Author Topic: Electric Car Experiences  (Read 14965 times)

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Online nctnico

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Re: Electric Car Experiences
« Reply #175 on: August 20, 2019, 05:39:27 pm »
But in the end it is the amount of power the motor can output which determines how fast it can accellerate. Torque is only interesting for dimensioning the gears. A gearbox is just like a impedance adaption transformer. It maximises the power transfer. Also if you have a motor with a constant power output then the torque will decrease with increasing RPM. That is very counter intuitive because more is usually better.
Read what ahbushnell wrote. When the car is moving slowly it takes a huge amount of torque for even a small amount of power to be applied to accelerating the car. Any real world motor has a maximum torque that it can produce, and this limits the power it can apply to accelerating the car, and therefore limits the acceleration.
No. Taking off is a limit situation which is bridged by the clutch or torque converter. Once the clutch or torque converter are fully engaged it is all about power. If you keep talking about torque then you are severely fooling yourself. Look at the formulas and imagine an engine with a constant power output (my previous car had a flat spot in the rpm/power curve where the torque becomes lower with increasing RPM). I can prove mathematically that the accelleration is faster when staying in the maximum power output range rather than switching gears to get back into the maximum torque area.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline tom66

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Re: Electric Car Experiences
« Reply #176 on: August 20, 2019, 05:54:10 pm »
No. Taking off is a limit situation which is bridged by the clutch or torque converter. Once the clutch or torque converter are fully engaged it is all about power. If you keep talking about torque then you are severely fooling yourself. Look at the formulas and imagine an engine with a constant power output (my previous car had a flat spot in the rpm/power curve where the torque becomes lower with increasing RPM). I can prove mathematically that the accelleration is faster when staying in the maximum power output range rather than switching gears to get back into the maximum torque area.

Well, duh. Of course it is faster if an engine produces a fixed 100 hp. 

But the reality is for the *vast majority* of internal combustion engine cars, it is not possible to get fixed power output at any given vehicular speed because the gearbox will never be able to keep the engine there.  The exception is a CVT gearbox, which can probably most closely replicate the performance of an EV with an ICE.  CVTs are unusual, though Honda and Nissan do make some, and the Prius has an e-CVT planetary drive set which replicates the performance of a CVT but uses two electric motors instead of a belt and cone drive.

In most EVs, the torque available from near zero rpm is full, limited only by the traction control system and the gearbox.  So, say from 2mph upwards, the car can output full torque.

At the point of maximum power (when RPM * Torque = Pmax) the torque begins to ramp off.  Attached graph shows this behaviour.  Depending on the EV this is somewhere between 30 and 50 mph.  For the Model S RWD85 it is around 45 mph. I believe for the P100D it is a bit lower because the vehicle is limited to 1300A from the battery. The power then remains constant until the Vmax of the motor is reached, typically this is due to commutation limits or sometimes just software.
 


Compared to an ICE vehicle, you would need to be at an RPM of ~2500-3500rpm to obtain maximum torque.  So if you want to gain maximum torque on launch, you need a system like launch control, which holds the clutch just at bite point for longer so the engine does not stall as the car launches.  This is of course terrible for the longevity of the clutch packs and in many owners manuals the vehicle manufacturer recommends avoiding frequent launches.  But, the option is there.

EVs have this same launch characteristic (minus the slightly lower power output typically) without risking shredding the clutch plates.  BUT, ultimately, it does not matter. Until you hit the peak power rpm of an engine,  you don't get that peak power. So the initial launch is always torque limited, and therefore most EVs will do better than most ICE vehicles on launch.

My GTE has a "psuedo-launch-control" system.  Pressing both the brake and accelerator when stopped when in the sport-GTE mode will start the engine and run it for 30sec.  The car will still launch with just the e-motor, then at sufficient speed clutch in the engine, around 2000rpm and combine the power output of the two systems. The net effect is that the total system power of the car (~220 hp) is approximately fixed until 100 mph. The electric motor applies an opposing power curve to compensate for the engine's varying power output across the rev range. 

Ultimately, torque is what moves you.  That is the force in F=ma.  Power is a necessary expenditure because you are moving a heavy car up to some speed, with air resistance and friction. 

I do ask ... how many EVs have you driven?  I have driven probably about 10 models, and ALL of them launch better than any ICE vehicle I have driven.
« Last Edit: August 20, 2019, 06:06:27 pm by tom66 »
 

Offline ahbushnell

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Re: Electric Car Experiences
« Reply #177 on: August 20, 2019, 06:25:08 pm »
But in the end it is the amount of power the motor can output which determines how fast it can accellerate. Torque is only interesting for dimensioning the gears. A gearbox is just like a impedance adaption transformer. It maximises the power transfer. Also if you have a motor with a constant power output then the torque will decrease with increasing RPM. That is very counter intuitive because more is usually better.

The point isn't that EV's have more torque, the e-Golf has approximately the same torque figure as a 2.0L petrol TSI engine.

The point is that an EV can deliver the full torque to the motor shaft from zero rpm, without a clutch or torque-converter slipping to regulate speed, and without having to be at a high-rpm.  Most petrol engines deliver peak torque around 3000 rpm, which means that for first gear in a regular car, you need to be around 15 mph before the vehicle is maximising its torque capability.

One side effect of this if is you have ever tried to park a car with an automatic gearbox.  It is a lot harder in my car to park (torque control is worse so creeping into my short driveway is hard) when the EV battery is "flat" and the car is using the engine exclusively for drive functions. When in the electric mode, I can creep forward less than 1cm at a time because the clutches are fully engaged and there is no need to slip anything.

Climbing up a hill is also interesting, at low speeds.  The "power meter" in the GTE will go to about 30%, which would indicate that the car is pulling 25kW or so, but the actual power consumption is around 3-4kW. (At low speeds the power meter seems to map to torque instead, perhaps to give a consistent appearance.) Most ICE vehicles to climb a hill need to run the engine quite aggressively in a low gear to get the torque required, but my car will climb almost anything in 4th or 5th gear in E-mode, and burns relatively few electrons doing this.
Sorry but this is utter nonsense from a physics point of view. It doesn't make any sense at all.
A permanent magnet motor can generate it's full torque at zero speed.  Not so a combustion engine.

That's a well known fact.  Look it up on a book on electric motors. 
 

Offline james_s

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Re: Electric Car Experiences
« Reply #178 on: August 20, 2019, 08:22:37 pm »
Is it too much to ask that people who have never even driven an EV refrain from posting in an "Electric Car Experiences" thread? I mean if you don't have any direct experiences, then what exactly can you contribute?
 
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Online nctnico

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Re: Electric Car Experiences
« Reply #179 on: August 20, 2019, 08:23:08 pm »
and ALL of them launch better than any ICE vehicle I have driven.
Because they have more power at low RPM but that doesn't really matter. In order to move a mass you need to supply energy. Power is energy per time unit. Sure you can derive torque but your graph shows precisely why speaking about torque is so bad. Usually more is better but at a certain point the torque starts to drop and suddenly less gets better?? How does that match climbing uphill needing more torque when you are in the area where the torque is declining? Even worse, you can fit a different motor with half the torque and the same power rating and still get exactly the same accelleration. Only this motor will run at double the RPM and the gear ratio needs to be changed as well.

And there is more to it. At the research institute where I used to work they did tests to determine how people perceive accelleration of a car. Interestingly the fastest accelleration isn't perceived as fast at all. A constant torque (and thus increasing power) is perceived as a faster accelleration than using constant (maximum) power. That is why most car motors/engines are controlled to have a constant torque (the graph you posted is a nice example). You can try it yourself if the engine in your car has such a characteristic. When the maximum power is reached it will feel like a dud when you want to accellerate further. In an ICE (usually with a turbo-charged engine) this effect will tempt you to change gears but in reality you are making things worse.
« Last Edit: August 20, 2019, 08:35:32 pm by nctnico »
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Offline boffin

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Re: Electric Car Experiences
« Reply #180 on: August 20, 2019, 08:33:32 pm »
Is it too much to ask that people who have never even driven an EV refrain from posting in an "Electric Car Experiences" thread? I mean if you don't have any direct experiences, then what exactly can you contribute?

As the OP of this thread, I would ask people to try and stay on topic.
 

Offline sokoloff

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Re: Electric Car Experiences
« Reply #181 on: August 20, 2019, 08:33:54 pm »
Is it too much to ask that people who have never even driven an EV refrain from posting in an "Electric Car Experiences" thread? I mean if you don't have any direct experiences, then what exactly can you contribute?
Though I daily drive a LEAF, I think there are people who've never driven an electric who can contribute their own relevant points of view in reaction to range anxiety, TCO, purchase price reaction, etc.

It's extraordinarily rare that an internet forum thread stays singularly focused and narrowly scoped to the subject line of the thread.
 

Offline james_s

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Re: Electric Car Experiences
« Reply #182 on: August 20, 2019, 08:50:05 pm »
I'm not saying a topic can't wander, I mean I'm certainly guilty of that myself. I just find the constant posts ripping on EVs with baseless arguments, previously debunked myths, deflect, deflect, move the goalpost, wash, rinse, repeat from a person or persons who have never even driven one to be rather tiresome and predictable. 

And on the topic of experiences, while I haven't owned an EV, I have driven a Meyers NMG, Nissan Leaf, Chevy Spark and a Kia Soul EV. The NMG did not impress me greatly but it was novel at the time as one of the first highway capable EVs. The Leaf felt remarkably like any other modern compact car I've driven. The Spark is crazy fast, at least off the line is feels quicker than the BMW M3 I drove a few times years ago. The Soul is similar to the Leaf in feel, not exciting to drive but smooth and quiet and a lot quicker and more responsive than a Prius. With exception of the NMG, the friends and family members with the other EVs are still driving them after several years and still rave about them. All are used for commuting and driving around town, picking up groceries, dropping off and picking up kids, etc. I can only roll my eyes when someone starts listing reasons EVs don't work when so many people are somehow able to manage with them just fine.
 

Offline tom66

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Re: Electric Car Experiences
« Reply #183 on: August 20, 2019, 09:20:11 pm »
Because they have more power at low RPM but that doesn't really matter.

I think you may misunderstand something here, an electric motor generally has very high torque at low RPM but power is determined by the torque and speed of the motor, so power output at low RPM is low by definition.

? How does that match climbing uphill needing more torque when you are in the area where the torque is declining?

If you are accelerating up hill in a car that is in the power-limited region you will find your acceleration is limited by the need to put more energy in to climb the hill. Power rating is the most meaningful figure as for most intents and purposes at speed you will be above the constant torque region and in the constant power region. Therefore more power means more acceleration, for the same given weight.

In my car I can use the full 85kW to climb a steep hill at 70 mph.  The battery depletes rapidly and the power limit starts kicking in.  It is enough to get to the end (4 miles) without any problem but I expend over 4kWh doing that.  But that is fine - the car was always designed as a short range PHEV so you will never really be able to overheat the car for any meaningful climb (I have tried it on a very long climb, once the power limit gets to 50%, the engine starts automatically.)  A more efficient vehicle with a higher power motor and a larger battery will have no problem accelerating up hills, nobody complains about a Tesla overheating when driving up steep 3000ft highway climbs at 70 mph. Of course the range is poor, but an internal combustion engine will have similar drop in fuel economy.  At least the EV going down hill will get good economy because the e-motor can act as a generator. 

I took a different route home today and used the motorway. I sat in the slow lane at 60 mph with the adaptive cruise on, and still achieved 15kWh/100km, with the car regening nicely down the steep hills and putting a bit of power out up the hill.  I managed to achieve the rated mileage of the car, and with adaptive cruise, felt almost no stress compared to trying to weave in and out of lanes at 70-80 mph. But, perhaps the electric car "economy bug" is getting to me.  I keep trying to achieve better than that figure.  I suspect in winter, with winter tyres on,thicker & colder air, and the heating running, I will struggle to get as good as I do in summer, but it will be interesting to watch.  I want a really cold winter this year so I can test that.

Even worse, you can fit a different motor with half the torque and the same power rating and still get exactly the same accelleration. Only this motor will run at double the RPM and the gear ratio needs to be changed as well.

I'm not sure what your point is here.  If you change the gear ratio, you change the torque. All torque figures are given after any gearbox effects. Unless I am misunderstanding you?

And there is more to it. At the research institute where I used to work they did tests to determine how people perceive accelleration of a car. Interestingly the fastest accelleration isn't perceived as fast at all. A constant torque (and thus increasing power) is perceived as a faster accelleration than using constant (maximum) power. That is why most car motors/engines are controlled to have a constant torque (the graph you posted is a nice example). You can try it yourself if the engine in your car has such a characteristic. When the maximum power is reached it will feel like a dud when you want to accellerate further. In an ICE (usually with a turbo-charged engine) this effect will tempt you to change gears but in reality you are making things worse.

An ICE will have low torque at low RPM.  Almost by definition it cannot have a constant torque region.  Most ICE vehicles with auto gearboxes (in acceleration or sport profile) are keeping the engine in a high power region, where power fluctuates about 10-20% over the RPM range between each gear. The GTE compensates for this fluctuation by having the e-motor provide less assistance at higher engine RPMs; this means the output power is roughly constant over the RPM range, 2000-6200rpm. But the GTE is unusual in this design, most ICE hybrids are not designed for performance, and most ICE vehicles are not hybrids...
« Last Edit: August 20, 2019, 09:32:22 pm by tom66 »
 

Online nctnico

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Re: Electric Car Experiences
« Reply #184 on: August 20, 2019, 09:40:47 pm »
I'm not sure what your point is here.  If you change the gear ratio, you change the torque. All torque figures are given after any gearbox effects. Unless I am misunderstanding you?
It is my understanding that with cars in general the torque figure is that of the motor/engine and not of what gets on the wheels. Or at least any torque/power graph I've seen so far shows the torque of the motor/engine and not what ends up on the wheels. I think your graph also shows engine torque and not torque on the wheel. A long time ago I calculated what kind of torque you can produce on the wheels in 1st gear. I ended up with a crazy number like 1000Nm or something like that for a relatively small ICE engine. So around 400Nm seems a low-ish number to me to drive the wheels. If I assume the graph is for a Model S with wheels with a 25" diameter then I get to a power output of about 100kW at 50mph (2400 RPM) based on the torque so it seems the graph shows the engine torque and not the torque on the wheels. Again, you can make the motor/engine torque anything you want. As long as the power output stays the same, the accelleration will be the same.
« Last Edit: August 20, 2019, 10:17:54 pm by nctnico »
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Online dr.diesel

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Re: Electric Car Experiences
« Reply #185 on: August 21, 2019, 01:07:26 am »
It is my understanding that with cars in general the torque figure is that of the motor/engine and not of what gets on the wheels.

In the US anyhow, this is correct, all power figures are at the flywheel, not wheels.

Offline james_s

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Re: Electric Car Experiences
« Reply #186 on: August 21, 2019, 01:38:47 am »
The power figures are at the flywheel, but obviously what you feel is the power at the wheels. I'm not sure what difference this makes though, ultimately specs only tell you part of the story anyway, to get the rest you have to evaluate the car as a whole.

To me the EVs I've driven certainly feel quicker and more responsive than comparable ICE cars. That instant response and tremendous low end torque makes them really leap from a stop.
 

Offline bdunham7

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Re: Electric Car Experiences
« Reply #187 on: August 21, 2019, 04:05:31 am »
The power figures are at the flywheel, but obviously what you feel is the power at the wheels. I'm not sure what difference this makes though, ultimately specs only tell you part of the story anyway, to get the rest you have to evaluate the car as a whole.

To me the EVs I've driven certainly feel quicker and more responsive than comparable ICE cars. That instant response and tremendous low end torque makes them really leap from a stop.

My EV is torque-limited or torque-reduced up to 10-15MPH and then it takes off.  I have 3 rather ordinary cars, and of the 3 the EV definitely feels the quickest, but by the numbers it is the slowest.  It seems to be programmed so that if you put your foot down 1/4 of the way it feels strong, but if you go further there's nothing else left.
 

Online Jeroen3

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Re: Electric Car Experiences
« Reply #188 on: August 21, 2019, 07:31:36 am »
Your discussion is going straight past each other saying the same things.
Yes an EV is limited by torque. But only because the motor is limited by current, or because the drivetrain/tires can't take the torque.
Torque is force without work. If you add work before speed the motor burns, so you limit torque current.

In an ICE you have to wait for explosions to get torque. Torque is a derivative of the explosion. Yet only many explosions support lots of work. If you add work before speed you stall it. Same problem.

An EV doesn't need to wait until enough explosions are happening to do work. Thus it's faster and better.

It seems to be programmed so that if you put your foot down 1/4 of the way it feels strong, but if you go further there's nothing else left.
It's probably limited for safety. Otherwise you'd be going 80 kmh/50 mph before the end of the intersection. You should enable "ludicrous" mode if you have it.  :P

My hybrid is extremely laggy when the battery is low. It needs 5000 rpm just to drive off.
 

Online nctnico

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Re: Electric Car Experiences
« Reply #189 on: August 21, 2019, 10:31:36 am »
The power figures are at the flywheel, but obviously what you feel is the power at the wheels. I'm not sure what difference this makes though, ultimately specs only tell you part of the story anyway, to get the rest you have to evaluate the car as a whole.

To me the EVs I've driven certainly feel quicker and more responsive than comparable ICE cars. That instant response and tremendous low end torque makes them really leap from a stop.
Either way be aware that motors/engines in a car are setup to fool your senses.

Still I think that an electric motor has another advantage over an ICE engine. On an electric motor it is easier to control the traction. IMHO this also helps to get a car quicker from it's place. I recall reading about diesel locomotives in which the diesel engine is not driving the wheels directly but a generator which in turn drives an electric motor because an electric motor has easier/better control over traction.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline tom66

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Re: Electric Car Experiences
« Reply #190 on: August 21, 2019, 11:22:23 am »
Still I think that an electric motor has another advantage over an ICE engine. On an electric motor it is easier to control the traction. IMHO this also helps to get a car quicker from it's place. I recall reading about diesel locomotives in which the diesel engine is not driving the wheels directly but a generator which in turn drives an electric motor because an electric motor has easier/better control over traction.

The traction control response is much better in EVs. Depending on the vehicle of course.

Most ICE vehicles traction control just interrupts spark/fuel or otherwise tells the ECU to not supply power for that revolution. So the power is "choppy".  Some newer cars can modulate the power better but are limited to the response rate of the engine.

The GTE does not have spectacularly great traction control (can judder on a start), but the Model 3 is fantastic.

Diesel locomotives have been using the setup you describe for quite some time.  Braking is often rheostatic, and different speeds are utilised by selecting taps on either the motor or generator. Some vehicles are hybrid, using the 25kV overhead line and a diesel generator to supplement this (or running the diesel engine on sections without overhead lines or between points.)
 
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Offline SilverSolder

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Re: Electric Car Experiences
« Reply #191 on: August 21, 2019, 11:58:48 am »

The traction control response is much better in EVs. [...]


Even under manual control, I have found electric traction motors (on a hybrid in my case) are much better for driving in snow -  it is just so much easier to get the force "just right" to move the car without spinning the wheels.  Even if you do manage to get stuck, electric is far easier to control to get you out of the hole - you can do the "rocking" trick starting with just a few mm movement and eventually get out.  This can be done with an ICE too, of course, but it is not as easy to control "just so" as with electric drive.
 

Offline tom66

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Re: Electric Car Experiences
« Reply #192 on: August 21, 2019, 12:16:49 pm »
Yeah, 100% agreed there.  The electric mode on my car can creep at very low speeds precisely. I can creep up a 1 in 3 hill at less than 1 mph and the car feels perfectly controllable, try doing that in a manual or automatic ICE and it is nearly impossible. 
 

Offline mcf12

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Re: Electric Car Experiences
« Reply #193 on: September 01, 2019, 03:13:26 pm »
I drive a BMW i3, love it. perfect metro commuter car. simple tweaks like wheel spacers dramatically improve handing over stock.

And I also convert old gas motorcycles to electric, which is crazy fun. You can see more
here: http://nightshiftbikes.com
and
here https://www.instagram.com/nightshiftbikes/

Happy to answer any questions. Much to learn....
 


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