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Discussion Starter #1
I tried the Car Scanner app a little while ago but wasn't convinced over what it was reading since the motor speed seemed high for any given road speed.
The brochure states peak power is at 8,000 rpm and on an ICE that would normally equate to the top speed which in the eNiro's case is 104mph.
However, that doesn't seem to be so.
The tyres on the car are 215/55/17 which need 768 rotations/mile. The gear ratio is 2.263 and the final drive reduction is 3.526. This means that 1,000 rpm on the motor translates to 9.8mph road speed. This matches what I have seen, allowing for the error in the speedo reading, in Car Scanner. Sorry Car Scanner, you were right all along!
So, if you get up to the top speed in the car then the motor is running at 10,622rpm and the peak torque (0-3,600rpm) is low down in the speed range, I wonder if anyone has seen a graph of torque and power for the eNiro - like the sort of thing you get for ICE cars?
Are the power and torque curves much flatter (linear) on an electric motor so the figures for the peaks are just given like they are because that is what people are used to?

Definitely a black hole in my knowledge here!
 

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I tried the Car Scanner app a little while ago but wasn't convinced over what it was reading since the motor speed seemed high for any given road speed.
The brochure states peak power is at 8,000 rpm and on an ICE that would normally equate to the top speed which in the eNiro's case is 104mph.
However, that doesn't seem to be so.
The tyres on the car are 215/55/17 which need 768 rotations/mile. The gear ratio is 2.263 and the final drive reduction is 3.526. This means that 1,000 rpm on the motor translates to 9.8mph road speed. This matches what I have seen, allowing for the error in the speedo reading, in Car Scanner. Sorry Car Scanner, you were right all along!
So, if you get up to the top speed in the car then the motor is running at 10,622rpm and the peak torque (0-3,600rpm) is low down in the speed range, I wonder if anyone has seen a graph of torque and power for the eNiro - like the sort of thing you get for ICE cars?
Are the power and torque curves much flatter (linear) on an electric motor so the figures for the peaks are just given like they are because that is what people are used to?

Definitely a black hole in my knowledge here!
Can't find anyone who's done a dyno test, but broadly speaking the torque is level and then drops fairly linearly above a threshold.
Closest example I can easily find is the graph from the Volt here: Road tests – Electric Vehicle Wiki (click "road test").
The torque at lower speeds is also reduced as the car is traction limited.
From feel, you don't get full torque until about 25mph. The leaf certainly did the same thing too.
The top speed is restricted. Definitely won't be lack of power.
Likely mainly to prevent overheating.

The numbers you quote sound right but where did you source them?
It would be nice to see what the curves are like across the eco/normal/sport modes.
 

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Top speed of any vehicle is rarely the torque peak or the horsepower peak, but the point where the wind resistance overcomes the available power.

Greg
 

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Top speed of any vehicle is rarely the torque peak or the horsepower peak, but the point where the wind resistance overcomes the available power.
That used to be the case on ICE vehicles prior to electronic speed limiters, and the top gear would typically have the right ratio to reach the top speed at or near the peak power output point of the engine to maximise the advertised top speed... but on most high performance ICE vehicles these days the top speed is electronically limited (155mph) rather than being set by running out of power.

In the case of BEV's, pretty much all BEV's are electronically speed limited and still have quite a bit more power in reserve than necessary to reach their "top speed". The top speed is based on motor RPM as the motor is spinning pretty fast at top speed - typically between 8,000 to 12,000 rpm depending on car, but a few (Tesla I think) go as high as 18,000 rpm.

Apart from limiting the centrifugal forces that would cause the rotor in the motor to fly apart at excessive speed, the efficiency and power output of some types of electric motor start to fall off above a certain rpm, and that sets a usable maximum rpm. Top speed is then determined by this maximum rpm and the gearing chosen. A higher ratio will give a higher top speed but will reduce low speed torque so a balance between off the line performance and top speed has to be reached.

This is why most low to medium power BEV's have relatively low top speeds compared to similarly powerful ICE vehicles - they choose a lower gearing that gives more acceleration at lower speeds at the expense of limiting top speed.

For example the Kia e-Niro has 201 bhp and a top speed of 104 mph while my old Xantia V6 petrol has 194 bhp and a top speed of 140mph...(which being an old car with no speed limiter is indeed limited by the power needed to fight drag as you describe)

You can test whether an EV is power limited or speed limited quite easily - just drive up a moderately steep hill and see if you can reach the same top speed as you can on the flat - if you can the speed is electronically limited because if power was the limiting factor you would not be able to reach the top speed up a hill like you could on the flat.

Even my old Peugeot Ion which was 66hp with a top speed of 83mph could reach the same 83mph up a fairly steep hill - it took a little bit longer to get there but it could make it relatively easily. This wouldn't be the case if 66hp was the limiting factor, so it clearly has quite a bit more power than it needs to reach that speed on the flat.

As for typical torque and power curves of an ICE vs electric motor, this article has a couple of representative graphs:

Typical ICE: (of course the curves vary a lot from one engine to another, this one looks like a petrol engine)

134046


EV:

134047


So in the case of an EV it's usually constant torque up to some road speed (about 40mph seems common, but it depends on gearing) at which point peak power is reached, then it's constant power and falling torque from that point. Out beyond 8000rpm or so depending on the motor the power will actually drop a small amount, maybe 10-20%. (Not shown in this graph)

It's hard to directly compare the curves of an ICE and a EV however because with a single gear ratio the EV's rpm curve directly maps to road speed, whereas in the ICE case the curves of the engine are repeated multiple times, once for each gear and form a "composite" power and torque curve with lumps in it where each gear change occurs.

It's also difficult to directly compare the peak torque of an EV with an ICE - in both cases it is refering to motor torque not wheel torque unless otherwise stated, however without knowing the gear ratios the torque figure comparison is a little meaningless especially when the EV has one gear ratio and the ICE has multiple ratios available.
 

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Responding to the OP and Greg, in the case of an EV the top speed may be limited more suddenly than wind resistance by electromechanical factors, in particular pack voltage v.s back EMF, or simply mechanical containment of the rotating mass.
Electric motors normally have a precisely-defined working envelope and the vehicle will be geared to exploit that as best as possible, noting that electric motor efficiency tends to be far more consistent than an ICE over the working range so "lower" single-ratio gearing is practical. There may be variations in the nominal continuous performance curve based on short-term thermal considerations, e.g. a higher than normal torque or power available for a few seconds.
The eNiro has a drive ratio of 8.206 according to published specs (noting that the Kona EV is 7.891). Other EVs are similar and most motors will reach or well-exceed 10,000 rpm at top speed.
An owner over at InsideEVs posted this power curve for a Kona. You can see the constant torque ramp (based on maximum current) and the flat constant power region, generally thermally limited.
The dip in the middle is likely an artifact due to use of a chassis dyno.

134048
 

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The eNiro has a drive ratio of 8.206 according to published specs (noting that the Kona EV is 7.891). Other EVs are similar and most motors will reach or well-exceed 10,000 rpm at top speed.
I'm surprised to find the Kona and e-Niro have such different ratios. Do they have the same rolling radius for the tyres ?
An owner over at InsideEVs posted this power curve for a Kona. You can see the constant torque ramp (based on maximum current) and the flat constant power region, generally thermally limited.
The dip in the middle is likely an artifact due to use of a chassis dyno.

View attachment 134048
That dip is not a measurement artifact - that's typical of a permanent magnet syncronous motor working near it's maximum usable RPM. Power output and efficiency start to drop off a bit near the top. A relatively small drop off compared to what you'd see in most ICE engines above their peak power point though.
 

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Sorry, I thought the case of a rev/top speed limiter was obvious.

Greg
Indeed, but it's not what you said was it...

Almost without exception (Twizzy maybe ?) all EV's have an electronically speed limited top speed, and aren't limited by power vs drag. Not even close in most cases. (An e-Niro has the power to go well beyond 140mph, but its gearing and motor rpm simply wouldn't allow it)

So indeed not exactly at the horsepower peak....
In a BEV, no, because there is a small sag in power at the top end. But in the case of old ICE vehicles without speed limiters, it was actually very common for top gear to be geared precisely to optimise the top speed of the vehicle (for bragging rights) by ensuring that peak power was available at the precise road speed where that power just overcame drag. Gear the car any higher or lower and the top speed would actually drop as you'd slide down either side of the power curve peak...
 

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I have 2 ICE cars, and they are both limited below the top speed of the vehicle, so I also do not concur in this claim. Both cars will run up to the limiter, and both are not exactly at the horsepower peaks. In my case, the manufacturers are not worried about optimizing the top speed or bragging rights, they certainly go fast enough.

Instead of trying to make overly broad/general "rules" about cars, perhaps it would be more interesting to confine ourselves to our specific vehicles, your charts are nice hard data.

I liked seeing the hard data about the electric motors, and interestingly enough, the slight flattening of the power....

Can you explain why the horsepower goes perfectly flat as the torque drops off, your second chart... something does not make sense there...

Greg
 

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And that flat power curve doesn't have the slight notch seen in the measured dyno curve. Because the chassis dyno curve is done dynamically and in a matter of seconds, I think the notch (but not the rolloff at the end) is the result of running through a system torsional resonance.
 

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Can you explain why the horsepower goes perfectly flat as the torque drops off, your second chart... something does not make sense there...
Power is electronically limited in the constant power region of an EV. Simple as that really.

The motor, battery and drive inverter all have maximum continous power dissipation ratings that set a safe operating area which the system has to remain within.

Since power is speed x torque, at low speeds the maximum power can't be reached without exceeding maximum torque (which is set by other factors) hence you have the constant torque, climbing power region, followed by the constant power falling torque region.

Something that's very important to remember about electric motors vs combustion engines is that there isn't really any hard limit on the instantaneous power or torque that an electric motor can produce.

In an ICE (especially a naturally aspirated one) it's usually the case that for a given state of tune the engine simply can't produce any more power than a certain amount, even at full throttle, and this is usually well below the point where the engine will self destruct. The power curve of an ICE shows the maximum power the engine can produce at full throttle. To get more power you have to change the tune of the engine by physically modifying it. (Add turbo, intercoolers, all kinds of other things)

In the case of an electric motor it does't generate power, it simply converts electrical power into mechanical power - the job of converting chemical power into electrical power is in a separate device - the battery. If you feed the motor more power it will output more power and torque - right up until the point where it overheats or disintegrates.

So an electric motor rated to produce 100hp for 10+ years can almost certainly produce 200hp for a short time, assuming something mechanical like a drive shaft doesn't break, but over the longer term it may overheat and ultimately fail prematurely.

So power and torque are both electronically capped in EV's to levels that the manufacturer deems safe and which should provide many years of trouble free operation from the motor. (And battery!) That electronic capping of torque and power to maximum values is what gives you that characteristic flat torque/power curve of an electric motor.

But the motors are always capable of more - sometimes a lot more, for short periods, as most EV motors have a large performance headroom - and as discussed in another thread recently for low to medium performance EV's I would say that the maximum power output is usually limited by the battery not the actual motor.

Take the Leaf as an example - the 24/30kWh models only have an 80kW motor, while the 40kWh model has a 120kW motor and the 62kWh model has 150kW motor. The motors in the different models of Leaf aren't very different if at all - the difference is the larger battery is capable of producing more sustained power without overheating.

Electric motors are truely remarkable and at the current point in time they are limited largely by the performance of the batteries which have to supply them. As battery tech improves expect to see the performance of the average EV go up even higher.

Unlike an ICE where a much higher power output means a more expensive, more complicated engine (more cylinders, valves, extra addons like turbos etc) with a much worse efficiency at low power (poor MPG) there isn't really any penalty for a higher power motor in an EV - it's not significantly bigger, its no more complex and it's usually still very efficient at low power levels. At most the cooling system might need beefing up slightly. So if the battery can supply more power, why not ?
 

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Electric motors generally have constant (maximum) torque up to a certain rotor speed, hence they have maximum torque at 0 speed.

This region is called the 'linear V/Hz' region, because every volt that goes in is worth a certain number of rpm. But the 'load' is variable as the motor increases, and the current in is roughly constant (even thought the voltage is changing), and the torque is roughly linear to current.

At some point, above which volts in and rpm out no longer match linearly, either, or multiply;
  • the back EMF from the motor exceeds the input, and/or
  • the power the motor can withstand is exceded, and/or
  • the power the drive electronics can withstand is exceeded
then the system reverts from 'constant torque' to 'constant power' and can then generate that power upto its maximum speed, which is again limited by the electronics and/or the motor.

The numbers usually quoted for max torque and max power are the same speed, and it is the point of transition above linear V/Hz. It is not a 'peak', but the language has been co-opted from ICE because that's what people are used to seeing.

So you'll probably find the peak speed is usually 2 or 3 times higher than the quoted 'max power' entry in the consumer literature.

HTH.
 

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At some point, above which volts in and rpm out no longer match linearly, either, or multiply;
  • the back EMF from the motor exceeds the input, and/or
  • the power the motor can withstand is exceded, and/or
  • the power the drive electronics can withstand is exceeded
Or the power the battery can safely produce for a prolonged period of time is exceeded. Especially batteries which have a small capacity (leading to a high C rate) and/or are not actively cooled.

For power handling/output I don't think the motor is the limiting factor on many BEV's and has plenty of headroom above that which the battery can provide for prolonged periods of time. So the maximum power in the constant power region is often dictated by what the battery can handle.

When was the last time we heard of the motor in a BEV failing ? Almost never. If you check the coolant temperature in a BEV for the coolant loop that runs through the motor (and inverter) it's usually very low temperature (low 40's) even after a drive on the motorway. The cooling system for the motor is barely ticking over unless it's a high performance monster.
then the system reverts from 'constant torque' to 'constant power' and can then generate that power upto its maximum speed, which is again limited by the electronics and/or the motor.

The numbers usually quoted for max torque and max power are the same speed, and it is the point of transition above linear V/Hz. It is not a 'peak', but the language has been co-opted from ICE because that's what people are used to seeing.
This wide flat torque and power curve is important - people looking at headline numbers like "peak torque" and "peak power" trying to draw comparisons between ICE and EV are going to be in for a surprise. In an ICE it literally is a narrow peak in most engines (as in my example image) while in BEV's it's a wide flat region with peak torque and peak power usually meeting in the middle.

So for a given "peak" power/torque an electric motor is going to out accelerate an ICE not just because there isn't the delay of gear changing, but largely because the motor is either at peak torque or peak power the whole time if your foot is flat to the floor. Not the case for an ICE where you only get peak power as you climb to the top of each gear. The area under the curve is much greater for the same peak values with a BEV.
 

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Discussion Starter #15
Just picking up on the comment from MetalHead "The numbers you quote sound right but where did you source them?" then I got the info for gearing from the Kia eNiro brochure. These two values give a drive ratio of 7.979 which is very similar to the Kona. The tyre data was taken from the web but I did do a sanity check by measuring the diameter of the tyre!

Thanks for all the input guys on electric motors and in particular, the positive upbeat information that there is potentially more power without penalty to come once the battery/electric source improves. .
 

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Or the power the battery can safely produce for a prolonged period of time is exceeded. Especially batteries which have a small capacity (leading to a high C rate) and/or are not actively cooled.

For power handling/output I don't think the motor is the limiting factor on many BEV's and has plenty of headroom above that which the battery can provide for prolonged periods of time. So the maximum power in the constant power region is often dictated by what the battery can handle.
Quite so.

But key is 'some'.

In the 'early days' the most that would fit and could be afforded was around the 20 kWh mark. Limiting output to 4~5C makes 80~100kW and this was perfectly acceptable for a passenger car, no-one wants to be making enormously powerful inverters.

So in those days, yes, limited to battery, then the motor was sized to match the inverter of the power the battery could deliver.

Now, I am not so sure. A 60kWh battery is good for 250kW, way more than is sensible for a FWD, so they put in a 'slightly bigger' 120kW inverter or so, does the job, nice increment, doesn't stress the battery, and size the motor accordingly.

What they will then do is specify the motor for 120kW for 60 seconds, so in effect it is governed by the power of the inverter, but real-world limits end up being on the motor overheating.

.... and they will overheat if you run a typical EV motor at 120kW for more than a minute.

When was the last time we heard of the motor in a BEV failing ? Almost never. If you check the coolant temperature in a BEV for the coolant loop that runs through the motor (and inverter) it's usually very low temperature (low 40's) even after a drive on the motorway. The cooling system for the motor is barely ticking over unless it's a high performance monster.
It is not so much the net temperature but local heating in the centre of the coils, the thermal impedance is not super good to transmit the 10kW or so of thermal flux efficiently out of a small compact motor. There are limits.

So not so much the total temperature of the coolant, but of course, you know this, the coolant temperature has to have the biggest temperature difference with the heat source to maximise the thermal transfer.

In an ICE, the difference is a 200~400C head/liner, with 80C water. If the coolant was the same temperature as the thing it is trying to cool, then it won't cool! So the centre of the motor coils are probably up at 150C or so peak and the coolant at 40~60C. The polyamide magnet wire coating begins to degrade over time over that, although it is capable of handling 300C in short goes, such temperatures if allowed to saturate the motor would conduct into the magnets (if a PM machine) which will demagnetise if they get too hot (over Curie temperature).
 
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