Honda CRV vs 2020 Prius Prime for trip to Rocky Mountains

It's a blended braking system. My understanding is that even the lightest pressure on the brake pedal will drag the friction brakes slightly, even if all conditions are right for regen. But I have no way to confirm.

Also, electronic brakeforce distribution just means using the ABS to remove braking force from a wheel that has too much. Or just a marketing term for the ABS doing its normal job. It's a crude way to do it, but the advantage over a proportioning valve is that it's reliable and reacts to the road surface. As far as I know it does nothing during braking light enough not to activate the ABS.

 

The blending ratio is dynamic, the computer picks for you, varies the ratio continuously in response to received data and doesn't share a full report of what it is doing. You can use aftermarket tools and phone apps to record or display that data.

 

Yes, I know all of that....or most.
The question IS:
Is there really that much MORE intake vacuum with the throttle closed and no fuel versus the throttle closed and minimal fuel (coasting) ?

I suspect that there is little or no significant difference in the intake vacuum........and the overall "pumping losses" becomes the significant factor, led by the compression stroke and not the intake.

Because you have no GOOD way to know IF or how much friction braking is happening or when it actually cuts in.
And we are talking about going down a steep grade.

Under normal driving conditions, I see nothing wrong with that.......if it somehow makes you feel better.

 

When idling, the throttle will be open some to allow enough air to burn the fuel. With fuel cut off, the throttle can be completely closed. Then without fuel, part of the expansion stroke will be pulling vacuum as the spring effect of the compressed air isn't enough for 100% return. Downshifting then increases the rpms, magnifying the engine braking.

Engine braking with a diesel is about as just as effective as in a gasoline engine. Actually less in cars, or any diesel without a butterfly valve exhaust brake. Jake brakes are an add on built into the engine that adjusts exhaust valve timing when activated. It opens at the end of the compression stroke, releasing all that energy from compressing air. The Wikipedia article is titled compression release engine brake. Then the valve closes, and the expansion stroke is pulling vacuum in the cylinder.

In theory, Jake brakes can work with gasoline engines. There just isn't any reason to make such an engine as the brake system is only used class 8 semi trucks. There are CNG engines that have it.

 

I knew about HA. I see it all the time in @john1701a's videos and wish they had a version for iOS. I have an old Android phone. I might just experiment with that.

Edit to add. It's on the old HTC One and tested. We have a 20 mile out and 20 mile back trip coming up in a half hour. Good opportunity to try it in the real world. It sure looks impressive after seeing the video on their website.

 

This has been mentioned the whole time I've been a member here, and was shown in some Toyota diagrams that accompanied the Gen3 rollout. And it may have been increased a bit by the recall that addressed the braking 'pause' over bumps.

But during normal regeneration, the friction fraction is usually quite small, so the heat produced is also quite small compared to similar non-regen braking on a conventional vehicle.

 

You can install a ScanGauge-II or other OBDII-port engine monitor, and watch manifold pressure for yourself. (It has been a few years since I looked at that display, so I can't give you accurate numbers right now.) Just remember that a Prius with normal "coasting" also drops to idle RPM with possible fuel cutoff.

Pumping loss is also proportional to RPM. Downhill engine braking screaming at 4600 RPM is thus a lot stronger than at an idling 992 RPM, not even accounting for vacuum and valve timing differences.

In Otto cycle engines without variable valve timing, the near-adiabatic compression and expansion strokes offset each other. The first compresses the "spring", the second relaxes it and recovers the energy, for no net braking. Atkinson cycles and valve timing may shift things a bit so the offset is no longer perfect, but not greatly.

Thus, the only large pumping loss portion is the intake stroke drawing vacuum from the intake manifold. This energy is then 'wasted' when the exhaust valve opens, allowing atmospheric pressure to flood the cylinder without any energy recovery. From simple physics, the power needed to draw this vacuum is Power = (intake vacuum) * (RPM) * ( 0.5 * engine-displacement). Those who understand the physics can also figure out the unit conversions needed to get a meaningful numeric answer in familiar units.

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P.S. On the intake stroke equation, effective engine displacement on the Atkinson cycle engine is less than the 1.8 and 1.5 liter sizes commonly advertised for various Prii engines. But as Trollbait pointed out, that full displacement volume still gets used on the expansion stroke before the exhaust valve opens, so still contributes. Maybe even more so, as the vacuum ends up stronger than the intake manifold vacuum by the end of that expansion stroke.)

 

I still have the codes for the friction brake sensor, but it’s for my 2012 Prius - they may not be the same as on the 2020. But so you can try them if you want:
TXD: 07B02107
RXF: 046145070000
RXD: 2808
MTH: 00020001FFCE

I’m not sure if I put this in my original post but it also seems to start relying on the friction brakes around 114F BT2 temp and completely at 124F and above.

 

Thanks. I'm not sure how to integrate those into or find them in OBD Fusion since it just lists names of the various functions.

The easy thing to do is probably just use Hybrid Assistant. But that's going to involve buying a $60 OBDII adapter to go along with the other two I already have that don't work with HA as well as haul around two phones since they still refuse to port it to iOS.

 

Yeah I used these for the scangauge II a few years ago. I’m honestly not sure what the latest tech for this stuff is as I haven’t been looking into it lately. I will say the scan gauges are still going strong though! They definitely don’t look as slick as the mobile apps but they work.

 

It wasn't intended for "hills".
It was intended for places where you NEED engine braking to help slow down.

Those who use B all the time need to get used to the "feel".......and if you have to give it gas to maintain your speed going DOWN a grade, then it is kind of self defeating.

 

The PLX DM-100/200 were the bomb for this. You could completely customize the graphical presentation of the data, and the displays were easy to mount 60mm round OLEDs. You could pull data from the native PIDs in the car, or add your own sensors and encoders.

That product line seems to have faded away, now they just make some bridge box so you can get that data displayed on... you guessed it, your android phone.

I still like my scangauge though I've never even tried it in the Prius.

 

I normally use it (not a Prime) on 5% to 7% grades.

Depending on desired speed, the car should roll freely at constant speed in D gear on roughly a 1% to 3% grade, there is no point in using B at or below such a point.

" ... and just about came to a stop" ... Did you take your foot completely off the gas pedal? If so, then put it back on the pedal and apply some "fuel", really just reducing the amount of B braking.

He has a Prime. It is materially different than our non-Primes in this regard.

 

Descending from Stevens Pass on Highway 2 after a hike today, I pulled up the MAP (manifold air pressure) display and got some fresh numbers, expressed in psi ---

Home, engine not running (500 feet elevation): 14.4
On surface street at lowest elevation (<100 feet elevation): 14.6

Construction delay (<1000 feet elevation, 0 RPM): 14.2
Significant highway cruising power, lots of fuel burning (similar elevation, rolling terrain): 11.x - 12.8
Very low power (almost idle), minimal fuel burning: high 9.x to 10.x.

Estimated air pressure at 1500-2000 feet: 13.7-13.9
D-mode cruising shallow downhill at that elevation (992 RPM, no fuel): 6.8
B-mode braking steep downhill at that elevation (screaming 4600 RPM, no fuel): 2.8

Tomorrow I'll try to get a true stationary warm-up idle reading. But as the numbers above show, screaming B-mode engine braking produces considerably more manifold vacuum than does minimal D-mode engine braking. And remember that the 'pumping loss' component of that engine braking is also proportional to RPM.

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P.S. Warm-up idling, quite variable but mostly 7.5-7.9 (at home ambient pressure 14.4).

 

Did an 1800 mile through Colorado last summer, which included RMP. My PP did fine. The roads are paved and it's a comfortable car to drive. I also just got back from a 4000 mile trip through the western states. This time taking in some dirt roads as well. Again, the Prime did spectacularly well. Plus I got 60 mpg.

 

Beautiful! More pictures please!

 

Su-weet!! Yes, more pictures por favor!