Newb Asks: How Do You Dyno An Electric Car?

[Gregski]

The Greg recently took his freshly converted 1971 GMC truck to a dyno shop to have it tested for horse power and torque. What he found out is that you can not get a torque reading on electric cars at this time! In this companion thread to Damien takes Zombie GS450h on dyno I would like to share the lessons learned, ask some questions, and find a way to trick the genius reverse engineers on this platform to come up with a solution.

PROBLEM:

• Electric cars have no spark plugs, no coils, no ignition.

• Pre 1995 cars have no OBDII port.

QUESTIONS:

• So how do you obtain the tach/RPM signal from an EV?

• Can you use the inductive pickup probe on the weak tach signal in your gauge cluster?

• Can you use the VSS Vehicle Speed Sensor signal?

Please forgive the redundancy with my build thread, I feel this is a worthy topic and didn't want it to get buried in my other nonsense.

[Zieg]

I've been wondering about this ever since you posted the video. Is this the case for all chassis dynamometers? No optical pickups that you can point at a sticker on the axle, or something like that?

My first thought was that if you know the dyno's reported speed and the tire rolling diameter and all the gear ratios, can you calculate torque based on that info? Horsepower = Torque (in lb.ft) x RPM/5252

It looks like it might be possible to get OI to talk to an OBDii adapter if you can configure both ends correctly? If so I might have to add "grab a random OBDii port" to my junkyard shopping list...

[Gregski]

Vehicle RPM Hook Ups with Dynojet Automotive Dynamometers

this video goes over the five different ways you can send RPM signal from the vehicle to the dynamometer (maybe two of these could work for the EV, what do you think?)

1. Inductive Pickups (this is the method we already tried and it did NOT work)
2. Optical Pickup
3. EZ-RPM Module
4. OBDII Data Link (vehicles older than 1995 in the US do not have this port yet)
5. Calculating RPM

[Gregski]

I've been wondering about this ever since you posted the video. Is this the case for all chassis dynamometers? No optical pickups that you can point at a sticker on the axle, or something like that?

You are asking all the right questions?

I don't think it is a dyno limitation it is a paradigm shift in the way we need to dyno cars. I think all dynos will be the same. I think the optical pickup method may work, but instead of placing the reflective tape on the harmonic balancer of the engine I would place it on the output shaft of the Lexus GS450h transmission (in my case, your placement will vary accordingly, maybe put it on the CV joint, etc) the thing is that, that is already after the 1.9 high gear reduction in the tail of the transmission, so how do we offset that, or do we?

Dyno shops should already have this component as it is commonly used for diesel applications:

[Jacobsmess]

You don't offset anything for the gear ratio. You just have to allow for it in whatever calculation takes place. For the GS450H you know the shaft RPM, and you know what gear you are in so you just use 1.9 or 3.9 in the calculation for mg2 motor rpm.

[Gregski]

You don't offset anything for the gear ratio. You just have to allow for it in whatever calculation takes place. For the GS450H you know the shaft RPM, and you know what gear you are in so you just use 1.9 or 3.9 in the calculation for mg2 motor rpm.

that's literally doing the offset, lol to get us a 1:1 that I was talking about

[Jacobsmess]

I see, the term offset threw me, but that's the fun of terminology!

[Gregski]

I see, the term offset threw me, but that's the fun of terminology!

yeah, sorry, I make shiet up as I go along, if there is a more better term, have at it, adjustment maybe?, or compensation, you know what I mean, either way, when future humans read these threads 50 years from now they will go why didn't they just use their lunar organic flux capacitors coupled with their mind readers to get torque?

[arber333]

Hm...
Old tachos are of two kind usually.
1. Ignition coil pickup. This one is sensing the surge from the ignition coil secondary. This happens 3x per revolution with V6 and 4x per revolution for V8 engines.

2. Inductive pickup from the main driveshaft pulley or flywheel. Normally there is a toothed wheel there which has a sensor to pick up the transitions from teeth and valleys. BUT there is a caveat. To know the start of RPM top position VCU needs to distinguish where the starting position is.
They get this by cutting away two or three of the teeeth from the sensor wheel. Algorithm will see the missing teeth and interpret this as starting position.

My mazda had V6 befoere. What i did was put alu casing on the rear motor shaft and put 3 magnets into it at 120deg. A hall sensor (magnet pickup)
https://mazdamx3ev.wordpress.com/2015/0 ... er-senzor/

EDIT: I just remember for Peugeot 406c i used a wheel on one of the driveshafts.
https://leafdriveblog.wordpress.com/202 ... l-trouble/

And RON made a software for arduino
viewtopic.php?f=9&t=52

[uhi22]

What I do not get: To calculate the power, the dyno needs just to measure the force and the speed, and multiply both. Doesnt have the dyno its own sensors for this? Which one is missing?

[jrbe]

"Dyno offset" sounds like dyno fudge.

Should just be the wheel torque divided by the gear ratio between the tires and the motor. None of this extra stuff is necessary if you have those 2 things, if it's a fixed ratio. I think all dynos have a gear ratio input mode but not positive.

I wonder what the inductive clamp would do around one phase leg. Set the # of poles as cylinders and it might work.

[Zieg]

"Dyno offset" sounds like dyno fudge.

Mmm.. dyno fudge

.

[arber333]

What I do not get: To calculate the power, the dyno needs just to measure the force and the speed, and multiply both. Doesnt have the dyno its own sensors for this? Which one is missing?

He is just saying that particular dyno cant produce exact results as it does not get the RPM from the car. But this shouldnt be a problem as the rollers should have their own rpm sensors...

[Aragorn]

What I do not get: To calculate the power, the dyno needs just to measure the force and the speed, and multiply both. Doesnt have the dyno its own sensors for this? Which one is missing?

Nothing is missing, which is why you have a chart showing wheel speed and power...

The dyno would usually additionally use engine RPM to calculate a gear ratio, and then provide engine torque, producing a typical dyno chart of power and torque against engine RPM.

If the dyno system doesnt know the ratio, it cant do that, and thus you only get power and speed.

If theres no tach input available, i thought these systems had a mode whereby it asked you to hold the engine at say 3000rpm, and used that to figure out what the ratio is.

The dyno chart is a slightly weird shape compared with Damiens. Damiens had a clear constant torque region, and then transitioned over to constant power, which it held fairly flat. Yours on the other hand hits constant power fairly quickly, then shortly afterwards the power starts dropping off. They started the dyno run at a fairly high speed though, which is perhaps hiding some of that constant torque region? but why the power drop off? Batteries?

[Gregski]

The dyno chart is a slightly weird shape compared with Damiens. Damiens had a clear constant torque region, and then transitioned over to constant power, which it held fairly flat. Yours on the other hand hits constant power fairly quickly, then shortly afterwards the power starts dropping off. They started the dyno run at a fairly high speed though, which is perhaps hiding some of that constant torque region? but why the power drop off? Batteries?

both of my pulls were done from rolling starts, the first one from almost 40 miles per hour, and then I think it held for about 10 miles till 50, and yeah I am using a very small 9 kWh hybrid battery pack, one that I shouldn't even be running around with let alone driven 25 miles to a dyno shop and then tried to do some power pulls, lol

also both of my graphs are just horse power, one is from almost 40 miles per hour the other from about 10 mph

[jrbe]

How did they get hp without rpm or torque?

[Gregski]

How did they get hp without rpm or torque?

math-zack-galifianakis.gif (181.3 KiB) Viewed 19094 times

[uhi22]

Let me guess: The dyno has sensors for the torque and the rpm of its roll. And just needs to multiply these.

[muehlpower]

There are roller dynamometers with known inertia, roller diameter and speed detection. The performance is then calculated from the acceleration based on the driving speed. If the gear ratio is known (measured or entered), the power and torque can be specified based on the motor. I myself have a small test bench for motorcycles based on this principle.
Other test stands also have a brake with torque recording. Of course, the inertia must also be calculated if the speed changes during the measurement

[jrbe]

My understanding is because it's an inertia dyno it calculates hp from accelerating the drum. There are different input methods for rpm input.

From: https://dynojet.zendesk.com/hc/en-us/ar ... o-for-use-

RPM CONFIGURATION:
In the RPM Configuration tab there are settings that affect how the dyno reads RPM from the vehicle.
“RPM Source”: The method the dyno uses to read RPM.

Inductive 1 or 2: The inductive inputs on the dyno, for using with a clip over wire style inductive sensor.

Digital 1 or 2: The digital inputs, for using with digital input devices (like an optical sensor).

OBD2: for use with the Dynojet OBD2 device connected to the dyno by CAN.

Power Commander 5: For use with a Dynojet PCV device connected to the dyno by CAN.

Gear Ratio: For use when other rpm sources are unavailable. The dyno will estimate the RPM based on it’s speed. Uses the gear ratio shown in the gear ratio value editer on this tab.

A few options for speed input. I'm not positive the dyno will allow a manual rpm input or not. The dyno guy may have not known about all of these.

I've used mostly load holding dynos and was able to manually input gear ratio if needed (coil on plug with standalone.) Being able to hold a fixed rpm helps immensely while trying to tune an internal combustion engine. I'm sure they could help tuning electric motor parameters as well if you have that level of control over the system - an OI inverter board.

I don't understand why they started the run at 40mph.

Do you have a timing light that you can try on a phase wire to see if it lights when the motor turns? The clamp might not fit. Could work with an optical pickup.
A few coils of wire around a phase leg with the manual input might work with the digital input too. Probably would need some electronics to convert it to a square wave though.
They also have a stick on pickup to attach to the top of coil on plug coils. I wonder if one of these would work on a phase leg?

If you decide to dyno again, try to find one that has the ability to manually input rpm and make sure the operator knows how to do it.

[muehlpower]

There are other points to consider when testing an electric motor. Dynos are primarily built for ICE engines. In order to measure this in a comparable way, the air pressure and the air temperature are measured and the determined performance is corrected to sea level and 20°C. In addition, the transmission losses are determined in a coasting test and subtracted to get the crankshaft performance.

[Aragorn]

both of my pulls were done from rolling starts, the first one from almost 40 miles per hour, and then I think it held for about 10 miles till 50, and yeah I am using a very small 9 kWh hybrid battery pack, one that I shouldn't even be running around with let alone driven 25 miles to a dyno shop and then tried to do some power pulls, lol

also both of my graphs are just horse power, one is from almost 40 miles per hour the other from about 10 mph


HP Curve.jpg


HP Curve 2.jpg

Ahha your 10mph pull looks much more like i'd expect! You can see the constant torque region now.

If we know tyre size we can calculate wheel torque from the chart.

Your 215/65r17 tyre has a diameter of around 28"/711mm and will do around 721 revs per mile.

At 20mph therefore the wheel is doing (721*20/60) 240rpm.

Its making ~73hp at 20mph/240rpm so we know its making 1597lbft at the tyre. (Power*5252/rpm)

At 30mph its making 110hp, doing the same math we can see that again its making similar torque (1604lbft)

Thereafter it transitions into power limiting and torque starts to fall off.

We also know your rear diff ratio, 3.73 and the 1.9 reduction in the trans, so we can calculate motor torque.

1575/3.73/1.9 gives us around 222lbft, or almost exactly 300nm at the motor.

[Gregski]

please allow me to preface this post update with a motivational quote from my wife:

"well if you can't figure it out maybe I will just have to ask my dad"

[pause to allow that to sink in with our reverse engineers on this forum]


here is what I got from the Dynamometer Support Technician

"I wanted to give you the information about the ability to both export Dynojet dyno data and to read external data into Power Core. As of version 2.3 we have implemented the use of the JETDRIVE library to allow for two-way communication with other software. If this other software "listens or broadcasts" using the JETDRIVE standard, Power Core will be able to share and receive data.

This is the JETDRIVE library – this should contain everything someone would need to implement JETDRIVE support in their software. There’s a DOCX file in the repository that describes the standard fully as well."

https://bitbucket.org/DynojetRDTeam/jetdrive-sharp/