EV Acceleration Efficiency

Newton's second law of motion.
According to Newton "Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object)"

Of course some of Newton's laws are not totelly true.

 

That doesn't actually answer the question. Mass is almost the same (200lbs). Acceleration can be the same too. Energy is not in there.

 

electric motors have power efficiency curves too, like combustion engines. The curves might even look similar. You should also remember that the heat lost from a wire is proportional to the square of the current.

 

I agree (and using Newtonian physics, F=MA, working backwards) increased force would derive increased acceleration. Increased energy per unit time from the pack would in turn increase torque and thereby force. So while the energy is increased you would expect that the time would be less so to me either way you're storing the same amount of energy from the pack into the vehicle in the form of kinetic energy. The only losses I can see are the losses associated with the increased current flow through the system (assuming no wheel spin ).

So, maybe I'm not thinking about this correctly and I'd love to see more conversation on this but I think it shouldn't matter much how hard you accelerate in EV mode.

 

Your first premise is wrong: With a gas engine, you get better mileage from moderately aggressive acceleration. Granny driving will cost you gas. This is because of the excess capacity of the typical automotive gas engine. When you accelerate slowly, you are working in an inefficient power range for the engine, as it is required to draw air through a mostly closed throttle plate. Pumping losses are greatly increased. For best mileage you should accelerate at the engine's most efficient power level, which means aggressive but not wide open throttle.

Electric motors don't suffer from this problem. The efficiency curve for an electric motor is much flatter than that for a gasoline engine. Within reason, the efficiency won't be affected by slow or fast acceleration, other than the obvious fact that fast acceleration causes you to spend more time at higher speed.

Tom

 

You are correct. From classical physics, acceleration does not affect the work required to get a mass to a given speed. It does affect the rate at which work is done, so faster acceleration equates to more power, but the total energy required to accelerate a given mass is the same in either case.

The difference in a practical application comes from the efficiency of the prime movers and the non-linear nature of drag, not from fundamental physics.

Tom

 

I'm thinking this through…

Analogy:
Electric Motor (EM) coil as a wire
Internal Combustion Engine (ICE) throttle body as pipe.
EM – As you increase current through a wire, it’s internal resistance increases. This means more electricity is converted to heat. EM becomes less efficient. Assuming RPM independent.

ICE – As you open the throttle (pipe has less resistance). ICE actually becomes more efficient!!! To a certain point. As RPM goes up, eventually the full open pipe becomes a throttle (throttles the air), engine is once again experiencing suction loss. ICE becomes less efficient.

The reason aggressive “driving” sucks more gas is more because of abrupt braking. Not the aggressive acceleration.

I think the answer is YES, you waste more electricity with aggressive acceleration. Though, I think the effect is not as drastic as suction loss.

 

I just read the refresh, when i posted. and Qbee42 described the same senerio, with the ICE.

 

This pretty much says YES. (It waste more electricity accelerating agressivly)

And NO, you won't get more EV miles accelerating agressivly. Sorry I didn't ready your question.

 

I think that summary is that in a 100% efficient system, the rate of acceleration does not impact the total amount of energy used to get a given mass to a given speed. But in practical systems, there are losses in the motors (electric or ICE) related to how much force they are producing and at what speed. And for ICE, the factors suggest reasonably aggressive acceleration, but for an electric motor, the heat losses suggest going easy.

 

This is a solid answer and leads to the next question, what are the average losses from the increased current flow through the system for a minimal EV acceleration as compared to a high EV acceleration?

Anyone have any ideas?

 

The electric motor is part of a system which includes the power source and inverter. If we are talking about pure EV, then the power source is the HV battery. Battery losses increase with higher current, as the internal temperature of the battery rises and increases heating loses. These are non-linear losses, so they take a bigger bite at high current levels.

High current levels are less of a problem with wiring. Unless drastically undersized, the losses in the cable will be nearly linear, meaning it doesn't cost more at high levels. In other words, the heating losses are higher, but they happen for a shorter period, so it all cancels out.

Inverter losses are harder to quantify. They depend heavily on design choices, and will normally be set to optimize normal driving. Inverters are generally not efficient at low power levels, so creeping will take a toll.

Motor losses are interesting. While liquid cooled, the motor will generally run much hotter than the supply wiring. This is a cost and space issue: Wire is cheap, but motors are expensive and bulky. This means that motor losses will be non-linear as the power goes up. Speed is also an issue: A fast turning motor suffers aerodynamic or hydrodynamic drag which is non-linear. Adding to this is the design of the Prius motor. The Prius uses a hybrid-synchronous type design. At lower power levels it functions as an efficient synchronous motor. At higher power levels it runs as an induction motor. The induction mode is more powerful, but not as efficient.

Adding this up, it is more efficient to take it easy with the electric motor.

Tom

 

If you get an answer to your question, what good is that information?

The only question I care about is, "How do I drive to minimize my use of electricity (battery charge)"? In other words, "How do I drive to get the most number of miles from a full battery pack"? I don't care about losses from current flow.

 

Well, while I do think your point is spot on: "How do I drive to get the most number of miles from a full battery pack", I also would want to know how much of an impact would it be if I accelerate quicker. Today with my G3 Prius I accelerate just barely faster than walking to stay in EV mode (I just like staying in EV mode) but with a new PiP I'm hoping to accelerate more in line with other traffic. And from time to time I will want to accelerate with a little enthusiasm for fun, so the question would be what's the impact to the energy in the pack from that?

Finally, I'm really interested in the overall technology and how it all works so this is something I'd like to understand a bit more...

Hope that makes sense -

 

My best guess is jack squat. Tesla claims the efficiency from battery to axle is 88% on a much higher current system. The prius motor is built for 60kw, which is much higher than the max 38kw that we think the car will do in ev mode. Higher speeds make this motor less efficient, and drag at higher speeds requires more power to overcome. The prius phv gets less efficient in kwh/mi as speed increases, but I honestly doubt it makes much difference accelerating to that speed Since you get to speed faster, you might be going at that higher rate of speed for a longer period of time. Now decelerating fast using friction brakes will definitely hurt mileage versus a coast down.

This is different than in a hybrid, where ice efficiency varies with power levels. In the gen 3, I believe 15hp-30hp was the most efficient range which is a slow but not creeping acceleration. Someone with bsfc maps handy can probably tell us exactly what this range is.

 

I think the question you want answered is, "How much power is required for the car to go from EV mode to ICE mode"? John1702a just got his Plug-In yesterday or the day before. I just saw where he said that with the Gen III Prius a power demand of ~15 HP causes the ICE to come on, with the Plug-In that power demand goes up to 55 HP. That's quite a difference.

 

If you accelerate quickly most drivers end up going fast then stopping. If you accelerate slowly you end up going slower between lights and use less energy. Each way accelerating and decelerating you are wasting some amount of energy since neither is 100% efficient. You won't be accelerating quickly ever in EV mode since it cannot.

 

That is true and I totally agree losses in the system will bleed pack or kinetic energy away as the velocity changes with the vehicle even though an EV system is dramatically more efficient than any other combustion based system. And truthfully, I don't plan on rapid acceleration or deceleration but I am interested in 1) being able to accelerate with the flow of traffic and stay in EV mode and 2) I am curious about how the system functions and I'd like to understand more about it.

The great news here is that some of the data already captured by the many early adopters as well as some of the video already recorded proves that my first concern is very much satisfied. I don't believe it will be a problem at all to accelerate with the flow of traffic and stay in EV mode!

Thanks a bunch for the comments! I look forward to learning more about the PiP...