an electric fan in my 2nd gen and find it very at the track between runs to the engine bay cool.

Perhaps the advantage of the electric fan is cost. It an affordable alternative to replacing a stock clutch fan. The unit is very expensive purchased new, so it is possible to several hundred dollars by an electric fan instead. However, advantage is not as great if you have a wrecking yard in your

Used clutch fans are cheap.

There you have it. For the part, electric fans are not as an upgrade unless you have a specific reason to do so. They may inferior cooling to the stock and may create other problems if installed.

If you want to install an fan, I have done a writeup of the generic process. How To Properly Install An Electric Fan .

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