Honda CRV vs 2020 Prius Prime for trip to Rocky Mountains

A question for other Prime owners: how much braking drag does battery charging produce in this model? It seems that it ought to be considerably more than in a regular Prius, due to the more powerful battery and M/Gs.

My Liftback's battery charging drag can produce all the braking I need on many of my regular hills -- until the battery is nearly full, which isn't long enough.

Judicious use of the brake pedal should also produce maximum charging, when one selects a braking level that is predominantly regenerative, not friction. I am quite familiar with how this is displayed on the Gen3 Liftback HSI display, but other models are different.

When the battery is not filled, riding the brake for regeneration (not friction) on a Prius should be just fine. Only after the battery fills does it shift to the same problem as in conventional cars.

On a regular Prius, the battery will fill fairly quickly, typically just a few hundred vertical feet of descent. But for a Prime starting without a nearly full battery, it should go many thousands of vertical feet.

 

OR if you mis-judge how much to push the pedal.

I am pretty sure, with a regular hybrid Prius, that when you are zipping down a mountain grade AND the engine is screaming because it is exerting max. compression braking.......that pushing the brake pedal will NOT result in any additional regenerated energy being produced.

 

It is well displayed on certain models, easy for drivers to use. But I can't speak to your 'c' model.

Even if misjudged, it should still be better off than most conventional cars.

Damned straight ... because the engine screaming bit happens only after the "battery is not filled" condition that I mentioned, has ended.

And this is a Prime thread. The Prime will go much much farther before its battery fills. A great many states don't have the necessary road topography, unless the driver charges the battery close to the top of the hill.

 

The Hybrid System Indicator lies to you...

Long before the battery fills up, the car will start to reduce the charge current (or power) limit. Normally it is 40kW, but when descending a long downhill in the Prime it will often drop to 20-30kW. The CCL is shown on the Hybrid Assistant app.

As the CCL drops, the transition from Regen to friction braking happens much sooner, to avoid putting too much power into the battery. But the bar on the HSI doesn't necessarily indicate any difference.

In addition, if you're on a long and steep enough downhill, the engine will start providing engine braking well before you exceed the CCL. That could happen in as little as 1 minute on a steep downhill at highway speed.

The worst CCL I've seen was on a ski trip where my battery was frozen from being parked in 10 degree F weather overnight for a couple nights. I wasn't able to charge to use the battery heaters, and the drive out of telluride is basically all downhill, so there's no opportunity for anything to warm up. On that drive the CCL quickly dropped to about 1.5-2kW and stayed there until I stopped paying attention in Ridgeway. Engine braking is also not super effective at 45mph, so I had to use the brakes a lot.

 

Thanks for the 40kW charge rate limit. It is about a 50% boost over the regular Gen3 cap.

Was any of this bad enough to run into brake overheat problems, as compared to conventional cars on the same grades?

 

I've never seen signs of the brakes overheating. B mode is usually adequate except at lower speeds (and then the risk of overheating them is lower anyway). It is good to be cautious, because the Prius brakes seem to be relatively small and weak.

I have seen someone warp their rotors in a regular car on a single mountain descent. They didn't know to downshift.

 

My approach on long downhills is to just put the car in cruise. The problem with B is that is applies a fixed amount of regeneration, whereas cruise control varies the amount of regeneration or throttle as required to maintain the speed. What I don't know is what cruise does when the battery gets full. Does it apply the friction brakes, or engage the engine braking as per B mode?

BTW, just to be pedantic, engine braking with gasoline engines is not compressive braking; it is vacuum braking. The manifold vacuum creates the drag. Large diesel engines can use compression braking ("Jake Brake"), which make a horrible amount of noise, and are thus banned in many places. No fuel is used either way, but the difference is in which engine cycle the braking occurs.

 

Cruise control works well on roads straight enough to allow constant speed, but falls apart where repeated curves and corners demand pre-braking, which disengages CC. E.g. when descending from Sunrise in Mt. Rainier National Park, as we did yesterday after a hike to Fremont Lookout and Burroughs.

B mode strength is easily modulated by the gas pedal, and is not disengaged when friction brakes must momentarily but frequently be added.

I'm noticing some signs forbidding only "unmuffled compression braking". Is this a nod to the practice of the loudest offenders being the rigs that replace mufflers with straight pipes?

 

It will definitely employ engine braking. I don't believe the cruise control can make use of the friction brakes in any version of the Prius.

When I've used CC on hill descents I notice that the car will overrun the set speed by a small amount. It's not terrible, but it just doesn't seem to be as authoritative when it is asked to slow the car as when it is asked maintain headway against resistance. Not sure if this would be similar in a Prime model, would be curious to learn.

 

Pikes Peak has (or did when I was there) a mandatory brake checkpoint, where an attendant with an IR remote thermometer checks brake temperatures of all descending vehicles.

While waiting for the spouse to skim through the adjacent gift shop, I suddenly became aware of a really horrid brake smell. A smoking F-250 with Florida plates was being directed into the 30 minute cooling off parking zone. Not surprised that it was from a state that has no place where downshifting is necessary.

 

That is Jacobs Vehicle Systems(inventor of the Jake brake) stance. The difference between muffled and straight pipe trucks can be 20dBA; 80 to 100 dBA.
https://www.jacobsvehiclesystems.com/sites/default/files/2018-08/Jacobs%20Vehicle%20Systems%20Engine%20Brake%20Noise%20Brochure_0.pdf

Heard a story that the heat from the brakes caused the rubber brake line to melt, spraying fluid on the out rotors, which then ignited.

 

DRCC will use the friction brakes if it needs to slow down or stop for a car in front of you. So it has the ability, but I don't know if it ever will apply the friction brakes just to maintain speed on a downhill with no cars in front of you.

 

Unless you screw with the valve timing, ALL of the engine motion creates "drag"......if you don't feed it any fuel.
And.....the engine is built to withstand high pressures inside the cylinders, not necessarily so in the intake manifold.
Upon what do you base that conclusion ??

 

In gasoline engines, it is called "pumping loss", which has been discussed a lot over the years here in PriusChat. The intake side of the engine is effectively a large vacuum pump. You can easily compute how much power this drag component consumes simply from intake manifold vacuum, engine RPM, and effective (i.e. adjusted for valve timing) displacement.

Reduced pumping loss is one of the reasons Atkinson-cycle engines in hybrids are more efficient at normal cruising loads than are Otto-cycle engines in conventional cars.

During engine braking, this drag component is boosted (by higher RPM, increased vacuum from closing the throttle, and setting valve timing to maximum effective displacement) while fuel-burning power production is eliminated.

 

Worth making that distinction. The radar cruise control gets friction brake authority, but not the regular cruise control.

 

In a vehicle with regenerative braking available (not already maxed out), what would be the reasoning for not riding the brakes, even a little?

I understand in terms of conventional vehicles. For long steep downhills, engine braking is needed to prevent overheated friction brakes, regardless of vehicle type. Hybrids too.

And for lesser hills, light riding caused problems when brakes were unbalanced or mismatched, selectively overheating the positions that engaged with the lightest touch, while the others contributed nothing. This was common with old-era vehicles, and trucks, and still is the norm for towing combinations. E.g. tractor-trailers where the tractor truck gets hitched to a different trailer every day. Heavy pulsed braking spreads the heat more evenly among all the brake positions.

But modern passenger cars with individual brake proportioning, and now electronic brake force distribution too, ought to be far less susceptible. With proper balance among the positions, light riding or heavy pulsing should be a wash, the total heat is the same.

Then move up to regenerative brakes on hybrids and plug-ins. During primarily regen, there is no meaningful brake heat. So lightly riding them on moderate downhills should then be keeping well within regeneration limits, reducing conversion losses and battery I-squared-R heating, and even allow staying within regeneration when the allowed battery charge rate is reduced for whatever reasons. Thus maximizing total energy recovery, while possibly reduce battery heating too. These advantages end if/when the battery is filled and engine braking become necessary.

Some hybrid drivers habitually brake too hard to stay within regeneration limits, thus reducing the regen benefits. And not getting the doubling of brake life that others gain. My spouse is one of them, having been originally trained to use heavy pulse braking instead of longer lighter braking, based on those old era rules. She just won't change to fit the new hybrid architecture, so her pulses greatly exceed our Prius's max regen limit, thus a bunch of energy is necessarily (but needlessly, from my viewpoint) thrown away as friction brake heat.

Am I missing some other reasons to avoid lightly riding my Prius brakes?

 

There are about a gazillion PIDs for the braking system. But, from their names, I can't tell which one (or ones) give notice that the hydraulic brake system is applying pressure to the brake pistons. If I knew which one it was, it would be simple to add another gauge to my phone app. In fact, OBD Fusion has two identical categories in Network A for Toyota called ABS/VSC/TRAC. Would it be stroke sensor? Stroke sensor 2? Status of the Stroke open? Status of the Stroke2 Open? Status of the Solenoid Relay? Status of the RR-RL-FR-FL Wheel Sensors? Status of the Regenerative Cooperation? And the list of possibilities goes on.

I found out that there's an X-Gauge for a friction brake sensor. According to @Kramah313 in post #14 in this conversation, Regenerative vs. Friction Braking | PriusChat, the friction brakes do not engage until the CHG bar is full except for special situations like slow speed, full battery, etc.

I spent way more time than I should have searching through the available PIDs in OBD Fusion trying to find the friction brake sensor there. Haven't found it yet. It would be nice to have a light or something to confirm what Kramah said.

 

That's where the Hybrid Assistant app (available only for Android OS) can help you. I don't usually look at the HA in realtime while driving, but if you set up a phone holder to show the dashboard, it can show you the realtime brake operation along with other useful hybrid information.