Purplemeanie

1 minute ago, aerobod - near CYYC said:

I'm assuming if you take 20% linear driveline and tyre rolling resistance losses (i.e. add 25% to the power required at the wheels at a given speed to determine motor power), then as drag is 0.5 x ρ x CdA v^2 in Newtons, then work done (in Joules) per second is power in Watts, i.e. P = 0.5 x ρ x CdA x v^3, with torque (in Nm) at the wheels being P / (2π x RPS). RPS is v / (π x d).

So wheel torque required to overcome drag is 0.25 x d x ρ x CdA x v^2, (or torque at the motor is about 0.0267 x d x ρ x CdA x v^2 with 20% loss). This seems to correlate with a top speed of 128mph (57.2m/s) at a wheel torque of 606Nm (ρ=1.225kg/m^3, d=0.576m, Cd=0.7, A=1.5m^2).

Hmm. I had some of those assumptions in my spreadsheet but not all. For me, being within two significant figures was all I was aiming at. 🤣
 

When I get some time I’ll update my spreadsheet with your factors included.

This is why it’s so good to share thoughts, somebody (often you James! 😉) has something to teach me! 🙏
 

Thanks!

Here's a quick screen grab of my spreadsheet...

While I’m on a role here. I thought it would be worth showing some images I’ve shared on socials recently. I’m looking at a motor option now that would have to sit in the transmission tunnel of a Seven… where the gearbox normally sits.

The prospective motor would be an extremely tight fit… with just a mm or two on each side as clearance.

[ Has anyone got experience fitting anything into a car with that tight a tolerance? And I appreciate a regular 5 or 6 speed box probably only has that sort of clearance… at least the 5 speed in my car is only a mm away from the chassis in places! ]

To this end, I spent a lot of time in CAD (first image) trying to determine if the motor will fit in my scanned digital twin. But it’s so close that I wanted to get a better physical confirmation… so I 3D printed the motor.

I’d like the motor to sit further back in the chassis than shown in that picture, but as we all know the transmission tunnel tapers and so my concern about it fitting where I want it.

This is a 320bhp motor that I’ll probably only be able to realize about 60% of that in my first iteration of this project.

The second image below is of the 3D print of this motor. I don’t create the model, that’s from the motor supplier.

If you’re wondering, then the perforations in the stator casing (pink bit) are for two reasons. Firstly so I can “see through” it when I put it into the chassis and secondly to save a bit of print time and filament (though that may have not been the case in the end - infill density vs number of wall layers).

I just need some time, space on the drive and good weather to take the cover off the project car and offer up this motor to see what works.

John

1 hour ago, aerobod - near CYYC said:

From a drag perspective John, I have previously estimated a 7 with roof off having a drag coefficient around 0.65 to 0.70. The frontal area is about 1.5m^2 for an SV or about 1.4m^2 for an S3, leading to an estimated CdA of 1.05m^2 for an SV. Power required is of course is closely related to the cube of velocity at high speed due to the predominance of aerodynamic drag.

I agree with your calculations James. I think I posted speed vs drag (and power??) in a Low Flying article last year. It’s surprising how closely my graphs correlated with max speed at max power. with my drag calculations showing drag power equal engine max power. 🤔😉

17 hours ago, aerobod - near CYYC said:

I think it is a good comparison, John. Just looking at your code, you have taken a 2.0m wheel diameter, I think 1.7m (85%) is much closer to the typical rolling circumference of a rear tyre on a 420R (typically 195/45-15 in the original size).

With a 0.85 multiplier on the speed shown in the graph, it would seem the 550A motor would be turning about 14,400RPM when peak power is reached at 85mph, or 150kW (201bhp) at 100Nm motor torque. The power would then drop off to 113kW (152bhp) at about 21,600RPM at the max speed of 128mph, likely also being the drag limited speed at that power level.

This behaviour seems to be as expected from a BEV, strong mid range and a drop off in power at the top end.

James, also picking up on your point about drag. It's probably also something I should try and graph... motor power output vs power required to overcome drag. As you are well aware, but I'll state for others reading, it's important get a balance of peak motor power and drag. Though TBH... in my first incarnation of this project I'm almost certainly going to be current limited (current determines torque -> T ~ BI). I'm going to struggle to get enough current to the motors AND enough voltage to also get past the knee of the torque curve (which equates to needing increased voltage due to field weakening). Though I do have a cunning plan (cue Baldrick voice) to cheat a statically set current and voltage tradeoff that you normally get with BEV battery packs.

John

Following some tinkering last night and more pondering today, I've updated the code and created a new plot. The plot in the first post was a statically pasted image to this site. The image below is a link to the file on GitHub, which should therefore change as the code and image changes on GitHub...

6 hours ago, Jonathan Kay said:

Neat.

Initial thoughts.

1 How can you fill in what happens below 10 mph for the clutched ICEV?

2 What are the useful on-the-road outcome measures? I was brought up on 0 to 60 mph figures and the repeated simultaneous insistence that they weren't very relevant (see also GDP). Motor or Autocar advocated 30 to 50 mph times and distances as being much more informative, possibly in non-overdrive top. 

3 When you've decided on those measures how much effect does the need to change gear have on them? There were always allegations of gear ratios specifically chosen to make it just possible  to reach the final speed without another shift. The Lotus Elan Sprint might have been mentioned. 

Jonathan

Hi Jonathan,

To answer in turn…

1. I had this conversation with someone on WhatsApp last night about this. I started off writing the code to work out a 0-100 (or other arbitrary similar stat) but decided it made more sense for an overall picture to start with wheel-torque vs speed. I may come back to acceleration graphs (including gear change estimates and starting clutch slip).

2. Hmm. I think all performance measures only capture some of the driving experience. I’m also a fan of Autocars more realistic in gear acc.n metrics. The graph I’ve created here first was a stab at putting a nail in the ground on what sort of performance I might expect. I think it shows an EV based on this configuration would be really punchy from 20 to 80 but might not be quite as quick up to 20 as say this 420. I should probably try and create an acceleration graph, that might give a better sense of how it would feel to drive 🤷‍♂️

3. Hmmm, yes. People often don’t appreciate the careful planning that goes into gear ratios. There’s the basic misunderstanding that with more power you can pull a bigger gear, but fail to appreciate the cube law of power needed to overcome wind resistance. Thee are many many more twists to the gearing conundrum.

EDIT: And of course when you get this "increased gearing" really wrong you find that while you might have enough max-power to achieve a higher top speed, you now find that at slightly lower speeds you now sit off the peak power and so don't have enough power to make you go faster and therefore can achieve this new theoretical top speed. Gearing needs careful planning.

4. Electric Porsches. Well it goes back to point 3. Even though EV motors can spin fast, the torque multiplication through the gear sets, power and top motor RPM didn’t give Porsche both the 0-60/100, top speed and range they wanted without needing two gears. Gearboxes in EVs are especially tricky as there’s not the same inertia as in an ICE car (which is a twin edged sword) so the gear change timing has to be made in association with probably a partial powering of the drivetrain to match revs as the gears change. Downshifts also need to be thought of in this context. 

16 hours ago, aerobod - near CYYC said:

I think it is a good comparison, John. Just looking at your code, you have taken a 2.0m wheel diameter, I think 1.7m (85%) is much closer to the typical rolling circumference of a rear tyre on a 420R (typically 195/45-15 in the original size).

With a 0.85 multiplier on the speed shown in the graph, it would seem the 550A motor would be turning about 14,400RPM when peak power is reached at 85mph, or 150kW (201bhp) at 100Nm motor torque. The power would then drop off to 113kW (152bhp) at about 21,600RPM at the max speed of 128mph, likely also being the drag limited speed at that power level.

This behaviour seems to be as expected from a BEV, strong mid range and a drop off in power at the top end.

Thanks for the comment. As it happens I knew the wheel/tyre circum. was just a guess. If you look at the code now you’ll see it calculated out correctly - and I even went to the garage and wrapped a tape measure around to verify I was making things better! 🤣

Hi All,

  As part of my EV conversion project I wanted to get a better grip on how an electric motor would perform against an ICE powered car. Lots of people have commented on how any Seven with an electric motor will probably be a crazy accelerating beast. But while that could be the case, the amount of space in a Seven chassis (even an SV) may be prohibitive.

And what matters is torque at the wheels. As we all know "power" is just torque multiplied by RPM. So its torque that determines the performance of your vehicle.

Then you have to take into account any gearing between the motor and wheels. Any gear reduction will "amplify" the torque created at the output (and gearing up will decrease output torque of course).

But in general a Seven has a gearbox going one stage of gear reduction followed by a differential doing a second stage. And then the differential also splits the torque in two to each wheel.

Therefore, the torque to each rear driven wheel at any given motor speed is: wheel_torque = motor_torque * gearbox_ratio * differential_ratio / 2.

So it should be simple to come up with a Wheel Torque vs Speed graph, showing the torque produced by the motor.

I tried to come up with a way of doing this in a spreadsheet but found it was a bit of a blunt instrument. So, I wrote a script.

I made the script so it can be reasonably easily modified to provide different "motors" and "gearboxes". The script runs through each motor configuration and plots each gear it finds in the gearbox on a chart. 

THIS CODE IS NOT PRODUCTION GRADE: I threw this together for my own benefit. If I'd have wanted to get paid for the code then it would have looked quite different. It also evolved and could probably do with a refactor and prune.

Anyway, here's a plot form the code as it stands at posting date. I created a benchmark set of in-gear plots for a Caterham Seven 420 (I found the data at the link repeated in the code). I then created two motor variants based around a motor I'm thinking of using in my conversion project. The two variants vary by the current that can be supplied to the motor and by a gear reduction I'll need to include in the design.

You can see from the graph that for the variants I've configured, the 420 beats the EV configurations in 1st gear but the EVs then take the lead from 2nd gear onwards.

If you have any comments or suggestions then please let me know below.

The link to the code is here:

https://github.com/Purplemeanie/TorqueAndPowerComparisons

John

Afternoon all,

  I've just uploaded the first progress video on this EV project of mine. The video covers the purchase and road registration as an ICE car.

Aft'noon all,

JK has twisted my arm to throw my experience of Lithium batteries into the ring here.

Firstly, after the second Banner battery died on me in the space of 3 years, I decided I'd go Lithium. I nearly did when the first Banner died but I needed something quick and so just went with what I knew - Banner.

But when the second one went, it was time for Lithium. 

My first purchase (yes there's more than one), was a Varley Li-5. This had been recommended to me so that's what I went with. 

Specification

• Model: 7162-0002
• Battery Chemistry: LiFePO4
• Nominal Voltage: 12.8V
• Capacity: 5.5Ah
• Short Circuit Peak Current: 736A
• Cranking Current*: 335A
• Dimensions (LxWxH): 99 x 82 x 95mm
• Weight: 1.1kg
• Terminals: M6 Female
• Operating Temperature: -10ºC to +50ºC
• Storage Temperature: -20ºC to +60ºC

Unfortunately after installing it and playing around with mounting options, including buying the Varley Mounting Bracket for the Li-5, it was clear after a few weeks that the Li-5 wasn't up to the job for a 420 2.0L Duratec. Turnover was very sluggish and it didn't take many cranks for the battery to then twist the engine even more slowly. It never actually let me down starting the car, but that was probably only a matter of time.

So, I went back to the Demon Tweeks website and had a look around for something bigger. I landed on the VT-900.. here's the specs from the Varley site:

Specifications:

• DMS Part No: 7162-0068
• Voltage: 12V
• Capacity: 16Ah
• CA:160A
• CCA:900A
• Terminals: M6 Female
• Weight: 2.5kg
• Dimensions (mm) LxWxH: 175 x 87 x 155

As you can see this has nearly 3 times the cranking current and lights my 420 up like it's been struck by lightning. The engine cranks really quickly and fires immediately. Much much better than the Li-5 and much better than the Banner.

The battery has been installed for about 9 months and has held its charge over the winter, even without being on a battery conditioner.

To be honest, I think this battery is overkill for a 420, but it certainly seems to do the job, for the moment. And there are as many solutions to this problem as there are cars. 🙂 

Mounting

I also recently modelled and printed a battery tray so the smaller VT-900 sits in the space that the Banner originally sat in. See pictures below. The battery needed to be located in the centre of the tray, but also needed raising. The 3D print solves that problem in all three dimensions.

STEP And STL files attached and linked here...

420 Lithium Battery VT-900 Base v2.step 

420 Lithium Battery VT-900 Base v2.stl

The mount has been installed for a few months and is working well at the moment.

For those still following (awake), I've also posted a more in-depth look at this project here:

https://purplemeanie.co.uk/index.php/2024/03/06/ptevis-suspension-logger-mini-project/

John

Hi all,

  a quick note to publish the initial results of my Suspension Logger Project. 

  Here's the image you want to see, but there's more information on the project below:

The plot above shows time on the horizontal axis and  suspension travel on the vertical axis. The suspension travel is measured from the TOF sensor, so increasing numbers on the plot mean the suspension ears are moving downwards and decreasing numbers mean it's moving up. The plot above is of a short part of a trip out in my 420R. The whole trip can be seen below... you can clearly see me getting into the car (twice) at the start, and then getting out at the end. The first plot is towards the end of the one below...

There's clearly a lot of noise in the plots, but I got what I wanted out of it, and that's the suspension compresses about 40mm and extends by about 20mm. I'm probably not a lot better off than the maximum allowable travel for the suspension which is +/-50mm, but it was worth this mini-project just to tell me that.

Background

The idea behind the project was to try and get a ball-park limit on how far the suspension travels on a Seven. I need to know this so I can reliably set limits on the rise/drop angle of two new drive shafts I think I'll be needing in my project. If the real-world rise/drop is too much then the drive-shafts will pop out of the tripod end assemblies.. and that wouldn't be a pretty result!

  The project electronics consists of a few Adafruit Feather prototype boards and about 400 lines of Arduino code: 

• RP2040 CanBus CPU board (didn't need to be the CanBus version, but that's what I had to hand)
• VL53L0X Time-of-flight sensor board
• 128x64 OLED Feather Wing (Just so I could see what the boards were doing and to give me a few switches to start/stop recording etc)
• Adalogger Feather Wing (with SD Card to record results)

  After getting the software and hardware running I also 3D printed a widget to attach the T-o-F sensor to my car. Image below:

Here's a link to a short video showing my waving my hand over the Time-of-flight sensor.

Hi All,

  just a quick post to say I've released two "explainer" videos. I've put both together so I can refer back to them in future "progress" videos and I don't need to keep explaining the same things over and over again. You can also find my blog posts about the videos on my website (linked at the bottom of this post). Those blog posts are more about how I made the videos rather than the content of the videos. 

Here are my website blog posts:

https://purplemeanie.co.uk/index.php/2024/02/25/the-electric-in-electric-vehicles-youtube-ptevis-2-1/

https://purplemeanie.co.uk/index.php/2024/03/03/electric-vehicle-components-youtube-ptevis-2-2/

1 minute ago, Jonathan Kay said:

If there's going to be a mechanical reduction gearbox could you use a design where the output isn't inline with the input? That could reduce the angulation of the drive shaft.

Jonathan

Hi Jonathan, yes, I’d thought about that, and others have suggested things like belts etc. However, an epicyclic gearbox is the perfect solution. They’re small, light, can run in both directions and can deal with the power. The downside is that their complexity increases cost… and I’ll need a custom box designed. But I’ve got a couple of companies giving me what look like reasonable quotes for a design and supply. So, all in all it’s my currently preferred option.

Hi all,

  unfortunately I can't make this month's SIG meeting next Tuesday. So in lieu of that meeting, here's an update on the project with a few images. I'm also in the process of writing a piece for March's Lowflying that I should hopefully get written before the deadline later this month.

  As those of you that attended last month's meeting will remember, I've now started to make significant progress with my project. I've been sidelined a little by trying to get two "explainer" videos published to YouTube, but they're very close to being finished now and should go live in the next couple of weeks. The first gives an "explainer" on what forms of electricity are used in an EV (DC and AC) while the second goes into what components I'll be needing in my project. They're meant to be resources that novices can look at to understand what's in an EV, and that I can keep referring back to. Hopefully they'll be useful to anyone wanting to know a bit more of the behind the scenes.

Link to a short YouTube video showing how a motor works... this is part of the second video.

As for project updates...

  The big change is that I've decided to explore more fully the idea of a twin rear motor configuration for the car. Each will be a smaller motor of around 120kW, mated to a small 6:1 epicyclic gearbox to reduce the ~12,000rpm of the motors down to ~2,000rpm needed at the wheels. This has the advantage of removing prop-shaft and differential while also improving packaging - meaning I can have more space for batteries up front. See image below...

  The second big change is that I'm planning to use a high-power DC-to-DC converter that will sit between the battery pack and the twin inverters. This means I can de-couple the voltage of the pack from the voltage required for the motor. One of the critical conundrums of an EV project is to be able to get enough voltage supplied to the motor without significantly increasing the size of the battery pack to accommodate that voltage. A high-power DC-to-DC converter will allow a low pack voltage of say 250V but still drive the motor at 400V. This gives me much more flexibility on the battery options up front. See image below...

  Finally in big ticket items, I've finalised on how I'll do all the electronics control of the car now. I'm going fully CANbus for the control interfaces and removing the standard gauges to be replaced by an electronic dash. I appreciate that some may baulk at the idea of an electronic dash, but for my "prototype" car I need a much higher density of debug information on a screen. And this should allow me to do that. Another benefit of going completely CAN, is that I can use a Power Delivery Module. This will replace my current fuse box (that would have needed rewiring anyway) and replaces it with 32 programmable "soft" fuses. I expected to have been "blowing" a lot of fuses if they are standard fuses, so having a soft-resettable fusing system will be a big bonus. The PDM can also do a lot of logic functions for me and seems to be a big win-win for the design of the car. It also means there's a lot less wiring to do. Image of AIM dash and switches below...

  This last CANbus decision has also meant I can replace more of the wiring in the car. This isn't perhaps ideal, I'd have rather keep the current loom as "stock" as possible, but the reality is that an awful lot of it will change anyway and I might as well go all on with it. A consequence of this is that I'll be replacing the wiring to the lights at each corner with a CANbus driven lighting system (of my own design). Prototype board shown below...

  And finally, finally, I've been diverted a little by a project to determine actual rear suspension travel. I need to know how much suspension travel I'll need so I can get a good handle on the rise/drop angles of the rear suspension - I'll need new half-shafts with the twin motor arrangement. So... me being the type to dive in with a way over engineered solution, I've created a way of "instrumenting" suspension travel using a RP2040 processor and some data-logging prototype boards. See image below and link to YouTube short. The processor samples a distance measurement 30 times a second and records it to an SD-card. The distance is taken using a laser based Time-of-Flight sensor which I attach to the frame of my 420R above the De-dion "ears". As the ears move up and down I record their travel. More info on my results in further reports.

Here's an image of the "Suspension Logger", link to video below it...

As well as the electronics to capture suspension position, I've also modelled, and printed, a "widget" to attach the T-of-F sensor to the frame of my 420. The processor and data logger sit in the boot of the car (in the dry) and the sensor clips onto the frame in the wheel arch. This arrangement isn't meant to be permanent. And in fact even "temporary" is stretching its use. It will be attached for a few minutes to gather suspension travel information - if I wanted it attached for any longer, and for all weather, then I'd have done something more robust. But this should get me to the data I need.

Link to suspension logger video: 

That's all for now. If you have any questions then please shoot them my way.

John

As part of a bigger project I've put together an animation of a simplified Permanent Magnet Synchronous Motor.

If you're interested you can find more about it and a link to the video here:

https://purplemeanie.co.uk/index.php/2024/01/12/pmsm-motor-internals-video/

If you have any questions or if I've got something wrong then please let me know.

John

We aim to Educate! 🙂

For those of you that are interested, Caterham has sent out an email with a link to their latest Project V video. The video is unlisted at the moment, so you can only find it using the link below:

John

From today’s Times. I’m not sure it will open the flood gates to new charging stations but I guess any progress is progress… all be it slow.

John

I thought both of these news points were interesting but the crate motor concept will certainly be good for conversions where we don't always want to use crash damaged components. (Article from the 2023-11-15 Autocar).

3 hours ago, 700newtons said:

I wonder if the heavy battery would be too much for older people

I guess the trolley wheels and handle are supposed to help there, but you do wonder about door thresholds and stairs.

I wouldn’t normally post about any old EV. But I’ve (and I suspect a lot of people have) been banging on about personal transportation with swappable batteries for ages. This looks promising for all of those short journeys.

John

Caterham is now promoting a Project V Deep Dive with Bob, to be aired on November 17th @ 18:00

I've been working on some explainer videos over the past few weeks (then there'll be an update video).

Here's a couple of test rolls I made to get my head around trying to explain AC vs DC and then the flow of electricity around an EV. They will be part of a 3 part explainer series, each probably about 10 minutes. They will be on:

1. Electricity in an EV
2. The different operational modes of an EV - Motoring, Regenerating, Charging, etc
3. Overview of the components in a (simple) EV

Both of these videos are created as templates in Apple Motion and then imported into Final Cut Pro where I can adjust the timing of the animations and add text.

These are unlisted videos, so won't appear on any searches.

The Final Cut project for the Oscilloscope test looks like this...