[Dale_K]

My first post here.

I kind of facinated why engineers make the choices they do when they design vehicles. I just traded in a 2010 GMC pickup on a 2012 Ram pickup and I was kind of surprised to see a mechanical cooling fan on the Ram instead of electric fan(s) like I had on the GMC. I know this site has some ex Chrysler folks and I wondered if anybody could explain why Chrysler made this choice. It seems like the electric fan has some advantages. It can run full speed at idle where the mechanical fan can only spin as fast as the motor. Is it just a cost saving choice or is there something deeper going on?

 

[KOG]

Cost. Fuel economy standards are eventually going to push all engines to electric fans. As well as electric oil and water pumps. I think the 2013s are electric fan, but can't pin that down yet.

 

[ImperialCrown]

Many trucks use both. Are you sure that there isn't an electric fan mounted deep inside the fan shroud against the radiator core?
The Cummins diesel uses a thermal-controlled hydraulic fan motor run off of the power steering pump fluid pressure, just as the power brakes are run off P/S fluid pressure (Hydro-boost).

 

[TWX]

Sometimes automakers use electric cooling fans on light duty applications and mechanically driven, clutched fans on heavy duty applications. I expect that it's less costly to move that much air mechanically instead of electrically.

If the truck has a factory towing package that could explain it.

 

[AC TC]

cooling capacity....
it would take some really big electrical units to even come close
to a simple shrouded mech unit.
simplicity....
parts left out cost no money and makes no trouble.

 

[Dale_K]

Thanks. I knew the Cummins had some kind of advanced mechanical fan. I had a very troublesome 2007.5 Dodge pickup with the Cummins (Chrysler bought it back eventually) but there was no trouble with the fan. I think that fan essentially had variable speeds by modulating the fan clutch. On that application I could understand a mechanical fan because it was so powerful you'd have needed a huge electric motor to get the same airflow.

If cost was the driving factor then I don't understand why GM uses the electric fan. And I doubt an electric fan provides fuel savings. It seems like you loose efficiency converting mechanical energy into electrical energy and back to mechanical energy again when the end result is already within 6" of a rotating shaft.

This isn't the only place where GM and Chrysler go different ways. I don't understand why Chrysler doesn't use mass airflow sensors or DRLs or pressurized coolant reservoirs.

 

[ptschett]

Dale_K, in all three cases I'd say it's because they're not necessary technologies.

For example, Chrysler doesn't use mass airflow sensors because they don't need them: http://www.sae.org/mags/aei/power/8539

DRLs aren't mandated in the US market. They are mandated for Canada cars and Canadian cars have them. IIRC it's a simple change in the computer to enable them on a US car.

A surge tank is generally only necessary when the highest point in the cooling system is somewhere other than the top of the radiator. My Challenger has a surge tank, though my Dakota doesn't.

 

[Bob Lincoln]

It appears that a typical electric cooling fan moves somewhere around 3,000 CFM (cubic feet per minute) of air. They also draw about 4.5A of current at about 13.5V, or about 60 watts of power.

60 watts of power equals 0.08 horsepower.

I think a mechanical fan will use more than 0.08 hp to spin at full speed. Electric wins.

 

[Kevin1990]

I love that they still use mechanical ones. Almost unbreakable and reliable, pulls down some net. HP, but a electrical fan that size would also cost a lot of energy.

 

[Dale_K]

Thanks for that SAE link. That site looks like it might answer a lot of the questions I have. With regard to the DRLs, Chrysler puts it into the vehicles, they simply don't enable it as the default delivery choice. The only ones they turn on seem to be for the styling reasons, like the 300. I just had the dealer enable the DRLs on my new Ram and they finished the task in no time and didn't charge me anything. It seems like a proven safety improvement and I'm surprised they sort of hide it.

I can't see how an electric fan can be more efficient. You lose something in every switch between energy conversions and it requires a transformation from a rotating shaft to electricity, then to a electric motor and back to a rotating shaft. Now you can charge the battery primarily during deceleration or coasting, like my GM vehicles and in that case the energy is stuff that would have been wasted anyway. Maybe I answered my own question, lol. Sort of thinking out loud.

 

[Bob Lincoln]

You forget that a fan is cycling on and off, its duty cycle can be low enough that it uses less energy than a mechanical one, which is always spinning at some speed. And newer cars have PWM fan designs, that vary speed according to the need. My car turns 2600 RPM at 65 mph, a mechanical fan would be spinning fast for no good reason, whereas my electric fan never comes on during the vast majority of my commute (unless I put the A/C on).

Consider also that the mechanical fan needs a pulley and a belt, its parasitic losses are greater than an electric motor spinning.

The 60 watts that I reference is an actual measurement off my car. That's NOT a lot of power. It's the same as what this laptop uses.

 

[ImperialCrown]

With a cooling fan controlled by PCM software, it tightens up emission and engine load/temperature management controls that much more. They want the PCM to have its hand in controlling as much as possible.
Variable fan speed is controlled easily by a duty-cycle driver. Ramping the speed up instead of just turning it on helps to prevent the headlamp flicker a surge would cause. Some electric cooling fans ran a while after shut-off to cool the engine compartment down.
The scan tool can be used to select or deselect a lot of features during new car prep. The EVIC can also allow the customer to configure many personal settings themselves.

 

[Doug D]

My car turns 2600 RPM at 65 mph, a mechanical fan would be spinning fast for no good reason, whereas my electric fan never comes on during the vast majority of my commute (unless I put the A/C on).

You are not taking into account that the belt driven fan has a clutch and only truly engages when enough heat eminates from the radiator (heavy stop-n-go traffic). My '06 Ram (Hemi) has the belt driven fan and is basically free wheeling at highway speeds - it's not truly engaged. Basically the only time I hear it engaged is after a cold start. Once I get over 25 mph once, I hear it disengage and rarely hear it engage - even in stop-n-go traffic.

I do agree an electric fan uses less energy and is far more efficient in helping to control emissions. On DodgeTalk.com I have read of some Ram owners removing the mechanical fan and installing an electrically controlled fan.

I just had the dealer enable the DRLs on my new Ram and they finished the task in no time and didn't charge me anything. It seems like a proven safety improvement and I'm surprised they sort of hide it.

I would think it has more to with what is required. DRL's are not required in the US, but are mandatory in Canada. As you posted, the dealer can simply enable the feature.

 

[TWX]

I would also expect that when the fan isn't needed, like when running down the road, the electric fan can be turned off, dragging less on the engine. Since clutched fans aren't actively controlled (relying on how the clutch automatically works) there isn't that degree of control.

 

[Marcos]

When a mechanical fan clutch engages to bring up fan speed at idle and pull more air, it doesn't disengage immediately when you accelerate from a stop, and will create extra drag on the engine. In the case of my Jeep ('93 Grand Cherokee) it can take up to a quarter of a mile or more to disengage. It's worse in the summer heat when cooling demand is greater, or when first driving in cold weather when the clutch is "sticky". In stop and go, this repeated extra drag adds up in extra fuel consumption. With an electric fan, the load from the fan is independent of engine speed, and the demand can be better matched to cooling needs.

On the flip side, an electric cooling fan setup requires many more pieces to operate, which is many more links in a chain that can fail.

 

[AC TC]

"I think a mechanical fan will use more than 0.08 hp to spin at full speed. Electric wins."

-this says something about how much air thats moved also..mech wins.
Remember mother has to guarante that the truck can tow the biggest load at the biggest climb in the highest temperature.
Now how the manufacturer interpret these demands may well be the diffrense between a mech or electric unit.

 

[Bob Lincoln]

Please explain to me how a mechanical fan inherently will move more air than an electric one.

 

[tazdevil]

The aftermarket electric fan in my Demon draws 20 amps and flows 2000 CFM.

 

[Bob Lincoln]

Well, that's not optimal. Maybe you should grab an OEM one for a later car out of the junkyard.

 

[TWX]

Please explain to me how a mechanical fan inherently will move more air than an electric one.

I found a study on the Internet:

Energy Loss and Belt Efficiency

CONCLUSIONS
Median efficiency of the surveyed industrial and agricultural
belt types and constructions is 96 per cent. Within
rated and application power levels, efficiency ranges from
90 to 99 per cent depending on belt type, construction, and
application parameters. Both median and range agree with
historical data.

Compared to some alternator information:

From Wikipedia's Alternator page, which in turn cites Horst Bauer (ed.) Automotive Handbook 4th Edition, Robert Bosch GmbH, Stuttgart, 1996, ISBN 0-8376-0333-1, page 813

Efficiency of automotive alternators is limited by fan cooling loss, bearing loss, iron loss, copper loss, and the voltage drop in the diode bridges. At partial load efficiency is between 50-62% depending on the size of alternator and varies with alternator speed.

I did not find any information about efficiency of electric fan motors.

To me, that would indicate that the conversion process is inefficient with all things equal. However, I know that not all things are equal, in that the electric fan can be possibly cut off by a controller or a computer when not needed. A clutched fan will also let go in many circumstances, but not as well, and a fixed, shaft fan will be least efficient as it is always running.

If a mechanically driven fan and an electrically driven fan have the same mechancial power source and they have to provide the same airflow, the mechanical fan will be more efficient than the electrical fan, as the alternator's inefficiency will mean more parasitic horsepower drain just to get power to the fan than the belt drive.