400h Braking

[speedyg]

I just picked up my RX400h from the dealer today! The braking sensations on the 400h are interesting. It's supposed to be different because of the regenerative braking. I am wondering if other people who have gotten their 400h have noticed this same thing or not: under certain conditions, if you apply a certain level of pressure to the brake, the vehicle doesn't decelerate at a constant rate, but seems to decelerate in spurts. It's as if I was stepping harder on the brake, then letting up a little, then stepping harder again. It's subtle and doesn't happen all the time (might be related to what speed you are going at and how much pressure you apply to the brake pedal.) Anyone else notice this? I noticed this in regular city driving while slowing down for traffic lights while it was raining (and no, it's not the ABS kicking in.)

[RX400h]

We noticed this as well. It takes a little bit getting used to - regulating the speed at which you depress the pedal, but after driving the RX for a while, I hardly notice it. I get a big kick out of cruising parking lots and seeing people stare at us, not believing their eyes and ears as the silent SUV slips by them!

[wwest]

400h MPG??

[LexKid630]

We noticed this as well. It takes a little bit getting used to - regulating the speed at which you depress the pedal, but after driving the RX for a while, I hardly notice it. I get a big kick out of cruising parking lots and seeing people state at us, not believing their eyes and ears as the silent SUV slips by them!

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I can't wait to hear how quiet it is!! :( jealous

[speedyg]

400h MPG??

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With cruise control set to about 67 mph, mostly flat freeway (a couple rolling hills), A/C off, CD player on, I got 27.3 MPG. That was over about 20 minutes, on the freeway only. Then after I exited the freeway and drove for about 3 more minutes in the city (stopping at stop signs and traffic lights etc) the average MPG went up to 27.8

While the non-constant braking action is still a little disconcerting to me, it's fun looking at the power consumption bar chart trying to see how much energy you can regenerate :P

[stiksandstones]

We are noticing some different braking as well, not bad, just different.

Takes some getting used to, but I do like the braking.

[kh046]

That MPG doesn't seem as impressive as I once thought. I just took my 04 RX330 to Los Angeles and back this weekend. On the computer meter, it stated that I was getting about 22 highway.

[scoobie]

That MPG doesn't seem as impressive as I once thought.  I just took my 04 RX330 to Los Angeles and back this weekend.  On the computer meter, it stated that I was getting about 22 highway.

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Never mind the mpg, going to Los Angeles and making it back with vehicle intact is impressive enough :P . But seriously, my (or any) RX330 can can easily get 26-27 mpg on the freeway, however, when you get off the freeway the mileage gets back to the low 20's very quickly, I suppose that's when the hybrid earns its keep. And why are we so concerned with the mpg anyway, honestly, are people buying the 400h in order to make a stand on the environment or similar issues, or is it more for the 'gee-whiz' factor.

[RX in NC]

I believe that most folks who decide to purchase hybrid vehicles like to think that they are doing their part for global conservation. That helps them to swallow the financial calculations a little easier when they learn that it may require as long as seven or eight years of operation in order to justify the premium they paid to purchase such a vehicle compared to its conventional stablemate. Only a small percentage of American drivers keep their vehicles that long.

But hybrid gas/electric is only a stopgap in terms of the true technology that will eventually replace fossil fuel-powered automobiles. Hybrids will have their run over the next few years but hydrogen-powered vehicles will eventually be the catalyst that frees the motoring public from our crude oil addiction. Unfortunately, the technology still has years to go, as does the establishment of a national fueling distribution system that will eventually become global.

Hydrogen technology is already quite safe, but the industry must find ways to manufacture and market it cost-effectively and allow drivers to refuel as easily as we now pump gas into our dino-machines. Perhaps hydrogen will be the norm in 10 to 12 years or so, but the entire automotive industry must begin to evolve in this direction and that will be far from easy....

[RX400h]

That MPG doesn't seem as impressive as I once thought.  I just took my 04 RX330 to Los Angeles and back this weekend.  On the computer meter, it stated that I was getting about 22 highway.

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Never mind the mpg, going to Los Angeles and making it back with vehicle intact is impressive enough :P . But seriously, my (or any) RX330 can can easily get 26-27 mpg on the freeway, however, when you get off the freeway the mileage gets back to the low 20's very quickly, I suppose that's when the hybrid earns its keep. And why are we so concerned with the mpg anyway, honestly, are people buying the 400h in order to make a stand on the environment or similar issues, or is it more for the 'gee-whiz' factor.

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My Corvette gets 30 MPG on the highway, but those of us who live in large cities have to contend with commuter traffic every day. And yes, hybrids shine under these conditions. However, there are a few other reasons why hybrids make sense now.

1) $2000 tax credit taken off the top

2) Possible use of ROV lanes

3) 650 lbs-ft of torque to blast by Excursions in a blink of the eye (that alone is worth the extra $5000 in my book)

By the way, I average 17 MPG in the Corvette; that lofty highway mileage figure doesn't do me much good most of the time.

[scoobie]

I believe that most folks who decide to purchase hybrid vehicles like to think that they are doing their part for global conservation.  That helps them to swallow the financial calculations a little easier when they learn that it may require as long as seven or eight years of operation in order to justify the premium they paid to purchase such a vehicle compared to its conventional stablemate.  Only a small percentage of American drivers keep their vehicles that long.

But hybrid gas/electric is only a stopgap in terms of the true technology that will eventually replace fossil fuel-powered automobiles.  Hybrids will have their run over the next few years but hydrogen-powered vehicles will eventually be the catalyst that frees the motoring public from our crude oil addiction.  Unfortunately, the technology still has years to go, as does the establishment of a national fueling distribution system that will eventually become global.

Hydrogen technology is already quite safe, but the industry must find ways to  manufacture and market it cost-effectively and allow drivers to refuel as easily as we now pump gas into our dino-machines.  Perhaps hydrogen will be the norm in 10 to 12 years or so, but the entire automotive industry must begin to evolve in this direction and that will be far from easy....

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If I recall correctly, it required some legislation to spurred the actions that brought the hybrids to market, I think the technology was there (in the labs), but the manufacturers needed a push to make it widely available and cost effective. Maybe the same thing need to happen to bring about hydrogen-fuel technology. Also, as you had mentioned, hydrogen-fuel has a bigger hurdle because there is no infrastructure to distribute to the consumer (I'm no expert, but I assume regular gas pumps won't be adequate to 'pump' hydrogen fuel).

[JonathanIT]

I'm wondering from all you lucky RX400h drivers already on the road... do you use the transmission/engine braking lever very often during typical city/highway driving? I haven't seen much discussion on it yet. It's one of my favorite features on Synergy/Hybrid vehicles; I plan to use it all the time! It seems also that it would help keep the mileage up and save on brake wear. I haven't seen the Owner's Manual yet... do they give any tips/hints about this feature? Any do's/don'ts to be conerned about?

TIA,

[speedyg]

I'm wondering from all you lucky RX400h drivers already on the road... do you use the transmission/engine braking lever very often during typical city/highway driving?  I haven't seen much discussion on it yet.  It's one of my favorite features on Synergy/Hybrid vehicles; I plan to use it all the time!  It seems also that it would help keep the mileage up and save on brake wear.  I haven't seen the Owner's Manual yet... do they give any tips/hints about this feature?  Any do's/don'ts to be conerned about?

TIA,

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I use the B engine braking position every so often. Not out of necessity, more out of fun. The deceleration is very slight though so I'm not sure what it gains you over just using regenerative braking. From what I recall the manual doesn't go into detail about how B works but says that you shouldn't drive in B all the time because you will get worse gas mileage. I'm pretty used to the regular braking now and I can get the power meter needle to stay deep in the blue while and yet not have that unusual pulsing sensation that I first mentioned. I'm assuming that the deeper the meter tips into the blue the more regeneration is taking place. From what I've noticed, using the B position only makes the meter dip into the blue very slightly.

Lexus should come up with a FAQ or something to answer all the technical questions about how to get the most mileage and what the actual algorithms are for when the electric motor kicks in, etc. :)

[patt]

The B position uses engine braking. In other words, the transmission connects the gas engine to the wheels without the engine being powered by gas so the turning the engine takes energy from the wheels. This energy is wasted (turned into heat). It is better to use regenerative braking which turns the motor as a generator so that the energy can be saved and used again. As long as you are braking gently, the brake pads aren't being used so you aren't saving the brake pads by not using the brake pedal.

The time to use the B position would be going down a steep slope where you need both that and the regenerative braking to slow you down.

[speedyg]

The B position uses engine braking. In other words, the transmission connects the gas engine to the wheels without the engine being powered by gas so the turning the engine takes energy from the wheels. This energy is wasted (turned into heat). It is better to use regenerative braking which turns the motor as a generator so that the energy can be saved and used again. As long as you are braking gently, the brake pads aren't being used so you aren't saving the brake pads by not using the brake pedal.

The time to use the B position would be going down a steep slope where you need both that and the regenerative braking to slow you down.

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Does it use only engine braking or does it also use regenerative braking (capture energy with the flywheel)? From observing the energy gauge it looks like some regeneration is going on. So am not really sure yet what the B position actually does. Engine braking only, regenerative braking only, combination of both, or some special algorithm that chooses either/both based on the prevailing conditions.

[patt]

According to what the manual says, B does engine breaking. That doesn't mean that it disables the regenerative breaking. If you look at the energy display, when you coast (meaning your foot is off the brake and off the accelerator) in Drive, there is regeneration going on. Presumably when you coast in B, that same regeneration goes on plus engine braking.

[JonathanIT]

The B position uses engine braking. In other words, the transmission connects the gas engine to the wheels without the engine being powered by gas so the turning the engine takes energy from the wheels. This energy is wasted (turned into heat).

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Wasted!?? Are you sure this is accurate? Did it actually say this in the manual??

It seems that the entire purpose of this "B" feature on a hybrid vehicle would be to NOT waste the inertial energy while slowing down. In a traditional gas powered vehicle, with or without an automatic transmission, you can downshift to achieve "engine braking" also, but the energy there is totally wasted. I doubt the engineers would design an entire "engine braking" feature just for a hybrid vehicle (note the "B" transmission position appears only in hybrid vehicles) that would completely waste all the energy used to stop!! This was part of the fundamental concept of the hybrid vehicle theory, to allow regenerative properties from all means of stopping the vehicle.

I think in the hybrids when using the transmission to slow down, the energy is used to recharge the battery more efficently than even the "regenerative" properties of the regular brakes. As I mentioned, I have not read the manual yet, but this is what I understood from past knowledge of the hybrid/synergy system.

[patt]

Jonathan,

Yes, I'm sure that is what B is for. Using the generators to slow you down by running the generator and storing the energy in the battery is what happens in the hybrid everytime you step on the brake or even when you just coast. You don't need to go into a special transmission position for this.

When you need to slow down more quickly than the load of the generator can accomplish, you have two choices, step harder on the brake and the brake pads will be used or use B. For normal driving, you will just step harder on the brake. When you do this, as much as possible is used to run the generators and the brake pads will take up the slack.

If you need to brake heavily continuously - going down a long downgrade too steep for the regeneration alone to do it - you can use the B position. Just like in a non-hybrid, this will add engine braking into the mix to reduce your need for brake pads. In this situation, there will probably be enough regeneration going on to keep the battery topped up. You need the engine braking plus the generator load.

The manual explicitly says that using B to slow you down instead of the brake can reduce your mileage.

I was talking to a friend who did some electric car conversions. To allow for the situation of going down hill where the battery got fully charged, they had to put in a big power resistor to dump power from the generator so that the brake pads didn't have to take the full load. In the hybrid perhaps they don't need to do this because they have the engine braking but perhaps they still do. The manual mentions that the engine braking in the hybrid is not as strong as in a standard car. I assume this is because of the CVT transmission which doesn't simulate the lowest gearing of an n-speed transmission.

[JonathanIT]

Ok, thanks for setting me straight on this! Now that I think of it my knowledge was coming more from what I had heard (not read from any reliable source), and was also combined with my own assumptions (and hopes)!

I just did some research (the abundance of which seems to be on the Prius, but from what I understand they are very similar). What I did not get before was that the the primary reason for the big difference in the "B" setting on the hybrids had less to do with hybrid aspect of the car than with the new-fangled planetary CVT transmission. It seems that the dynamics of the transmission would not allow it to have the traditional "1" & "2" low gear settings that would enable downshifting, so they decided to "simulate" the effect of "engine braking" with the new "B" position.

The good news is that while the engine is freely spinning (without fuel usage) and the compression is used to slow the vehicle, at the same time the secondary electric motor/generator is also being used to recharge the battery. So, the kinetic energy that is dispersed is not completely wasted. This is why the power monitor shows the batteries being charged when "B" is used to slow the vehicle.

The question I am still wondering is, what ratio of this energy is put back into the system? i.e., If the vehicle is about to descend a very long mountain downgrade, would if be more "efficent" (from a battery recharging standpoint) to "ride" the brakes all the way down? Or would "B" transfer just as much recharge energy (while saving brake wear)? Or perhaps would both methods used simulateously increase the recharge factor than either one by itself?

I disagree with the philosphy of "just drive it like a regular car and hope for the best"... I'm totally into maximizing fuel efficency/battery usage with the most advantageous driving methods!!

[patt]

The power meter (the dial on the left side of the gauge cluster) shows how much energy is going to or from the wheels if my understanding is correct. (I haven't gotten to that part of the manuals yet.) As you brake, you should see it go into the region below zero (the direction it goes when energy is flowing from the wheels to the generator. As you brake harder, there should be a point where the maximum generation is going on so it stops going any lower and the brake pads start to be applied. I've been meaning to watch to see where this happens.

If you are going down a long grade and having to brake to at or near the maximum regeneration, it is probably time to put it in B because you don't want to overheat the brakes.