Tuesday 4-30-09: Battery Testing:
The long tedious testing of the individual batteries has begun. After talking to our contacts at A123 and other EV teams, we decided that the two most important indicators of a battery's health are voltage and impedance. We plan to first measure the voltage of all the cells that A123 donated to the team and throw out any cell that are not between 3.296 and 3.340 volts. The cells have been sitting for 3 years so if the voltage is within this range, it is a good indicator that the cell is sealed well and that there is very little current leakage through the cell. This range was given to us in the A123 battery pack design manual that is available to the public. This design manual has a lot of very useful information about building a pack and particularly about building a pack with lithium iron phosphate batteries. Today I checked over a 1000 cells. My goal is to get all the cells checked before the car arrives next week. Today, Mike and I also went to the electrical engineering lab and used their impedance tester to check a box full of good cells (cells that passed the voltage test). The average impedance of the cells was 7 milliohms with a very small standard deviation. It seems to me, and the other team members agree, that we only need to measure the voltage of the cells in order to tell if they are good or not. In other words, if a cell passes the voltage test, it should pass the impedance test as well. If we decide that this is a reasonable assumption to make, we will be able to save a lot of time by just testing the voltages of all the cells.

Cell Level Battery Pack Fuses:
Today I ran a couple of tests to determine if the individual cell fuses are sized correctly. Our nickel sheet is .010" thick and the tab fuses are 5.5mm wide. A123 sizes their fuses to be 3.6 mm wide and .007" thick nickel sheet. They rate there fuses to blow at 2100amps in .1 seconds. Our fuses are currently about twice the cross sectional area of A123's. We wanted to investigate this further so we ran a few tests using an A123 26650 cell. We estimated that an A123 cell can output 300 amps in a short. We cut two test strips of nickel. One that was the size of the fuses that we are currently using on the motorcycle packs and one that was the size of the A123 fuses. Both blew when we shorted the battery across them. This was a good sign because the purpose of the fuses is to protect against a short. After talking it over, we decided to stick with the 5.5mm width rather than reducing it to 3.6mm because we do not want to risk the nickel heating up when we rapid recharge these packs. I went ahead and ordered enough nickel to finish the motorcycle packs and also build one of the car modules. We bought our nickel from National Electronic Alloys Inc. They have a very good selection and they also have good prices.

BMS Updates

After several meetings over the past week, we have decided to use a LTC6802 BMS IC as the main BMS circuitry for the slave boards. We will interface them to a main controller via an optically coupled interface. The main controller will be designed around an MSP430. The data sheet for the LTC6802 can be found here:

http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1037,C1134,P86662

More info to come soon...

-Eric

The Battery Pack

The past week or so has been spent designing a prototype battery pack for Lennon's electric motorcycle, eMoto (http://www.electricmotion.org). We'll be replacing his 240 pounds of old-school lead acid batteries with a 30 pound pack of A123 LiFe cells. Of course, this pack will only have about 75% the capacity of the lead acid stack, but the power and weight improvements will be significant.

This pack isn't a favor for Lennon though. It'll serve as a test platform for future pack design for the elEVen, as well as demonstrate rapid-recharge capabilities on a smaller scale. Consequently, the pack has a very similar architecture to that of the proposed elEVen pack. We've run into a lot of problems during the design process which we never anticipated. It's very likely we'll run into the same problems on the full sized elEVen pack, so hopefully we'll be able to get a lot of bugs out with this small scale version.

Here's an overview of the basic design, both for the elEVen and the eMoto:

Modularity: It is rather difficult to make an electric vehicle without using high voltage, which can be very dangerous to work with if proper precautions are not taken (just like gasoline in ICE cars. It's just a property of high energy systems). The obvious solution for dealing with the dangers of high voltage is to eliminate the high voltage itself. We are accomplishing this by building several independent low voltage modules, which will be wired in series to create the high voltage required only at the very end.
The eMoto pack will consist of two 19.8 volt (6 cells in series) modules and two 16.5 volt (5 cells in series) modules wired in series to reach the nominal voltage of roughly 72 volts (required for the motor controller). Each module will have 10 cells in parallel to achieve a capacity of 1.6 kWh for the entire pack. We originally intended to use three 24 volt modules, since the elEVen will be using 24 volt modules, however, we decided to use the existing battery mounting on the eMoto. This made arranging seven of the ten cell batteries in series very difficult. Thus, we arrived at the much simpler pack design we currently have.
We anticipate that the elEVen pack will consist of 24 volt modules with 72 cells in series. However, the issues we ran into with the geometric constraints of the motorcycle could arise when designing around the existing chassis of the elEVen. Hopefully the experience with the eMoto will help us keep our future designs a little more flexible. Additionally, the eMoto will have the four modules wired directly to each other. We plan to connect the elEVen modules with contactors, allowing the packs to disconnect and drop the maximum voltage to 24 volts. This will provide a safer working environment as well as making the vehicle safer if it is ever involved in an accident. Finally, each module will have a built in Battery Management System (BMS), which will then communicate to a master module to balance all of the modules together.

Inter-cell Connections:The individual cells of the modules have to be connected some how. We took a lot of advise from the likes of Bob Simpson (http://www.evdrive.com) and Bill Dube (http://www.killacycle.com/) and have decided to use nickel plates spot welded to all of the cell terminals as the interconnects. These nickel plates will be bent to help arrange the cells in each module. They also have tabs for individual cells with integrated fuses to protect the pack should a cell internally short.

Cooling:As we rapidly recharge the cells and continuously discharge them, they will heat up. Though warm cells have a lower internal impedance and are therefore more efficient, running the cells too hot will damage the internal structure and reduce the life of the pack. We'll be air-cooling the cells across the electrical contacts, since they are both electrically and thermally coupled to the inside of the cell. We don't anticipate that a lot of cooling will be required (see the section on battery data which will be posted soon), but we want our design to be flexible in case more cooling is required. We accomplished this by spacing the cells vertically to allow airflow. However, we have not finalized the method of spacing (the leading idea can be seen below). We're also looking into various methods of cooling, including passive cooling and using various arrangements of computer cooling fans for active cooling. We'll be testing the effects of various cooling systems once we finish the prototype eMoto pack.

Cell enclosure: This has surprisingly been one of the more difficult aspects of the pack design. The cells need to be restrained somehow, and in the case of the eMoto, the enclosure also needs to be able to take a compressive load. We were originally looking at using FR4 as the primary cell enclosure for its insulating and fire retardant properties, and an aluminum secondary enclosure to provide rigidity. Since then, we've looked at a single FR4 enclosure design, an FR4 primary and polycarbonate secondary, polycarbonate primary and secondary, and a single polycarbonate enclosure. Currently, we've decided on a 1/8” polycarbonate primary which will likely be encased in a polycarbonate secondary with possible rubber shock-mounts in between the two layers. The shock-mounting will reduce the effects of vibration on the cells, hopefully preventing any of the connecting welds from breaking.
After figuring out the enclosure materials, we had to design a good way of creating a box. This sounds trivial, but there is a lot more that goes into it than meets the eye. In our case, we were limited to making the box out of sheets of polycarbonate. This was the only cost effective way of obtaining all of the necessary features, such as cooling slots and cell dividers. Machining a solid piece of polycarbonate into the shape we wanted would take too much time, cost too much money, and waste too much material. Therefore, we had to come up with a good way of joining the polycarbonate sheets together into a box. We looked into drilling and tapping screw holes into the thin face of the polycarbonate, but in order to do so, we would need to use #0-80 machine screws. That method of fastening seemed impractical and labor intensive while possibly being insufficiently strong. We also looked into chemically bonding the polycarbonate together. We've decided to do so on the prototype eMoto pack, with the addition of interlocking tabs for added mechanical reinforcement. We are considering using polycarbonate angle stock to reinforce the corners. As of now, the box seems pretty rigid.



As of now, we've assembled the primary enclosure with dead cells, and we are waiting on the battery welder. We've received the weld samples, so the welder should come in sometime this week. Once we have that, we will begin matching cells and electrically assembling the prototype pack. To prepare for this, Paul and I have been testing boxes of cells to weed out the obvious bad ones. We're looking to manufacture the secondary enclosure this week, as well as experimenting with various cooling designs. Additionally, Shane and Lennon have been working on our first rapid-recharge charger for the motorcycle. If we're lucky, we'll be rapid-recharging our first pack by the end of the week.



Look for test data and more images soon.

-Mike

Saturday 6-29-09:
Mike and I started mass testing the A123 batteries. We tested the voltage of about 500 batteries. This is enough to do all of the packs for the motorcycle. We are planning to test the impedance of the batteries after testing the voltage because voltage is easier to measure. The good batteries are around 3.3030 +/- .0050 volts.

Tuesday 6-23-09:
I finished up the CAD drawing for the motorcycle battery pack in the morning and we had a battery review meeting in the afternoon to critique and improve the design. Many great suggestions came out of the meeting such as ways to improve the battery cooling, ways to simplify the design, and a way to reduce vibration when the battery is on the bike. We decided that we are not going to use FR4 initially and instead are just going to make the battery enclosures out of only Polycarb. We also decided that we are going to have a double enclosure design so that we can put foam in between the boxes to dampen vibration and so that we can use the outer box as a manifold to pull air through the pack. I will integrate these suggestions back into the design later this week. Mike and I are planning to water jet the casing tomorrow or Thursday. We are also going to water jet the tabs for one of the modules this week.

Wednesday 6-24-09:
Mike has a friend who offered to help us water jet so we cut the battery tabs today. They look very good. In order to cut the nickel, we sandwiched it between two sheets of Aluminum. We test fit the battery tabs onto the batteries and they fit well. We did not have enough time to water jet the polycarb battery enclosure today.

Thursday 6-25-09:
Today, I helped Matt look for a spline shaft for the electric motor. We have an engineering drawing of the shaft so I thought it would be relatively easy to find. I spent several hours calling up junkyards and transmission rebuild shops and did not have any luck. We do not know what transmission has the spline we are looking for so I tried to look up the spline by the number of teeth and the outer diameter. In the afternoon, I drove out to the junkyard that Matt went to last week and talked to them about the shaft we needed. The junkyard was called Nissenbaum's Auto Parts. They were very helpful and interested in our project. I first looked for the spline shaft in a dumpster they had that was full of busted transmissions. Then I looked for the shaft in their stock of good transmissions. I thought I found the shaft we were looking for so I took down the information for the transmission. The guy in charge at Nissenbaum's had a good suggestion that we look for the shaft at a transmission rebuild shop. He recommended New England Transmissions in Alewife. I called them up and made an appointment to come in around 9am on Friday morning.

Fridays 6-25-09:

I took the T early in the morning to New England Transmissions. The guys were very helpful and told me I was welcome to look through their shelves of used transmission parts and see if I can find the shaft I'm looking for. I looked for about 3 hours and finally found the shaft that fit the engineering drawing. I took it back to the shop and it did not fit. It turns out, I had been looking for the wrong size shaft. The CAD drawing showed a shaft with two ends and we had just assumed that the one on the left side of the shaft was the one we were looking for but it turns out the right end was the correct spline. So I drove back out to the shop and looked for another hour and was unable to find the correct spline. When I got back to the shop, I walked over to the central machine shop and they said they do not have the right tooling to make a spline shaft. While I was out, Mike finished water cutting the battery enclosure and he partially assembled it. The clear polycarb casing looks very nice. It makes it very easy to see the tabs so in case a battery internally shorts, we should be able to see if the cell fuse has blown. I spent a couple more hours in the afternoon trying to find a machine shop to cut the spline for us. Over the weekends, I will continue to look for the spline and also work on getting the battery pack ready to assemble when the battery welder comes.

Sizing up the Ford Fusion Hybrid

Dropping an electric bus motor and a Ford 9" differential into a modern mid-sized sedan is no easy task. While we're waiting on delivery of our conversion vehicle (locked in, will finally get here in two weeks), we're doing everything possible to prepare the new drivetrain outside of the vehicle.

So while Kevin is bench-testing the motor and controller system, Matt has been gettting the drivetrain components together that will connect from the motor to the wheels, and he's had an amazing find in picking out a differential that has exactly the gearing we need.

Electric motors can spin at high RPM (ours goes to 12,000) and have a very broad torque range; our vehicle will only need one gear ratio to reach 100mph. To match this speed with the top speed of our motor, we need a gear ratio of 7.3:1. The Ford 9" differential has a huge aftermarket following among the rock-crawler crowd and there are a few companies that make very low gearing for it. Luckily, the lowest one we've been able to find is 7.3:1, exactly what we need. This means we can drive the motor directly to the differential, eliminating the need for a high-speed gear reducer (an expensive piece of equipment that would have otherwise needed to have custom made).

To double check that all of the components will fit before we order them, we took a couple of measurements off of a 2010 Ford Fusion Hybrid (Ford is donating us a Mercury Milan Hybrid, but they're the same vehicle mechanically). We have a few options for fitting the motor and differential into the car; they will be in a T-shape, so the motor will be sitting in the car longitudinally. The motor with the differential coupled to it will have a length of 90cm. The distance from the center of the front axle to the radiator is 65cm, too short for the system. We'll need to connect the differential to the front wheels, with the motor sitting behind it.

However, this mounting also leads to a few other problems: we need to get by the steering rack, a few subframe mounts, and make room for the motor in the exhaust tunnel. Fortunately, the tunnel is already relatively large. We also have plenty of interior room in the car's center console location to enlarge the tunnel with no visual modifications to the vehicle interior. This may also mean connecting the motor to the differential with a CV joint at an angle to not disturb the steering rack; we will find out for sure once we remove the drivetrain. Until then, we're looking to order the differential, gearing and an IRS center housing with CV style ends.

Initial schematics for the BMS circuitry

Over the past few days, initial schematics for the BMS circuitry have been designed and redesigned.  To monitor each battery voltage, we have decided to settle on a difference amplifier topology which will then feed into an Analog to digital converter on a TI MSP430 chip (Fig. 1).  The MSP430 will then run an algorithm to sort out the minimum battery voltage within the module and start bleeding the other batteries until they are balanced.  The bleeding circuit may be created using an optically coupled p-channel device which turns on an n-channel transistor (Fig. 2).  Eventually, individual temperature monitoring for the slave boards will be added and a master module will be designed to include inter-module balancing as well.  Please keep in mind that this circuitry is still preliminary and not everything has been simulated to ensure that everything works as expected. 

- This circuitry was designed by Bin Lu and Eric Winokur

(figure 1)

(figure 2)

Updates: Wednesday June 17th and Thursday June18th

After Arya and Matt picked up the lift on tuesday, the team has been working hard to set it up. Most of Wednesday was spent getting all the necessary parts for the lift together and assembling it. Though the lift has a few bugs to be worked out, it is operational and  can lift the Porsche.

Arya:

I spent today mostly focused on finishing the installation of the lift. In order to ensure that the lift does not buckle under load, Radu and I braced the overhead crossbar with two pieces of U-shaped steel. After the overhead beam was reinstalled, we ran the cables and hydraulic line across it and attached them to their respective insertion points. Then, the two of us installed the safety cable (which stops the lift if the car gets too high), and we tested the lift. In testing, we noticed that the latches on the lift arms were not engaging the safety catches at the same time, so I tightened one of the overhead cables in order to make the latches engage simultaneously. The installation was complete by the day's end, we even managed to raise the Porsche 914 BEV. Even so, we encountered two problems. First, the hydraulic fluid tends to bubble and foam, which is troublesome because it indicates the presence of air in the system. Second, the fluid tank is precariously attached to the pump by nothing more than a horizontal hose clamp, which is troublesome because the tank could fall and spill if knocked a little too hard. We are looking for an additive to solve the first problem, and I fixed the second by putting a support bracket underneath the fluid tank.

Kevin:

Today I worked on the wiring between the motor controller and the vehicle.  The donated system came with a large wiring harness that integrates all of the wires between the motor controller, the 12 VDC relay box, and the vehicle interface.  Unfortunately, the documentation only has the pin descriptions on one end of the harness, so it was necessary to map out where all of the wires from the controller exit the harness.  After labeling all of the wires, I worked on planning out a switch control box which will be used as a testing tool to run the motor before the system is completely integrated into the car. 

Paul: I worked primarily on the battery pack design for the motorcycle. Space on the motorcycle is limited so designing an efficient battery layout is crucial and tricky. I also looked into what materials we can use to make the enclosures for our battery packs and how to manufacture the nickel connector plates that will connect a group of batteries in parallel.

 

Matt:

Today I spent the majority of my time contacting machine shops that will make custom spline shafts.  The SatCon motor has a 1" 23 spline output that has been difficult to find.  I was able to find several shops/companies that make custom spline shafts.  I am waiting to hear back about pricing from most of them.  Another option is to take a shaft out of late 90's model Plymouth/Chrysler van.  SatCon originally outfitted these motors to fit the drive train of these vans.  I began looking for local junk yards that have these vehicles.  I also began writing an outline of the drive train that includes descriptions of the components and why we are choosing each one.  

Mike has been working on getting sponsor decals and trying to find a place for the Porsche. He is also working with Paul on a battery pack design for a motorcycle. On Wednesday he continued cell testing and helped out with the lift. 

On Wednesday, Radu had a meeting with MIT lawyers. He was also part of a meeting during lunch about battery management systems and setting up for a motorcycle pack that will be configured to be similar to the automotive pack

The Porsche on the lift

Motor Vibration Evaluation part III - a sour note

After some further research on other users' experiences with vibration problems with the VoltsPorsche kit, we've discovered a few more contributing factors. Many thanks to TimK and his excellent blog at http://914ev.blogspot.com/.

Tim did very thorough measurements with an accelerometer and notes that a major conspiring factor in his vibration issues was the resonant frequency of the entire motor/transmission assembly. The entire assembly weighs close to 200 lbs. and is only supported at either end, so the entire thing is able to flex pretty freely. With the coupling between the motor and transmission made by a 1/2" aluminum plate, there's a pretty flexible member smack dab in the middle of everything. Also, the fact the bolts into the motor were only engaged by 1.5 threads can't help either. This explains why the vibrations are so significantly reduced when the motor & flywheel are spun up without the transmission - no resonance.

In layman's terms, the transmission is able to vibrate just like a guitar string. If you hum the note 'G' next to a guitar, you'll notice the G string starts to vibrate on it's own without even plucking it while the others really don't do much. So, if the motor/flywheel vibration is the humming, then the motor and transmission are the guitar string... when you hit the right frequency, the string starts vibrating with a much greater magnitude than what's exciting it.

So, the vibration is really caused by a wide array of small factors, all conspiring to cause a large problem. We're now pursuing multiple tracks to fix each of these problems:

Sing softer: reduce the vibrations coming from the motor/flywheel assembly by shortening the distance from the motor bearing to the flywheel, eliminating the flywheel and using a small diameter lightweight racing clutch to reduce the exciting mass, and by chasing out the dozen or so tiny misalignments that could be adding up and putting things off balance and out of alignment

'Tune' that guitar string to a note higher than we can sing: stiffen the coupling between the motor and transmission to increase its resonant frequency, which may involve machining a thicker adapter plate and cutting that massive 'doughnut' down, in addition to making sure that all parts are mating flat and flush, and the bolts are fully engaged in the motor housing.

Mute: like pressing your finger gently to the guitar string, add rubber bushings to the motor mount to absorb or 'dampen' the vibrations so they want to die out rather than build up. We'll do this last since it really just masks the problem rather than solve it.

On Sunday after a team meeting to discuss all these findings, Dave, Tyler, and I measured the mating surface of the transmission. It wasn't round, flat, perpendicular nor concentric (oh boy!). We put sandpaper on the adapter plate and used it to knock down the front mating face of the transmission to get rid of that lip and make it a flat surface to mount to. In the end, it's very flat and perpendicular except around the top right bolt which is still 10 thou low.. we shimmed it with razor blades and the adapter plate sat eerily flat and perpendicular.

The adapter plate itself is not perfectly flat anymore, perhaps due to the unevenness of the mating face on the transmission, and the alignment ring is slightly undersized, so there's a good 10 thou of play in that plate. With some experimentation, the closest to concentricity we could get was 5 thou too high by pushing the plate towards the bottom of the transmission as we tightened it up. Perhaps we could knock that much off the plate's alignment ring and get it perfectly aligned. We also took precision measurements of the concentric surface on the transmission, which is slightly oval shaped (narrower horizontally than vertically, leading to greater vertical than horizontal play in the mounting plate). If we end up making a new, thicker plate, we could use these measurements to make it fit the tranny perfectly (we could also build in the 10 thou shim).

Tyler is looking for that racing clutch, which has to fit the transmission's spline. We need to consider the torque rating of the clutch... we're only dealing with 90 ft/lbs from the motor, while racing clutches are often rated very high because of the high power engines, so we may need to reduce the clamping force so we don't have too much impulse torque transferred back to the motor (then again, the rotor has so little mass, it'd be hard to get much back torque). Racing clutches also have the a shorter pedal throw, and no springs, so we need to moderate the clamping force to smooth out the clutch engagement.

A glimpse of Tuesday June 16th, 2009

Highlight of the day: The lift was picked up and is in the process of being assembled. 

• Kevin has finished  securing the motor for testing. He is now focusing on ordering parts necessary for wiring the motor and the motor controller.
  

• Paul is working on a solidworks model for the battery pack geometric design and spatial arrangement.
  

• Mike is doing more cell testing at 15 amps. The discharge at pass was rated 4. He will be doing more testing on the bad cells to see what they do. He will be working on a set up to test all the cells and help Paul on the battery pack design
  

• Matt went with Arya to pick up the lift. He has also been emailing battery manufacturers in order to get lead acid batteries donated for the charging array.
  

• Arya met Gary Bloon and picked up lift from him. He went to hardware stores to get items necessary to assemble the lift.
  

• Radu  picked up the components to begin assembling the lift, and began putting it together. 
  

The team arranged the space in the shop for the lift and began drilling the anchors into the concrete (4” deep, ¾” holes). 

 

•  The team had a big meeting after hours for BMS system design, and will be building a smaller battery pack – very similar in configuration for the battery management electronics with what they are doing with the elEVen project – for Lennon’s lead-acid electric bike. This will be a smaller-scale version, and will be a large step in the development for the full-size battery pack.
  

A glimpse of Monday June 15th, 2009

• Mike is testing cells for temperature rise.
  

• Arya is working on updating the team proposal and sponsor package.

• Matt is contacting battery manufacturers for information about making a lead acid battery pack for charging. He is also working on couplings.
  

• Kevin is still working on securing the motor to a table for testing.

• Paul is reading up on articles and looking at battery pack space management and design.
  

Motor Vibration Evaluation part II

It was a big day for troubleshooting the motor vibrations, though unfortunately it's mostly bad news. I showed the clutch/flywheel to Dave Breslau (a machinist by day, helps the formula SAE team on weekends) and explained our issues.

He said the adapter ring bolts into the motor should go deeper still into the motor and should have washers under them (otherwise the aluminum will break up under the bolt head when torqued down). We should also set the max RPM limit to the same as the original engine's redline, since over-revving a flywheel (especially with our balance problems) can cause it to explode with deadly results. There's an odd lip on the inner mating surface of the transmission, about 5 mil thick by 50 across, we couldn't figure out why it's there. Dave wasn't concerned about the mating surface issues between motor/flywheel and motor/ring pointed out by the F-SAE guys.

We mounted the flywheel to the motor mount and used a dial indicator to measure the trueness of the flywheel.

The mating surface was very square and flat (+/- 1.5 mil), except for a spot eaten away by corrosion. The friction surface was out of perpendicular with the shaft by +/-4 mil near the outer edge. This was roughly the same as the rear face of the flywheel (indicating the friction surface and rear faces are paralell with each other, but slightly not perpendicular with the shaft, while the pressure plate mating surface is on a different plane, perpendicular with the shaft. The outer diameter was +/- 2.5 mil, though the distribution was irregular, with the min near 12 o'clock (with the top-dead-center notch on top), and the max at 10 o'clock.

We then removed the flywheel to measure the flywheel mount on the motor shaft, and here's where it gets interesting. The mounting face is only 0.5 mil from perpendicular at the outer edge. Then we moved to measuring the outer diameter of the mating surface (which is not a press-fit like it was a few days ago, don't know if that's from the new chamfer or because we cleaned it off with a paper towel). The O.D. there was virtually dead-on, but we came upon a very significant finding. Applying some pressure to the shaft results in significant deflection, without a scale handy, I'd say about 20 lb = 1 mil (and still half that right next to the bearing). We both pretty much ghasped at this point. The motor shaft was never designed to take a load of any significant mass, it was meant to be coupled to the transmission by a small diameter gear.

The clutch assembly weighs a good 30 lbs, so deflecting the tip of the shaft 1.5 mil (and the shaft inside the motor deflecting up probably twice that) could cause quite a lot of problems at low revs, all that flexing could explain some of the heat. At high revs it's a self-reinforcing error, the heavier side of the flywheel will pull on the shaft, which then pulls more mass to its side of the center of rotation, increasing the imbalance further. This explains why the vibration doesn't appear at low revs, and gets exponentially worse at higher revs. This also explains why the wear in the bell housing goes all the way around... with one side of the flywheel flexing out, it basically increases the effective diameter of the flywheel. The gouges in the bell housing are about 20 mil at their deepest.

Now that we understand the phenomenon, we can quantize it: the sides of the housing where the gouges on both sides are 5 mil deep, the housing is 11" wide, giving an effective flywheel diameter of 11.01", over the nominal flywheel diameter of 10.88. This is a difference of 130 mil!! This means the flywheel is roughly 1/16" off-center at speed.

We are talking motor shaft displacements great enough to grind the rotor against the stator inside the motor. We're also talking enough flexing and side loads to burn out the motor bearings (also possibly explaining the thermal issues). You can also see some discoloration of the bearing, also indicative of the bearings getting overheated.

Compare that dangling motor shaft to the flywheel mount on the stock engine.

Solutions we discussed:

Rebuild the flywheel mount to shorten its cantilever from the motor bearing. This is probably the easiest, but should be combined with one of the flywheel solutions below.

Removing the clutch altogether and using encoders and motor feedback loops to force rev-matching on shifting (basically my auto-synchro shifting idea, which does make the clutch unnecessary) the (big) downside to this is there is nothing to soak up impulse torques from bumps in the road (which have been known to snap motor shafts).

Getting a racing "button" clutch. These are ultra-low mass, small diameter clutches that use multiple friction surfaces to get the same grip force as larger diameter clutches. An example are those at JB Racing. They run in the double kilo-buck range, but perhaps there are some available used. This would be the best solution, and the only one that could potentially allow us to exceed 6,000 rpm.

Cut down and swiss-cheese the existing flywheel. This will absolutely require the re-balancing of the flywheel by pros (which could take a week), but could considerably reduce the mass and rotational inertia. Dave suggested Lindskog Balancing.

Get a lightweight replacement flywheel. These use the same clutch/pressure plate, but are made of aluminum with steel friction surfaces. Probably the least desirable solution, since it won't reduce the weight or diameter all that significantly, but still cost a lot.

Friday June 12th, 2009

The elEVen team had a meeting with Bill Dube from Killa cycle. Bill Dube builds a123 drag bikes and has extensive EV experience. 

Radu Talked to Bill Dube from killa cycle and gave advice on general EV design. It was an informative meeting, and they received two weeks worth of worth of research in one hr from Bill Dube.

Kevin is currently working on mounting the motor to a sturdy table in order to test it.

Matt discussed plans with Bill Dube and the rest of the team. He began solid modeling a coupler for drive train and finished specing out drive train components. The team will be using a ford 9 differential for the drive train. 

Mike was also present for the talk with Bill Dube. Mike received a lot of information about battery packs, charging, transmission, and the motor. He has begun working on testing voltage on the cells. He has tested about 30 cells out of a total 10,400 that we received.

Arya focused on getting information about insuring the Fusion and contacting people at insurance offices. He is also working on setting down a timeline for the project.

The Porsche team is still working on the transmission.