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Author Topic: Radiator fan consumes over 10% of engine HP!  (Read 9034 times)
zubzub
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« Reply #15 on: June 10, 2011, 08:33:39 AM »

Some buses, even  the later 4104, had an engagement system for the fan so that full power was only applied to the fan when the engine needed it.  Most new cars that have an engine driven fan (as apposed to electric fans/more common) have a thermo clutch drive in the fan hub so that the fan does not spin below a set temp.  It is likely that your bus already has some kind of thing like this so that full hp losses are only realized when you need all the cooling you can get......and you will need full cooling at some point.  My old bus has a fixed fan with no variables...it takes forever to get to operating temp, I see shutters in the future....my bus is pretty slow and I could really do with the extra hp but it is a simpler system and not prone to catastrophic failure so I will continue slowly on my way.
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Iceni John
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« Reply #16 on: June 10, 2011, 12:33:46 PM »

the next best thing to a front mounted radiator.
Yup, I've often wondered this as well.   Older British rear-engine designs, such as the Bristol RE and VR, had their radiators up front, and a good-looking radiator grill makes for an attractive front (in a slightly traditional way).   The coolant is already piped up to the front for the heater/defroster, so what's the problem in also running it through a radiator there?   Ironically, the more moderate temperatures in Britain are where inherently-better cooling designs have less benefit, compared to the much higher summer temperatures in North America.   As has been discussed here before, airflow is the key.   A truck with the same radiator and engine as my bus never has cooling issues, so radiator placement seems to be the obvious culprit for our cooling woes.   I may try one of those fancy new multi-blade fans in an effort to persuede more air to flow through my radiator, but otherwise it's mister time, or an expensive re-core.
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Jeremy
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« Reply #17 on: June 10, 2011, 01:21:01 PM »

My bus is mid-engineed and has the radiator underneath the bus behind the front axle.

How is the cooling on modern rear-engined buses arranged? It wouldn't surprise me if they also relied on big, power-consuming fans, but I can't help think that a more intelligent solution exists by exploiting the pressure differences around or underneath a bus to naturally draw air through a radiator. Think of the way air is drawn through the radiator of a mid-engined, low-front supercar such as a Ford GT-40 - no big fans or ugly ducting required, just clever design

Jeremy
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« Reply #18 on: June 10, 2011, 01:48:45 PM »

At the rear of the bus there is relative  high pressure along the sides, and low pressure behind and under.  But it's not a big pressure differential that promotes strong cooling flow in the same way as the front mount rad on a car or pickup truck does.  You still need to pump air at speed, while on a front rad the fan becomes relatively unimportant above around 40 mph.
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kyle4501
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« Reply #19 on: June 10, 2011, 04:21:57 PM »

It is the design of the radiator that determines the amount of air required (& at what pressure drop) to remove heat from the coolant. The design of the fan determines how much pressure it can develop & how much air it can move. I find it interesting that building pressure takes very little power when compared to moving it.
If you deny the radiator the air it needs, the engine will overheat.
For normal driving conditions, you may not need much cooling. It is during those times when you are in a situation that requires max power for a sustained period that you will realize the limits of your system & the benefits of having more cooling capacity.  Grin
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RoyJ
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« Reply #20 on: June 10, 2011, 06:06:29 PM »

Why don't buses blow the hot air out through the radiator instead?  I'm just guessing here, but perhaps if that much air was being pushed outward while at cruising speed, that same perpendicular air flow that resisted inflow, when confronted with a high volume of air pushing out into it would form back pressure restricting outflow.

I have wondered why they didn't put the radiator on the back of the bus and have the fan(s) push the air through it with the aid of the natural vacuum on the back like they did the AC condenser on my RTS.

Well, the outside of the bus, especially towards the rear where flow is highly turbulent, generally has a higher pressure than the engine compartment (which is usually negative, especially if full width mud flap is retained). To blow air out, you'll have even greater pumping losses. If you open a window towards the rear of the bus, you'd see just how much wind naturally flows in.

However, on a mid engine coach like Crown, the radiator may indeed be in a region of low pressure / vacuum, depending on speed. But then again, the air underneath the chassis may have even lower pressure, so it may still be better to suck air in.

Finally, I believe rear radiators aren't used for two reasons: engine accessability and grease buildup. Accessability is pretty self explanatory. As to grease build up, all you have to do is look at some rear engine Class A motorhomes. Engine oil mixed with dirt gets caked onto the radiator over time, and really messes up cooling efficiency. Imagine that on a Detroit!


Think of the way air is drawn through the radiator of a mid-engined, low-front supercar such as a Ford GT-40 - no big fans or ugly ducting required, just clever design


If you look closer, almost every mid or rear engine sportscar has a FRONT radiator, with long coolant hoses running to the back. The side or roof scoops are for air intake, or turbo intercooler.

But, if necessary, a rear radiator can bedesigned on a sportscar, due to the following:

The much shorter length, and better drag coefficient, of sports cars meanings flow is often laminar (attached to the body). If you look at wind tunnel tests of Vettes and Porsches, flow normally seperates AFTER the body. Given this, it is easier to design a side intake, with gentle curvature, so that airflow stays attached to the skin, and guided into a radiator mounted at the mouth of the scoop.

Again, the same concept won't work well on a 45' bus, because the flow is already seperated. It'll certainly be better that a sideways radiator, but is the tradeoff in cargo and seating capacity worth it? Not to mention a Prevost with a giant Viper/Porsche style side will be very expensive to design or fix after an accident.

So I guess to conclude, we're pretty much stuck as far as radiator location goes. I even thought about a front radiator with long coolant hoses, but even that may reduce the front interior space too much. Remeber, the air would also require a path to escape. And then the question becomes: how to you drive the fans? And we all know how much electric fans are frowned upon  Wink
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« Reply #21 on: June 10, 2011, 07:10:32 PM »

Think of the way air is drawn through the radiator of a mid-engined, low-front supercar such as a Ford GT-40 - no big fans or ugly ducting required, just clever design


If you look closer, almost every mid or rear engine sportscar has a FRONT radiator, with long coolant hoses running to the back. The side or roof scoops are for air intake, or turbo intercooler.


Yes I know, but the point is that it's not a traditional front radiator that sits vertically right at the front. The radiator on a GT40 sits at a shallow angle a little way behind the front of the car, with airflow through it being generated by the venturi effect created by air passing over a large slot in the top surface of the bodywork:



Jeremy
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« Reply #22 on: June 10, 2011, 09:02:02 PM »

  The GT-40, Ferrari's, Porsche's, Lambos, etc., they all use and have used NACA ducts, as do all modern aircraft. NACA ducts were deveoped to induce higher airflow with minimal drag by generating a vortex. Those cool door vents on the Ferrari 308 were modified NACA ducts, and do actually promote high airflow.

  Perhaps if the air scoops used on MCI's had a large NACA duct placed ahead of them they would flow more air? Could make one out of cardboard and duct tape it on as a test.
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RoyJ
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« Reply #23 on: June 10, 2011, 10:40:26 PM »

Yes I know, but the point is that it's not a traditional front radiator that sits vertically right at the front. The radiator on a GT40 sits at a shallow angle a little way behind the front of the car, with airflow through it being generated by the venturi effect created by air passing over a large slot in the top surface of the bodywork:



Jeremy


Ok, now I see what you were trying to say.

I agree it's a cool aero design, but again, I think it's just about impossible to implement similar designs on a bus body.

The closest I can think of is the Nova transit buses, if you look at the roof radiator's angle, it almost matches that of a GT40:



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But sometimes I wonder if the added benefit of ram-air is offset by the lower efficiency of hydraulic drives. Riding on these buses feel slightly slower than side fan models, but that could also be the CVT  Huh
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luvrbus
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« Reply #24 on: June 11, 2011, 04:51:21 AM »

Didn't MCI starting with the D model mount the radiator and air charger in the rear on the back above the engine ? I was looking at a MCI that the Amerex Fire System had went off and the radiator was above the engine at the rear huge fan 16 or 18 blades

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« Last Edit: June 11, 2011, 05:02:12 AM by luvrbus » Logged

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buswarrior
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« Reply #25 on: June 11, 2011, 05:02:28 AM »

Yes, the D model MCI has the rad and charge air cooler mounted flat against the back. The fans are belt driven and clutch controlled, the air intakes are from the grated sides, with a cavity ahead of the rad/cooler.

Best of all worlds? belt drive, clutched separate fans for rad and cooler fans, minimal intrusion into the coach interior, engine accessible from both sides.

And then the E and J went to a driver's side low mounted combined rad/cooler single fan with a mitre box set-up.

Who knows?

happy coaching!
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« Reply #26 on: June 11, 2011, 05:10:59 AM »

Thanks BW I thought it was a D model but this old mind plays tricks on me from time to time  LOL

good luck
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« Reply #27 on: June 11, 2011, 07:48:38 AM »

On the E and J models, do the fans blow air out the side or suck it into the compartment?  Is the compartment ducted separate from the engine area?

Brian
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« Reply #28 on: June 11, 2011, 10:48:07 AM »

On the E and J models, do the fans blow air out the side or suck it into the compartment?  Is the compartment ducted separate from the engine area?

Brian

Though I've never driven one myself, I can almost gurrantee it sucks air in, due to the following:

1) It's easier that way aerodynamically, due to the reason I mentioned in a previous post

2) The intercooler is on the outside. If air was blown out, hot engine bay air would first pass through the rad, making it even hotter, before hitting the intercooler. By then it would all be but useless. You always want ambient air to hit the charge air cooler, so air needs to be sucked in.
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« Reply #29 on: June 11, 2011, 11:40:10 AM »

I maintained a MCI D for a hockey team for a few years. What I found is the rear mounted rad and intercooler and fans and associated hardware gets corroded from all the road grime blowing back into that compartment. Following a run in winter conditions, the rear of the bus is covered with dirty slush and gunk. I wonder if that is why they went back to side mounted rads with solid rear cap?

JC
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JC
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