Hybrid with "stuck" revs

[TedTX]

Giving it gas is the only way to stop the over revving (except for my "trick" above). When you experience it, you gladly give up some gas to get it to stop revving. Remember this occurs on significant mountainous downgrades - areas where losing your engine could be very, very bad.

Ok this is the first time on the internet I am going to call someone out and be careful.
You got this @ss backward. The truck is pushing the engine with the electric motors to keep the brakes from overheating when going downhill. The revving is safe because its not using gas.
The Truck is trying to protect YOUR ability to slow and stop, ie protect your life in a dangerous situation. It doesn’t need you to protect Truck from a mode it put itself in.
Granted its loud! All air braking/ engine braking is, Have you noticed the signs < no engine braking > <no air braking>. Truckers use it all the time to slow and save their brakes.

Sponsored

 

[Chicora]

Ok this is the first time on the internet I am going to call someone out and be careful.
You got this @ss backward. The truck is pushing the engine with the electric motors to keep the brakes from overheating when going downhill. The revving is safe because its not using gas.
The Truck is trying to protect YOUR ability to slow and stop, ie protect your life in a dangerous situation. It doesn’t need you to protect Truck from a mode it put itself in.
Granted its loud! All air braking/ engine braking is, Have you noticed the signs < no engine braking > <no air braking>. Truckers use it all the time to slow and save their brakes.

Understand, appreciate and accept your considered tone. Until it is experienced it is hard to understand. Once experienced it is impossible (apparently) to describe. Come up to the mountains and I'll show you.

 

[TedTX]

Accelerate down a mountain, you say...? This very important feature is designed to prevent exactly that, without command from the right pedal. To prevent a runaway truck.

Was not recommending this!! We actually agree it’s stupid. But that, was what the other guy I quoted was doing. He seems to think its helping and steadfast believes its a great idea. I just wanted to know where the extra-extra energy was going.

 

[jsus]

Was not recommending this!! We actually agree it’s stupid. But that, was what the other guy I quoted was doing. He seems to think its helping and steadfast believes its a great idea. I just wanted to know where the extra-extra energy was going.

The post I quoted of yours didn't quote anyone...

Reality is it's just an overreaction and misunderstanding that we've already cleared up for OP.

The reason why the ICE would quiet down (stop) when you get on the "gas" is that you're no longer coasting, so in the case of descending a mountain, it no longer needs to use the hill descent control when coasting (or with cruise control set).

Even on level ground, with an ICE vehicle, when you're coasting, the engine keeps spinning which consumes some of your kinetic energy and slows you down a bit. Modern engines don't burn gas while doing this; everything is already spinning so that's where the energy comes from. That allows it to keep powering the alternator and other systems like climate control and steering.

A hybrid is a bit different, since even climate control can be powered fully by the HVB. The hybrid design allows the ICE to stop spinning entirely while you continue to move, with full control to everything else. To replace those "parasitic losses" when coasting, a hybrid will apply some amount of regen braking - the exact amount depends on your selected drive mode.

Further, both when coasting and with cruise control enabled, a hybrid will use the regenerative braking system to keep the vehicle from accelerating past the current or set speed. Basically, keeping you from speeding up downhill.

However, a FHEV (full hybrid) has a rather small HVB, as compared to a PHEV (plug-in hybrid) or a BEV. The result is that if, say, you're descending a steep mountain, the HVB will quickly fill up. Now how is it going to keep you from speeding up uncontrolled?

Yep, that's what OP experienced. It's part of the Grade Assist feature. It just allows the engine to spin up just like any other ICE vehicle, except there's no alternator or other belt-driven accessories to power. The engine isn't adding power to the system - not speeding you up nor is it burning any gas - it's helping to take energy away. It's just engine braking. And as an engine, complete with oil for lubrication, it's fully designed to do a lot of spinning. It's not going to spin fast enough to do damage.

Yes, it is going to make some noise. No, it does not mean there's anything wrong. Except, of course, that you could use a larger HVB if you're regularly descending steep grades.

If you're really that bothered by the noise, occasionally apply the brakes instead. That'll get the friction brakes engaged (since regen isn't available at that point), meaning there's less energy for the ICE to dissipate. But then you run into the other issue with mountain driving, which is overheating your brakes. Hot friction brakes + full HVB == you're going to have a bad time.

Or, just think, wow, this is some impressive engineering. And then go back to watching the road instead of dwelling on something operating as designed, in a way that will do no harm.

 

[TedTX]

Dude, been there/ done that, this is my second hybrid. I have experienced West Texas mountains. Its jarring and makes a scary situation even more terrifying.

 

[TedTX]

Did find it odd that I had the engine break happen in the Maverick in the city on a route with just normal overpasses. Only occurs on a set of mountains in West Texas for my Prius.

 

[MakinDoForNow]

Understand, appreciate and accept your considered tone. Until it is experienced it is hard to understand. Once experienced it is impossible (apparently) to describe. Come up to the mountains and I'll show you.

You can experience engine braking in city driving do not need BAM (Big A$$ Mountain). In STD or ECON push the "L" button = maximize Regen Braking. Drive one pedal except when need to come to complete stop. After 10-15 min you hear engine brake happen say at 20-25 mph (the HVB will be at 70% so Regen braking will be retarded and ice braking is active) take the PUPPY up to 45 mph or so and let off the go pedal = (4500+/- rpm) LOUD ENGINE BRAKING!

 

[Chicora]

You can experience engine braking in city driving do not need BAM (Big A$$ Mountain). In STD or ECON push the "L" button = maximize Regen Braking. Drive one pedal except when need to come to complete stop. After 10-15 min you hear engine brake happen say at 20-25 mph (the HVB will be at 70% so Regen braking will be retarded and ice braking is active) take the PUPPY up to 45 mph or so and let off the go pedal = (4500+/- rpm) LOUD ENGINE BRAKING!

I never thought about intentionally trying to over revv. But it sounds like you got it! Congrats (I think?)

 

[TedTX]

Crazy Maverick Tricks, lol, maybe we can show off our skills to the younger noobs with making our engines really loud.
Some one with good ForScan skills could match the RPMs to the radio Bass range and get an awesomely loud thump-a-thump.
I am old enough to remember the trick to get your floppy drive motor to play a song.

 

[TedTX]

The post I quoted of yours didn't quote anyone...

Reality is it's just an overreaction and misunderstanding that we've already cleared up for OP.

The reason why the ICE would quiet down (stop) when you get on the "gas" is that you're no longer coasting, so in the case of descending a mountain, it no longer needs to use the hill descent control when coasting (or with cruise control set).

Even on level ground, with an ICE vehicle, when you're coasting, the engine keeps spinning which consumes some of your kinetic energy and slows you down a bit. Modern engines don't burn gas while doing this; everything is already spinning so that's where the energy comes from. That allows it to keep powering the alternator and other systems like climate control and steering.

A hybrid is a bit different, since even climate control can be powered fully by the HVB. The hybrid design allows the ICE to stop spinning entirely while you continue to move, with full control to everything else. To replace those "parasitic losses" when coasting, a hybrid will apply some amount of regen braking - the exact amount depends on your selected drive mode.

Further, both when coasting and with cruise control enabled, a hybrid will use the regenerative braking system to keep the vehicle from accelerating past the current or set speed. Basically, keeping you from speeding up downhill.

However, a FHEV (full hybrid) has a rather small HVB, as compared to a PHEV (plug-in hybrid) or a BEV. The result is that if, say, you're descending a steep mountain, the HVB will quickly fill up. Now how is it going to keep you from speeding up uncontrolled?

Yep, that's what OP experienced. It's part of the Grade Assist feature. It just allows the engine to spin up just like any other ICE vehicle, except there's no alternator or other belt-driven accessories to power. The engine isn't adding power to the system - not speeding you up nor is it burning any gas - it's helping to take energy away. It's just engine braking. And as an engine, complete with oil for lubrication, it's fully designed to do a lot of spinning. It's not going to spin fast enough to do damage.

Yes, it is going to make some noise. No, it does not mean there's anything wrong. Except, of course, that you could use a larger HVB if you're regularly descending steep grades.

If you're really that bothered by the noise, occasionally apply the brakes instead. That'll get the friction brakes engaged (since regen isn't available at that point), meaning there's less energy for the ICE to dissipate. But then you run into the other issue with mountain driving, which is overheating your brakes. Hot friction brakes + full HVB == you're going to have a bad time.

Or, just think, wow, this is some impressive engineering. And then go back to watching the road instead of dwelling on something operating as designed, in a way that will do no harm.

Really good write-up, now I am going to re-edit my post as well and add the missing quote….

 

[MakinDoForNow]

I never thought about intentionally trying to over revv. But it sounds like you got it! Congrats (I think?)

I discovered LOW when many were complaining about their brakes grabbing, etc when starting and stopping in heavy traffic. I put mine in LOW and found it is excellent in stop and go traffic and not having to pay close attention to Regen braking pedal pushing. Everyone needs to try LOW to know it's there. FWIW if your HVB has some room left for Regen current when you are coming to a turn say at 50 mph let off the go pedal and push the L button instead of the brake pedal. Regen goes to max and when Regen pointer get to 50-60% or less press the brake pedal to control your speed down to where you want to turn at. If you come to a complete stop this makes 100% brake really easy to attain while stopping as quickly as possible. Does take a little practice.

 

[GPSMan]

I think it's all be covered. Very normal.
The truck has a tilt sensor. So it knows you are on a hill even if you are not riding the brakes. In the Prius and older model hybrids, you only got the engine brake effect after the battery was "full" and you were riding the brakes.

The Mav will do it based on tilt sensor so you will see the effect more often.

But normal, and by design.

 

[abcinv]

I appreciate all of the follow-up. I feel like I should have known this after reading the replies...

 

[jsus]

I appreciate all of the follow-up. I feel like I should have known this after reading the replies...

No worries!

Sponsored