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Novak Conversions Jeep Wrangler TJ engine mounts

Using PCM to control aftermarket electric radiator fan?

In addition, I've seen quite a few internet posts claiming that a mechanically-driven fan is 100% efficient in its power conversion. There are two problems with this logic:
  1. For a fan driven with a viscous clutch, this is simply not true. A significant portion of the power taken off the water pump shaft is lost as heat. More details later.
  2. For a fan solidly connected to the water pump shaft, this is technically true. However, as per the fan affinity laws, the fan requires a stupid amount of power to run at high RPMs. More on this later.
Regarding Problem #1, a fan clutch is designed to slip both when locked and unlocked. According to Newton's 3rd law, the torque on the fan clutch at the water pump end must equal the torque at the fan end. (Otherwise the fan clutch would start rapidly spinning until it explodes.) However, we are concerned with power, and not torque. Power is proportional to the rotational speed of a shaft multiplied by the torque on the shaft.

A typical fan clutch states that it slips 20% when unlocked and 80% when locked. I've never seen anyone state at which engine speed this occurs at, so I'll take the conservative approach and assume this occurs at high idle (1000 RPM). For simplicity for this example, I'm also going to assume that water pump RPM equals engine RPM. If you change this RPM value based on a different pulley ratio, you will note that it does not affect the rest of the calculations. In fact, you could substitute in any variable of your choice and it will ultimately not change the final answer of the following calculations.

So how much power does an unlocked fan + clutch pull at 1000 RPM? We'll have to make a few assumptions here. Imagine if you were to take a typical household box fan and duct tape it behind the radiator. Due to the restrictions of the radiator, condenser, and grille, it would have to be roughly at 50W power consumption. 50 watts isn't much in the realm of things. Way lower than our electric fan at full power. But in order to spin this fan at the unlocked speed, the clutch slips to 20% of shaft speed. Torque on both ends of the clutch is the same. But power is proportional to torque times RPM. So where does the extra power off the water pump go? 80% of the energy leaving the water pump is lost as heat in the fan clutch! This means we are using 250 watts, 200 of which is being wasted as heat! That's 1/3 of a horsepower we are using to drive this fan and clutch.

But what if the fan locks up? Surely it becomes more efficient! Assuming the 20%/80% rules apply here, the fan spins at 4X the speed when the clutch locks up. According to the fan affinity laws, power scales with the cube of shaft RPM. Head increases with the square of shaft RPM. Airflow scales linearly with the shaft RPM. The head assumption is perfectly valid in this case, as the force required to pull air through the radiator increases with the square of the airspeed as per the drag equations. But in order to spin our fan at 4X the speed, we need to provide 64X more power! Hence our fan is suddenly demanding 3,200 watts, or roughly 4.3 horsepower! In addition, the fan clutch is still burning up 20% of the energy as heat, while passing 80%. So our fan + clutch combination is now burning 4,000 watts (5.4 HP) of engine power! And we have 800 watts being converted straight to heat! (Ever wonder why fan clutches have so many heat fins?)

So what happens when we start revving our engine higher? If it were a mechanically locked fan, power would increase by the cube of the input RPM, and we would be burning an insane amount of horsepower at redline! Fortunately the mechanical clutch helps us out here, but perhaps by not as much as we might like.

The torque transmitted by a fluid coupling is proportional to the square of the slip RPM. Hence, the faster it slips, the more torque it can transmit. Or in other words, as torque increases, slip RPM increases by the square root of the applied torque. Given the fan affinity laws, the torque on the impeller shaft is proportional to the square of the RPM driving it. So (to my actual surprise), it turns out that the slip ratio between input and output speed is actually roughly constant!

So what happens when we double engine RPM to 2000? Our fan/clutch combinations are now taking 8X as much power! Our unlocked fan is now taking 2000 watts (2.7 HP) of power, and our locked fan is taking 32,000 watts (43.2 HP)!

That said, few fan clutches will remain locked past a few thousand engine RPM. In addition, as the fan clutch oil heats up, the fan begins to lose viscosity, and the fan slip ratio actually increases. Realistically, you will never see a clutch fan take more than about 40 HP off the engine when locked at high RPM.

But our unlocked fan still takes a lot of power. Even if we assume the slip ratio doubles to 90% (10% shaft speed), at redline (5300 RPM), that fan is pulling 4,650 watts, or 6.2 horsepower! 90% of that loss immediately becomes heat! And if somehow it is still locked, you could see a loss as high as 40 horsepower! (These numbers line up well with a lot of dyno results you can find on various racing forums.)

So there are really two things that prevent the engine clutch fan from being efficient mechanically. (1), a large amount of power is converted to heat in the clutch. You've probably noticed that a fan clutch has many heat fins. All that heat is wasted energy. (2), with a lack of control over the input shaft RPM, the power demands increase rapidly and unnecessarily. The fan has to be sized to cool at idle, and as such at high RPM it is way overpowered.

Meanwhile, the brushless electric fan always has a loss coefficient of about 50% (roughly 60% alternator efficiency multiplied by the 85% efficiency ratio of the electric motor), and only uses the power it needs to when it needs to.


TL;DR Fan clutches are great and save us a lot of fuel over directly driven fans, but they are still very inefficient and a well set-up electric fan will absolutely dominate the clutch fan in terms of power (and as a result, fuel) conservation.
 
Interesting work @Steel City 06. Like Nash said I like peeling back layers. Seems like there may have been consideration to use the electric setup at one point, I’d be interested to learn why the decision was made by Jeep to use the viscous clutch on the 4.0.
 
Meanwhile, the brushless electric fan always has a loss coefficient of about 50% (roughly 60% alternator efficiency multiplied by the 85% efficiency ratio of the electric motor), and only uses the power it needs to when it needs to.

This is the key right here and why electric fans are better for street driving.

Ever drive a car with no fan at all? I had a little s10 pickup with a high-compression 350 crammed in. Had cooling troubles and pulled the stupid flex fan and old radiator and put in a new single-row aluminum, and drove it with no fan for a bit while I figured out what fan to put in. Sitting at stoplights was nerve-racking, watching the temp gauge slowly creep up, but as soon as I got to 30 or above the temp quickly dropped back to the t-stat control. It never got to boiling. This was early summer in Arkansas and outside temps were 80s and low 90s. Taught me that a fan is only needed at low speeds when the rest of the cooling system is good.

As to the efficiency of a mechanical fan vs electric, yes a mechanical connection is more efficient but it's all for naught as 90% of the time it's running the fan when it isn't needed or running way faster than needed.

But this assumes street driving. If you're plowing a field on a tractor or crawling around in the woods and rocks (low vehicle speed and low engine rpm) a mechanical fan is a more-efficient fit. Like so many other things it comes down to how you use you jeep.
 
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That is one option.

The primary goal here would be to get the high airflow of the 11-blade Explorer fan and clutch at idle and low speeds without sacrificing engine power and fuel economy at higher engine and higher road speeds.

Thus you can boost the performance of the A/C without it hurting something else.

In addition, this is a mod that would eventually pay for itself. A typical clutch fan uses about 4 hp on average (a lot more when locked and a good amount less when unlocked). So let's do that math:
4 hp = 10,200 btu/h power
Assume 30% thermodynamic engine efficiency
33,920 btu/h gasoline
At 116,090 btu/gallon of gasoline
0.292 gallons per hour
Assume 100,000 miles remaining vehicle life at an average of 50 mph
2,000 hours of operation
584 gallons of gasoline burned for clutch fan
Assume gas is $3.50/gallon
$2,045 in gasoline is required to run the clutch fan.

Next, the electric fan:
Upfront costs
Shroud $100
SPAL 30107101: $300
A few random wires: $10

Assume fan is PCM controlled and only engaged when needed by coolant, A/C, or trans temperatures. Assume fan is 2-speed.
Average current draw: 8 amps (32 at max power, 4 amps at half power, 0 amps off). Note that per the fan affinity laws, power increases with the cube of shaft speed. Shaft speed and flow are linearly related. Amperage rated at 13V.
Alternator efficiency: 50% (typically closer to 60%)
Engine HP to run fan:
0.278 HP
= 709 btu/h engine power
= 2,364 btu/h gasoline
= 0.0204 gallons per hour
At 100,000 miles at 50 mph average:
40.7 gallons of gasoline
$142 to run fan
Add in the installation costs, and you're at $552 total to install and run the electric fan.

Even if you assume the clutch fan uses less than a third of what I did you still end up saving money.

But even if you're not concerned about the money, there are several other distinct advantages to the electric fan:
1. Frees up horsepower at higher speeds
2. Engages based on A/C demand
3. Can be easily "tuned" to increase or decrease cooling based on various inputs like road speed, ECT, A/C pressure, transmission temperature
4. Can be manually turned on or off if appropriately wired for user control and for water crossings
5. Slightly increased range
6. Less risk of punching a fan through the radiator in a collision or off-road impact

That said, the MPG gains are going to be small and probably not noticeable on a day to day basis. Assuming a 15 gallon tank fill cycle (19 gallon tank with 4 gallons reserve) and 13 mpg, you would see about a 1 mpg gain (to 14 mpg) and about a 15 mile increase in range.

Note, to calculate the MPG gain you must convert both units to gallons per mile and back. So in this case it is MPG = 1/(1/13 - .290/50).
7. More room in front of the engine and less hazardous, too, making some maintenance easier.
8. Possibly BETTER cooling at idle
9. Reduced heat-soak issues by running after shutdown
 
As to the efficiency of a mechanical fan vs electric, yes a mechanical connection is more efficient...
This was actually the point I was trying to argue against in that post. (I agree with everything else though.)

Whe a fan clutch is unlocked it is only 20% efficient. 80% of the power taken off of the engine becomes heat in the clutch. (When it's locked, it is 80% efficient.) Since a fan clutch spends most of its time unlocked, its efficiency is far below the 50% of an electric fan.

There are ways to make it more efficient. If you delete the clutch and bolt the fan straight to the water pump, it is now 100% efficient. (However it would require an insane power demand and absolutely destroy fuel economy.)

The fan clutch is merely a compromise that saves a lot of fuel over a permanent connection because it lowers the overall power demand, even though by itself it has terrible efficiency.

A good comparison to think of is the blower motor resistor in your HVAC blower. The system is most efficient at high speed, because at lower speeds some amount of energy is being burned up in the resistors. However, the system overall still uses less power at those lower speeds.
 
I know that JK uses electric fan.
When it comes to electrics, my knowledge end on positive and negative terminals. But, I always wondered if JK models (especially early ones) deviated that much from TJ, if they use it, why can't we?
Or have they borrowed everything from cryslers minivan line?
 
I did an e-fan conversion on my old S10. The popular fan to use is a '95ish Ford Taurus. I didn't notice any gains in power or economy, and I track my mileage in a notebook. I think the viscous clutch works better than folks give it credit. I did however have several relay failures that resulted in overheating and trips to the freeway shoulder.
 
This was actually the point I was trying to argue against in that post. (I agree with everything else though.)

Whe a fan clutch is unlocked it is only 20% efficient. 80% of the power taken off of the engine becomes heat in the clutch. (When it's locked, it is 80% efficient.) Since a fan clutch spends most of its time unlocked, its efficiency is far below the 50% of an electric fan.

There are ways to make it more efficient. If you delete the clutch and bolt the fan straight to the water pump, it is now 100% efficient. (However it would require an insane power demand and absolutely destroy fuel economy.)

The fan clutch is merely a compromise that saves a lot of fuel over a permanent connection because it lowers the overall power demand, even though by itself it has terrible efficiency.

A good comparison to think of is the blower motor resistor in your HVAC blower. The system is most efficient at high speed, because at lower speeds some amount of energy is being burned up in the resistors. However, the system overall still uses less power at those lower speeds.
Thanks for the thought put into this @Steel City 06 ! Just like everything life is full of compromises.
Millions of FWD cars have used this technology for 40+ years , however I can understand the reluctance to run down this road on a
Jeep. Many AWD cars have the PCM decide when to engage the rear wheel drive only when absolutely needed , saving fuel and " wear and tear ". However, here in TJ land we have the whole front drivetrain spinning around all the time ready to be engaged.
Sure there have been attempts on early XJs , eta.. to address this , but it wasn't reliable as it could have been. and the fuel savings were virtually non existent.
In many ways a TJ is very primitive ! To me that's part of the draw , simple and reliable. But I like your fan thinking and am curious to see where you go with it !
 
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Now an axle disconnect I can see having an actual effect on fuel mileage and power. I spun the front wheel on a JT the other day. The tire went around 15+ times with just a light push. Try that with a stock TJ you would be lucky to get one revolution.
 
Interesting. I’ve been wanting to figure out a way to do this with an electric fan. The PO put dual electric fans on the tj controlled by a toggle switch on the dash that’s on or off. It would be nice to have a better way of controlling it.
 
There are aftermarket controllers. I use a Dakota digital, but there are many options. I would love to make it integral to the pcm though. The switch method is going to fail you at some point though. I'm sure that's why you are researching something better.

BTW--dual fans? I'm imagining some pretty small fans in my head.
 
BTW--dual fans? I'm imagining some pretty small fans in my head.
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Our Unlimited has a stock cooling system with engine driven fan.

Now the 1980 FSJ (360 V8) has a Ford Couture fan ( Rated at 3500 cfm) conversion with a 185 degree temp switch.

I also have a manual on off switch that allows me to turn the fans on and override the temp switch.

When I know that it will be idling for a period of time like going into the post office or store I turn on the fan to constantly run.

This way when it is 115 out the cooling system is pulling the max heat out of the radiator at all time and the AC Condenser is getting full air flow.
 
Unfortunately I haven't yet had time to upload the tune and test the harness pins. There are visible pins in those locations but I need to tear apart the harness to actually access then while the engine is running.

Writing the tune was easy; I just need to upload that to the PCM.
 
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Well I had the chance to write some new tunes with various modes enabled, and to poke around the empty spots on connector C3.

I got a null result. I could not find any change in the pinout operations based on enabling each of the different fan modes.

So either the tuner isn't capable of enabling the PCM to do these tasks, or the PCM isn't capable of it.
 
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The one simple problem with electric fans is that they cannot match the airflow of a mechanical fan with a stock TJ radiator so you loose peak cooling capacity which is a fatal flaw in warm climates. If you don't need that extra flow then go for it
 
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The one simple problem with electric fans is that they cannot match the airflow of a mechanical fan with a stock TJ radiator so you loose peak cooling capacity which is a fatal flaw in warm climates. If you don't need that extra flow then go for it
That vastly depends upon what fans you compare. The largest SPAL brushless fan can move more air than a stock TJ clutch fan at idle to low RPM. But the Explorer 11-blade fan moves more than both. A cheap Derale isn't going to come close to any of the above.

Often, maximum airflow is needed at idle or low RPM. Only an electric fan can do that. Mechanical fans compensate by being physically larger and more powerful, which works well at low RPM, but is unnecessary at high RPM.
 
Novak Conversions Jeep Wrangler TJ engine mounts