F355 Aircon Recirculation Flap

Thank you. It's a capable little chip costing less than $5 and there's an IDE with debugger. Of course it has its limitations on speed and capacity - this program is about 2/3 full - but there are others in the family and they are ideal for one-off projects and replacing bundles of low speed logic. It was only a lack of an extra I/O port which required me to use the TTL logic gate. As you might gather, I am retired now and my electronics knowledge is stuck in the 80's...

Still, it's all installed the car, the trunk is back together and it's working perfectly.

Interestingly, the biggest single difficulty was locating a power take-off for 12v switched by the ignition key. The Ferrari circuit diagrams are not good and there was no point that I could find close by after an hour of looking so I eventually tapped into the switched 12v supply for the radio by running a sleeved cable through a convenient hole in the bulkhead.

 

Yes, if you think about it, if you were designing an HVAC system for a car, you'd choose an air inlet position with only slightly positive pressure and as independent of speed as possible so that the airflow was controlled solely by the fan speed. Otherwise, any ram effect would be very noticeable inside and I've never noticed a hugely strong airflow at 150mph on the autobahn. Might have been useful to dry my sweaty palms as the front goes light...

 

First, you are looking at static pressure. Low static typically means high velocity. See my attached figure which shows Cp and velocity. Note that where Cp is lowest over the hood, V is highest. Now, stick a total pressure probe in that low static pressure area and what do you think the total pressure would be? Total pressure is static plus dynamic. Pt = Ps + rho x V^2/2, ignoring compressibility. Now, what ever it is, the pressure on flap is the total pressure because the air entering the vent is stagnated. Recall, I said this could result in a force as high as 2 lbs, probably less. Obviously it isn't 2 lbs. But that it is higher than the cabin pressure (which would be expected to be at Cp = 0) is a given otherwise when you opened the flap air would be sucked out of the cabin when we know that indeed air blows into the cabin, even with the fan off, quite vigorously. We can knit pick about the magnitude of the pressure force, but that it is present and increases with speed is a given. And I can guarantee, without doing a simulation, that the pressure at the face of the hood vent on a 348 or 355 is a higher pressure region than slightly before it or just behind it simple because when the flap is closed the hood vent is a step obstruction to the air flow and the air must turn up an over it. And there is only one way to turn the air, by an increase in pressure as the vent is approached.

Anyway, my next effort will be to try and determine if the closing force on the flap changes when the current is cut. At this time I am not saying it does or doesn't. But for it to change something must moved after the current is cut off. This is a mechanical system where the closing force is related to rotational and linear displacement and the torque applied by the motor. Once closed, something has to move if the force is to be reduced. It's apparent that the worm gear isn't going to rotate backward, something I have already confirmed. So that leaves the flap.
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Aside that this shows a complete lack of understanding of aerodynamic let me make it really simple that it changes with speed. Get in you car, start it up. Open the flap with the fan off. How much are blows through the vents when you are stopped? Now accelerate, 5 mph, 10 mph, .... 50 mph. Got air yet? Sorry to be so condescending, but for Christ sake, use a little common sense. It doesn't take an expert in aerodynamics.

 

I disagree, the motor can generate torque and apply stress to the gear train without moving; switching the motor off reduces the stress in the gear train without any movement.

As a matter of interest, aside from appearing to be a troll, why are you making such a big deal about this? My objective of implementing what I consider to be a better engineered solution has been achieved, just as it was by those who chose to install limit switches. It's working fine, I'm pleased with the result.

 

I have no objection to you disagreeing and I am always ready to state that I was wrong if shown to be. But for now I disagree. I'm not trolling, I'm just a retired scientist/engineer who finds this interesting and aside from being bored by winter, think that it makes an interesting discussion considering the usually nonsense that get's posted here, like what battery to buy? You know, the devil is in the details. And let me repeat, I do admire and compliment you on the design. I'm just on the other side of the fence as to whether is solves the failure problem.

Let me pose this in a different way. I mentioned elastic components. I'm sure you understand what I mean but for others, something that is elastic in the engineering sense means that for a given applied force the component deforms (compresses or expands) in a linear fashion. The common example is a linear spring. If you apply 2 lbs to compress the spring it compresses twice as much as if you only apply 1 lbs. Ok, now look at the flap mechanism. When the flap is closed against the seal what you have is a series of springs that are compressed by the force applied to the gears by the rotation of the worm. The amount that those springs are compressed is determined by one thing, the rotation of the worm gear. Once that worm gear has rotated to a certain angle and stays there the spring of the flap drive gears, etc, are compressed to a certain point and they remain that way unless the worm gears back up. The worm gear is a device that converts a torque or rotation around the axis of the motor to a linear displacement or force in the direction of the axis of the motor. The torque and resulting axial force are at 90 degrees to each other. Thus, once the torque is removed the only thing that is unloaded is the torsional stress on the motor shaft. The worm remains stationary and the gears remain loaded. The only way for the gear train to unload is for something in the flap mechanism, between the worm and what ever stops the flap from moving, to deform further under the applied stress. Something needs to give. It could be a gear or shaft breaking, or it could be something that just, over time, stretches or compresses a little more. Such a component would be in-elastic in that the amount it deforms is a function of both how much force is applied and how long it is applied. So unless there is something in-elastic in the flap mechanism, once the worm is rotated to a certain position, and remains there, the forces in the flap mechanism must remain constant, regardless of if the current is left on of cut off.

The idea of cutting the current off before the motor stalls, either with your device or limit switches, would reduce the load on the gear train components since the worm would not rotate as much. That may delay or prevent failure. But the problem in my eyes is that the integrity of the shaft and gears degrades over time due to the effects of age on the plastic.

So, again, I'm not trolling. It's an interesting problem and one which I think requires looking at from more than an assumed cause/effect relationship. But lets face it. It's a $100 part at most and lasts 15 to 20 years. If it brakes, fix it. My engaging in this discussion is like you designing the device, we both had apparently nothing better to do.

 

BLUESL did not say that the air pressure (or suction over most areas) over the bonnet surface does not change with speed. He said "choose an air inlet position with only slightly positive pressure and as independent of speed as possible". The "as independent of speed as possible" is the area where the suction transits into pressure - the two balance / neutralise each other and it appears that the 348's intake grill is right in this zone.

 

Yes, there is quite a lot of suction (lift) over the front 2/3 rds of the bonnet at high speeds. If it could somehow be eliminated, the front end would not get so light...

 

I totally agree with all of the above. What can relax the load is the further (plastic) settlement of the elasto-plastic foam over time. If the motor is under current, it will follow the plastic settlement and maintain the same load; if not, the secondary plastic relaxation in the foam will reduce the load. The type of foam I have on my flap (not sure if original) is, I think, elastic over shorter periods of time but goes into some plastic deformation after longer exposure to load which reverses itself when unloaded, again over a longer period of time.

Great thread, this has really been a thorough analysis from all aspects.

 

With that I agree, and stated previously, though I referred to it as the gasket or seal. However, I checked my car and I'm not so sure the flap seals against the foam. Anyway, I would really like to measure the closing force and see what happens over time.

 

I opened my hood last spring to see the flap had snapped off and was lying at the base of the fan. I took it out so it wouldn't interfere with the fan but haven't done anything with it yet as the dealer said there was nothing that could really be done. I was thinking about afixing it to the inside in the closed position with some silicone and calling it a day. Any other suggestions?

 

Unscrew the 2 phillips screws holding the flap motor on the outside of the air box.
You'll probably see that the short plastic shaft connecting the motor to the flap is snapped.
Buy the replacement Hill Engineering metal shaft.
Fit new shaft into motor...push fit
Push fit shaft into flap
Push fit other end of flap into recess in the air box
Screw motor back to box

Never listen to Dealers without double checking here!

 

and you can buy this at Ricambi......the part is called an 348/355 air re-circulation boss (cost $57.00)
and here is the link:
348/355 Air Re-circulation boss - Ricambi America, Inc.

 

Or take the broken piece to a machine shop and they can probably knock one out for less than $20.

 

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Not fully understanding what the above means? (from post #2)

With temps north of 85', engaging my AC compressor (STOP button) also activates/closes my recirculation flap. My recirculation button does not open or close the flap.

Does the owners manual information mean that with higher ambient temps, the ECU will take over the recirc flap operation?