A funny thing happened on the way thru an oil change

That is a funny one, Jeff. But I suppose one would never notice their lack of a fan unless, or until, they get stuck in traffic and need to stand stationary for a while.

Which brings me to another thought... why do we need fans that run directly off the engine, increasing and decreasing with engine speed, when in all reality, the biggest need for additional air flow thru the radiator will be at the lowest engine speed, and the least need for the fan will be at higher rpm when we are making good speed?

Seems like an intelligently controlled electric fan would be a better design, no?

-Fred W

Years ago, I had a 1971 Ford truck. the fan was bolted straight to the water pump. It didn't matter much since the engine didn't go much past 4000 RPM anyways.. However, in the newer cars. they have a fluid clutch, so when its cooler, the fan almost completely slips. As it gets hotter, the clutch will engage more and more until its fully locked. This has worked for many years, but just as you say, its not the most efficient way of doing it. The fan usually robs about 15-20 HP. So removing the mechanical fan and putting in an electric one with a thermostat is better, but also costs a fraction more. This is most common on front steel drive cars with the transverse mounted engines.

That's what I woulda thunk too. But, I'm in Southern California and we have already been well into the 90s and I've been stopped in traffic with the A/C on high.

Anything that won't break would be an improvement.

It isn't the engine speed that dictates the air flow, it's the ground speed. If the car is moving, there is air flow through the radiator, and if the A/C is on, there is an electric fan that kicks in, maybe this fan is enough ... Hmmm,

----- Original Message ----- From: "Malt_Hound" Newsgroups: alt.autos.bmw Sent: Wednesday, July 06, 2005 2:16 AM Subject: Re: A funny thing happened on the way thru an oil change

snip

BMW fans are viscously coupled - engine cold - fan slips ; engine hot fan spins.

Electric fan should cut in if engine get too hot (e.g. in traffic), but represent a potentially disastrous failure mode if relied on completely.

I should try and find out why the fan blades broke - any sign of the remains of the neighbour's cat that hid under the bonnet.

I had a friend with a large car in times past. It was some kind of Buick and had a huge fan and a large space between the fan and the radiator, with a giant cowling coupling the radiator to the fan.

Well, he parked it one day at another friends house, a cold autumn day and we went inside to suck back on some wobbly pops.

When we went out and started the car, the most horrible whining sound came from under the hood, we stopped the engine and a cat ran out from underneath the car. The cat was not injured, just scared to death.

It had sat in close to the radiator to get warm.

After that experience, any time a car pulled up in the driveway, the cat would run in the other direction.

Or a progressively driven mechanical one which idles when not needed...

I am pretty certain the new era of BMW's have electric radiator fans. I thought I read somewhere that is one of the means they used to lower engine loading to increase power yet maintain fuel efficiency (apparently better battery capabilities mean less load without the fan belt, but also no increased load for the alternator). The fan and fan speed are driven by signals from the engine controller. I also think they might have electric water pumps for the same reason.

And I know both my E39's had at least some semblance of an electric fan because it would run long after the engine shut down on hot days when the AC had been used.

That's the auxillary electrical fan for cooling AC condenser. The main fan is mechanical with a viscous clutch. Check out

the main fan (located between the radiator and engine) and the AC fan (located in front of the radiator).

Strictly speaking, no. It's the air speed only. However, we won't quibble for a motor car... :-)

For an aircraft it really matters whether there is a headwind or tailwind. The difference between ground speed and air speed can easily reach 50 mph or more.

DAS

For direct contact replace nospam with schmetterling

Thanks. I was wrong on the new cars' water pump too. The site clearly shows the water pump and alternator run from the same belt.

Thing is if you're going to drive an alternator from a belt and then use that electricity to drive a waterpump and fan, you might as well drive the waterpump and fan directly. When we get high voltage alternators that are part of the flywheel so no friction from the drive, things might be different. I dunno how much power a waterpump devours, and whether the ability to vary its output independant of engine speed would be useful or not, though. But I'd say viscous coupled fans are pretty efficient - and rather longer lived than the average electric fan - if the aux one on the E39 is anything to go by. ;-)

No signs of the cat. Well, there wasn't any fur on the plastic parts that I found. I did have some service recently, and I found that the fan shroud wasn't fully clipped into place. I don't know that it came unclipped as a result of the fan coming apart, or caused the fan to come apart. The shroud is in good shape considering the other distruction in the area.

Yeah, but my SAABs with 4 cylinder transverse engines all had electric fans, and they were pretty darn reliable. They worked simply on the thermostatic switch that measured the coolant temp in the radiator. So long as the coolant in there was OK then the cooling system should be able to regulate properly via the regular coolant thermostat.

The nice thing about that design is that when you are traveling at a decent speed (45mph+) the forced airflow through the radiator is more than the little fan can create so it does not need to run.

OTOH, there is a 2 speed control on the SAAB fan so that with the AC on, or if the temp gets above a secondary threshold the fan goes into warp drive.

This really seems to be a more intelligent design to me...

-Fred W

Well, sort of... My thought was with electric auxiliaries, you can turn them on and off with intelligent controls rather than just having them run all the time that the engine is running.

For instance, running a water pump full time, or the fan full time, even though the engine may not need them.

I also recall reading somewhere that a pusher fan is more efficient than a puller type such as one driven off the engine.

-Fred W

Well maybe the water pump. The heat generated inside the engine is generally proportional to rev's and a bit dependent on load, so the flow of cooling water should vary with engine speed, which it will do if the pump is directly driven.

The operation of the fan depends on the amount of heat generated, but also on how difficult it is to get rid of. Factors such as ambient temprature, current block temperature, speed of the car, wind, is it raining, use of the in car heater etc. all affect how fast the fan will need to go to dispose of the suprlus heat.

One idea I like with the electric fan and water pump is the car can then continue to run them after the engine is off until the engine temp gets below a set temp.

With a transverse engine and a front mounted rad it would be difficult to have an engine driven fan...

Hence the temperature sensitive viscous coupling? And I'd suggest a BMW one is more powerful when locked up than any electric one.

That is true and has worked for years, but given the need to enhance power while lowering fuel consumption, an electric driven water pump and radiator fan makes a lot of sense. I agree with Fred on this. I found the article I was lloking for, and BMW is using an electric water pump (engine cooling, not just heater core flow) in the E90...and their results indicate the savings from the pump compared to the increased load on the alternator, is well worth the change.