46 replies to this topic

[Cooperman]

Whatever the mathematics it really boils (  ) down to the following:

 

1.   For a road going Mini the ultimate bhp is largely unimportant as the car is on public roads. In terms of possible performance the Mini will always be a slow car compared with modern ones, so why take the risk of overheating a good engine for 1 or 2 bhp.

 

2.  On a rally car it would be totally unfeasible to remove the mechanical fan. Rather also add an electric fan.

 

3.  For racing/hill-climbing/sprints the deletion of the mechanical fan may give a couple of bhp and that might be enough to take a few hundredths of a second off a lap or timed climb. It doesn't matter if it is battery driven as the race/hill-climb won't be long enough to use all the available power in the battery.

[Dan]

Exactly what i thought.....and said in the first response I made to this post.  Then i got shot down by someone who knows what they are talking about....apparently.  You really can't win on here.

 

  Err, if you mean me I wasn't disagreeing with you.  Just adding more detail to what you said.

 

 

And yes Carbon Weezel make carbon versions of a couple of different fan types.

 

 

  Anyone playing with pulley sizes to minimise fan drag by slowing it down should remember it also slows the water pump down.  If it was previously spinning so fast in normal use that it was cavitating then slowing it will improve cooling but in most cases it will reduce cooling considerably.

[ANON]

bhp is only for pub talk anyway so not worth worrying about   

 

if you want to up the bhp just use smaller horses, problem solved!!

[madaboutcherry]

I,ve got one of these on my 68 cooper, it never overheats & it's quieter on motorway journeys.

[madaboutcherry]

http://www.ebay.co.u...=item1e7cd478c4

 

oops

[Captain Mainwaring]

If the mass of the fan causes power sap.........Has anyone developed a lightweight carbon fibre fan?

 

It doesn't. Makes not an ounce of difference.

[Captain Mainwaring]

 

 

I'm sure this subject was discussed on the forum a few months ago and the conclusion was that, for a road going car with a side mounted radiator, having just an electric fan was not a good idea. It would be likely that the fan would run permanently as there is not enough airflow through the grille and out into the inner wing to cool the coolant without the fan running. Hence why Rover fitted both the mechanical and electric fans, the electric fan helping in slow traffic when the engine speed and fan speed are too low to produce sufficient air flow for cooling the coolant.  

 

This is more like it. You fit an electric fan to boost a mech fan for in traffic use, and as revs rise, airflow also rises and you can select a mech fan that is just enough for full speed full power heat exchange.

 

In the past cooling was always a compromise, the mech fan that had to provide enough air at idle in traffic conditions to cool the engine could provide too much cooling at higher speeds with light throttle openings. 

 

You need to put a graph of heat into the cooling system at various engine speeds at full throttle versus air flow a various engine speeds - you'd end up with a sort of "S" shaped graph that would show you what the required airflow was - 

 

The only fans that have an "S" curve for airflow vs fixed static head are forward facing aerofoil centrifugal fans which are fitted in forced draft applications where they can be slowed down until the air temperature rises, otherwise they will back load. and run away until they stall.

 

I suppose if you were determined enough you might be able to develop a model to allow a system redesign and claw back a bit of power, but always at the possible risk of overheating if you get it a bit wrong.

 

Ideal efficient RPM for a propeller fan like the mini is 950-1050 RPM, maximum speed would normally be around 1450 RPM - you can see for yourself just what a compromise the mini system has to be.

 

Part of my work is specifying fan systems for use on thermal transfer equipment - one small machine can can 500 kW (670 HP) of puff and blow on it, and despite contracting for a scottish company, I tend to oversize by 15% on all fan sizings, working on the old adage "I'd rather be looking at it than looking for it" - to try and get an extra 5% air out of a fan on the limit can take a whole lot of time and money and use loads of energy when running.

[Cooperman]

Cap't Mainwearing said: The most sensible stuff yet quoted on this.

 

What on earth is the point in trying to gain a couple of bhp on what is one of the slowest cars on the road and which will show no measurable improvement to acceleration, speed or mpg, all at the risk of 'cooking' the engine.

[Ethel]

" trying to gain a couple of bhp on what is one of the slowest cars on the road" That's your answer, right there   

 

Quite agree it's not worth the risk of been sat on the hard shoulder boiling over, stopped is slower than slow.

[Captain Mainwaring]

Cap't Mainwearing said: The most sensible stuff yet quoted on this.

 

What on earth is the point in trying to gain a couple of bhp on what is one of the slowest cars on the road and which will show no measurable improvement to acceleration, speed or mpg, all at the risk of 'cooking' the engine.

 

That's the thing - we could spend load of time and lots of money and on the dyno you may just see a gain of 1 BHP on a good day.

And that's assuming you design the thing right !!

 

Lash together an electric fan on pull through thinking it'll do the same duty as it did on push through on a similar application, running in conjunction with a conventional modified fan with cut down blade say...and you might just see the same 1 BHP gain, at the risk of having the thing cook up after a hard run and the idle in trafic.

 

Everybody (particularly here) spends fortunes chasing the elusive horse power yet rarely if ever enjoys them - what they crave is power, ironically what the enjoy driving is torque.

[Captain Mainwaring]

 

Exactly what i thought.....and said in the first response I made to this post.  Then i got shot down by someone who knows what they are talking about....apparently.  You really can't win on here.

 

  Err, if you mean me I wasn't disagreeing with you.  Just adding more detail to what you said.

 

 

And yes Carbon Weezel make carbon versions of a couple of different fan types.

 

 

  Anyone playing with pulley sizes to minimise fan drag by slowing it down should remember it also slows the water pump down.  If it was previously spinning so fast in normal use that it was cavitating then slowing it will improve cooling but in most cases it will reduce cooling considerably.

 

 

 

It really doesn't matter what your fan is made out of - a bit of bent up tin or a beautifully crafted piece of carbon fiber, the same rules regarding power consumption and airflow hold true for both, the carbon fiber one may make you feel more trendy and your pocket lighter, but that's about it. 

 

Since you mention about messing with pulley sizes the reader should be aware that exactly the same rules apply to centrifugal water pumps as do to fans regarding performance and power consumption - reduce the speed 10% and the mass flow will drop by 10%..

[jaydee]

The metal fan weighs more than the later plastic fan

That said, the risk of cooking an engine for 2 bhp is simply ridicolous, unless you do hillclimbs

[Ethel]

It's interesting, even if we're all agreed it's not worth the effort.

 

I wonder how well matched the pump & fan are, on their common shaft? Does radial vs axial favour one or the other at various rpm? More effort seems to have been put into developing the fan over the years. I'd assume improving the pump's flow is futile unless heat exchanger bit also keeps up?

 

Perhaps the pump is more of the story, you'd think having a substantial mechanical fan  for a side mounted radiator would be an advantage when stationary: compared to a front mounted rad, deprived of the gale from being pushed headlong through the air.

[Captain Mainwaring]

It's interesting, even if we're all agreed it's not worth the effort.

 

I wonder how well matched the pump & fan are, on their common shaft? Does radial vs axial favour one or the other at various rpm? More effort seems to have been put into developing the fan over the years. I'd assume improving the pump's flow is futile unless heat exchanger bit also keeps up?

 

Perhaps the pump is more of the story, you'd think having a substantial mechanical fan  for a side mounted radiator would be an advantage when stationary: compared to a front mounted rad, deprived of the gale from being pushed headlong through the air.

 

 

Hehehe...generally people aren't interested in heat transfer - 

 

You mentioned "improving the pump's flow is futile unless the heat exchanger bit also keeps up?"  - well not really - heat exchangers are spec'ed usually something like this:-

 

Heater to have the capacity to raise the temperature of 50,000 Cubic Feet Minute of air measured at 221F from 180F to 221F when supplied with 1650 lbs/hr of dry saturated steam at 14 bar - total heat transfer 1,500,000 btu/hr - resistance to air flow 0.25" water column.

 

Sorry about the imperial measurements!!

Now, if we put that back into our model, what can we change? 

Well we can't change outside air temp, it's fixed, and we don't care what temp the exhaust air is, though we know it can never exceed the temp of the water determined by the rad cap less 6C - so for let's say a 1 bar rad cap 120C-6C = 114C, so now we have two constants we can't change.

 

All that is left is air mass flow(volume) or water mass flow, and they are directly proportional to each other because they are mounted on a common shaft. What I can tell you with a water to air heat exchanger, the smaller it gets, the less effect water flow has and the more effect air flow has - which is why my wife's Honda CRV has a big thin low water content radiator mounted directly in the airflow at the from of the car, and the cooling fan rarely comes on except when in slow moving traffic - yet my little mini has to puff and blow like anything because it has a small high water content radiator out of direct airflow.

what you can see for yourself is that if you want to really try and refine the cooling system on a mini is that you need to switch to a big lazy low water content radiator mounted where it can take some advantage of the airflow (ala MPI) with ideally two electric cooling fans blowing through, not sucking with two thermostats, one set at set 85C and the other at 95C (just guessing here to start with).

 

In reality I wouldn't bother - especially where I live, with a car fitted with AC too, I have the most efficient rad and cooling fan assembly I can find. 

We don't have the RAC available here when things go wrong!!

Edited by Captain Mainwaring, 16 July 2013 - 02:00 AM.

[Captain Mainwaring]

The metal fan weighs more than the later plastic fan

That said, the risk of cooking an engine for 2 bhp is simply ridicolous, unless you do hillclimbs

 

What has the weight got to do with it? weight=inertia/momentum, not power. if you need that bit extra throttle response, then shave 100gms off the flywheel. It's why high efficiency alternators have over run clutches on the drives - to stop  shredding the drive when the engine suddenly decelerates and the alternator wants to keep on turning due to rotor momentum.

 

It wouldn't be as much as 2 bhp anyway - even cooling at it's most efficient because of the compromises in the mini cooling system (small rad, high differential pressure to get air through the rad) - the mini system is by definition power hungry.

 

Work it out for yourself - the MPI with it's forward mounted radiator and electric fan doen't have a 2hp (1500 watt) motor driving the circulation fan) .

A look at the drawings shows the cooling fan hanging off a 15A fuse so the maximum it could draw is 180 watts or less than a 1/4 hp - So putting like that- buy yourself a really god radiator - as tall and wide as you can fit in (or better still an MPI front mounted rad) and fit an MPI electric fan - and I can guarantee you that I can get the fan side of your cooling system losses down to 0.25 hp !!!

 

Ask yourself why, with the same water pump does one engine need only 180 watts of cooling fan while the other engine needs a great big chip slicer.?

Edited by Captain Mainwaring, 16 July 2013 - 02:21 AM.