Fully Charged HV Battery==No Regenerative Braking?

Thanks

 

Absolutely not. This is a recipe for smoking the brakes, possibly leading to loss of brakes and loss of control. And it doesn't save any power. If the slope is long and steep enough to require any braking, the car must be left in gear.

The aroma of burning brakes was imposed on me yesterday by a heavy tractor-trailer rig descending a long grade without sufficiently downshifting at the start. Fortunately, it wasn't the steepest grade in the region, the smoke never got thick, he never lost control, and I switched to recirculated air quite early. But on other steeper grades in the region, I have witnessed extremely thick smoke from a truck's burning brakes (driver was able to stop), and have seen the crash scars left by others that did lose control, sometimes fatally. While light cars are less likely to suffer this problem, it can still happen.

 

Interestingly, I've always made analogies between spacecraft, aviation vehicles and the humble Prius. In this case, going a long way down hill in "B" mode (self or HSD induced), while hearing the ICE revving up to almost 3000 rpm reminds me of an aircraft trying to bleed off speed after landing, by invoking "Reverse Thrust".......or, alternatively, a spacecraft using "retro thrusters" to slow down!

The Prius.....a lot more than a car.....a little less than a Spacecraft

 

I have a quick question guys! Gosh i feel like i'm always asking questions. So last week I drove a small-ish mountain pass the up side really isn't that steep and it's very curvy so traffic goes from 70 mph to 45-50 on the way up, so my battery didn't drop one bar (good yes! Stayed at 6 bars SOC) but then that means it will fill up quickly on the way down where it's very steep and longer.

So I used B mode all the way down and of course less then half way down the battery filled because it was almost full to begin with! Well let me tell you, my engine revved like I've ever see it before. WAYYYYYY past 3000 rpm I can promise you. I can almost guarantee it was at its 5000 rpm redline for Almost 5 minutes. near the end I even saw a small cloud of smoke from my Behind. I felt terrible! My car doesn't use very much oil but after that trip I saw a drop in oil level. it seemed to not use any friction brakes (I didn't even use the brake pedal much) thanks! p.s I don't think it's even revved that high under full throttle acceleration

 

Traction battery 2008 with 125,000 miles perfect health. ICE has 241,000 miles and seems to run good and smooth except the 1st second of the cold start. Other than that, oil changes ever 3,000-4,000 miles. Pretty much a maintenance freak

 

Did you have an engine monitor (SacnGauge-II, Torque, etc.) to show you the actual RPM? I haven't seen my GenIII go above about 4500 RPM in this condition, well below its redline.

 

Sadly I don't have one. So i don't have proof Of my claims, it makes me never want to drive this strech of road again, but I know I will, not often but sometimes. It wouldn't be bad if the battery would drain on the way up, I've put the AC on max up and down to keep the battery cool and use power.

The real reason its bothering to me is that the poor little engine has so many miles and I'd never ask an egine with 241,000 to run that fast for such a long time. It breaks my heart! Lol

 

Hi Aaron. Yes the engine can rev 4000+ RPM in "B" mode, but remember that it's not under load and is only acting as an engine brake to dump energy. So it shouldn't be causing "smoke" as there shouldn't even be fuel injection.

It is possible however that during this period of engine rotation without ignition, that a small amount of oil is being drawn through the valve stem seals into the cylinders. I'm guessing that the puff of smoke was just this small amount of accumulated oil burning off as the power and fuel injection recommenced at the bottom of the hill.

You could try not using "B" mode, and this will certainly delay the onset of the high rev engine dumping, but on a long decent like that it will still eventually happen. In any case remember that the engine and oil are fully warmed at the time, so the engine is extremely well lubricated and under little load despite the revs. Hopefully that will at least ease your mind a little.

 

Hi Uart! I do believe you are exactly right on the smoke, that makes perfect sense. And yes your information does put my mind to ease (almost). Even though I still do like it revving so high. Next time I go up the mountain I'll try on off, on off of the throttle a little more, that can drain the battery more with the battery trying to provide instant acceleration on throttle depress.

 

If the hill is long enough so that engine braking will happen anyway it’s better for the engine and other vice to use B-mode from the beginning. Engine braking (and rpm) will be less in B-mode since engine braking is used for entire length of the downhill. In normal mode engine braking isn’t used until battery is full and then it has to be used more.

 

When in B mode, you can adjust the RPM of the spinning engine with the go-pedal. If you hold a little bit of pressure on the go-pedal, it will reduce the regen and the engine RPM will come down. You can combine this will strategic pulses of the brake pedal (a few seconds of application here and there, as needed) for a safe descent with reduced engine RPM.

I agree with valde3: engaging B mode earlier in the descent seems to help reduce engine RPM toward the end by helping regulate speed and HV SOC the whole way down. Activiting B mode most of the way down, at 80 mph with 80% charge, will really spin the engine up.

I like to think of a descending Prius as a little power plant. Run the A/C, charge some accessories... "free" power!

 

Great advice! Thank you so much

 

I drove up and down a long winding hill a few days ago. Elevation change is ~3500ft, ~12mi, 8% grade. On the way up, I was worried that the the traction battery SOC would drop dramatically to assist the ICE. Nope, from watching the MFD the ICE was doing most of the work and SOC stayed at 5-6 bars. On the way down, SOC was at 8 bars and I didn't notice the ICE kicking in to dump the extra energy. I try to avoid this by turning on the AC, fan blower, headlights, stereo to help consume some of the energy. If the grade is not that steep, don't ride the brake pedal as it would kick in regen more aggressively. Braking is best done is pulses... brake gently.... release to cool.... brake....

 

When descending a hill, has anybody noticed if the battery will flash when fully charged and still going downhill?

 

ECU does it for you. It will kick ICE on and use it for braking along with friction brakes. There is no loss of power except you'll feel small hiccup.

 

Not to the same degree. While conceptually the ECU will do the same thing, the quantity of engine braking is different. B can provide substantially more engine braking than D ever will.

I had understood pulse braking to be necessary only for mismatched and unbalanced brakes -- normal for tractor-trailer rigs that are continually swapping trailers, and common on old fashioned cars. The goal is to more evenly distribute the heat among all the brakes, not allow a few more sensitive ones take more than their share. But this is not normally an issue on modern cars with good proportioning systems and EBD.

On a car with regeneration, pulse braking can be wasteful. E.g. my spouse regularly applies pulses well above the regen limit, unnecessarily throwing away energy in friction. While it doesn't matter on your long hills that fill the battery regardless, it does matter on shorter hills where the battery does not fill. But I've given up trying to retrain her to make best use of HSD.

 

Agree. I've had the experience of coming down a local ski hill, a couple of times recently. Shifting to B causes a real hells-a-popping increase in rpm.

 

I didn't know Prii could ski!

 

On this coast, our snow is very thin this year. That means our Prii can navigate the local ski slopes without getting high centered, unlike downtown Boston.

 

There was ZERO snow at the top. Don't ask...: my wife and son have their own agenda, wanted to go up there anyway.