Uncomfortable Over-Revving Sound on Long Hill

So was the 4L used by Jeep.

 

It rev's more than my Gen 2 would do. I just notice when I came down a hill with DRCC on the high rev's sounded so bad I turned off the cruise. The DRCC mode controls the speed downhill, unlike the normal cruise control mode.

 

Normal cruise control, controls speed downhill on the Gen3, spinning up to high RPM to produce substantial engine braking when needed. I can't speak for Gen2.

Some other vehicles also have cruise controls that control downhill speed, applying brakes as needed.

 

I would guess that our DRCC mode applies the brakes on hills or what ever is necessary to precisely control the speed. It does this following the car in front of you. I have used it on steep up and down hills right to 0 speed. Normal CC mode just lets off the gas and allows minimum regeneration. The car speed goes to what ever the hill size will push it to.

 

Gen 2 doesn't rev the engine or engage more than the normal minimal regen braking with cruise control on going down steep hills. The cruise control will back all of the way off the throttle, but the engine will then just sit at it's minimum rpm (e.g. 960rpm between 42-72mph) and there will be some very gentle regen braking just like if you were completely off the gas pedal on a downhill. It will allow the car to go faster than the set cruise speed.

(To OP)
Like other Pri-i, once the battery is at 8 bars, it will rev the engine to use the excess battery charge and keep the battery below 80% to extend its life. For this reason, when going up and down steep hills (e.g. I-70 in Colorado) I turn off cruise control and use B mode on hills that will cause me to get a full battery. In B mode it will go up to and hold 4992 rpm while in S4 mode, according to my Scanguage.

The major takeaway is that when the car is showing a full battery, the engine reving up is normal behavior. Also, when you have a full battery, regen braking will be turned off, so if you're going down a long steep hill, you should use B mode to minimize the amount of braking you need to do. The Prius' brakes are sized in consideration of using regen at the same time, so they're easy to overheat on a long steep decline if your battery is full.

 

That is quite different than my Gen3's normal CC mode. Downhill, it can provide far more regeneration than simple D mode. I use it for regenerative braking frequently (e.g. today, descending from the North Cascades passes), on hills short enough to not need B mode compression braking. On longer hills it also revs the engine after the battery is filled, but B mode is more appropriate for that, revving the engine even before the battery fills.

 

It does. It is gearing down, using the motor's internal frictional drag to control speed, rather than burning up the brakes to maintain the cruise control set speed. My former Prius c did the same thing.

 

To add to Kevin's point, you don't need to use B for braking unless the battery is full or the engine is starting to race. Take advantage of normal regen in D for normal braking and normal downhill conditions.
If you can anticipate a long downhill and use up battery beforehand, then you can take advantage of the regen before the traction battery fills up.
On a long downhill you could also max the A/C, turn on the defogger, turn up the sub-woofer to use 12V battery, which gets charged from the traction battery and reduce the need to select B.

 

If the hill is long enough that the batter will fill up regardless, then you may as well engage B from the top. This will delay the filling of the battery somewhat, spinning the ICE at moderate speed until the battery is filled and reducing the amount of time spent spinning at high RPM. The slower battery charging will also leave it less hot.

 

To add even more to this - the car will run the engine at about 1700 RPM if it only has one bar, and 1280 rpm if it has two bars to recharge the battery, even fully off the gas pedal, so if you hit one of these hills, a good strategy to reduce fuel use if you're below 3 bars is to use the brake pedal until you have 3 bars and then switch to B mode. I tried B mode last time I was driving I-70 with one battery bar and it didn't go into fuel cutoff (shown by a reading of 2000+ instantaneous mpg on my scanguage) until I got up to 3 battery bars.

 

It would be nice to know the RPM of the engine when it does this. At 40 to 50MPH the engine RPM is a lot faster than I would rev an engine to. I have a few hills I encounter this on daily. I tried this this yesterday. As the engine was screaming I put the car in neutral. Seamed fine, the car coasted. The engine braking disengaged and the car didn't gain much speed. I applied a bit of brakes, which I assume was mechanical. I need to do some more testing, but It felt better than the engine screaming for a minute down a hill.

 

Perception seems to be the crux of the matter. The OP titled this thread "Uncomfortable Over-Revving Sound..." and this recent post is about the engine "screaming"....

The energy burn-off, like anything else the powertrain does, is under full control of the ECU, which knows the engine's rev limits. My old Gen 1 I never saw rev above 4500 (which is the max-power point in its operating curve); in the Gen 2 (same basic engine) they stretched that to 5000. Neither of those would really qualify as "screaming" ... keep in mind the Echo used the same basic engine, in a non-Atkinson variant, and its max-power point was at 6000.

I don't have the New Car Features Manual open for Gen 3 or Gen 4 right now, but Wikipedia puts the max-power point for the 2016 at 5200 rpm, which I think is about the max I've ever seen my Gen 3 turn, too.

If at first you're uneasy about how it sounds, just get a ScanGauge or some OBD phone app, like Torque, and watch the revs, and you'll see the car knows exactly what it's doing at all times.

Then it goes from being an "uncomfortable" sound to a beautiful sound ... it's the sound of your car calculating the most effective way of combining regen, friction braking, and engine braking for the specific conditions you're in.

If you go shifting into neutral or otherwise fussing at it, you're just trying to outsmart the car, quite unnecessarily in this case.

-Chap

 

I suppose people are used to the sounds of cars with conventional transmissions, which have always coupled engine speed to road speed to some extent. As your conventional car accelerates, the engine sound winds up.

Prius sound works like boat or airplane sound, telling you how much power is being produced (or dissipated, in the engine braking case), irrespective of road speed.

Sit in your airplane at takeoff. You hear the engines wind up to their takeoff power level, you sink back in your seat, the plane begins to roll, and you just hear that engine sound hold constant as the plane rolls faster and faster, eventually taking off, then the crew adjusts the engine level for the climb and the cruise.

Pretty much exactly how a Prius under constant-power acceleration sounds. (Or any car with a CVT for that matter, even the mechanical kinds.)

But people still hear vroom and connect it to road speed.

-Chap

 

Yes, if you put it in neutral, I believe it won't do any regenerative braking. I thought I read in the MANUAL not to leave the car in N but can't find it now.

 

Right, since the Prius has no shiftable gears, and highway regs require a 'neutral' setting, the only way that's accomplished is by cutting all electrical paths in and out of the motor/generators, so no regen for you.

In Gen 1, it was possible to damage the transaxle that way, having taken away the computer's ability to enforce the MG1 rev limits. Starting in Gen 2, they addressed that possibility by defining a few special cases where the computer can temporarily override your selection of "neutral". Those 3 cases are on page TH-46 of the 2004 New Car Features Manual, so they reduced the risk of that kind of damage, at the cost of turning the already sort of weird idea of Prius "neutral" into an even more, um, flexible, concept.

But this is a bit into the weeds from the main point. The engine isn't screaming on your downhills, it is spinning at a fully controlled speed well within its safe limits, so you don't need to worry about just learning to recognize that sound as normal operation for your car, following its own calculations of how to best reclaim the energy it can reclaim and dissipate the rest. Fussing and interfering isn't necessary, and isn't likely to improve on the car's own choice of strategy.

-Chap