The case for the scratch-built electric sports car

With batteries slowly becoming "real", it's only a matter of time before someone wants to build an electric Midlana. More power to them!
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rennkafer
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Re: The case for the scratch-built electric sports car

Post by rennkafer » 13 Dec 2019, 07:13

John wrote:
12 Dec 2019, 17:02
Enter such things as ground effects and wings to increase the force pressing the car onto the road and you have effectively decoupled "g" from the coefficient of friction when the car is travelling fast enough for those things to be effective. Enter another idea, the fan car. This has the advantage of offering consistent down force irrespective of speed. This is important because the suspension loads don't change by a large factor as they do with more passive types of down force generation.
A couple things here. Tire grip isn't as simple as force normal x CoF, as the tire interacts at a molecular level with the road surface. We've known about this stuff since the '70s (Carroll Smith talks about it briefly in his "tires" chapter in Prepare To Win). Second, while suspension loads don't change much with a sucker car, it also has all the drag (both parasitic from motor draw, and from the additional force normal it generates) all the time, unlike vehicles with passive aero which typically have less drag at lower speeds. Could be an advantage or disadvantage depending on the particular track. Race shop I worked for did some significant work on the Chaparral 2J getting it ready for Monterey Historics a while back, and we got to talk to Jim Hall about some of the tradeoffs. Working in a vintage race shop that ran a lot of Can Am and F1 cars from the birth of real race car aero was an interesting experience, as we got to talk to some of the designers/engineers and find out why they did the things they did.
Bill J

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Re: The case for the scratch-built electric sports car

Post by freakynami » 16 Dec 2019, 03:52

I'm not digging too deep into this field... yet... Maybe once I get the first Midlana finished...

So apologies if this has already been mentioned, but I just saw this section of Rich's latest youtube and thought readers might be interested in a quick explanation of electric crate motors.
https://youtu.be/x-OY1hwrqqs?t=342

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Re: The case for the scratch-built electric sports car

Post by Midlana1 » 16 Dec 2019, 06:51

I was talking with a designer (who worked for Colin Chapman at Lotus!) and he said that right now, figure an electric drivetrain/batteries will run "4-8" times as expensive as a internal combustion engine of similar power.

My reason for discussing electric drivetrains is just a thought experiment at this time, partly because building anything new means getting rid of what I currently have, and I have no plans to do that!

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Re: The case for the scratch-built electric sports car

Post by roy928tt » 18 Dec 2019, 18:29

I saw an interview with Gordon Murray the other day, he spoke about E.V.'s, his issue was that with an EV the battery supplies all of the power and must be carried around all of the time. Whereas liquid hydrocarbon fuel is carried by the car but the air it is mixed with is not, and we only use 1 part fuel to 15 parts air so the weight of energy that is actually carried by the chassis is lower.....

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Re: The case for the scratch-built electric sports car

Post by bgkast » 19 Dec 2019, 10:18

true, but even the most efficient internal combustion engines are only using 40% of that energy to move the car, and most are closer to 20%.

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Re: The case for the scratch-built electric sports car

Post by John » 19 Dec 2019, 14:23

I think the Prius with its Atkinson cycle engine has cracked 40% peak efficiency. Atkinson does make the engine less powerful. Don't also discount that only 20.9% of air is oxygen so if you are talking chemical potential energy that would be 3 lb of oxygen for 1 lb of fuel at perfect stoichiometric ratio. The rest of the air is just working fluid for the heat engine and could be recycled through the engine were it not for the need for fresh supplies of oxygen. The EV can be very efficient. An EV would come close to extracting 4 times as much work from its energy reserve as an ICE car. Actually one thing I like about EV's is that the CG doesn't shift as fuel is used up.

The case for or against isn't going to be purely economic especially if the end result isn't directly comparable. Bill's desire for a symphony of pleasing mechanical noises illustrates the point. Others might prefer silence and maybe pay extra for it. Others might see the environmental aspect as a moral imperative.

When making an economic case for EV's the old principle was to define the mission the EV had to fulfill and then fit the minimum battery required to full fill that mission. Take a look at A123 20Ah cells. I think they can be had for $12 US per cell. In a 30S 3P configuration cell weight would be 45kg. Capacity would be 6kWh. Pack could make 118kW. Cost would be $1080 for just the cells. Nominal voltage would be 99V. Motors 2x Motenergy ME1115, 15.5kg and $1000 each. Controllers 2x Kelly KLS-96601-8080l, 5.5kg and $800 each. $4680, 87kg for core components. Gear so maximum motor RPM occurs at maximum speed. Wont exactly set your hair on fire but costs are sane.

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Re: The case for the scratch-built electric sports car

Post by John » 14 Jan 2020, 19:08

I want to introduce the idea of scaling. If you were to double the number of motors, batteries, and controllers you would double the available power but you would also double the weight of these components (and cost). If you assume a highly optimized vehicle, then you would also have to double the power absorption capabilities of the brakes and the weight of the structure required to support all the components and bear the dynamic and static loads. Except for one factor the vehicle would wind up with very similar performance. That thing is the weight and dimensions of the driver and passenger (the payload). These dimensions to a large degree dictate the dimensions of the car. Power to weight ratio becomes a key metric by which you should judge the performance potential of a propulsion system rather than power alone, and when I say system I mean system complete with the supporting systems that have to be carried by the vehicle. With an EV you have the opportunity to separately optimize two major components of the system for power to weight, the battery and the drive. With the battery the balance of the competing requirements of range and performance come into play. At some point the limitations of rubber and road will impose a ceiling on performance for at least the lower part of the speed envelope. I think scaling down and accepting some limitations on range could allow a vehicle that is performant, light, fun to drive and cheap to build and very energy efficient. Light enough to dispense with power brakes and steering should make the car very simple and responsive. Ideas aren't very advanced yet. A target weight of about 400kg would be 1/4 Nissan leaf and 6kWh battery would be 1/4 of a 24 kWh Leaf. Power to weight ratio about 5 times an 80kW Nissan Leaf assuming sufficient cooling could be arranged to allow it. Could make an ideal test bed for some suspension ideas.

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Re: The case for the scratch-built electric sports car

Post by Midlana1 » 20 Jan 2020, 07:21

I pretty much agree with all your points; my issue is, when it's all said and done, what about "Performance/cost." That's my current hang up right now, that at the end of the day, it may have less range and be slower—but more expensive—than a car with a gas engine. This situation will improve over time as more used EV components hit the market. The advantage, for a while at least, is that the parts "should" become fairly cheap because no one knows how to use them— and they're afraid of electricity.

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Re: The case for the scratch-built electric sports car

Post by John » 20 Jan 2020, 19:14

You are right it would be extremely difficult to better a salvage gas unit on performance/cost bassis. The current crop of EV's wouldn't, at least in my mind, yield suitable parts to build my idealized machine. The more optimized bits need to be the less likely standard parts will be suitable. I do geek out on the unrealized possibilities and potential of the tech.

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