Braking regeneration

It definitely has an effect. For cars more so than planes. Drag and airspeed are the differences in how they are specified. Drag is technically the same but that's because the airspeed is increased because it has to be for the necessary lift. Lift isn't a factor for cars.

Air density is a thing and it very much affects how easily something travels through the air. You can even notice less wind speed at high altitudes. If you play tennis like I do, you'll notice how different the physics are simply by the difference in how the balls move through the air. Even on a tennis ball, it is extremely observable with the naked eye. It is why tennis tournaments regulate playing at a relatively low altitude for the more prestigious tournaments.

It affects cars. It just isn't really talked about because the difference in oxygen outweighs the benefits of decreased drag for gas cars.

 

Air density is about half at a mile above sea level. So in terms of wind resistance, which gets increasingly significant after about 55 mph for most cars (depends on aerodynamics) the loss off efficiency specifically due to drag would be about half as much at similar speeds. Should easily translate to what would be the equivalent of about 2-5mpg depending on the car. But again, this would only be relevant for electrics and that would be a kwh measurement, which again, depends on the car.

Would be cool to see a test of this done. I know it would make a measurable difference. Each car deals with drag differently so there isn't a simple formula but there is (generally) for air density or atmosphere pressure at whatever altitudes.

 

Well not in a good way.

Why Nascar uses roof flaps Bad

Better

I don't think a Prius can get fast enough to get light.

 

*Does double take, thinks "can that be right? what are planes flying on?"*

Looking in the Imperial units chart over here, density at sea level is 23.77 (✕10^(-4) slug/ft^3). At a mile up (a shade over 5000 feet), it's about 20, not nearly down by half. To get close to half (11.9-ish), you're up between 20,000 and 25,000 feet, four miles and some change.

Looking in the metric chart on the same page, density of 1.225 kg/m^3 at sea level drops to half (0.612) between 6000 and 7000 meters ... 3.7 to 4.3 miles.

 

I'd use Pikes Peak or Mt. Evans. But the margin in your conclusion leaves plenty for them too.

My gasoline-powered cars always get very noticeably better MPG at the high altitudes in Colorado, Utah, Wyoming and Montana, something I've noticed since the late 1980s, from my first fuel injected car with automatic fuel-air mixture control. Many others have found the same.

Two reasons for this better MPG at high altitudes:

(1) At highway speeds, air drag commonly consumes more than 50% of the engine's power output. Cruising on high plains at 5000 feet elevation, air density is about 16% less than at sea level, significantly reducing drag and increasing MPG.

At 7000 feet, air density is 22% less. At 10,000 feet, 31% less. Cruising along the top of Trail Ridge Road in Rocky Mountain National Park, probably 35% less.

(2) The atmospheric composition doesn't change with elevation, at least within the range humans breath and operate their cars. The oxygen percentage is the same. Only the air density changes.

Traditional gasoline engines never put full density air into the engines except when the gas pedal is floored and throttle butterfly valve wide open. All the rest of the time they work at various levels of intake manifold vacuum, which is very substantial when idling or at low power, deeper vacuums than found at the tops of Colorado's 14ers. This turns the engine into a vacuum pump, wasting some power to maintain that vacuum. (This intake vacuum is how ordinary gasoline engines regulate their power. Diesel engines don't usually do this.)

At high altitudes, the vacuum difference between engine cylinder intake and outside ambient, shrinks. This means less wasted power as a vacuum pump, saving energy and boosting MPG.

The Atkinson-type engines on Prius and other hybrids, use other means to reduce this sea level 'pumping' loss, so have less to gain from this factor.

The thin air at high altitudes does cause engines to lose considerable maximum power capacity, because the maximum air-oxygen charge that can be put into the cylinder is reduced. But this matters only when trying to operate at or near maximum power. Normal level cruising at legal speeds doesn't use anywhere near this much power, so it is not a factor.

 

Are you confusing meters and feet? One half of sea level density happens at closer to 6,000 METERS. At a mile high, the air is obviously thinner. You just have to watch the punters and place kickers at a football game in Denver to see that. A professor at the University of Nebraska did a study that showed lickers averaging about 8 yards more per kick in Denver than at their home fields at or near sea level. So I'm not going to stand pat on "I don't think air drag at any highway elevation is much of a factor." But I'm not going to totally retract it since it all depends on where you draw the line at "much."

@fuzzy1 has a good point about gs mileage at higher altitudes. My primary experience was with carbureted engines. I can see that computerized fuel injection could greatly reduce that impact. And, in fact, we got pretty amazing mpgs in the Colorado high country last May. But using a Prime is kind of cheating. Pretty hard to sort out how much of that was the big battery and how much was reduced drag.

 

Back to braking regeneration. I am trying to understand how it actually slows the car. If the traction motor acts as a brake while generating current to charge the battery, does that mean that on a front wheel drive car only the front wheels are being braked with regeneration and the rear wheels are being braked conventionally?

 

I think yes. At least sometimes. Maybe the computers are able to just use the front (regen) braking, under some circumstances, but for sure the rear friction brakes are often used.

I think the front friction brakes are often employed as well, in conjunction with regen: it's not one or the other.

I am just winging it, but that's my take.

 

There's a computer in the car that calculates how much of the car's kinetic energy needs to go away (whether by storing it in the battery, turning it into heat at the brakes, or turning it into noise and heat by using it to spin the engine).

It then calculates how much of that can be done by each of those methods (taking into account such things as how fully charged the battery already is, temperature, etc.). It picks proportions, and then tells other computers in the car (the engine control module, the brake ECU, and an "MG ECU" inside the inverter box) to make it so.

It also monitors the results and updates its orders continually for as long as you are braking.

And yes, the braking by regen and the braking by engine spinning both rely on the power train, so they both apply only to the front wheels. The friction braking component applies at all wheels, and the brake ECU can control all four brakes separately.

If you are braking moderately gently and happen to go over a rock, manhole cover, or anything else that makes the car think the traction might be sketchy, it will drop the regen out completely and go full friction, which gives it the best control at all four wheels. A lot of people notice that effect and say "yikes, my brakes let go for a split second!" It does feel like that a little, but just for instant.

That's repeatable enough you can make it happen on purpose; when I bought my first Prius, people here on PriusChat advised me to do so a few times to learn what it feels like, so it wouldn't startle me some other time. It's good advice.

 

Yes but as @“Mendel Leisk” pointed out it uses both depending on speed and pressure. You would need a motor on the rear wheels to regen like my wife’s NXh.


iPad ? Pro

 

70% to 80% of braking force comes from the front, so front regen braking can get by without rear brakes in many situations. I can't say for sure, but the Prius may engage the rear friction brakes before the front ones in non panic stops. ABS systems now allow the car finer control of the brakes

RWD BEVs get past needing the front friction brakes through brute force. The motor and battery can apply more regen force than what rear friction brakes can.

 

Yes, he must be. Otherwise, air pressures on top the popular mountain hikes on Colorado's 14ers would be so low that even supplemental oxygen masks and canisters would be insufficient, hikers would need full astronaut-style pressure suits.

I am aware that carbureted engines have mixture issues at higher altitudes and often need different jets or other methods of adjustment. But these are beyond my experience.

Computerized fuel injection, with oxygen sensors in the exhaust stream to provide closed loop control of the air-fuel mixture, turns all that into a nothingburger. Nothing seems different until, on the steeper hills, the engine simply doesn't have as much power available as at sea level, so earlier downshifts are required.

 

Lower ambient air pressure effectively makes a naturally aspirate engine a smaller displacement than it is.

 

And in our world of generally much more powerful engines than actually needed, smaller engines ==>> greater MPG.

 

And if I'm not mistaken, it meant a richer mix in the days of carburetors, which hurt MPG. On the other hand, when we moved from Denver to Ohio in 1981, my carbureted car jetted for the mile high city started knocking. Gotta love computerized fuel injection.