Car Chat

[Laconian]

Isn't there something like an accelerator pedal that you back off on when you disengaged the clutch.

Right, but electric motors make huge torque at very low speeds, so feathering it without electronic management could still be nasty.

[Soup DeVille]

You'd need logic in the motor which would limit torque output when the clutch is disengaged. Otherwise you'd nuke the clutch every time.

The way I understand it, motor RPM is commanded to be like 200 whenever the clutch is engaged and the accelerator released. I assume there's also an acceleration profile it follows when the clutch is released.

[Soup DeVille]

I wonder if you could do the whole thing without a clutch at all.  Just logic to rev match the electric motors to the gear for each shift so it goes right into gear.

It would be like playing Cruis'n USA or something.  WOAAAH WOAH

Yes. You can just leave it in third the whole time.

[FoMoJo]

Yes. You can just leave it in third the whole time.

Would a battery discharge quicker if kept at a higher ratio/gear?  It seems that it would, but I really don't know if an electric motor running at higher rpms with less drag would use more battery energy than when running at a lower rpm with more drag.

[MX793]

Would a battery discharge quicker if kept at a higher ratio/gear?  It seems that it would, but I really don't know if an electric motor running at higher rpms with less drag would use more battery energy than when running at a lower rpm with more drag.

Comes down to the efficiency curve of the motor.  Higher speed (RPM), lower torque may or may not consume more power.

[Eye of the Tiger]

I wonder if you could do the whole thing without a clutch at all.  Just logic to rev match the electric motors to the gear for each shift so it goes right into gear.

It would be like playing Cruis'n USA or something.  WOAAAH WOAH

Program the motor control to rev match for you.

[Soup DeVille]

Would a battery discharge quicker if kept at a higher ratio/gear?  It seems that it would, but I really don't know if an electric motor running at higher rpms with less drag would use more battery energy than when running at a lower rpm with more drag.

It depends- Maybe.

[CaminoRacer]

Comes down to the efficiency curve of the motor.  Higher speed (RPM), lower torque may or may not consume more power.

EVs really make you re-think how cars work with regards to efficiency, power, etc. There are a lot of assumptions that average people have about vehicles that are only true for ICEs.

Here's an example I've been wondering about - ICE cars use more gasoline to accelerate quickly. Do EVs use more juice? Or does it use a very similar amount of KW, just in a shorter amount of time? If you take the area under the curve how does it compare to drive 0-60 slowly and 0-60 quickly?

https://cleantechnica.com/2018/04/16/how-ev-range-is-affected-by-quick-acceleration/

This guy does an experiment that showed faster acceleration = less range. But I think there are other factors that play into the real world. Over the course of the drive, faster acceleration increases your average speed since you get to your cruising speed faster. That means your average air resistance is also increased over the course of the drive. If you were to isolate just the acceleration periods, I think the energy is the same (or at least very similar).

[FoMoJo]

Under hard acceleration with black smoke pouring out of the tailpipe, one can assume that the efficiency is not very good...IC engines do not work efficiently under hard acceleration though the more modern they are, the better efficiency there is.

EV motors, presumably, work just as efficient under higher a heavier load??

[MX793]

EVs really make you re-think how cars work with regards to efficiency, power, etc. There are a lot of assumptions that average people have about vehicles that are only true for ICEs.

Here's an example I've been wondering about - ICE cars use more gasoline to accelerate quickly. Do EVs use more juice? Or does it use a very similar amount of KW, just in a shorter amount of time? If you take the area under the curve how does it compare to drive 0-60 slowly and 0-60 quickly?

https://cleantechnica.com/2018/04/16/how-ev-range-is-affected-by-quick-acceleration/

This guy does an experiment that showed faster acceleration = less range. But I think there are other factors that play into the real world. Over the course of the drive, faster acceleration increases your average speed since you get to your cruising speed faster. That means your average air resistance is also increased over the course of the drive. If you were to isolate just the acceleration periods, I think the energy is the same (or at least very similar).

In a perfect world without friction or losses, accelerating from 0-60 consumes the same energy whether that takes 3 seconds or 3 minutes.  Real world has losses.  Harder acceleration in an EV means more current, which means more heat and more resistance, thus more losses.

I assumed the question was whether cruising in a lower (shorter) gear consumes more power than a higher (taller) gear in an EV.  In an IC car, higher engine RPM generally means higher fuel consumption.  More RPMs means more air, more air needs more fuel.  EVs aren't bound by stoichiometric air/fuel ratios, so unless the motor loses efficiency with higher RPM, an EV may not consume any more energy cruising in 4th gear instead of OD.

[MX793]

Under hard acceleration with black smoke pouring out of the tailpipe, one can assume that the efficiency is not very good...IC engines do not work efficiently under hard acceleration though the more modern they are, the better efficiency there is.

EV motors, presumably, work just as efficient under higher a heavier load??

ICEs are actually most efficient, in terms of thermal efficiency and BSFC, at WOT.  Fuel mileage isn't great, but you're getting the most out of every bit of fuel.  Some hypermiling strategies revolve around accelerating at WOT (but not necessarily high RPM) and trying to get up to cruising speed relatively quickly.

[CaminoRacer]

In a perfect world without friction or losses, accelerating from 0-60 consumes the same energy whether that takes 3 seconds or 3 minutes.  Real world has losses.  Harder acceleration in an EV means more current, which means more heat and more resistance, thus more losses.

I assumed the question was whether cruising in a lower (shorter) gear consumes more power than a higher (taller) gear in an EV.  In an IC car, higher engine RPM generally means higher fuel consumption.  More RPMs means more air, more air needs more fuel.  EVs aren't bound by stoichiometric air/fuel ratios, so unless the motor loses efficiency with higher RPM, an EV may not consume any more energy cruising in 4th gear instead of OD.

I think this would be a really good college class. Comparing ICE and EV assumptions and strengths/weaknesses on a physics/engineering level (not a fanboy level).

More current to the electric motor will eventually create more heat and resistance, but how long does it take for that to affect efficiency? Will it affect short 0-30 bursts around town or just 0-60+? I'm sure it's a curve, so I wonder where the sweet spot is. And with quicker acceleration, you have higher current for a short time but then it's reduced to cruising level current, where the motor can cool down a bit. With slower acceleration, the peak current is lower but more spread out, and still 2x the cruising current.

For cruising, Porsche Taycans appear to do better with 2nd gear. I don't know what speed it starts to show benefits at, but presumably at normal 70 mph highway speeds. My Bolt's efficiency goes down a lot over 70-75 mph and the car is limited to 92 mph since it's just a single speed. But it's hard to disconnect air resistance from the mileage numbers. I guess you'd have to test it on a treadmill or something.

[Soup DeVille]

Under hard acceleration with black smoke pouring out of the tailpipe, one can assume that the efficiency is not very good...IC engines do not work efficiently under hard acceleration though the more modern they are, the better efficiency there is.

EV motors, presumably, work just as efficient under higher a heavier load??

Actually, a lot of this is wrong.

ICE engines, for the most part; and in terms of energy produced and consumed, are most efficient at Wide Open Throttle and a specific RPM. The problem is that engine efficiency (as defined above) and transportation efficiency (defined as distance traveled and energy consumed) are two different things entirely.

As for electric motors, they are typically most efficient at around 50% of their load. The variable here is what's called back electro motive force, which in simple terms is the difference between where the magnetic field which drives the rotor is, and where the rotor actually is.

Imagine a normal permanent magnet that you use to drag around a paper clip. Right up close there's no problem; but only a few centimeters away its almost impossible. Load on an electric motor effectively increases the distance between the electromagnetic stator and the rotor (our paper clip). At zero load, a motor is doing zero work; at max its working too hard.

[FoMoJo]

Actually, a lot of this is wrong.

ICE engines, for the most part; and in terms of energy produced and consumed, are most efficient at Wide Open Throttle and a specific RPM. The problem is that engine efficiency (as defined above) and transportation efficiency (defined as distance traveled and energy consumed) are two different things entirely.

As for electric motors, they are typically most efficient at around 50% of their load. The variable here is what's called back electro motive force, which in simple terms is the difference between where the magnetic field which drives the rotor is, and where the rotor actually is.

Imagine a normal permanent magnet that you use to drag around a paper clip. Right up close there's no problem; but only a few centimeters away its almost impossible. Load on an electric motor effectively increases the distance between the electromagnetic stator and the rotor (our paper clip). At zero load, a motor is doing zero work; at max its working too hard.

Is this theory or in practice.  I've driven the same route at speed and then at more leisurely paces.  It seems that pushing it, I have less gas left in the tank than when driving leisurely. 

[Soup DeVille]

Is this theory or in practice.  I've driven the same route at speed and then at more leisurely paces.  It seems that pushing it, I have less gas left in the tank than when driving leisurely. 

I tried to cover that. There's a big difference between vehicle and engine efficiency, and how each is defined.

You get better mileage when you drive slower and accelerate slower because you're not requiring as
much kinetic energy to be produced.

[MX793]

Is this theory or in practice.  I've driven the same route at speed and then at more leisurely paces.  It seems that pushing it, I have less gas left in the tank than when driving leisurely. 

You're conflating efficiency types.  In terms of miles per gallon, you will do better taking it easy than pedal to the metal.  You will travel further per unit of fuel consumed.  But in terms of thermal efficiency (how much energy you extract and convert to propulsion from each unit of fuel burned), you do better at WOT.

Just pulling numbers out of my butt for examples sake:

Taking it easy down a stretch of road may only consume 1 gallon of fuel.  However, of that gallon of fuel burned, only 15% of the energy released from burning that gallon of gas may have actually gone to propelling your car.  The rest is just dumped off as waste heat.

Running hard down over that same stretch might consume 2 gallons of fuel, but 40% of the energy released by burning that fuel actually went to propelling your car.

So you burned more fuel, but you wasted less of the energy liberated from it.

[FoMoJo]

I tried to cover that. There's a big difference between vehicle and engine efficiency, and how each is defined.

You get better mileage when you drive slower and accelerate slower because you're not requiring as
much kinetic energy to be produced.

Okay, resistance, etc.

Are we saying that an I/C engine functions most efficiently at its peak power rpm?

Does an electric motor care about rpms because maximum torque is available at any rpm?

[MX793]

Okay, resistance, etc.

Are we saying that an I/C engine functions most efficiently at its peak power rpm?

Does an electric motor care about rpms because maximum torque is available at any rpm?

IC engines don't necessarily perform at peak BSFC (Brake specific fuel consumption, or hp per fuel burned) at peak power.  Frequently it's somewhere closer to peak torque.  They are more efficient at WOT than partial throttle regardless of the RPM.

[Soup DeVille]

Okay, resistance, etc.

Are we saying that an I/C engine functions most efficiently at its peak power rpm?

Does an electric motor care about rpms because maximum torque is available at any rpm?

Lots of things at work here. Often, peak efficiency is at or near peak torque- it may help to think of peak torque as the point where the most instantaneous energy is extra from the combustion cycle (people who know better are already screaming "hey, wait a minute..." but lets let the analogy slide for a moment, OK?). But of course there are other factors at work here; friction and valvetrain momentum and so on and so forth. Peak power may happen at a different point, because power is also dependent on speed (in this case RPM).

Electric motors do care about RPM, but to a far lesser extent.

[FoMoJo]

Lots of things at work here. Often, peak efficiency is at or near peak torque- it may help to think of peak torque as the point where the most instantaneous energy is extra from the combustion cycle (people who know better are already screaming "hey, wait a minute..." but lets let the analogy slide for a moment, OK?). But of course there are other factors at work here; friction and valvetrain momentum and so on and so forth. Peak power may happen at a different point, because power is also dependent on speed (in this case RPM).

Electric motors do care about RPM, but to a far lesser extent.

Is it more or less correct to say then that because electric motors have a broader range of almost maximum torque that the overall efficiency on a journey of varying speeds is superior to that of an I/C engine?

[r0tor]

This makes it easy to see...

Electric motors efficiency is very forgiving.  Massive island of peak efficiency.



ICE engine have much smaller islands of peak efficiency located at hig loads near peak torque.  If you convert the units to efficiency, peak efficiency would be around 30-35% and lowest would be half of that.

[Laconian]

Wow, that's a remarkably edifying graph.

Also note that the electric motor efficiency has a pretty broad efficiency plateau, with falloff only happening near the limits. ICE has a peakier peak.

[Soup DeVille]

Is it more or less correct to say then that because electric motors have a broader range of almost maximum torque that the overall efficiency on a journey of varying speeds is superior to that of an I/C engine?

Efficiency of an electric engine is always-Always vastly superior to an internal combustion engine.*

*when compared from battery to wheels and from gas tank to wheels.**


**Getting the energy to the battery is the more lossy part.

[CaminoRacer]

Efficiency of an electric engine is always-Always vastly superior to an internal combustion engine.*

*when compared from battery to wheels and from gas tank to wheels.**


**Getting the energy to the battery is the more lossy part.

If we could have a gasoline powered electric motor the total system efficiency would be outstanding!

[Laconian]

If we could have a gasoline powered electric motor the total system efficiency would be outstanding!

What about a *hybrid* ICE driving an electric motor? When the ICE isn't efficient enough, the hybrid electric motor turns the output shaft to supply power the electric motor.

[CaminoRacer]

What about a *hybrid* ICE driving an electric motor? When the ICE isn't efficient enough, the hybrid electric motor turns the output shaft to supply power the electric motor.

Too realistic.

You described a Chevy Volt. And it's recommended to switch to the ICE or hybrid mode when driving on the highway with Volts, because the EV-only mode is less efficient in that situation and it's better to save the small battery capacity for around town

[Soup DeVille]

What about a *hybrid* ICE driving an electric motor? When the ICE isn't efficient enough, the hybrid electric motor turns the output shaft to supply power the electric motor.

The problem hybrids are solving is not efficiency, but energy density (or specific energy for those pedants amongst us).  Gasoline contains basically 48MJ/kg. There are 3.785 kg in a gallon of fuel.

The best lithium-ion batteries hold about 0.87MJ/kg.

In other words, a pound of gasoline contains about 50 times the energy that a pound of battery does.

What hybrids really shine at though is their ability to run the ICE engine at or near its peak efficiency in order to charge the battery, and to use the greater efficiency at a larger range of speed and loads to actually spin the wheels.

[Laconian]

I was 100% joking. I should've used a winky face!

[Soup DeVille]

I was 100% joking. I should've used a winky face!

And I'm far too obtuse.

[Laconian]

Skip to 12s.

https://www.youtube.com/watch?v=zOFIDfd03IY#t=12s