How Often Do You Use The B Mode In The Tranny?

[Colo Springs RX400h]

Below is an answer given on another forum regarding the "B" Gear Position. I believe it to be correct based upon experiments I performed as stated earlier in this thread. I believe if you perform the experiments that I mentioned, you can see for yourself that putting the transmission into B mode increases the regenerative capability. I believe that the reason Lexus discourages the frequent use of the B Mode is to prevent the overheating of the rear motor - just as this is the reason they discourage heavy off road use.

"Deceleration in “B” Range

While the vehicle is being driven with the shift position in the B, and decelerates, the wheels drive MG2

and MGR, causing MG2 and MGR to operate as a generator, charge the HV battery, and supply electrical

power to MG1. Accordingly, MG1 maintains the speed of the engine and applies an engine brake. At this

time, the fuel to the engine is cut."

[wwest]

The answer is:

Regenerative braking is not as effective when used in conjunction with ("B" mode) engine braking. Yes, the batteries will continue to be charged but since engine compression is providing some of the braking HP the regenerative "efficiency" will be lower.

If the batteries are almost fully discharged and the roadspeed is fairly low then engine braking might not even come into effect even in "B" mode.

[skyfish400h]

The answer is:

Regenerative braking is not as effective when used in conjunction with ("B" mode) engine braking. Yes, the batteries will continue to be charged but since engine compression is providing some of the braking HP the regenerative "efficiency" will be lower.

If the batteries are almost fully discharged and the roadspeed is fairly low then engine braking might not even come into effect even in "B" mode.

I think you guys are confusing the control system details with the physics. The first pharagraph is true. The second one is not. The engine compression is what is providing the decel. The electrics are only used as a signaling devise to cut off the fuel.

The "B" mode should be used on long decents precicely because it will continue to slow the vehice even when the batteries are full... because its the compression...

[jditom]

The answer is:

Regenerative braking is not as effective when used in conjunction with ("B" mode) engine braking. Yes, the batteries will continue to be charged but since engine compression is providing some of the braking HP the regenerative "efficiency" will be lower.

If the batteries are almost fully discharged and the roadspeed is fairly low then engine braking might not even come into effect even in "B" mode.

Not true! Try it yourself. Look at your power meter when going down hill in B mode you will dip into the blue. Now when in B mode apply some braking and it dips almost to the bottom of the blue really generating lots of E!

But I see what you are saying in that if you are using B mode to slow you down you may not need as much regen. braking. This all depends on the grade of the hill and length. Where I live in Oregon it is real easy for me to do both B mode and regen. braking going down step hills everyday.

Below is an answer given on another forum regarding the "B" Gear Position. I believe it to be correct based upon experiments I performed as stated earlier in this thread. I believe if you perform the experiments that I mentioned, you can see for yourself that putting the transmission into B mode increases the regenerative capability. I believe that the reason Lexus discourages the frequent use of the B Mode is to prevent the overheating of the rear motor - just as this is the reason they discourage heavy off road use.

"Deceleration in “B” Range

While the vehicle is being driven with the shift position in the B, and decelerates, the wheels drive MG2

and MGR, causing MG2 and MGR to operate as a generator, charge the HV battery, and supply electrical

power to MG1. Accordingly, MG1 maintains the speed of the engine and applies an engine brake. At this

time, the fuel to the engine is cut."

Finally some real science and data!

Thanks :)

[wwest]

Think of it this way...

Lets begin with the hybrid battery needing to be seriously (re)charged.

As you apply pressure to the brake pedal the regenerative braking system will start charging the batteries and as you apply more and more pressure to the brake pedal the battery charge rate due to regnerative braking will also increase incrementally. At some point the system will decide to start bringing in friction braking and at that point regenerative braking will stop increasing in effectiveness.

If you, instead of applying the brakes, switch into "B" mode the question becomes which "braking" effect will be the majority source for slowing the vehicle? Should the hybrid battery happen to be already fully charged then the answer is quite obvious, you will be left with only engine compression braking.

So, as a general rule the use of "B" will, on the average, reduce the effectiveness of regenerative braking.

Obviously if the hybrid battery charge is already "topped-off" then forget about regenerative brakingat any level.

If

[SC1911]

I finally had an opportunity to use the B trans mode today. On a cross country trip, I was making my way westward, across the I-90 at the downhill side of the peak- somewhere on the Montana side between Coeur D'Alene and Missoula. It started snowing harder and the word Blizzard came to mind. Big rigs were slipping and pulled over to the side of the road. There was 2-3" on the roads. The truck ahead of me stopped behind 3-4 other big rigs who stopped, apparently not wanting to risk losing control and sliding into another rig on the downhill side of I90.

A BMW 7-series (who obviously has done this before) came from behind and went around several trucks and thru an opening between the crooked line. I followed, and not wanting to use brakes on this snow covered 6% grade at near 7000ft in 28 deg temps, wanted to drop it into a lower gear, but B was my only choice. It felt like engine braking. I turned the nav display to look at the energy screens. It showed the battery was charging. I applied some gas to see if the display changed from charging to supplying juice to the electric motors- the display continued to show battery charging. Handy in this situation. On that 6% grade, B kept speeds at about 20-25mph. I had my RX400 loaded with myself, wife,and 200-300 pounds of gear. Dang spooky for a while there. But I got down the mountain just fine. So, for me, the "B" gear is for Battery. ;)

S. Chang

07 RX 400h AWD

[wwest]

I finally had an opportunity to use the B trans mode today. On a cross country trip, I was making my way westward, across the I-90 at the downhill side of the peak- somewhere on the Montana side between Coeur D'Alene and Missoula. It started snowing harder and the word Blizzard came to mind. Big rigs were slipping and pulled over to the side of the road. There was 2-3" on the roads. The truck ahead of me stopped behind 3-4 other big rigs who stopped, apparently not wanting to risk losing control and sliding into another rig on the downhill side of I90.

A BMW 7-series (who obviously has done this before) came from behind and went around several trucks and thru an opening between the crooked line. I followed, and not wanting to use brakes on this snow covered 6% grade at near 7000ft in 28 deg temps, wanted to drop it into a lower gear, but B was my only choice. It felt like engine braking. I turned the nav display to look at the energy screens. It showed the battery was charging. I applied some gas to see if the display changed from charging to supplying juice to the electric motors- the display continued to show battery charging. Handy in this situation. On that 6% grade, B kept speeds at about 20-25mph. I had my RX400 loaded with myself, wife,and 200-300 pounds of gear. Dang spooky for a while there. But I got down the mountain just fine. So, for me, the "B" gear is for Battery. ;)

S. Chang

07 RX 400h AWD

Well, you're wrong, and luckily not DEAD wrong.

"B" mode most definitely enables engine braking.

Engine braking on a FWD vehicle on slippery LEVEL roadbed can be faught with hazards. But on a downhill slippery slope, run, you are damn lucky you didn't lose control on one of those downhill turns.

Granted, "B" mode is to be used to prevent the friction brakes from being overheated and fading in almost EXACTLY the circumstance you describe. The exception is NOT on a downhill SLIPPERY roadbed! In that case it would probably be best to use the e-brake judisously, lightly, to provide the light REAR braking you need to keep your downhill speed within reason.

Advice from someone having accrued THOUSANDS of miles in MT winters.

And by the way, that BMW-7 was RWD, a totally different animal on the downhill side of a snowy/icy mountain pass. All he has to do is lift the throttle slightly to attain light rear braking and "pull" the car back into "line" should it begin to wonder. And even better if it had a manual transmission.

Golden rule, never follow a "LION" into its natural habitat.

Oh, and one more thing.

Those "rigs" were most likely stopping at the top to remove the chains from the tractor, driving, wheels where they would be inherently unsafe, but for an even improved safety factor leave them on the rearmost axle of the trailer to provide "drag" braking on the downhill side, just as you might wish to do with the e-brake.

[skyfish400h]

Going down mountain roads, I use the B position. It keeps you from having to hold your foot on the brake for the entire decent.

You all are more mechanical than me. But when I look at the guage on the left and put the transmission into B mode, the gauge moves into the "Blue" zone which shows that it is generating electricity. Also, adding brakes to the B mode gets you farther into the "Blue" zone than just brakes. Are you sure the B mode has no generating capacity?

Nobody has said the B mode does not re-generate. It does. What it ALSO does is use engine compression so you don't have to use the brakes as much to suplement...

Think about what would happen when the batteries are full and cannot take any more charge. Normally in D the system would tell the motors to stop regenerating and then it would fall onto the brakes to slow the car. In the B mode when the batteries are full, the engine compression comes in to fill the gap so you don't rely on the brakes as much. This is needed for long decents to prevent the brakes from overheating.

The confusion comes from the statement that the system will not be as effecient under B mode and this is true. Rather than taking advantage of the body in motion when you are at speed to cover ground without using any energy, the B mode is contantly trying to slow you down. This back and forth is not effecient, but it does come in handy on long downgrades.

[SOMEONE ELSE'S GOT MY NAME]

Wow! Seldome do we all get to experience so many varied "positively certain" - type opinions ~ that are diametricly opposed. Not wanting to get my feelings hurt here, ( ) with stern contradictions about B mode ... I'll only say, "ask your service tech". I'm guessing he knows what he was talking about, when he told me that, "B mode does such & such". I'm such a coward.

[wwest]

Wow! Seldome do we all get to experience so many varied "positively certain" - type opinions ~ that are diametricly opposed. Not wanting to get my feelings hurt here, ( ) with stern contradictions about B mode ... I'll only say, "ask your service tech". I'm guessing he knows what he was talking about, when he told me that, "B mode does such & such". I'm such a coward.

Actually I have overhead salespersons tell customers that "B" mode stands for battery and it increases the hybrid battery recharge rate. In the few cases I have stepped in and challenged this statement there was one time that the salesman called a service technician who verified the salesman's statement to be true.

Page 414, left column of the 2006 RX400h drivers manual...

"{B} Using engine braking.

(picture)

To use engine braking intentionally, downshift the transmission to "B" position and take your foot off the accelerator pedal.

During high speed driving you may feel that engine braking is less effective than that of an ordinary vehicle.

Do not continue normal driving with the transmission in "B" for a long time. This may cause decreased fuel economy."

While not exactly explicit the last sentence clearly implies that the use of "B" mode will degrade the ability to use regenerative braking for recovering energy.

[retired00]

Wow! Seldome do we all get to experience so many varied "positively certain" - type opinions ~ that are diametricly opposed. Not wanting to get my feelings hurt here, ( ) with stern contradictions about B mode ... I'll only say, "ask your service tech". I'm guessing he knows what he was talking about, when he told me that, "B mode does such & such". I'm such a coward.

Actually I have overhead salespersons tell customers that "B" mode stands for battery and it increases the hybrid battery recharge rate. In the few cases I have stepped in and challenged this statement there was one time that the salesman called a service technician who verified the salesman's statement to be true.

Page 414, left column of the 2006 RX400h drivers manual...

"{B} Using engine braking.

(picture)

To use engine braking intentionally, downshift the transmission to "B" position and take your foot off the accelerator pedal.

During high speed driving you may feel that engine braking is less effective than that of an ordinary vehicle.

Do not continue normal driving with the transmission in "B" for a long time. This may cause decreased fuel economy."

While not exactly explicit the last sentence clearly implies that the use of "B" mode will degrade the ability to use regenerative braking for recovering energy.

and in the previous paragraph, in the "D" section, it leads into this section by saying it's for use on long downhill grades.

[SC1911]

I finally had an opportunity to use the B trans mode today. On a cross country trip, I was making my way westward, across the I-90 at the downhill side of the peak- somewhere on the Montana side between Coeur D'Alene and Missoula. It started snowing harder and the word Blizzard came to mind. Big rigs were slipping and pulled over to the side of the road. There was 2-3" on the roads. The truck ahead of me stopped behind 3-4 other big rigs who stopped, apparently not wanting to risk losing control and sliding into another rig on the downhill side of I90.

A BMW 7-series (who obviously has done this before) came from behind and went around several trucks and thru an opening between the crooked line. I followed, and not wanting to use brakes on this snow covered 6% grade at near 7000ft in 28 deg temps, wanted to drop it into a lower gear, but B was my only choice. It felt like engine braking. I turned the nav display to look at the energy screens. It showed the battery was charging. I applied some gas to see if the display changed from charging to supplying juice to the electric motors- the display continued to show battery charging. Handy in this situation. On that 6% grade, B kept speeds at about 20-25mph. I had my RX400 loaded with myself, wife,and 200-300 pounds of gear. Dang spooky for a while there. But I got down the mountain just fine. So, for me, the "B" gear is for Battery. ;)

S. Chang

07 RX 400h AWD

Well, you're wrong, and luckily not DEAD wrong.

"B" mode most definitely enables engine braking.

Engine braking on a FWD vehicle on slippery LEVEL roadbed can be faught with hazards. But on a downhill slippery slope, run, you are damn lucky you didn't lose control on one of those downhill turns.

Granted, "B" mode is to be used to prevent the friction brakes from being overheated and fading in almost EXACTLY the circumstance you describe. The exception is NOT on a downhill SLIPPERY roadbed! In that case it would probably be best to use the e-brake judisously, lightly, to provide the light REAR braking you need to keep your downhill speed within reason.

Advice from someone having accrued THOUSANDS of miles in MT winters.

And by the way, that BMW-7 was RWD, a totally different animal on the downhill side of a snowy/icy mountain pass. All he has to do is lift the throttle slightly to attain light rear braking and "pull" the car back into "line" should it begin to wonder. And even better if it had a manual transmission.

Golden rule, never follow a "LION" into its natural habitat.

Oh, and one more thing.

Those "rigs" were most likely stopping at the top to remove the chains from the tractor, driving, wheels where they would be inherently unsafe, but for an even improved safety factor leave them on the rearmost axle of the trailer to provide "drag" braking on the downhill side, just as you might wish to do with the e-brake.

I followed the 7-series, a RWD vehicle, same as my BMW 528i and Z3, thru the hole he found between the rigs. From my vantage point, I couldnt see there was a clear path between the trucks. Tho my RX-400h is labeled AWD, it has Michelin all season tires, or more accurately, NO SEASON tires. I had no intention of trying to keep up with the 7-series, who prob had real snows on and knew how to drive in those conditions, as I didnt see his car in the ditch or piled up on the guardrail. I've only put a couple 1000 miles on my RX400h and am still figuring out what's what. I crawled along (in retrospect, my speeds were 1-5-10 mph thru the worst of the storm, and gradually increased as conditions improved). After I got past the stopped big rigs, I stayed in "B" till the road surfaces improved. I didn't see anyone on the top of the pass mounting chains, but it was apparent they weren't going anywhere without them. In a mile or two, the snow stopped falling, and the road surfaces were getting better. I was white knucked, going slow -- and taking hands off the wheel to do e-braking is out of my skill set.

I'll give the e-brake technique a try when I find a nice safe slick place to play with it.

[wwest]

I finally had an opportunity to use the B trans mode today. On a cross country trip, I was making my way westward, across the I-90 at the downhill side of the peak- somewhere on the Montana side between Coeur D'Alene and Missoula. It started snowing harder and the word Blizzard came to mind. Big rigs were slipping and pulled over to the side of the road. There was 2-3" on the roads. The truck ahead of me stopped behind 3-4 other big rigs who stopped, apparently not wanting to risk losing control and sliding into another rig on the downhill side of I90.

A BMW 7-series (who obviously has done this before) came from behind and went around several trucks and thru an opening between the crooked line. I followed, and not wanting to use brakes on this snow covered 6% grade at near 7000ft in 28 deg temps, wanted to drop it into a lower gear, but B was my only choice. It felt like engine braking. I turned the nav display to look at the energy screens. It showed the battery was charging. I applied some gas to see if the display changed from charging to supplying juice to the electric motors- the display continued to show battery charging. Handy in this situation. On that 6% grade, B kept speeds at about 20-25mph. I had my RX400 loaded with myself, wife,and 200-300 pounds of gear. Dang spooky for a while there. But I got down the mountain just fine. So, for me, the "B" gear is for Battery. ;)

S. Chang

07 RX 400h AWD

Well, you're wrong, and luckily not DEAD wrong.

"B" mode most definitely enables engine braking.

Engine braking on a FWD vehicle on slippery LEVEL roadbed can be faught with hazards. But on a downhill slippery slope, run, you are damn lucky you didn't lose control on one of those downhill turns.

Granted, "B" mode is to be used to prevent the friction brakes from being overheated and fading in almost EXACTLY the circumstance you describe. The exception is NOT on a downhill SLIPPERY roadbed! In that case it would probably be best to use the e-brake judisously, lightly, to provide the light REAR braking you need to keep your downhill speed within reason.

Advice from someone having accrued THOUSANDS of miles in MT winters.

And by the way, that BMW-7 was RWD, a totally different animal on the downhill side of a snowy/icy mountain pass. All he has to do is lift the throttle slightly to attain light rear braking and "pull" the car back into "line" should it begin to wonder. And even better if it had a manual transmission.

Golden rule, never follow a "LION" into its natural habitat.

Oh, and one more thing.

Those "rigs" were most likely stopping at the top to remove the chains from the tractor, driving, wheels where they would be inherently unsafe, but for an even improved safety factor leave them on the rearmost axle of the trailer to provide "drag" braking on the downhill side, just as you might wish to do with the e-brake.

I followed the 7-series, a RWD vehicle, same as my BMW 528i and Z3, thru the hole he found between the rigs. From my vantage point, I couldnt see there was a clear path between the trucks. Tho my RX-400h is labeled AWD, it has Michelin all season tires, or more accurately, NO SEASON tires. I had no intention of trying to keep up with the 7-series, who prob had real snows on and knew how to drive in those conditions, as I didnt see his car in the ditch or piled up on the guardrail. I've only put a couple 1000 miles on my RX400h and am still figuring out what's what. I crawled along (in retrospect, my speeds were 1-5-10 mph thru the worst of the storm, and gradually increased as conditions improved). After I got past the stopped big rigs, I stayed in "B" till the road surfaces improved. I didn't see anyone on the top of the pass mounting chains, but it was apparent they weren't going anywhere without them. In a mile or two, the snow stopped falling, and the road surfaces were getting better. I was white knucked, going slow -- and taking hands off the wheel to do e-braking is out of my skill set.

I'll give the e-brake technique a try when I find a nice safe slick place to play with it.

On my '01 RX the e-brake would latch into a braking position once applied, until I removed the latching mechanism that is.