Faster acceleration vs slower acceleration

Let's say you want to reach the speed of 45mph in city driving before you start gliding. Accelerating with 3 bubbles to the right of the center bubble will get you to the 45mph mark faster than accelerating with 1 bubble to the right of the center bubble. So while it expends more energy, it does it in less time with the faster acceleration. On the other hand, slower acceleration may expend less energy, but it takes long to reach 45mph. So if you calculate the total energy it takes to reach 45mph in both ways, wouldn't they come out to roughly the same anyway? Assuming that the energy efficiency of the motor doesn't change much in either scenario.

If so, why wouldn't you want to do the faster acceleration to save time off your trip? Time saved off your trip would also mean less AC time and less overhead energy consumption time, so you'd come out ahead in the end.

What am I missing here? I know conventional wisdom says that slower acceleration is better than faster acceleration. But I'm not sure I can explain why. Can you?

There was a thread on this very subject a while back. I can't remember what they said, but for me, I definitely get better m/kW h using zero to one bubble to accelerate to my desired speed. I've tried both methods.

All things equal, a slower acceleration will keep you more time at slower speeds, so the average drag is lower and it will spend less energy.

While I have no numbers to back this up, I suspect that the battery is more efficient at providing electrical energy at lower discharge rates.

At greater discharge rates from the battery to the ECU (higher current, as the voltage out of the battery is the same), there is more resistance in the wires creating more heat and wasting energy. On the low side of the ECU, it is difficult to estimate as the ECU regulates both, voltage and current to the motor, not just current.

Slower acceleration will be energy efficient, Faster acceleration is time efficient.

This is not an ICE where it might help to get into overdrive gear a bit faster.

And in ICE cars without Valvetronic technology or the equivalent, reduce pumping losses through a larger butterfly opening and thus lower intake manifold vacuum...

smkettner said:

This is not an ICE where it might help to get into overdrive gear a bit faster.

Click to expand...

TomT said:

...Valvetronic technology ...reduce pumping losses ...butterfly opening ...intake manifold vacuum...

Click to expand...

:shock:?? that is where I realize I know EV's way better than ICE's...

You definitely increase losses in the battery and motor controller with increased accelleration (current). I calculated the ESR of the battery to be ~70mOhms. At full accelleration 80kW you are pulling about 200A from the battery. This means 200*200*.070=2.8kW. This is only ~3.5% loss. Assuming similar losses in the motor controller we have 7%, maybe a little more loss at full accelleration. This is a noticeable factor if you are trying to get every last mile out of your charge but is a much smaller factor than speed (70mph uses 100% more power as 50mph but only 40% more speed) or hard braking (up to 100% loss).

Yep - another issue is the Peukert efect. Basically, most batteries will waste extra power at higher loads.

Sorry, I must sincerely disagree. Faster acceleration is better (not necessarily floor it, but maybe halfway).

Here's why: Let's say I can go 1 mile between stoplights. If I accelerate faster, I can travel slower once I reach a constant speed. If I accelerate slower, then I have to travel at a faster speed when I'm not accelerating.

Maybe that didn't make sense. Ok, let's put this a different way. Suppose we're still going one mile. And traffic wants to go 55 mph. I want to conserve energy so I would rather go 45 mph. If I accelerate quickly, I leave the other cars behind and can cruise along at 45 mph. By the time the traffic catches up to me, I would be getting close to the stoplight and would have to slow down anyway. So I have not held up traffic. The other cars would have spent more energy because their top speed was higher.

The point is: To get there in the same amount of time, it is more efficient to accelerate rapidly and then maintain a slower top speed, than to accelerate slower and therefore have to maintain a higher top speed. The top speed makes the difference.

Hope that made sense.

abasile said:

While I have no numbers to back this up, I suspect that the battery is more efficient at providing electrical energy at lower discharge rates.

Click to expand...

That may be true, but we do have a LEAF electric powertrain graph from a presumably reliable source, SAE International 2011 Nissn Leaf Vehicle Overview which shows that the inverter+motor is less efficient at low power, and less efficient at lower speeds.

I share Volusiano's puzzlement here, though not perhaps to as high a speed as the 45 mph he mentions. By then air resistance will definitely be taking a toll. But up to, say, 25 mph and using only three or four bubbles of power, shouldn't you actually be ahead of the game to get out of the inefficient part of the powertrain graph as quickly as possible? That is not a high discharge rate at the battery, relative to what it is capable of.

On the other hand, facts are more persuasive than theory, and LEAFfan's facts are tough to ignore.

Ray

I think we need to call Mythbusters on this one...
All, go to discovery.com and ask them!
That's the only way we will know for sure

planet4ever said:

abasile said:

While I have no numbers to back this up, I suspect that the battery is more efficient at providing electrical energy at lower discharge rates.

Click to expand...

That may be true, but we do have a LEAF electric powertrain graph from a presumably reliable source, SAE International 2011 Nissn Leaf Vehicle Overview which shows that the inverter+motor is less efficient at low power, and less efficient at lower speeds.

Click to expand...

Do we know the conversion factor between RPMs and the car's speed?

abasile said:

planet4ever said:

abasile said:

While I have no numbers to back this up, I suspect that the battery is more efficient at providing electrical energy at lower discharge rates.

Click to expand...

That may be true, but we do have a LEAF electric powertrain graph from a presumably reliable source, SAE International 2011 Nissn Leaf Vehicle Overview which shows that the inverter+motor is less efficient at low power, and less efficient at lower speeds.

Click to expand...

Do we know the conversion factor between RPMs and the car's speed?

Click to expand...

The conversion slope is reported to be 94MPH at 10000 RPM.

TickTock said:

You definitely increase losses in the battery and motor controller with increased accelleration (current). I calculated the ESR of the battery to be ~70mOhms. At full accelleration 80kW you are pulling about 200A from the battery. This means 200*200*.070=2.8kW. This is only ~3.5% loss. Assuming similar losses in the motor controller we have 7%, maybe a little more loss at full accelleration. This is a noticeable factor if you are trying to get every last mile out of your charge but is a much smaller factor than speed (70mph uses 100% more power as 50mph but only 40% more speed) or hard braking (up to 100% loss).

Click to expand...

The thing is, I'm not debating full acceleration at 80kw vs minimal acceleration at say 10kw. I'm just debating 1 bubble acceleration (usually shows about 10kw), versus 3 or 4 bubble acceleration (that usually shows around 20-25 kw on the motor energy indicator).

planet4ever said:

I share Volusiano's puzzlement here, though not perhaps to as high a speed as the 45 mph he mentions. By then air resistance will definitely be taking a toll.

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I thought that the air resistance difference is more relevant when comparing one constant speed to another constant speed. But I didn't think that air resistance differs that much going from the same 0-45mph distance, albeit just slower or faster. I understand that pushing through the same air thickness faster may be harder than pushing through the same air thickness more slowly. But we're not talking about bullet train going at warp speed here. We're only talking about going 0-45mph in 20 seconds versus 0-45mph in 40mph in 40 seconds. Surely the air resistance in both cases can't differ that much to significantly affect efficiency. Maybe a 1 or 2 percent difference, if any.

planet4ever said:

But up to, say, 25 mph and using only three or four bubbles of power, shouldn't you actually be ahead of the game to get out of the inefficient part of the powertrain graph as quickly as possible? That is not a high discharge rate at the battery, relative to what it is capable of.

Click to expand...

This makes sense.

johnr said:

The point is: To get there in the same amount of time, it is more efficient to accelerate rapidly and then maintain a slower top speed, than to accelerate slower and therefore have to maintain a higher top speed. The top speed makes the difference.

Hope that made sense.

Click to expand...

Perfectly. I drive trains for a living, we do exactly the same. Always full acceleration (typically 2-9000kW), to be able to travel as slowly as possible to keep up with the schedule. If the rails are slippery, acceleration suffers and I might have to accelerate to 100mph between two stops instead of 80mph. This uses more energy in total.

jkirkebo said:

johnr said:

The point is: To get there in the same amount of time, it is more efficient to accelerate rapidly and then maintain a slower top speed, than to accelerate slower and therefore have to maintain a higher top speed. The top speed makes the difference.

Hope that made sense.

Click to expand...

Perfectly. I drive trains for a living, we do exactly the same. Always full acceleration (typically 2-9000kW), to be able to travel as slowly as possible to keep up with the schedule. If the rails are slippery, acceleration suffers and I might have to accelerate to 100mph between two stops instead of 80mph. This uses more energy in total.

Click to expand...

I learn something every day here it seem. I admit I doubted Johnr's statement at first but I plotted the energy required to reach a light 1 mile away in 80 seconds (assumes they are timed for 45 mph) at different acceleration rates and it looks like the minimum energy used (blue curve) occurs under pretty hard accelleration (need to reach 47mph in 6.7 seconds, or accellerating at ~7mph/s). This includes the losses in the battery+motor controller and square law increase in energy to maintain speed based on measurement in my Leaf. In the graph below, s (red) is the time is takes to reach the cruising speed (green). Blue is the total energy expended and the x axis is accelleration in mph/s.

What if the time was flexible? What if the 80 seconds could be 100 seconds?
Sitting at the next red light longer does not always increase the commute time.