Full Battery & going downhill - Engine coming on?

Soooo, what does the Volt and/or Bolt do if you are using the paddles to brake down a hill with a full battery?

 

volt probably does the same as prius, bolt must switch to friction braking?

 

The Bolt allows hilltop owners a setting to not fully charge their battery at home in anticipation of downhill regen on the next drive.

I don't have hills but I can experience this ICE on for regen in my Energi when my battery is fully charged and I apply the brakes fairly hard while in L gear at the end of my block. Using my scangauge, I have determined that it does go into a warmup cycle burning gas to use up that excess regen charge.

 

This is something I hoped they would have fixed between the PiP and Prime.
There's no reasonable reason for the car to fire up the ICE and go through the whole warm-up cycle just because you brake with a full battery. Toyota should definitely have programmed the car to simply use normal friction braking instead. Brake pads are cheaper than fuel.
A way to manually avoid this behavior is to shift to Neutral when going down hill with a full battery.

 

Doing that is unsafe on a large hill. I've seen many people on the side of the road with their brakes on fire from descending I-70 into Denver by using their brakes instead of engine braking.

 

now we need brake temp sensors.

 

I remember a former colleague from Colorado, working we me out here in Texas, saying that Coloradans can always tell when they're following a Texan down a mountain pass by the "eau de burning brake pads" trailing off behind them! (I'm a Texan, so I can say that!)


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Does Norway have any roads with escape ramps for runaway trucks? If not, then your reply is easier to understand.

The same concept applies to cars. On certain long steep mountain roads, brakes alone will overheat and fail. The engine is needed to keep speed down to a safe level.

 

(After you recharge the battery.)

Conceptually though, compression braking doesn't require any fuel be burnt. Hmmm... I think at least.

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There are two kinds of engine braking. One has to do with compression on the upstroke and then just releasing the compressed air, and the other has to do with creating a manifold vacuum that makes it difficult to move the pistons down. Wikipedia suggest that it is the latter that gasoline engines do by closing down the throttle, and the former which diesel engines do ("jake brake"). Neither involve burning fuel. But as to which mechanism is used in the Prius, I cannot say.

 

Yip, definitely not going to get much manifold vacuum on a diesel...

From what I've seen at least, taking my foot off the gas on a conventional automatic transmission only helps a bit, even in low gear, but downshifting a manual is a very different story.


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I have driven automatics, often towing a boat, over the divide since before I was 16. Down-shifting an automatic is VERY effective. So much so, that even towing I could usually make a long 6-7% grade without touching the brakes.

I recently drove our automatic Mazda5 from Denver to Breckenridge and back without touching the brakes on any hill.

 

That is true for modern fuel injected engines. Typical carbureted engines, and the original fuel injected engines, still burned some fuel during compression braking.

Prius very definitely uses the later, creating a strong intake manifold vacuum.

While my household hasn't owned a convention automatic in over three decades, I clearly remember good downshift compression braking, both in mine back then and in the others I've driven since.

 

I'm wondering how you know that, considering it has variable-valve-timing.

 

It was described here and in Toyota product details, many times during and after the Gen 3 product reveal, shortly after I joined PriusChat. And I watched the manifold pressure on my ScanGauge-II.

The VVT system is primarily for the intake valves. While the exhaust valves move too, the adjustment range is nowhere enough to produce a diesel jake-brake type pressure release.

 

So in gasoline engines, it's really not "compression" braking. It is "vacuum" braking? I understand how energy is dissipated with "compression" braking in diesels: the air is heated by being compressed on the upstroke, and then is simply vented, rather than the energy being recovered by expanding in the downstroke. But with gasoline engines, the braking happens on the downstroke, pulling against the manifold vacuum. Why isn't the energy recovered on the upstroke? Or is this the "exhaust" upstroke cycle, where in normal operation, burned fuel is is exhausted, not the "compression" cycle, where the fuel/air mixture is compressed before being ignited? In gasoline engines, the energy is dissapated by "uncompressing" the air and just releasing it, whereas in the diesel scenario, there is both the energy used to compress the air, as well as the heating caused by the compression, that contributes to the braking?

Just trying to understand the physics here.

 

This is probably a semantics issue.

My mind is still in partial zombie mode, so is yet ready to review the whole situation. But the gasoline engine does create a large vacuum flow rate, then re-compresses it back to atmospheric pressure. Because of valve operation, the process is not balanced or 'reversible'.

In the first stroke, the intake valve is open the entire stroke, exposing the piston to full manifold vacuum for the entire stroke.

The next two strokes (the normal compression and power strokes) keep all valves closed, so cylinder pressure varies through these strokes, but ends up as a wash.

For the fourth stroke, the exhaust valve is opened at the start, letting air back into the cylinder and exposing the piston to full atmospheric pressure for the entire stroke. This creates the irreversible vacuum/compression asymmetry that consumes power.

 

My observation is even after I finished downhill engine would still be on, for a few min.

 

We Priusers are not idiots. Regular cars don’t use low gear automatically, then why Prius?

 

My Mazda5 does shift down when descending a long hill in cruise control.