Parachute effect, underside of vehicle, undesirable

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electronchaser

Well-known member
Joined
Nov 8, 2019
Messages
80
Not sure how many people have gotten under the car to take a look, but my BEV has a rather large triangular cut out below the motor area all the way to the end of the battery pack. I thought it might be for cooling, then remembered our motors are cooled by sealed looped refrigerant.

Anyway I don't see or need a purpose to this opening, other than letting more road noise reflect into the rear of the vehicle (rear seat area is directly above this opening, and creating an open parachute effect when the air goes in and gets trapped by the bumper.

Trying to find some suitable material (plexiglass type stuff). Anyone suggest any other materials that might be lightweight and rigid enough to use to close off the bottom underside of the i3?
 
I've seen this opening myself, and wondered the same. However, I can't imagine that the Bavarian Aerodynamikers in charge of this project missed this little detail?

Regarding road noise, I will tell you this: I've driven a few trips with the cover removed, and the electric motor is incredibly loud. Earplug-wearing loud, to be exact. I wondered if, as opposed to keeping road noise OUT, this area was left open so as not to keep motor noise IN.

Whatever you try, be sure the front lip is attached well to something of substance. I drove one of my cars with the front splash guard removed and the piece immediately behind the splashguard became a giant wing with its front edge suddenly exposed, and pulled down at 40 MPH making some sort of "oh sh*t" racket. You wouldn't want to send a plexi Frisbee through the windshield of the car behind you!
 
i3 BEV rear underside showing large triangular opening:

bmw-i3-underbody-10.jpg


This has to be intentional. Visible are openings in the panel below the bumper that would allow some air that enters the open triangle to exhaust. It would be nice to understand why BMW didn't enclose the entire underside as with many very aerodynamic vehicles. I've wondered about this since purchasing our new BEV in 2014 but decided against closing it because I don't understand why BMW left it open. Not sure that we will ever know…
 
eNate said:
I've seen this opening myself, and wondered the same. However, I can't imagine that the Bavarian Aerodynamikers in charge of this project missed this little detail?

Regarding road noise, I will tell you this: I've driven a few trips with the cover removed, and the electric motor is incredibly loud. Earplug-wearing loud, to be exact. I wondered if, as opposed to keeping road noise OUT, this area was left open so as not to keep motor noise IN.

Whatever you try, be sure the front lip is attached well to something of substance. I drove one of my cars with the front splash guard removed and the piece immediately behind the splashguard became a giant wing with its front edge suddenly exposed, and pulled down at 40 MPH making some sort of "oh sh*t" racket. You wouldn't want to send a plexi Frisbee through the windshield of the car behind you!

hehehe, yes! Definitely no throwing 90mph plexi frisbees! But I do really think the opening was designed to provide cooling to the ICE and associated components.

Makes sense that it lets motor noise dissipate. However I've already addressed the area with sound treatment, so hopefully noise is absorbed and cancelled instead of getting amplified. I'll report back on findings once everything's fab'd and mounted. Nice thing is the triangle opening has predrilled holes for attachment points for a cover. Just need to leave a bit open for that one engine mount that hangs below the motor.


alohart said:
i3 BEV rear underside showing large triangular opening:

bmw-i3-underbody-10.jpg


This has to be intentional. Visible are openings in the panel below the bumper that would allow some air that enters the open triangle to exhaust. It would be nice to understand why BMW didn't enclose the entire underside as with many very aerodynamic vehicles. I've wondered about this since purchasing our new BEV in 2014 but decided against closing it because I don't understand why BMW left it open. Not sure that we will ever know…


Awesome picture, thanks Art!
Intentional yes, for ICE cooling. I brought a few friends to get under and check things out with me. We couldn't figure out a valid reason for having the opening. If anything I've found the opposite of beneficial when the opening is left open. Obviously water and debris gets into the 'motor bay' and I've seen countless posts of people finding corrosion in the connectors that go in and out of all the components on the motor.

But when all that air rushes into that opening, and only has a few little slits in the rear bumper to escape from, you create a very beautiful parachute effect. by definition :idea:
 
electronchaser said:
But I do really think the opening was designed to provide cooling to the ICE and associated components.
That would be a reasonable explanation if all i3's had ICE's. It would have been easy for BMW to have blocked off this opening on the many BEV's being sold, but it didn't happen. So there must be another explanation for this opening.
 
alohart said:
electronchaser said:
But I do really think the opening was designed to provide cooling to the ICE and associated components.
That would be a reasonable explanation if all i3's had ICE's. It would have been easy for BMW to have blocked off this opening on the many BEV's being sold, but it didn't happen. So there must be another explanation for this opening.

All i3's did have ICE. at least per the original design plans. The powertrain and gen unit were designed to be shoehorned into that spot. Cooling was the biggest challenge. better said porting the air to cool down the ICE power plant. BMW just skimped on adding a cover for the BEV models. There's nothing in there in a BEV model which would require that amount of opening/airflow. If we say to keep the cables/electronics/etc cool, you can accomplish the same thing with an opening 1/10 the size. Besides the motor also 'breathes' from the rear fender areas as well. PLenty of openings and airflow. And isn't the motor also cooled by the a/c loop as well?

More cost cutting. Cheaper to leave it as is.....Same reason they left the speaker connections in the back for the non HK systems. ;)
 
vreihen said:
One person's parachute effect is another person's low pressure cavity for developing under-tray downforce.....

wouldn't the faster air, with less pressure creating an area of low pressure between the vehicle and road surface, sucking the vehicle to the road.

Instead, in its current form it creates turbulence and vortices. What a drag...
 
The i3 spent a large amount of time optimizing it for efficiency. I'd be really surprised if there's not a good reason for that opening to be shaped as it is. At least according to information I've read, they sweated grams and minimized drag. If there were a significant benefit to closing that off, I think they'd have done it. FWIW, I don't think it was for cost savings, but then, that's me. Other things in the car just don't smack of cost savings without benefits in optimizing the thing for range. Every gram extra you're carrying around requires power to accelerate the car and decreases range. If they determined the drag cost was less than the extra weight penalty, there'd be no reason to close that off. Some complain of lack of power seats. I'm the primary driver, so in nearly six years now, it's only been moved a few times...IOW, no big benefit, and when it did need to be moved, no big deal.

BMW has the advantage of lots of test equipment and a wind tunnel along with CAD and system modeling. Second guessing them may create an unforeseen problem if you close that off. May not notice it in the winter, might in the summer. Closing it off could also trap moisture that could create problems.
 
I suspect the opening does not create a lot of drag, but I hope one of us will get around to testing a block off panel. BEV owners can appreciate a bit of extra highway range!
 
I've similarly read that the engineers were trying to shed weight wherever they could with efficiency as the target.

For example, they added the weight and complexity of a heat pump to the BEV because they realized that those pounds were a worthwhile sacrifice to achieve a range increase.

While I agree that the opening in that panel looks terribly inefficient, it doesn't make much sense they'd leave a big, drag inducing hole that could have been plugged by a BEV-specific $1 vacuum-formed underbody tray.


From Technical training, Product information, I01 Heating and A/C Systems:
The heat pump heat exchanger is installed in series between the coolant pump and electric heater. The energy consumption of the electric heating is reduced effectively by using the heat pump and can be saved under certain circumstances.

In the following efficiency comparison it is clear how high the energy saving is with the heat pump. In order to obtain about 5 kW of heater output, about 5.5 kW of electrical power must be used for the electric heating due to resistor losses.

A system with a heat pump only requires about 2.5 kW of electrical energy in order to provide the same heater output of about 5 kW.

The EKK consumes this electrical energy in order to compress the refrigerant and generate the heater output at the heat pump heat exchanger.

By using the heat pump electrical energy of up to 3 kW can be saved in favorable conditions. This energy can then be used for the electric motor of the vehicle, thereby increasing the range.

The heat pump is not an individual component, but a complex adaptation of a refrigerant circuit with an equally complex control structure.

The highly complex system of the heat pump, adapted to the heating and air-conditioning system, places high demands on the workshop personnel during diagnosis, but when it comes to its application the customer is unable to distinguish it from the conventional heating and air-conditioning system in I01 with the range extender.
 
Is there any proof there's an actual parachute effect? I was more concerned about the crud getting into the engine compartment. I'm thinking it's necessary for some cooling, ability to allow heat to escape.

frictioncircle said:
I've similarly read that the engineers were trying to shed weight wherever they could with efficiency as the target.

For example, they added the weight and complexity of a heat pump to the BEV because they realized that those pounds were a worthwhile sacrifice to achieve a range increase.

While I agree that the opening in that panel looks terribly inefficient, it doesn't make much sense they'd leave a big, drag inducing hole that could have been plugged by a BEV-specific $1 vacuum-formed underbody tray.


From Technical training, Product information, I01 Heating and A/C Systems:
The heat pump heat exchanger is installed in series between the coolant pump and electric heater. The energy consumption of the electric heating is reduced effectively by using the heat pump and can be saved under certain circumstances.

In the following efficiency comparison it is clear how high the energy saving is with the heat pump. In order to obtain about 5 kW of heater output, about 5.5 kW of electrical power must be used for the electric heating due to resistor losses.

A system with a heat pump only requires about 2.5 kW of electrical energy in order to provide the same heater output of about 5 kW.

The EKK consumes this electrical energy in order to compress the refrigerant and generate the heater output at the heat pump heat exchanger.

By using the heat pump electrical energy of up to 3 kW can be saved in favorable conditions. This energy can then be used for the electric motor of the vehicle, thereby increasing the range.

The heat pump is not an individual component, but a complex adaptation of a refrigerant circuit with an equally complex control structure.

The highly complex system of the heat pump, adapted to the heating and air-conditioning system, places high demands on the workshop personnel during diagnosis, but when it comes to its application the customer is unable to distinguish it from the conventional heating and air-conditioning system in I01 with the range extender.
 
panamamike said:
I was more concerned about the crud getting into the engine compartment. I'm thinking it's necessary for some cooling, ability to allow heat to escape.

Definitely some "crud" getting in there. All of the orange bits on and around my motor are surprisingly dirty, probably a combination of brake and tire dust mixed with assorted road grime. I wonder about the survivability of my fiberglass trunk extension hanging down there near the axle shaft vs. a random rock strike, and whether I should put some sort of rubber coating on the face of it.

As for cooling, I can't really imagine that was much of a design thought, given that there's no outflow. Would a cooling engineer leave a big gaping hole that leads to nowhere, or design a small scoop that direct just the necessary amount of air across whatever requires the airflow, then out?
 
Without a wind tunnel, it's really hard to tell whether that gap is producing drag under normal operating conditions. Very small shaping could make the air just wisk by without affecting things. Cars these days spend lots of time in both virtual and real wind tunnels. IF covering it produced a measurable benefit in drag reduction, the cost to cover it would have been done already. IT wouldn't have cost much.

As to potential cooling...keep in mind, you have convection and radiation. Closing it off would diminish both types. IOW, you don't need a lot of wind airflow to affect heat flow, depending on the materials.

One reason they used narrow, tall tires can be seen between the i3 and the i3s. MPGE is 118:112 on the i3 versus the i3s which has wider tires.
 
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