Electric Engine Cooling Fans

[davidbrady]

The latest Prevost chassis is outfitted with 8 electric cooling fans. Gone is the belt driven, miter gearbox, clutch, and single 32" fan mounted on the inside of the engine compartment.


I went to the Prevost Car site and I downloaded a set of manuals for the latest coach. Prevost uses 8 SPAL fans mounted to the outside of the engine radiator and the CAC - 6 on the radiator and 2 on the CAC. The fans are in a pusher configuration and are fully shrouded. The cooling radiator and the CAC are side-by-side and the entire package is sealed tightly to the fan shroud with rubber gaskets.

The SPAL fan specs are:

SPAL Model: VA113-BBL504P/N-94A
Max fan speed rpm: 4750
Min fan speed rpm: 1200
Max air flow: 2500 cfm
Sound pressure level dBA: 85.6 - at 1 m ± 0.005 m from the fan module- lateral side
Weight Kg: 2.6
Operating supply voltage range V: 16.0 .. 32.0 at the Drive Connector
Supply voltage to reach max speed V: 26.0 .. 32.0 at the Drive Connector
Amperage: 30 .. 40A
Watts: 850
Operating ambient temperature range °C: -40 .. +110
Speed derating threshold °C: +65 (*)
Storage temperature range °C: -40 .. +125
Lifetime h: up to 40000 hours depending on mission profile

Each of the 8 fans draws a maximum of 850W or 1.1hp for a total max air flow of 8 * 2500 = 20,000 cfm.

How is it that 8 * 1.1 = 8.8hp of electric fans can replace the mighty 40 to 60 hp of a single engine belt driven fan? Most of us would discount the use of such power sipping electric fans as being an impossibility. Attached is a well thought out Masters Thesis which believe it or not describes the use of electric fans in Volvo buses! Check it out; it's a good read. 8.8 hp is equivalent to 6.5KW which is easily provided by a 24V engine driven alternator (6500W / 24V = 270A).

I'll skip to the chase and fill you in on how this is possible. It's all due to static pressure differences between the two setups. If you take a fan and put it in the middle of a room and turn it on, it'll blow a certain amount of air and require a certain amount of power. If you take the same fan and set it an inch away from a wall and ask it to blow an identical amount of air it needs more power. It needs to overcome the back pressure created by the wall; this is called static pressure and when it goes up so does the fan power required. Air flow and the subsequent cooling is the same, but power went up.

So why is the engine driven fan so bad? Because, according to the research paper, to get the same air flow as the electric fans the engine driven fan needs to create a much higher static pressure. We all know how incredibly cramped it is inside the engine compartment, especially the radiator side. That huge 32" fan is thick, the sandwiched or stacked 6 row radiator and CAC are restrictive, the clutch and miter box is in the way of air flow, and the Series 60 (which everything is shoved next to) is a big wall. Imagine removing most of that and putting the fans on the exterior of the radiator and CAC. Now we have a huge void in the engine compartment which dramatically lessens the static pressure needed by the fans to achieve the required air flow. In addition, Prevost reduced the thickness of the CAC and the radiator and placed them side-by-side maximizing frontal square area further minimizing system impedance; and hence, the static pressure of the system.

In short, if we could mount the engine driven S60 fan on the outside of the body, creating a huge air-flow friendly void inside the engine compartment, then the engine driven fan would not eat as much horsepower while achieving the same cooling! If we did this would it be as good as electric fans? No it wouldn't. It'd be better than it was but not as good as electric. To make it as good we'd need to take further steps to reduce system impedance like narrowing the width of the CAC and radiator and placing them side-by-side to maximize area and optimizing the fan shrouding and sealing all voids with gaskets. Then there are the other advantages of electric fans; for instance, with electric fans you can run them when the engine is off, you can run them forward or backward, you can infinitely adjust the speed independent of engine speed, multiple fans means redundancy and no single point of failure, and replacing a fan is quick with just a screwdriver!