Won't coast on battery

That's the reason for my doubt.

 

And coasting is only useful when you would otherwise NOT be using the brakes.
If you actually have to stop or slow down a LOT, then coasting is counter-productive.

 

This is normal and has nothing to do with the auxillary battery - the HV battery is disconnected from the rest of the vehicle when off. The vehicle won't let the HV battery deplete unless you've got lots of other issues (and codes galore) or you are ready-on in neutral for a while.

 

Well, the part the laws of thermodynamics play here is this; every conversion of momentum (kinetic energy) to battery power (chemical energy) is less than 100% efficient. This is why the inverter has a cooling system, and why the battery gets warm. You're losing energy in those components to heat. Hence, if you accelerate to some speed and coast with no regen, you will use less energy (because less is lost to heat) than if you accelerate and then brake lightly to get some regen.

 

What gives you the power to be able to coast? Gasoline.
What slows you down when you coast? Friction.

Regeneration takes some of that lost energy back.
Coasting does not.

 

The friction is the same either way due to rolling resistance. So is aerodynamic drag. The friction brakes are not in use in either case. The only difference is that during coasting you are maintaining as much kinetic energy as possible, while with regen you are converting some of that energy to potential energy in the battery and some to _additional_ loss.

 

It's all loss when you coast. Not only to friction, but the Hybrid Synergy Drive has to consume extra power driving the electric side of the power differential in reverse, closely matching the wheel speed, in order to keep the gas engine stopped.

 

Lets look at it like money (kinetic energy) going into a shredder (friction/conversion losses). At a given speed, you have $100 worth of kinetic energy. Rolling and air resistance are always working against you while the car is in motion, along with some energy to power the transmission (which always uses some power). Lets say its taking away $10 of you energy every unit of time and putting it in the shredder, so no matter what, we can't have more than $90 left after this time. If you applied the friction brakes in a regular car, that remaining $90 would be shredded as heat in the brakes. If you used regen in the Prius, about $63 of the remaining $90 would be shredded as heat in the transmission, battery, and other components and $27 would be put in the battery (I've seen in several places that the Prius captures about 30% of the car's kinetic energy). If you were to coast, the only losses over the time period would be to the original $10 loss, leaving you with $90 of kinetic energy moving the car down the road.

Basically, if you think its worth it to pay $1 to get $0.30 back, regen all the time. It really doesn't matter to me what you think and do with your car, I'm not paying for your gas. But please don't try to convince others that using regen at any time other than when you need to slow down (or maintain speed going down a hill) is more efficient. Regen is only more efficient than friction brakes.

 

[Hills excluded] With a rolling car, potential energy minus friction = distance. Braking lessens the amount of time during which friction is dissipating that energy. Therefore, when you press the brake, there's less time, less distance, and less friction-loss, so most of the energy goes back in the battery instead of heating the air, transmission oil, axle-grease, tire-rubber and the road surface (which is basically where most of the energy goes).

Add to that, the energy required to simulate "neutral" in the Prius, and it's completely pointless to shift a Prius into "N while driving.

I can't think of any better way to stretch a gallon of gas than to drive the speed limit with the cruise-control on.

 

But when you go to use the energy from the battery later, you're still going to have the same friction that you avoided earlier. The difference is that if you avoid regen and just coast, you're avoiding the loss in converting kinetic energy to electrical energy, electrical energy to chemical energy, chemical energy to electrical energy, and electrical energy to kinetic energy.

Also, the car doesn't have to do any work to keep the motors moving when coasting or in Neutral - it just lets them freewheel. There's a little friction there, sure, but it's not applying any current to the motor (or drawing any current from the motor).

Here's another way of thinking of it: I give you two cars, both moving at 60 mph on flat ground. One of them just coasts for as long as it can; its speed slowly drops from 60, until all its kinetic energy has been dissipated as friction with the air, road, drivetrain, etc. It probably goes a mile or two. The second one converts all its kinetic energy to chemical energy in the battery and comes to a stop. Then, using only EV power, it accelerates again until it has used all this new chemical energy. Because this process is lossy, it necessarily isn't moving as fast as before. Because of that loss, at its peak speed (let's say 40 mph), it's not as far down the track as the other car was when it was moving at the same speed. If it just coasts forever, it will stop before it reaches the other car. If you repeat the process and come to a stop again, you'll lose more energy in the conversion, and the car will stop even sooner.

One final note; I don't believe that most sources of drag are time-dependent; rather, they're *distance*-dependent. Aerodynamic drag is one exception, because it depends on the square of velocity, which is distance divided by time. (So actually, *increasing* the time spent going from point A to point B (reducing your velocity) will decrease your total aerodynamic drag.) Other kinds of drag depend on the work done by friction (work is equal to a change in kinetic energy), the formula for which is distance times force (which is mass times gravity times the coefficient of friction) - time just doesn't fall in that equation anywhere.

 

There's something to be said for pulse-and-glide. It's a proven way to stretch the most miles from gasoline. Engine drag isn't happening when it's turned off. Unfortunately however, the Prius isn't designed to disengage the drive-train at the wheels, which would be the best way to eliminate drive-train friction while coasting. Not only is there drive-train friction, when the Prius "coasts," but (as I said before) it also consumes energy just to do so.

When Jimmy Carter imposed the National Speed Limit of 55, the actual fuel savings were afterwards calculated to have been less than 1%, but that could fall well within the statistical margin error (depending on the sample size). It also fails to account for improvements in fuel efficiency by newer vehicles over that time.

What seems to have been overlooked in that plan, was that while fuel consumed per vehicle per hour was reduced; the time on the road for each vehicle also increased, thereby canceling out the advantage.

 

Do you have a reference for that claim of 1% ?

And that last statement sounds good but is totally bogus.
It is miles per GALLON, not miles per HOUR or gallons per HOUR.
If you travel 1000 miles and use less gallons, it doesn't make any difference in the fuel consumption HOW long it takes to do it.

 

It just happens to be much less energy than is wasted by regen.

 

Yes, this ^^. It is true that the car will draw about 200-500 Watts of power all the time, whether moving or not. In the grand scheme of things though, that's a fairly small number, relative to the power it takes to accelerate your car, or even relative to the power loss (double-digit percentages) when you then decelerate your car to get regen.

The 1% (or less) number may be true, but it's important to add some context - apparently less than a third of drivers were even obeying the reduced speed limits in the first place. It's straight physics - double your speed, and you quadruple your aerodynamic drag. You can no more avoid that truth than you can avoid gravity.

(Also maybe worth noting - the law predates Carter by about 3 years. Nixon was the one who proposed it and signed it into law.)

Anyway, if you're curious to learn, I'm happy to answer questions for you - but it doesn't seem like I'm likely to convince you. I'm an engineer, not a persuasive speaker.

 

Well, that's the expertise I defer to.

 

Greg, you may call yourself an engineer but regardless of your title you seem to have a fundamental misunderstanding of newton's laws in general and thermodynamics in particular.

 

That is not one of the reason(s). The causes were:

1. More cars on the road
2. Drivers who did not obey the limit. They were a two-fold problem; not only did they continue to use more fuel than expected, they caused disruptions in the flow of traffic. Disruptions = braking.

 

That sounds like driving in Russia.

 

I'd never call myself an engineer, but thanks if that's what you're doing.

 

Copulos, Milton R. (September 9, 1986). "The High Cost of the 55 MPH Speed Limit" (PDF). The Backgrounder (Washington, DC: The Heritage Foundation) (532): 1–8.