maseace

Good luck with that - the RXh is not the Accord PHEV. Lexus already designed the hybrid system to be as efficient as possible, so any attempt to trick it into better mpg overall is futile. The B mode on the RX will only hurt mpg since it engine brakes the car to slow you down. If you try to recharge the battery in traffic using B mode, you only use more gas, and the energy conversion to battery charge is inefficient. The only way to get "free" charging is downhill in B (gravity does the work) or regenerative braking.

spacecraft

Not necessarily.
I mean, if B is only used when it makes sense (i.e. in the situation I described in my previous post #12), the mpg should be unaffected. And on the positive side, you can save a bit of brakes wear.
But other than that, I agree that B mode is useless.

joedaddy1

iTrader: (1)

i thought it was for "engine braking"..

Like making sure your brakes don't get cooked coming down a mountain...
same as putting the gear to D3 (on a D4 automatic car).

I used it once because I swore I smelled something funny coming down a long mountain...

maximizese

I would disagree with you on Lexus designing the hybrid system to be as efficient as possible. It seems like Lexus and their dealerships don't recognize resetting the ISC. As for B mode, I really haven't experienced any noticeable difference. B mode doesn't seem to keep the ICE on, but it keeps the CVT in a higher ratio to slow the car down.

I drove my friend's 2015 Ford C-Max Energi and the information system displays the percentage of efficiency after each braking session when you come to a complete stop. I don't know of any way to measure the percentage or rate of regeneration on our Lexus; I was hoping there was a meter in Techstream, but can't seem to find anything like that.

MACRX400h

I have had my 2006 RX400h for 3 years but only recently started using B mode and noticed that the battery engages immediately when you back off the gas. Wow. More battery propulsion has easily helped my mpg jump as much as +5mpg commuting around town and +2 or +3 mpg highway compared to when I drive on the highway.

I have read about owners having had much better mpg for their RX-400h prior to some infamous Lexus “update.” If that is the case, it seems that the battery preserving update spared the B mode.

I wish the battery or electric propulsion engaged as readily in D mode. It would have made for a much better hybrid/gas conserving driving experience. Unfortunately, cruise control does not engage when I am in B mode.

Can anyone try to get similar results?

Full disclosure, I did replace my 12v battery and air filter, but it did not change my normal mpg in D mode. Thanks.

ukrkoz

1. there is no "battery propulsion" in B mode.
2. B mode is named B for Braking.
3. it is akin to engine braking in manual gear box. MG locks ICE, no fuel is supplied to ICE and ICE is electronically controlled to be used as ballast, so to speak, to slow down the vehicle. This is done by bleeding air through the cylinders, air flow controlled by VVT-i. Hence the noise. For what is can be called, it is hydraulic braking, as air is fluid.
At the same time, as MG is engaged, it does charge hybrid battery faster by virtue of producing higher amperage, than during regular regenerative braking. After battery is charged, excess energy produced is wasted. There is ongoing debate about if it charges battery faster or not. From experience with 2 hybrids, one of them being exect RX H mechanical twin, I can testify that it does. So I used B mode as much as I could. Both for battery recharging and for "pre braking'. Likely that's why my 12 HiHy averaged 32mpg.

MACRX400h

Hey ukkroz, thanks for the explanation and sorry about any confusion on my poor choice of words. I now understand that the regenerative braking is the emphasis in "B."

Could you shed any light on my perception that I am able to maintain current speeds much more readily exclusively using the hybrid system's electric power? Unless my gauges are misleading, it appears that I can use electric power more effectively and more often.

I have learned to accelerate to the speed I want and then get off the gas to let the gas engine stop working and carefully ease pressure onto the accelerator so as to engage only the electric motor. This process seems to work much better and more efficiently when driving in "B" than in "D"

Driving in B my RX-400h seems to get better mpg and at the same time feels more responsive. I rather like it. I just wanted to see if I could get confirmation from other users and to see if there were any reasons why I shouldn't be using it all the time.

Mando 06 RX-400h

mnewxcv

Be aware that in B, the gas engine runs *more* when you let off the gas pedal than it does in D. In D, if you let off the gas, the engine will typically shut off, while in B, it is used for engine braking. That being said, it is using little to no fuel at that point.

MACRX400h

Using B drive, I found that there is a sweet spot where I press on the accelerator to engage the electric motor and have successfully held cruising speeds up to 60mph at an insanely efficient 40-60mpg. This was not possible for me in D gear because the electric motor would not kick in as much or as strongly.

Anyways, thank you all for your input.

ukrkoz

NO FUEL IS SUPPLIED TO THE ENGINE IN B MODE. ICE is simply spun by MG into whatever RPMs suitable at the moment and engine braking is done by compressing air inside cylinders.

"B" mode -- it takes energy to turn a car engine that is otherwise not running. Friction in bearings and pistons and cams can be significant, as anyone who tries to turn an engine by hand is aware. But more significant is that it takes energy to pump air through the engine -- without fuel and spark, an engine is really nothing more than a big air compressor. The amount of energy needed to keep all that turning and pumping is what causes the "engine drag" felt in conventional cars when the accelerator is released. And if the transmission is geared down, the drag effect is more pronounced because the engine must then spin [i.e. be pulled around against air and mechanical resistance] that much faster.
Enter "B" mode. As in, "trucks use lower gear". By forcing the wheels to spin the engine and pump air, a good deal of that energy can be turned to heating the air going through the engine instead of heating the brake parts. Since fresh air is always coming into the engine, having it leave as much warmer air provides a convenient place to dump excess energy.
n a conventional car the wheels push the engine around through the transmission, but the Prius needs to help that process out a little bit by actually having its combination of electric motors spin the engine. In this case, the valve-timing in the Prius engine is advanced to increase the amount of air taken in and the suction against the throttle flap -- which uses much more energy than the coasting-in-"D" scenario above. Either way, stopping power now comes from a combination of things and the burden on the friction brakes is greatly reduced, allowing the hill to be descended safely.
"B" mode also increases regeneration current to 30 - 40 amps with no feet on the pedals, so the part about "more regeneration" is somewhat true. That is one of several mechanisms used to increase the "drag" feeling. That level also varies with the car's speed.
In general, the amount of extra resistance given by "B" mode is sort of staged upward depending on the car's speed and how charged the battery is. Some of these conditions can be utilized in entertaining ways. Under 20 mph, if the engine is not already running and your foot comes off the accelerator, B mode simply regenerates reasonably heavily [30A or so] into the battery. This drops off around 12 mph to a lower current, and is then similar to being in D until regen capability kicks out entirely around 7 mph. So between 19 and maybe 10 miles per hour, you can use "B" to slow down in an energy-productive way, and essentially drive around in electric-only mode with one pedal -- but be careful to not do something the person behind you doesn't expect without showing brake lights!
Driving around in "B" during warmup also tends to charge the battery a little faster, since electric-only mode is avoided, but again at the expense of burning more fuel to do it. Engine start/stop transitions are avoided. Sometimes this state feels more surefooted and responsive in snow and other tricky conditions.
Fuel usage in "B" is somewhat mitigated by the fact that when decelerating above some nominal speed, somewhere around 17 mph, no fuel is sent to the engine and it just spins "dry". It's still wasting energy and slowing the car, but there's no reason to throw away gas along with that. This is sometimes called "fuel-starve" mode, and is also used in some conventional cars during high-speed coasting conditions.
Many strange things happen when the battery pack gets up to "eight green bars" full level. The hybrid system begins doing several things to pull a little energy back out of the pack -- the engine will tend to spin in "D" mode even at low-speed, low-demand conditions, in fact just about in the same way as "B" mode does when the charge state is more normal. If "B" mode is selected in the full state during coasting, then the engine *really* screams and even more energy is pumped away. So while all the energy of a long descent cannot be captured, speed can be controlled in some interesting ways by creative shifting between "D" and "B" even after the pack is topped out.

mnewxcv

This is a contradiction. I assume you mean no fuel is used in b when not accelerating.

iMSQ

Because hills were too steep. B really helps to protect brakes from overheating.
PS: During my first travel to Colorado I didn't know how B works, and I felt the vibration while braking because brakes were overheated.

iMSQ

You are right. B is "engine brake"