Engine Braking

The car should be doing regenerative braking first, then engine braking if the former isn't enough.

The only other choices are friction brakes, or a crash.

 

Why? What speed were you doing? Is the battery full?. It will initially use REGENERATIVE braking (unless the battery is full), as well as engine braking and the friction brakes - (otherwise it'll run into the car in front).

If the battery isn't fully charged and not severe braking, it will use regenerative braking. The computer works out how far the car in front is and how severe the braking has to be, then applies the most appropriate braking.

Friction braking is the primary braking - like any other car, it will bring the car to a halt very quickly. Engine braking, I've only noticed when the battery is full. Regenerative braking won't completely stop the PRIUS (the friction brakes do the last few km/hr to being stopped - it is part of the HYBRID system, to capture energy and store in the battery to be re-used in driving later, and is probably used most.

But the PRIUS uses REGEN first - and sometimes almost till standstill if the braking isn't severe.

 

You could switch to the standard cruise control instead of dynamic cruise control, by pressing and holding the button on the end of the stalk for 1.5 seconds or more.

 

THIS. Other brands call it "adaptive" cruise control.
On my Ford, the activation distance is adjustable too.
I can't imagine a good reason to turn it off.......but to each his own, I guess.

 

Are you hearing engine RPM climb? That's the only clue I can think of, that would suggest engine braking is occurring. Try shifting to B on a good downgrade with some decent speed, apply the brakes. That'll give you a feel for when the car's engine braking.

I strongly suspect nothing's wrong here.

 

For what it's worth, my 2016 Four Touring does not act in this manner. When in CC, even in the event of quick slowing, the regen braking and the friction braking are the only means used. When in "B" mode (Engine Braking), CC should be inoperable.

 

My Gen 3 will divert regen braking amps to spinning the engine if the battery state-of-charge is high. They have to go somewhere....

-Chap

 

I guess we need to know from the OP if:
A) He wants the automatic braking to stop completely
OR
B) If he wants to change how the braking works.

A should be possible. B is not......unless he is driving in B already.

 

Mine does - in undulating hilly driving, if it's got a full battery, I've heard the ICE spinning on downhill runs. But, it HAS to do that (or WASTE friction brakes) above certain speeds, otherwise MG1 will over-speed.

 

I can't understand why anyone would use engine braking when regenerative braking is available. It seems like a waste of potential energy that you could be putting in your battery.

 

In D, the car does not resort to engine braking until regenerative braking becomes unavailable (because the battery has reached the programmed upper charge limit).

If you shift to B, the car will use more engine braking earlier, and less regen. This, as you say, is a waste of energy that you could be putting in your battery. So why would you ever select that mode?

If you are in the HEV (not the plugin/Prime PHEV), you have a rather small battery. There are downhills long enough that your battery is going to hit the charge limit even at the lower regen rate of B mode. If you are on one of those hills, B will reduce the stress and temperature of the battery, prolonging its life.

If you have the PHEV, you have a much larger battery, one that essentially won't reach full charge descending most hills on Earth (unless it happens to be near full already when you start). So in the PHEV, you might indeed prefer to capture all the energy you can. Starting with the gen 5 Prime, there is a different description of B mode in the owners' manual that reflects that difference in strategy.

 

I was doubtful that "B" mode was providing any regen, but it was confirmed for me today. Going down a long hill, my gauge indicated partial regeneration when I applied the engine brake. But you can always get the maximum regen from pressing the brake pedal.

But it makes perfect sense that the "B" position would provide some regeneration. It would be wasteful if all it did was dissipate energy by producing hot air (as an engine brake does). Why waste regen energy when your hybrid battery can store energy from it?

Of course, it's a different story when your hybrid battery has reached full capacity. There is simply no good way to dissipate the energy of the decline without burning the brake pads, except for engine braking. During engine braking, of course the engine is turning, even though it is producing no power. It has to turn for the engine braking to work, but it's not like a conventional vehicle, where there is always some drag from the compression of the engine, which is never stopped, like it is in the Prius. I don't see the Prius gas engine turning at all during a decline, unless engine braking is applied.

 

Actually the Prius engine braking is far superior to a standard ICE car. On the Prius, I believe the fuel injectors are OFF and the transmission is locked onto the engine crankshaft, providing more drag.
I've ran into some steep declines that required me to also use my brakes along with B to maintain control of my decent.

 

I'm not sure what you mean by that. The gas engine and the electric motor/generator are both connected to the drive wheels through a planetary differential. But that's not what creates drag. From what I've read, the "B" position manipulates valve timing to work as an engine brake, which dissipates energy by compressing air and then releasing it as heated air.

There's no such thing as free drag. Energy cannot be either created or destroyed; it can only be converted into another form of energy. In this case, it's heat.
https://i.etsystatic.com/39954128/r/il/012f42/4994972707/il_1588xN.4994972707_91lk.jpg

 

You've written that before, so it seems you may have read a misleading source somewhere.

Although Prius engines do have variable intake valve timing, that's for other purposes, and doesn't play any significant role in engine braking. The description you've just given sounds a lot like Jacobs braking, used on diesel engines lacking throttles. But Jake braking involves manipulating exhaust valve timing, which a Prius doesn't do.

The Prius engine's braking effectiveness is pretty much the same as the gasoline engine in grandpa's car. The only 'valve' manipulated to do it is the throttle, which is left at minimum opening. That forces the engine to pull a high vacuum against it, with a restricted flow of air producing turbulence and, yes, heat.

There isn't a whole lot of compression in the cylinder, because the high manifold vacuum lets only so much air into the cylinder to compress. That's ok, because with no Jacobs exhaust-valve hack, compressing air in the cylinder wouldn't help the engine braking much. It just uncompresses on the piston's way down and gives almost all the compression energy back.

The pumping loss across the closed throttle is where the engine is doing mechanical work that it doesn't get back.

Here, I agree with Paul that seems a strange way of putting it. The transmission isn't doing anything especially noteworthy here. Like pretty much all transmissions, it is able to transmit power from the engine to the wheels or vice versa. For engine braking it just does the vice versa part. The way our transmission does that inside is fun to think about, and follow the two power paths, one mechanical and one electrical from MG2 to MG1. But then, the way other transmissions do that inside is also fun to think about, with fluid torque converters and clutch packs and brake bands and stuff, in the end, what the transmission is doing (transmitting power from the wheels to the engine) is the same and doesn't require you to delve into the internals.

Our transmission, being continuously variable in ratio (a CVT) does have an advantage over transmissions that just have a selection of fixed ratios. Our HV control ECU can know what the engine's rev limit is, and how much braking torque it produces at different RPMs, and how much braking torque the current driving conditions call for, and effect a CVT ratio that twirls the engine at just the chosen RPM, whatever the road speed is. With a non-CVT, you'd have to pick the nearest gear one side or the other, and possibly worry about overrevving the engine, which Prius engine braking will never do.

But aside from the extra flexibility of the continuously-variable ratio, there's nothing especially magic going on. The CVT ratio that ends up getting picked will be somewhere near the conventional gear ratio you'd probably downshift to if you were driving a stick. On the engine braking test run I did in 2023, the ratio from wheels to engine worked out to about the same as 2nd gear in my last stick-shift passenger car, which is about what I would have downshifted to for that same hill and speed.

 

And I quote:
"What is the mechanism of the engine brake?



When activated, the engine brake alters the operation of the engine's exhaust valves so that the engine works as a power-absorbing air compressor. This provides a retarding, or slowing, action to the vehicle's drive wheels, enabling you to have improved vehicle control without using the service brakes*.

Source: Toyota Prius/Driving tricks - Wikibooks, open books for an open world.

It's easy to be misled by the internet, especially when the misinformation outnumbers the information.

 

Interestingly, you have colored much of that text blue—often the color of a link—only none of it is a link, except the Toyota Prius/Driving tricks - Wikibooks, open books for an open world part.

And following that link (despite you labeling it as "Source:") reveals that it is not the source of the text you placed above it, about the engine exhaust valves and the power-absorbing air compressor; that text isn't on that page.

A quick search for the actual source of the text you quoted locates it exactly, on a different page:

"When activated, the engine brake alters the operation of the engine's exhaust valves so that the engine works as a power-absorbing air compressor" - Google Search

The top hit is a page from Cummins Inc. They make diesel engines, and the page is a description of Jacobs braking. Here's the whole first paragraph, including what you left out:

The Jacobs Engine Brake® (also known as the "Jake Brake®") is an engine retarder that uses the engine to aid in slowing and controlling the vehicle. When activated, the engine brake alters the operation of the engine's exhaust valves so that the engine works as a power-absorbing air compressor. This provides a retarding, or slowing, action to the vehicle's drive wheels, enabling you to have improved vehicle control without using the service brakes*. This conservation results in reduced service brake maintenance, shorter trip times, and lower total cost of ownership.​

As #16 explained, Jacobs braking is used in diesel engines that don't have throttles. (It also makes that very recognizable, loud, 'compressed air being popped off at top of piston stroke' sound you hear from large diesel tractors, that leads to a lot of "no jake braking" signs in populated areas.)

A Prius, though, does not vary exhaust valve timing, and does not do jake braking. It does suck-against-closed-throttle braking just like the gasoline engine in grandpa's car.

 

When you can't refute the facts, resort to the minutia and deception. I tried to make it clear which words I was quoting and which were my own. You are free to believe what you like, but I was accused of not having read it anywhere, so I cited my source.

For some reason, you have developed a strong confirmation bias against my being right with what I believe to be the facts. If you have an issue with Wikibooks, take it up with them. They aren't known for spreading misinformation.

You certainly haven't refuted what I've said so far.

 

Were you? Or did I clearly say you might have read a misleading source somewhere?

After all, you did say this, which I totally agree with:

And because that is so true, I was completely prepared to be understanding and sympathetic if you came back and provided a source of misinformation on the internet that you had been misled by.

But that's not what you did.

You don't get to say "I cited my source" when you quoted words from a different source and then cited a Wikibooks article where those words do not appear, as if that were your source.

Why would I have an issue with Wikibooks? What's on that page (which is not what you "quoted") isn't wrong.

What you "quoted" (which is not what's on that page) also isn't wrong in the source where it really appears (which is a page about diesel engines and Jake brakes, nothing to do with Prius).

So this isn't a case of you innocently stumbling onto some pre-existing misinformation on the internet and being misled by it and passing it along.

It is a case of you finding multiple sources on the internet that are not, in themselves, misinformation—just talking about different things—and mashing them up yourself, deceptively citing one source for words that are not in it, and in that way manufacturing brand-new misinformation to mislead us.

That's just plain not nice.