Faster acceleration vs slower acceleration

[SanDust]

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?
...
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?

The energy it takes to go from 0 to 45 MPH will be the same regardless of the acceleration. The average velocity will also be the same. However, with greater acceleration you get to 45 MPH faster. Since you reach the final speed more quickly, you're traveling for a shorter period of time, and, since the distance traveled in getting to 45 MPH is simply the average velocity times time, you'll travel a shorter distance. Expending the same energy over a shorter distance is less efficient.

There will also be Peukert's Effect as mentioned though for efficient batteries the effect shouldn't be very large.

 

[mark13]

Hmm im totally confused..Are you saying that jack rabbit starts save energy..So ICE cars are different then battery power cars ..You must all be avg. 5.8 KWs with you fast starts then

 

[planet4ever]

The energy it takes to go from 0 to 45 MPH will be the same regardless of the acceleration.

That would only be true in a completely frictionless system; no air resistance, no internal resistance, no rolling resistance. (It also assumes that rates of heat loss - battery, wiring, inverter, motor - are directly proportional to acceleration, which they probably aren't.)

Expending the same energy over a shorter distance is less efficient.

That would not be true in a completely frictionless system. Once the object reaches its final velocity it can continue at that velocity forever with no additional expenditure of energy.

Since your two statements can't both be true for any given system, they can't be combined to "prove" that faster acceleration is less efficient.

Ray

 

[LEAFfan]

Hmm im totally confused..Are you saying that jack rabbit starts save energy..So ICE cars are different then battery power cars ..You must all be avg. 5.8 KWs with you fast starts then

+1! The proof is in actual practice, not theories. So you people who believe using 30-40kW to get up to speed, do your tests and please report back. I already know that zero (less than 5kW) to one bubble (5-10) gives me better m/kW h. I rarely exceed 38 now because our lights are really hard to make at 45mph. I've been making many more lights now at 38 without slowing down than I used to at 45 and that has helped me be at 6.4m/kW h/dash for OCT so far.

 

[Smidge204]

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.

Well, if you're not going to get through the intersection until it turns green, there's no sense in rushing up to it... especially when, if you take your time, it might be green by the time you get there and you won't have to stop at all. (That's probably what you were hinting at anyway...)

So yeah, sitting at a red light does not necessarily increase your commute time. The only time it does is when you just miss the light - if only you had been going a little bit faster you could've gotten through the yellow! :lol:
=Smidge=

 

[TickTock]

Hmm im totally confused..Are you saying that jack rabbit starts save energy..So ICE cars are different then battery power cars ..You must all be avg. 5.8 KWs with you fast starts then

Yes... if you hold the time to reach the destination a constant. Driving slower (and gettarriving later) will save power - no argument. If you cannot make the next light in 80 seconds (i.e. it turns red at 81), then you are better off slowing down and trying to reach it in 160 seconds (or however long before the next green).

(of course in heavy traffic it will be nearly impossible to implement this practice due to other cars overtaking you, rushing to the red and making you stop even when it turns green as you arrive)

 

[SanDust]

That would not be true in a completely frictionless system. Once the object reaches its final velocity it can continue at that velocity forever with no additional expenditure of energy.
...
Since your two statements can't both be true for any given system, they can't be combined to "prove" that faster acceleration is less efficient.

Actually it's still perfectly true. What I said was that hard accelerations use more energy per unit of distance. It doesn't matter what happens after the accelerations end. For example, even in your imaginary friction less system, if one car gets to 50 MPH is one mile and a second gets to 50 miles in two miles, both using the same amount of energy, and then both cars travel for 1000 miles, at then end of the next 1000 miles car one will will have gone 1001 miles on X amount of energy and car two will have gone 1002 miles on X amount of energy. IOW car two will have gone further using the same energy.

All you're saying is that if you go an infinite distance after the accelerations end the difference becomes infinitesimally small. True but that doesn't change the fact that harder accelerations use more energy per unit of distance.

 

[Volusiano]

The energy it takes to go from 0 to 45 MPH will be the same regardless of the acceleration. The average velocity will also be the same. However, with greater acceleration you get to 45 MPH faster. Since you reach the final speed more quickly, you're traveling for a shorter period of time, and, since the distance traveled in getting to 45 MPH is simply the average velocity times time, you'll travel a shorter distance. Expending the same energy over a shorter distance is less efficient.

Hmm im totally confused..Are you saying that jack rabbit starts save energy.

Actually, SanDust is saying that jack rabbit starts cost energy, NOT save energy. And I think what SanDust says makes total sense to me. That's the explanation I'm looking for.

 

[planet4ever]

even in your imaginary friction less system, if one car gets to 50 MPH is one mile and a second gets to 50 miles in two miles, both using the same amount of energy, and then both cars travel for 1000 miles, ...
<snip>
All you're saying is that if you go an infinite distance after the accelerations end the difference becomes infinitesimally small. True but that doesn't change the fact that harder accelerations use more energy per unit of distance.

That's just plain silly. I'm not talking about infinite distances and infinitesimal differences. If both cars travel five miles in this imaginary frictionless system, they will both use the same amount of energy to go that five miles. One is not more efficient than the other.

Your statement A: The energy it takes to go from 0 to 45 MPH will be the same regardless of the acceleration.
- True in an imaginary frictionless system (or outer space), not in a real system on earth.

Your statement B: Harder accelerations use more energy per unit of distance traveled.
- True in a real system, not in a frictionless system.

Your conclusion: Harder acceleration is less efficient.
- May be true, but can't be proved from the above statements, either on earth or in outer space.

Ray

 

[mark13]

Beam me up Scotty to the next thread

This is my finale thought ..Worm Holes if we only had worm holes to race into :shock:

 

[SanDust]

That's just plain silly. I'm not talking about infinite distances and infinitesimal differences. If both cars travel five miles in this imaginary frictionless system, they will both use the same amount of energy to go that five miles. One is not more efficient than the other.

Efficiency is measured by distance/energy. When saying both cars go 5 miles you're arbitrarily adding different distances. If car one accelerates to 100 MPH over one mile, and car two accelerates to 100 MPH over 2 miles, then, since both cars will use the same amount of energy getting to 100 MPH, car two will be twice as efficient. It will have gone a full mile farther than car one -- 2 miles vs. 1 mile -- using the same amount of energy.

Now if you want to arbitrarily add miles in an imaginary frictionless universe, that's OK, but you have to add the same number of miles to the distance each car has already traveled. You can't add four miles of extra distance to car one and three miles of extra distance to car two. Maybe this example will help. Say you have a Prius that goes 50 miles on a gallon of gas, and a Hummer that goes 10. If you add 50 miles to the distance the Prius goes and 90 miles to the distance the Hummer goes, you can claim that they're equally efficient since they're both getting 100 MPG. But you only get that result by adding different distances to the number of miles each vehicle has traveled. If you add the same number of miles, say 50 to both, then the Prius remains more efficient because it goes 100 miles on a gallon and the Hummer goes 60 miles on a gallon.

You seem to be getting confused because you're assuming that since in your frictionless system there isn't any more energy needed to travel an extra mile that miles can be arbitrarily added to the distance traveled by each car. But that's not the case. You can add any number of miles to the distance traveled but you're adding a constant so the number of miles has to be the same. Add three miles to car one and you have to add three miles to car two. When you do that car two will always be more efficient.

Worm holes are different!

 

[planet4ever]

Now if you want to arbitrarily add miles in an imaginary frictionless universe, that's OK, but you have to add the same number of miles to the distance each car has already traveled.

Wrong.
You have to make the total number of miles traveled by each car since the stop be the same.

You were the one who started this argument by pointing out that the faster accelerating car will use additional energy after it reaches its peak speed before it has traveled as far as the slower car does in reaching that speed. That is true, however based on what you just wrote, should I now claim that you should add the same number of miles, and hence the same energy loss, to both cars after they reach the maximum speed?

But, really, this whole "imaginary frictionless universe" business is a red herring. We need to focus on your first statement, the one I claim is untrue except in a frictionless universe.

The energy it takes to go from 0 to 45 MPH will be the same regardless of the acceleration.

Untrue.
With slower acceleration it will take longer to get to 45 MPH and you will have traveled farther before you get there. That means more loss from tires deforming and rubbing against the pavement; more loss in gears and bearings. It also means more time spent below 20 MPH, and a lower torque, both of which, according to the SAE graph, mean the electric powertrain is less efficient. Finally, it means more loss from air resistance, but perhaps I should explain that. Rather than go off into calculus, let's think of the speed as a bar graph plotted against time, each bar being 1 MPH longer than the previous one. (Obviously with calculus we would make this an infinite number of bars each infinitesimally longer than the previous one.) The slower accelerating car will have wider bars, meaning that for every speed, and hence every level of air resistance, it will be exposed to that air friction for a longer time, and hence lose more energy at that speed.

Bottom line: The energy it takes to go from 0 to 45 MPH will be significantly higher when accelerating more slowly.

Ray

 

[daggad]

Bottom line: The energy it takes to go from 0 to 45 MPH will be significantly higher when accelerating more slowly.

Rock bottom line: And you'll have a lot more fun getting there pushing your pedal all the way down !

 

[TickTock]

Well, FWIW, I tried the "jack turtle" method today (accellerate hard to get to speed fast but cruise 5 mph slower) and got my best efficiency ever for this drive (5.4mpkWh). Normally I get 4.8-5.2. I've always accellerated gently and set the cruise control to 54mph. This time I stomped it and set the cruise to 49. Got to work in about the same time (45 minutes +/- 2 min). I guess after trying this a few days I will see if gently accellerating and targetting the slower cruise speed results in measurably longer commute times (I'm confident lower speed and gentler accelleration can only help efficiency but the question is if, for constant travel time, does the jack turtle method save energy).

 

[tps]

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.

The "slowing down" part is interesting during "slippery rail" season, especially when the train hits an unexpected really slick spot and slides right through the station. Sometimes the conductor has to take the passengers to the end of the last car to get off; hopefully it has not passed the end of the platform...

 

[rmay635703]

Well, FWIW, I tried the "jack turtle" method today (accellerate hard to get to speed fast but cruise 5 mph slower) and got my best efficiency ever for this drive (5.4mpkWh). Normally I get 4.8-5.2. I've always accellerated gently and set the cruise control to 54mph. This time I stomped it and set the cruise to 49. Got to work in about the same time (45 minutes +/- 2 min). I guess after trying this a few days I will see if gently accellerating and targetting the slower cruise speed results in measurably longer commute times (I'm confident lower speed and gentler accelleration can only help efficiency but the question is if, for constant travel time, does the jack turtle method save energy).

There are a lot of misconceptions and imagination going on in this thread.

The most efficient acceleration method is the one that matches the motors torque/saturation curve the best while counterbalancing controller loss. Generally all motor controllers have something akin to a throttle and that is PWM, yes even AC systems need PWM, especially at lower power/rpm levels. On the other side if you are at WOT or high loads you have again controller losses, AC motor losses due to the field being too close to the saturation point and wire losses.
Nissan itself should be able to provide a plot of the most efficient acceleration curve for any given RPM/speed.
Like a gas car it will be somewhere around moderate I would guess depending on the RPM level. At low RPMs its probably near WOT, despite the wire losses but probably only to about 5mph then ramping down as your RPMs control current more effectively.

The reality is this is a very complex issue.

Next, almost always (within reasonable limits) a car like the leaf is going to have its best range at the slowest speeds and this extends right down to about 15mph because airdynamics become very significant above that point on a hatch. If you take longer to get to your destination, drive slower and accelerate near the most efficient point on the motor curve (whereever that is) you will use less current to get where you need to. I would guess once you get to about 15mph the motor efficiency will be overrun by aero issues so accelerating a bit slower from that point onward should save juice due to lower aero losses.

The kwib about running faster because you accelerated slower is pure rubbish, most people who are driving either accelerate fast and drive fast or accelerate slow and drive slow, combining the two is just silly.

 

[TickTock]

The kwib about running faster because you accelerated slower is pure rubbish, most people who are driving either accelerate fast and drive fast or accelerate slow and drive slow, combining the two is just silly.

I agree most people drive fast and hard or slow and gentle but I don't think combing the two is silly. This is a discussion about optimal driving styles - not popular driving styles. My personal priorities are:

• 1) I don't want a ticket so I limit my speed to <= 54mph
  2) I want to reach work as fast as possible
  3) I want to conserve energy

So if I can convince myself that I save energy with this approach, get to work in the same time, and not get a speeding ticket I will change my style. Similarly if just driving slower and gentler also gets me to work in the same time due to the light pattern, etc, then I will select that.