Is there a legitimate use for charge mode in the Prime?

It's not a situation you're likely to find yourself often, but when your hybrid battery is full, and you're on a long descent, there's a lot of energy to be dissipated. One way would be to burn up the friction brakes, but you'd need another option. Regen braking is out of the question because there's no capacity left in the battery to take it. I think that is why the Toyota engineers included the "B" (engine braking) option.

This is more often a problem for larger vehicles, like 18-wheelers; the driver can suddenly find themselves behind the wheel of a large load, going down a long slope, with a large amount of energy to dissipate. Brakes wear out, and in that case, they are no help. Engine braking can help, but if that's not available, or adequate, a dangerous situation exists. This is why runaway lanes are constructed on such slopes.

One might be forgiven for thinking that engine drag would be sufficient to keep the vehicle from picking up dangerous speeds, but it depends on the slope, and some are too much for that. Such slopes do exist, and a Prius driver should be prepared to use engine braking where necessary.

 

I hate it when it starts and runs the engine for 5-10 minutes in D mode for a small hill that lasted less than 15 seconds. It's stupid.

 

"I think that is why the Toyota engineers included the "B" (engine braking) option."

It certainly is among the reasons they equipped the car to do engine braking, whether you apply it early by selecting the B mode, or just wait for the car to apply it automatically as the battery approaches 80% SoC.

The typical reason you'd apply it early would be if you are beginning a descent that you know will be more than long enough to reach that charge by the bottom; selecting B in that situation will keep battery stress and temperature down by diverting some energy to engine braking for the whole descent.

On a shorter descent, you might also choose to remain in D to capture more of the energy into the battery, and the transition to engine braking won't begin until the battery approaches 80% state of charge.

 

Driving my Gen 5, I noticed a difference from my old Gen 3: Going down a hill, while setting the cruise control to a lower speed I hear the engine racing, as well as the regen gauge showing significant regen. I assume this is for safety reasons; sacrificing a bit of regen by employing a bit of engine drag to get the vehicle's speed in line with the lower cruise setting. With the Gen 3, it just slowed down gradually from regen braking alone, something that might carry you through a playground zone, speeding a bit too fast.

 

I drive a gen 3 regularly, and have already datalogged and quantified its engine braking behavior. You have seen the data before.

How much engine braking a gen 3 is capable of | PriusChat

TL;DR roughly 13 kilowatts. Descending a hill around 36 MPH it's about the same braking effect you would feel in a stick-shift car downshifted to 2nd (which is a pretty emphatic downshift at that speed).

 

On what degree of slope? Some are steeper than others.

 

Indeed they are. Which is why knowing the engine braking capacity in kilowatts is useful.

The energy gain from gravity depends not just on the slope but also the car's speed and loaded weight.

On a descent where that gain amounts to less than 13 kW, the engine braking will slow you. On a descent where the gain roughly is 13 kW, it will roughly hold your speed. If descending at a higher rate of gain, you will have to add some uses of the friction brakes to maintain speed. (But you will need 13 kW less friction braking than you'd need without the engine braking.)

The computation of the rate of energy gain based on slope, speed, and weight has been illustrated before in some posts made by, if I remember right, fuzzy1.

 

I suppose that if it were a simple matter; the combination of engine drag, regeneration and prudent use of friction brakes were enough to ensure safety in all situations, there'd be no need for an engine brake at all.

 

In #149 you've again written both "engine drag" and "engine brake" as if you believe them to be different phenomena.

The engine braking effect in a spark-ignited gasoline engine like the Prius is chiefly from the development of vacuum from pumping across a closed throttle. The Prius engine can give you a measured ~ 13 kW of that.

It can give you ~ 13 kW of that whether you have asked for it yourself by shifting to B, or the car has simply done so on its own because the battery has reached 80% SoC. The timing may be different; the mechanism is the same.

Does that make it "a simple matter"? Like anything else, it's as simple as it is, but no simpler. The math is reasonably straightforward.

 

It looks like you have had the Prime for more than 6 months. Have you ever encountered a drive where the car was not able to slow upon demand? I've only 15,000 miles on mine and about 48 round trips up and over mountainous passes as part of long distance travels up and down the length of California.

I've never had a situation when any of my 4 hybrid cars (used them for 23 years so far) were unable to handle the grade of California highways through the Northern Siskyews (bad spelling) or Sierra Nevada mountain range.

 

I'm not sure whether other cars have both or not in Paul-land. Do all cars have both? Or does only the Prius have both, so the other cars are unsafe?

In reality, it seems like they're equivalent from a control point of view - in both Priuses and other cars you do have to make a gear selection to get extra base no-brake-pedal-application drag. (I think that's a legal requirement, that a manual "more drag" selection exists).

But the Prius is smarter in that regardless of why the drag is selected (B, brake pedal or cruise) it will include high drag from the engine in preference to using brake pads. Most vehicles would not give you high engine drag without an explicit gear selection.

Conceivably B could have been different in that regard - not offering as many kW of engine drag as D did. But I guess they decided that would be pointless - having put in all that complex infrastructure required to apportion brake pedal application between actual brake pads and the HSD, there's no reason to not apportion drag requests between pads and engine just as you'd apportion between pads and regen.

A normal car couldn't make brake pedal applications request engine drag automatically, but the HSD makes it trivial.

Which is then a safety benefit, because it automatically protects drivers who fail to make the B selection on a long downhill slope - they're not going to overheat their brake pads like the driver of a normal car who failed to downshift.

So you could say the Prius is equally safe as other cars when correctly operated - equivalent amount of engine drag available to spare the pads. But it's easier to correctly operate. Only way you're going to get the pads to suffer downhill is to select N, which is unlikely.

 

all you need to do is learn how the car works. there are ways to avoid most of the stupid things we think the car does.

 

I've never had to use the engine brake, but if I found myself going down a long slope where I couldn't rely on engine drag, regen braking or friction braking, I'd be glad to have it. If it wasn't necessary, I'm sure it would have been eliminated to cut costs long ago.

 

Every car with a gasoline engine inherently has engine braking available, working the same way it works in the Prius.

They could cut costs by eliminating it, but people would miss having an engine.

 

What I do is brake harder a few times instead of keeping my foot on the brake. Seems to help but I assume people behind me aren't too impressed with my driving when doing that.

 

I'm old enough to remember when some people thought seat belts were stupid; "all you need to do is brace yourself with your hands."

 

I remember that too, though maybe I'm missing the connection to this thread, where I haven't seen anyone question the usefulness of engine braking.

Toyota agreed that it's useful, to the point that they made sure the car is able to do it, whether you call for it yourself by selecting B mode or wait for the car to do it when the battery fills up.

 

I could probably get along without it. I negotiated a steep 12 mile hill in my VW by judiciously applying the brakes for short periods to slow down, before much heat built up, all the while downshifted as much as I dared. It's possible to do away with the engine brake I suppose, but Toyota doesn't want to be criticized for doing away with a safety feature. Not to say being vulnerable to a lawsuit.

 

Of course no one has even talked about doing away with it. Toyota supplies it, both as-needed when the battery charge is high, and when you choose to ask for it with the B selection on the shifter.