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Looking for a motor for an EV conversion 3

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BenDorward

BenDorward

Automotive
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Dec 18, 2019
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I'm going to be converting a class b motorhome, most likely a sprinter, into an ev vehicle. I heard the new electric Mercedes have a 84kw motor (120hp) with 300 newtons (220f/lbs) torque so i thought I would model my needs off their specs. I found a few motors that were close on the hp but not the torque. Any help finding other brands would be greatly appreciated. Why would i need that torque if ev conversions for vw bus's are 71hp,120f/lbs.



(120hp,162f/lbs)
(165hp, 190f/lbs)
 
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More than a few Tesla motor EV conversion projects described on YouTube.

 
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1. Old VW busses are a lot smaller and lighter.
2. Sprinters are a lot bigger and heavier
3. You don't say anything about the final drive ratio(s). Your drivewheel torque would be torque/final drive ratio . If you're concerned about matching the design, match the output parameter as opposed half the drivetrain and you don't care about the other half.
 
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I've also commuted in cars with low power/weight, an 82 benz 240D and an 82 Benz 300D wagon. Peak torque for the 240D was 130 Nm and 160Nm for the 300D, both cars about 1600kg/3500lb. I wouldn't give the 240D to an unexperienced driver on today's roads, you need some brass and some planning to get around, especially merging. The 300D is slow, the manual transmission helps accelleration but 0-60 mph is around 15-20 seconds. You probably want to maintain highway speed, or close to it on an 8% grade. The 62hp/42kW 240 would barely do that, a sprinter weighs half again that much or more.
 
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The torque numbers with an electric motor are almost irrelevant because it depends on the design operating speed of the motor, which could be anything ... and you can compensate for that with gearing.

What you cannot compensate for with gearing, is if you simply don't have enough power to get the task at hand done. So, you need to look at the horsepower numbers first, and the target vehicle top speed and maximum rated motor RPM to work out what the gear ratio needs to be, THEN look at the motor torque rating and the gear reduction to work out if acceleration from a standstill and hill-climbing at low speed will be acceptable (and if not, you will need a multi-speed transmission). Part of why Tesla and other EVs seem overpowered is simply that in the interest of not needing a multi-speed transmission, they need a certain torque rating to have acceleration from a standstill and they need a certain motor operating speed (in combination with appropriate single-ratio gear reduction) to get the target top speed. Combine those, and it's a big horsepower rating.

I believe most of the old-VW electric conversions retain the rear-mounted VW 4-speed manual transmission and differential, which is rather handy if the motor that you plan to use is a smidge underpowered (or, under-torque-rated, if you will).

Now ...

That Sprinter, presumably extended length high roof, probably weighs 3000 kg on its own. Add another 1000 kg of permanently installed motorhome camper conversion stuff plus cargo plus something for your batteries, which are going to weigh something.

Climbing the above-mentioned 8% grade at (let's say) 25 metres per second (about 90 km/h or 55 mph if you prefer) will require 4000 kg x 9.807 x 0.08 x 25 = 78 kW JUST to get up the slope. That doesn't count aero drag (and it's a big vehicle), nor frictional losses in the tires, brakes, driveline, and what-not.

Mercedes' power rating for their Sprinter EV therefore make sense given that this vehicle is meant to be used in urban delivery applications ... not cross country road trips (you won't get very far on a charge).

The old VW buses were sllllloooooo
The chassis and suspension and tires on the old VW buses probably means it's just as well that they were that slow.
 
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Hey Brian, your response has been very helpful. Bill, thanks for the idea for the Tesla motor. I didn't know that some EV conversions don't have transmissions until i saw jehu garcia's youtube channel and started getting responses here. I understand that vw busses would be cool. It's changes some of my plans though, i'd have to do some research. Brian you said 78kw. I'd be looking at an 88kw motor right now. I'm more concerned about the torque. I could also live with a 15 second 0 to 60 time. I have a mechanic friend that will help me remove the old engine and possibly put in a new electric one but i want to take a big chunk of the labor and i know nothing about transmissions much less where i'd start putting a manual transmission into a sprinter.

Is the hill climbing thing a hp issue or a torque issue? I'm guessing the lower the torque the great the need for a transmission? I think to improve my overall power while going slow i will probably try and do something with a transmission. any ideas on truck models i can pull from?


Net Gain Hyper9 - 120 hp , 160 ft/lbs torque - 4,300$ Curtis 1238e - 125 hp, 213 ft/lbs torque - 8, 300$
 
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You need horsepower (or kilowatts, if you prefer) to climb the hill at a given speed. See my calculation earlier. Plug in your own numbers if you have more accurate ones.

You need horsepower to overcome aerodynamic drag at speed (and rolling resistance at speed and so forth).

You need torque to get moving from a standstill.

The motor's maximum rated operating speed combined with your target maximum road speed (taking into account the rolling diameter of the tires) will fix your overall drive ratio between the motor and the road wheels.

Then YOU need to crunch the numbers to see if you have enough power to get the job done for top speed and hill climb, and enough torque to get the job done for getting moving from a standstill. If it turns out that you need a different mechanical drive ratio in order to both (A) have sufficient top speed and (B) have sufficient torque to get away from a standstill while facing uphill ... then you either need (A) a bigger motor - Tesla's approach, or (B) a multi-speed transmission.

Keep in mind that "getting moving on a standstill" could also include getting moving from a standstill while stopped on a 30% (or whatever) grade ... or getting moving from a standstill with the wheels knee deep in mud or sand.

P.S. #1, The new Porsche Taycan contains a two-speed mechanical transmission inside its rear drive unit.

P.S. #2, for different reasons, a Proterra electric urban transit bus contains a two-speed mechanical transmission inside its drive unit.
 
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Well the good news is the curb of the sprinter is 4500 lbs and i probably won't be adding more than 1500 lbs including passengers. I also need to be able to get up the 13% grade i-80 outside slc. so thats:

2700 kg x 9.807 x 0.13 x 25 = 86kw

Does anyone know where i'd start trying to put a manual transmission in a EV? I heard from a youtube couple their tesla powered bus had 3 gears, i'm guessing that's not something out of an old car
 
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Really?


Short wheelbase low roof curb weight = 8550 lb GVWR - 3854 lb payload = 4696 lbs (not far off)

But ... you're building a camper out of a low roof van that you can't stand up in without ducking your head, and which runs out of floor space in a hurry if you put a bed for two people back there? Really?

Most campers are built from the longest (or maybe second-longest) configuration and with a high roof.

170" wheelbase extended (longest configuration) with high roof = 9050 lb GVWR - 3605 lb payload = 5445 lbs curb weight.

Even if you manage to do it with 1500 lbs payload, you're at about 7000 lb total = 3200 kg.

Aero drag will be significant, even if the drag coefficient is good, due to huge frontal area. At highway speed? Probably another 20 - 30 kW. This will be strongly dependent on how fast you drive ... and the wind direction. (I have a van. Strong enough headwind, and it won't do highway speed in top gear!)

We haven't even talked about battery capacity yet. You are going to need more power than you think you need with a vehicle that big. It's going to use more energy than you think it will. That means ... more weight.

So now you are at 3200 x 9.807 x 0.13 x 25 + (let's guess 20 kW for aero) = 121 kW (about 160 hp)

That's pretty plausible. The bare-minimum base engine in a van like that in Europe is somewhere in that range of power output. Going up a hill that steep at 90 km/h means foot to the floor ... probably acceptable. The official M-B EV version of this van doesn't have that much power, and won't make it up that hill at that speed ... it'll have to slow down. For urban delivery ... probably acceptable.

Another consequence is that if it uses 20 kW for aero to go 90 km/h (which is an ASSumption), that's 20 kWh of consumption to go 90 km/h for an hour i.e. 90 km ... 4.5 km per kWh. If we use 100 kWh of battery capacity it will do 450 km (bear in mind that you are puttering along at 90 km/h). If anything, that's optimistic for something of this size. Put a real driver behind the wheel and it's going to use more and not make it as far. I don't do 90 km/h on the motorway, not even in my van. I do more than that. Haven't talked about heat, haven't talked about air-con, either.
 
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Yeah, i realize going 55 on the highway isn't ideal. I'm looking at the high roof 144 wheelbase which i think is like ~5000 lbs curb weight depending on year. I'm thinking 2k lbs load is safe. And i can always avoid the steepest paved road in utah. I just need to make sure i can go slow up steep stuff and ideally hit the speed limit on the highway at least on the flat.

I'm also going to do propane heat. I'm not sure about ac but i wouldn't imagine it would be an issue while driving if i can just crack the windows. I realize this means i'm going to be making a few sacrifices to realize a dream of making this thing electric. I just need to make sure i'm not running into any serious calculation errors with what to expect.

Like I said before the new Mercedes electrics will have a 84 kw motor with some extra torque. I'm close to that now with the single motor, i just need to make sure my van will still go if i don't put a dual motor in like the Teslas

If you know anything about EV transmissions that's my next step.


edit: i realize a lot of the converted vans are 170" wheelbase. I think that's because people want to fit a bathroom/shower or have office, clothing, or storage needs that i don't have. check out this guys shower, i think it's the main reason i'm sticking with the 144" (high roof definitely)

edit: I'm pretty sure i'll have the necessary horsepower for the job: I don't know if the new electric mercedes has a dual motor but the output is 84kw. If i get a 2 speed transmission i think i can improve my initial acceleration. I just need to find a place that sells ev transmissions. here's a video exploring the issue in formula cars:
 
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There is no such thing as an "EV transmission" for an application like this. If it turns out that you need a transmission, adapt the existing manual transmission that is available in markets outside North America for the combustion-engine version of that vehicle ... just like other retrofitters do. The automatic is not suitable.
 
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I grant you I haven't found anything that seems to fit the size of my motor currently, however, it took me 20 seconds to find a larger EV transmission for commercial vehicles and truck applications.

I also found a 2 speed powerglide transmission that works with a slightly larger motor being sold on EV west.

Here's somebody discussing the advantages of EV transmissions

Here's a couple that has a 3 speed tesla powered ev van, they were talking about having 3 speeds but they also said they connected the motor directly to the gearbox for 4wd so maybe they are using the original tranny.

So i found this article from 2009. Assuming progress doesn't move in reverse I would know why there still wouldn't be a production model somewhere

I still think this is a better solution that trying to find a manual swap. I wouldn't even know where to start.
 
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Curb ? And have you factored in the tons of battery's that will be needed to get a few miles of range?
 
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the batteries i'm looking at have a density of 61 watts per pound. I'm going to be putting around 80kw of batteries so that's around 1300 pounds. nowhere near a ton. With my weight i should get close to 200 miles on a charge (atleast thats my estimate unless you see differently)

I'm also looking at a Tesla drivetrain now. I talked with some experts and i'm underpowered with that motor for this application.

thanks :-)
 
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It would be interesting to skip transmissions altogether and use wheel motors. I have no idea what they cost, but elaphe offers them with up to 1,106 lb-ft peak and 479 lb-ft continuous torque at wheel speeds up to 1480 RPM. You need to cool them, but as far as I know, you have to cool all EV motors (and batteries).
Wheel_Motors_qwzlig.jpg
 
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I sent Elaphe an email. I have a feeling they may be out of my price range but it looks cool and is definitely worth checking on.

I have a serious question. I'm looking at buying a hell of a lot of 18650 cells and connecting them together for a large battery system for the build. I heard from the youtuber jehu garcia that cooling isn't really necessary on batteries if their discharge stays at 1c (which i think is one amp per hour). I'm looking at around 4500 battery cells to make a ~70-80kw battery. The continuous power rating of the motor i'm looking at is 69kw. I put into the conversion calculator the voltage of the batteries and the discharge rate at max motor power (69kw). Am i doing this wrong? Can i just say that if i'm drawing 70kw from a 70kw battery that's 1 amp or as i got almost 17,000 amps from the calculator does that mean i'm running the batteries at 4 times their rating when i'm maxing out the motor (that doesn't seem right)?

Anyways, thanks for getting me closer to "hardware mode"!
 
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Trouble with wheel motors - and why no mainstream manufacturer is pursuing that route! - is the unsprung weight. All fine if you are rolling straight down a smooth road. Turn a corner on a rough surface if you have lots of unsprung weight, and you'll find out why it's a bad thing. Also why "live" rear axles have gone out of favour except in applications where ride and handling are low priorities (heavy trucks). The Ford Mustang was criticised for decades for that live rear axle resulting in skittishness on bumpy corners. They've gone to independent so that the differential no longer has to bounce up and down with the wheels. All the current-production Jeeps that don't have rock-crawler pretensions, are all-independent suspension ... to get the unsprung weight down. (I find solid-axle Jeeps to have uncomfortable ride quality ... that's why the street-oriented models went away from that.)

You're all confused about your volts and amps and power. You need to pick the nominal voltage of your battery pack. (Probably 3 or 4 hundred volts but it will be dependent upon the motor and drive that you select) Divide that by the nominal cell voltage and you get the number of cells that each segment of the battery pack needs to have connected in series. That will add up to a certain capacity in (let's say) kWh. Divide the total number of kWh that you desire, by the number of kWh in each segment of your battery pack, and that's the number of segments of battery pack that you need to connect IN PARALLEL. Each segment of the battery has a bunch of cells in series ... then a bunch of those segments are connected in parallel.

Whatever your max discharge rate is (in watts) divided by the nominal voltage of your battery pack, is the number of amps that your battery pack (and the entire power system) has to handle.

Look up Ohm's law, and look up electrical power calculations (volts x amps = power) ... you're going to need those ...

Real EVs that use lithium-based cells have additional control and monitoring and charge-balancing circuits for all those cells. In those battery segments that are connected in series, if you don't have that, one bad cell will take out that entire segment.

EVs are simple from the outside looking in. EVs that meet consumer expectations, are very complex!
 
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Crunching some numbers based on that wheel motor linked to above ...

Normal wheel rolling diameters for a normal road vehicle is around 600 - 700 mm, let's use 650 mm. Means the vehicle goes 2042 mm = 2.042 m down the road for each revolution. At 1480 rpm = 88,800 revs per hour that equals 181,000 m = 181 km/h. That's a good top speed for a road vehicle to not get mowed down on the autobahn and have some overtaking capability for the rest of us who live in a world of low speed limits.

Peak torque 1500 N.m is what a fast getaway from a stop would use. If the wheel rolling diameter is 650mm then the radius is 325mm = 0.325 m and it means this one motor would produce 4615 N of forward thrust. Obviously these motors would be designed for use in pairs, so two of them would give 9230 N of forward thrust.

Now F = M . A therefore A = F / M so with these two motors, if the vehicle weighed let's say 3000 kg, it will do a little more than 3 m/s2 acceleration (or, off a standstill, have a gradeability of about 30%). That's a wee bit sluggish; in a 2000 kg vehicle it would be decent but not earthshattering, if 4 wheel drive (4 such motors) were to be used then it would move that 3000 kg vehicle nicely along. "Continuous" gradeability would be a little less than half this, which is still OK.

Now, 4 such motors would have a continuous power rating of 308 kW ...which is quite a lot, and it's in the ballpark of what Tesla provides for their non-transmission-equipped vehicles.

Let's suppose you didn't want such a powerful vehicle (or your drive system or battery is not up to the task of delivering it). The 180 km/h top speed is still good, but you now select a lower torque rating (corresponding to your lower target power output). Except now ... the vehicle won't pull away from a stop well, or get itself up a steep hill.

And that's where having a transmission, even a basic one with just two or perhaps three speeds, comes into play.

No can do, if you are using wheel motors.

Or you just oversize the motor and drive unit and make sure the battery has lots of juice behind it, and let the vehicle be a rocketship (at least, until it runs out of gearing from the motor hitting rated revs or the drive hitting max voltage). Few people will complain and it means you don't need the transmission. That's what Tesla did. Even the lowlier EVs thus far have relatively high power ratings which come along with the required RPM rating for top speed and the required torque rating for takeoff from a stop.
 
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ok, cool, this helps a lot. I'm told I'm supposed to have a 300 volt architecture. Nominal voltage for each cell is 3.6 volts so thats around 84 cells per segment. I plugged the voltage architecture into the calculator and the number of cells into the amp section (i'm guessing one amp each) and got 25,200. My continuous power is 69kw but the motor is rated to 400 kw. So 400,000 watts divided by 25,200 watts is 15. so i need 15 segments of 84 cells each so i'd need at minimum 1334 cells. So replug the 1334 cells with a 300 volt architecture into the equation and i get 40.02kw. So that means i need a 40kw battery to supply the max rating of the engine using 1 amp per cell?

This seems right lol, hope i'm getting it correct
 
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