More control and easier on the machinery the more gentle you are. Works for all, repeat all, things mechanical or electric.Hi Everyone,
I am wondering what is the most efficient way to accelerate from a stop light. In gas cars, there is such a thing as accelerating too slowly, which results in using more fuel.
Is this the same for electric cars? So, is it better to gun it a little and use 30 kW until you reach your desired speed, or use 12 kw and slowly reach your speed?
Wow! Very useful. I read the map to show the motor should be most efficient between 25 and about 50 mph (assuming the Bolt's gearing is roughly 10 mph per 1,000 rpm).Driving Instructions:
A ramping launch (not full tilt, but say 3 seconds building up to 1/4 max pedal capability) up to 20 mph, followed by a slower accel 1/3 max pedal.
How do we get those approx instructions? Below:
Basic physics always applies: Isquared x R is always the power (energy rate) lost in the motor, batteries, and wires. (The other types of losses in the motor also increase a little due to the higher torque needed for brisk acceleration, but we can neglect that extra bearing friction usually.)
"I" is the electric current flowing, which is very high when flooring it. THAT is the main power loss, which creates extra heat in the battery, wires, windings, compared to taking it a bit easy on acceleration.
The main physics consideration above is a good starting place.
There are secondary effects (motor windage, bearing friction, iron losses,etc.), which translate into a motor efficiency map:
View attachment 34247
... that basically tells us to ramp up torque as our fixed-gear ratio vehicle gains speed into the redest region of the graph where we get the highest motor efficiency.
IOTW, make a bee-line to the middle of the red region until you are at the traffic-limited speed target.
Granted, air drag builds as you increase speed too, so best not to accelerate too rapidly into that energy-wasting situation too soon either. And keep currrent (amps) down to avoid the blue region on the left side of the graph during low-speed operation. ......So a ramping launch (not full tilt, but say 3 seconds building up to 1/4 max pedal capability) up to 20 mph, followed by a slower accel 1/3 max pedal for that particular motor, and we assume our motors are at least similar to that efficiency map.
Deeper (Tech Warning: The following article could cause headaches unless you're into physics-engineering.):
For sure! I am generally easy on my vehicles at my age. I have a basic understanding of getting good efficiency in gas engines but not electric motors hence I posted the question I did.More control and easier on the machinery the more gentle you are. Works for all, repeat all, things mechanical or electric.
Good point! But it is helpful for me to know the "why."I am surprised no one has yet suggested just watching the ring color around the speedometer. If you keep it green that is most efficient, if it turns yellow less efficient, the GM engineers know and provided the info to the driver. Keep the ring green for highest efficiency, or yellow for most fun ;-)
In theory. Maybe within 7% due to some small heat losses, as stated above.An EV uses the exact same amount of energy to accelerate to 60 mph whether it takes a second or a minute according to CleanTechnica.
Is that the basis of this question? To save a quarter?Put an extra PSI or so in your tires and you will probably save more energy. ... Some things are not worth worrying about... IMHO.
Disagree.Traffic affects considerate behavior. Go with the flow!
You're right the Bolt runs 1,000 RPM for every 10 mph (7:1 gearing). Electric motors spin faster than gasoline engines, true.Wow! Very useful. I read the map to show the motor should be most efficient between 25 and about 50 mph (assuming the Bolt's gearing is roughly 10 mph per 1,000 rpm).
One question: 1/4 of max pedal... am I right to understand you mean 3/4 pedal?
This approach is dependent on the accel pedal "capability" (angle? distance?) being linear with the motor torque. Is this true? We already know that GM limits torque at lower speeds, but is it linear after than?3 seconds building up to 1/4 max pedal capability) up to 20 mph, followed by a slower accel 1/3 max pedal
In my last reply to @Brad I said "I think pedal position could be more of a power command, not strictly torque or speed, at least apart from 1-pedal driving, I'm not sure what GM is doing exactly. "This approach is dependent on the accel pedal "capability" (angle? distance?) being linear with the motor torque. Is this true? We already know that GM limits torque at lower speeds, but is it linear after than?
Great! Thanks for answering!You're right the Bolt runs 1,000 RPM for every 10 mph (7:1 gearing). Electric motors spin faster than gasoline engines, true.
If you look back at the efficiency map graph above, that motor maxes out at around 300 torque, and the center of the red region is around 1/4 to 1/3 of that, approx.
I think pedal position could be more of a power command, not strictly torque or speed, at least apart from 1-pedal driving, I'm not sure what GM is doing exactly.
Acceleration is proportional to excess torque, meaning maybe the better rule of thumb is stated as:
1. Accelerate lightly from a standstill to keep the amps down, within reason, so maybe that's about 1/4 pedal travel gradually eased in over 4 seconds.
2. Once at 20 mph, accelerate a bit harder to reach your target speed, up to 1/3 or 1/2 the max capability.
Main thing is to avoid the blue region of the graph, which says don't floor it right away when going from a full stop, since the current (amps) flow is too high (IsquaredR losses).
To do that, you want to ease into the pedal at first, to keep the initial blast of amps down, but notice the 80 torque line (1/4 of max capability) cuts right down the center of the red high-efficiency region.
The Bolt already limits current some automatically to avoid wheelspin off the line, but they left a lot of low-rpm current flow "peppiness" for customer satisfaction, a selling point in sales test drives and Car and Driver reviews.
The engineers sized the motor & hence the efficiency map to maximize range in the standard EPA range test, which is representative of mixed city & hiway anyway.
I wonder how much quicker the Bolt would be with better tires and a re-map of the controller?Fun calculation: How much thrust (force, Pounds, lb) does a Chevy Bolt produce at maximum? Usually we think of an F-15 having 'thrust' not a car, but it does, just the same.
266 ft-lbs x 7.05 gearing = 1875 ft lbs at the half shaft. Acting over a 1.06 ft tire radius = 1,769 lbf at the front wheels = 7869 Newtons
7869 Newtons force / 1616 kg mass = 4.87 meters/s2 = almost exactly 0.5 g's acceleration.
Since current & torque is limited at low speeds, you may only get 0.5 g's accel at around 20 mph or so (?). And it's limited by tire grip too, so not sure if you can actually get there. Add a driver and mass goes up, so 0.5 g's is for a hamster driver only....
So the key point is to drive 'cheaply in the Bolt?....To minimize cost, though, it pays to accelerate and brake moderately and to take corners at moderate speeds - because driving aggressively will shorten the life of your tires and of some of the suspension and drivetrain components.