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-   -   What is MPG penalty for a running radiator fan? (https://ecomodder.com/forum/showthread.php/what-mpg-penalty-running-radiator-fan-14651.html)

ShadeTreeMech 09-23-2010 07:14 PM

What is MPG penalty for a running radiator fan?
 
I've started working on my grill block, and so far so good, I can already tell the difference with only half of it blocked. Was getting ready to work on blocking more when the adage of not letting the rad fan come on came to mind.

My question is, how much is the cost of letting the fan kick on at temp as necessary vs the aero drag of a half opened grill? I know the conventional wisdom is to make the fan stay off, but why? Especially if the temp the fan comes on is raised a bit?

talldudenumber5 09-23-2010 07:53 PM

i blocked off the full upper what does yours look like? i did not notice any differance with temps with the upper blocked. and i believe the temp thing is it gets too hot in the engine bay and melts things plus if the fan is running it is using electricity which mean the more the alternator has to work i e more gas used

ShadeTreeMech 09-23-2010 08:07 PM

I'm certain the underhood temps are high anyway, so not too concerned with that. But what is the cost of running the fan? The Max has a 30 amp fuse for the fans, so it would have to be less than that, so about 325 watts? (25 amps @ 13 volts) Assuming alternator efficiency of 50%, that would be a 750 watt pull, or close to 1 hp of the 50 I use at crusing speed.

I'm just not seeing a huge drop in mpgs from a radiator fan running in theory.....

talldudenumber5 09-23-2010 08:11 PM

it would be interesting to have a thermometer under the hood in the engine bay to see if it really got that bad.

ShadeTreeMech 09-23-2010 08:14 PM

On a side note, by using the current mpg guage on my scanguage, which is an average of the current drive, my wife and I have been perfecting our driving technique and have been averaging 30 mpg for the tank thus far. And this is on several trips of about 10 miles each. So far so good! Both of us getting into the spirit of things helps a lot!

talldudenumber5 09-23-2010 08:18 PM

it would be good to check engine bay temps just to see if it is really that bad but i am sure it may be a very small percentage loss in mpg more for true hypermillers not us in v6 sedans. sweet i am excited mine is around 26 - 28 here in branson which i see as impressive feet since we all know how traffic and hills are here lol.

ShadeTreeMech 09-23-2010 08:19 PM

I can guarantee it gets a bit warm. Ever tried to work on the top of the engine after it had been running?

Hmm, maybe unhooking the fresh air snorkel and using the Intake Air Temp gauge on the SG would give an indication......

ShadeTreeMech 09-23-2010 08:21 PM

Quote:

Originally Posted by talldudenumber5 (Post 195605)
it would be good to check engine bay temps just to see if it is really that bad but i am sure it may be a very small percentage loss in mpg more for true hypermillers not us in v6 sedans

You're right I'm sure. But I sure do love our v6 sedan. I saw a gold '98 Max at a dealership with a 5 speed and nearly started crying from desire!

Luckily I convinced the wife that replacing the auto with a manual was a good idea....now just need some parts :D

saand 09-23-2010 08:41 PM

ShadetreeMech, regarding the MPG penalty for running the fan I will be doing a test on current draw vs MPG for my car when i finally find a good enough test site.

but from previous tests (that weren't ideal) i think there was only a few percent penalty for running with full head lights compared to parkers. I have the standard fillament lights not LED so the current draw of my head lights is probably comparable to the fan current draw.

So i wouldn't expect to see more than 2 or 3 percent for running a fan also the fan wont always be on.

talldudenumber5 09-23-2010 08:59 PM

we will have to compare grill blocks once we both finish. same car, similar driving environment. let the testing begin lol

ShadeTreeMech 09-23-2010 09:03 PM

Which is kinda along my train of thought. Block the grill off completely, leave an opening under the bumper for the air draw as needed by the radiator fan, and enjoy the MPG savings, intead of worrying about controlling the variable air flow from the front of the car.

The Honda CRX Del Sol had a very small grille opening under the bumper. What if it had no grille opening and just pulled in air as needed with the rad fan?

ShadeTreeMech 09-23-2010 09:04 PM

Quote:

Originally Posted by talldudenumber5 (Post 195620)
we will have to compare grill blocks once we both finish. same car, similar driving environment. let the testing begin lol

Don't forget to put your numbers in, hard to compare when there are so few entries in your log :D

Weather Spotter 09-23-2010 10:27 PM

I like your questions :)

I did a full upper grill block, and have a lower one that can open and close as I want it :) :). This is the best of both worlds, I can block the grill at start up or when ever I do not need the cooling. Just before the fan comes on I open the lower grill and take the aero drop instead of using the fan.

RobertSmalls 09-23-2010 10:32 PM

ShadeTreeMech: An alternator is about 50% efficient, so if your fan draws 20A (275W), that's a 550W accessory load. You can use the calculator to turn it into a percentage if you like.

You definitely don't want the fan running ALL the time, because it is a pretty big draw.

nmgolfer 09-23-2010 10:53 PM

Well the aero drag for any of these solutions is the "sinking drag" which is equal to d(mV)/dt

mass (m in slugs) is the mass of air you're bringing up to speed of car (V - ft/sec) either passively or using the fan .... so you have the area (of your grill or opening) you have your speed (mph) and you have the density of the air P/RT (slug / ft^3) ... do the calc and you get in units slugs ft/sec^2 which is lbf ... that's your sinking drag required to cool engine.

So you block off the grill and let the fan suck it in (not too efficient at sucking these fans not designed for that) but it still wants the same heat transfer (mass flow) to cool the engine... but now instead of doing it passively you want to use electricity (juice) to get the job done. Not much sense in my opinion... I agree... take the aero hit instead.

The problem is not when you're cruising down the highway letting the good times roll though... the problem is when you're stuck in traffic in 100 deg heat going nowhere fast and the fan can't suck/push enough mass through the heat exchanger.

A variable area grill block seems like a good idea albeit an engineering problem... Personally I've got a permanent full upper grill block on (those are just for style mostly anyway) No discernible negative effect so far... and this winter I hope to see what a partial lower block can do...

This is one aspect the EV's definitely have going for them.

nmgolfer 09-23-2010 11:07 PM

Quote:

Originally Posted by Weather Spotter (Post 195645)
I like your questions :)

I did a full upper grill block, and have a lower one that can open and close as I want it :) :). This is the best of both worlds, I can block the grill at start up or when ever I do not need the cooling. Just before the fan comes on I open the lower grill and take the aero drop instead of using the fan.


Nice... is that thermostat controlled? or do you need to remember to open it... the latter not something I'd trust the wife with.

winkosmosis 09-24-2010 12:19 AM

Quote:

Originally Posted by ShadeTreeMech (Post 195621)
Which is kinda along my train of thought. Block the grill off completely, leave an opening under the bumper for the air draw as needed by the radiator fan, and enjoy the MPG savings, intead of worrying about controlling the variable air flow from the front of the car.

The Honda CRX Del Sol had a very small grille opening under the bumper. What if it had no grille opening and just pulled in air as needed with the rad fan?

Well it needs some opening to pull air in. The smaller the opening, the less efficient the fan is, and you can increase fan power without pulling in much more air.

busypaws 09-24-2010 12:44 PM

I blocked about 75% of my grill. Also ran an LED light on the dash to see when the fan cut on. On an 80 degree day, cruising at 55, the fan will cut on for a minute every 4-5 minutes. That seems reasonable to me. Not too many days over 80 here in San Jose CA. Then last night (around 60 outside) it only came on near home when I EOC'd into a light and then restarted when light turned green. Fan came on for 30 seconds on engine restart.
I also took the snorkle off the air intake so I am pulling air from about a foot behind the radiator fan. When the radiator fan is running I can get air temps of 90-100. When the fan is off then the air temps are only 5 or 10 degrees above the outside temp. This data is making me think about at better belly pan/more sealing around hood/lights. I would like to get the air intake temps up a little more before we head into those "brutal" winters here in the bay.

Weather Spotter 09-24-2010 12:53 PM

Quote:

Originally Posted by nmgolfer (Post 195664)
Nice... is that thermostat controlled? or do you need to remember to open it... the latter not something I'd trust the wife with.

No it is not automatic. If you want to make it wife proof, then you would need to automate it. Doax did that with his, check his thread out for how he did it.

talldudenumber5 09-24-2010 12:57 PM

Quote:

Originally Posted by ShadeTreeMech (Post 195621)
Which is kinda along my train of thought. Block the grill off completely, leave an opening under the bumper for the air draw as needed by the radiator fan, and enjoy the MPG savings, intead of worrying about controlling the variable air flow from the front of the car.

The Honda CRX Del Sol had a very small grille opening under the bumper. What if it had no grille opening and just pulled in air as needed with the rad fan?

in my max i have no opening under the radiator ( under belly pan) so that would not work for me i plan to block half of both the lower openings and go from there. lol i was thinking another advantage to an upper grill block is you could paint on a grill from another company( ie BMW split grill) thought it would be funny now to convince my wife to paint it on

i really did not like the log on here to track mpg so i use a program on my wife's ipod
here are my results from may till now these are full tanks
may 13 20.8
may 24 23.8
jun 11 20.1
jun 27 21.9
jul 7 22.6
jul 29 21.8
aug 13 23.3 ( started hypermiling practice again)
aug 23 30.4 (upper grill block and mud flap deletes, mostly highway)
aug 25 24.9 (back to the city)
sep9 27.1 (461 miles ,half tank in branson, half tank highway)

Phantom 09-24-2010 03:34 PM

If you know the coolant temp that turns the fans on you should aim to have enough flow to stay below that at all times when on the highway. For example on a 95F day cruising at 65-75mph going up and down hills my ECT will move between 197 and 204F the thermostat is a 195F. If I stay in that range my fans will never turn on when moving since the fan turn on temp is set to 212F.

One thing you will need to watch out for is Air intake temp as some cars have a fan turn on temp for that also on my car the turn on temp for that is 122F. So if it gets to hot at the intake or the sensor gets heat soaked showing high temps it can cause the fan to turn on.

ShadeTreeMech 09-25-2010 12:37 AM

I did a bit of an extreme hot air intake on my Max last week, and with our normal driving, it wasn't a big deal, but after driving it more than the normal ~10 miles the intake air temp rose precariously close to the water temp (maxed about 150') so I adjusted it to simply warn the air a bit. The engine didn't sound too happy to be pulling in that hot of air.

I hadn't considered how much air would be needed for the fans to work properly, sadly though my physics math is a bit, um, nonexistant. But it would seem opening the under belly pan under the front bumper would be sufficient to allow air in without needing to cause the aero drag of the front openings.


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