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Lexus RX 400h: Driving Tips & Tricks

110 messages,  Last post on Sep 10, 2007 at 8:21 AM

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What is this discussion about? Lexus RX 400h, Hybrid Cars, Fuel Efficiency (MPG), SUV

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#41 of 110 Has anyone tried using "B"? (tsotsi) by cyclone4 May 05, 2005 (12:06 pm)
Replying to: tsotsi (May 05, 2005 9:01 am) Here is what it states on the RX400h Owner's Manual: "Do not continue driving with the transmission in "B" for a long time. This may cause decreased fuel economy". It also states throughout this chapter that is used for braking. This to me implies that it should be used in mountainous terrain only. Also, does it not make sense that if braking is applied even when coasting, the vehicle will lose MPG? I think that the battery is getting recharged regardless even when coasting. Perhaps somebody that really knows the ins & outs of this stuff can explain it better. Replies to this message: karkus (May 05, 2005 2:45 pm) skyfish400h (May 05, 2005 3:21 pm)
#42 of 110 Has anyone tried using "B"? by karkus May 05, 2005 (2:45 pm)
Replying to: cyclone4 (May 05, 2005 12:06 pm) I don't think the C&D statement is entirely correct. First of all, the Prius (and 400h I presume) will always generate a small amount of electricity when you let off the gas pedal, regardless of whether you are in B or D. Applying the brakes lightly increases the amount of regenerative braking, and then at some point pressing harder will involve more and more friciton braking. B involves engine braking (compressing air in the cylinders) so you will lose that energy instead of putting it in the battery (plus maybe there's some extra regeneration). Using B will NOT improve your MPG under any circumstance, as far as I know, and should only be used on very steep long downhills where you would have to ride the brakes so hard that the friction brakes would be activated anyway.     Ideally you would let the car coast as much as possible, but that's not always practical, and the hybrids recapture some of your braking energy. However, generating electricity, storing it, and converting it back to mechanical torque all involve some energy losses, so really it's not anywhere close to 100% efficient.    Full hybrids (Toyota, Lexus, Ford) improve fuel ecomomy because 1) Engine is off when coasting, braking, or stopped 2) Some braking energy is recaptured 3) Battery power is used primarily when only a little power is needed - in those cases it's more efficient than running the gas engine at low RPMs 4) Gas Engine is used primarily when it can run at high (efficient) RPMs   Also, I saw people comparing when the gas engine comes on. It depends on various factors: 1) Temp: The engine will always run more if it's cold, especially during first 5-10 minutes 2) The battery level: Low = engine runs a lot, in part to charge up the battery. HIGH=engine stays off unless you need lots of power. (The computer tries to keep the battery in the upper-middle of it's range whenever possible) 3) How hard you hit gas pedal, and whether it's uphill or down. Replies to this message: skyfish400h (May 05, 2005 3:21 pm)
#43 of 110 Has anyone tried using "B"? by skyfish400h May 05, 2005 (3:21 pm)
Replying to: karkus (May 05, 2005 2:45 pm) I do not see any inherent contradiction between the C&D comments and the Owners Manual. Both could be true in certain cercumstances.   Assuming:   1) That there is always regeneration going on whenever your foot is off the acc. 2) There is more than one level of electric braking so that it can increase when the brake pedal is applied 3) The CVT allows seamless blending of the ICE and EMF torques in both acc and dec modes. 4) The D vs B mode determines whether or not the ICE is added to the mix for braking. In D it is only the EMF and friction pads, in B its all three.   The first one (1) just makes sense to take advantage of the cars inertia whenever possible. (2) seems plausible given that it works that way in the acc direction. We know that (3) is true from the Lexus literature bragging about the simple beauty of the CVT design. The last one (4) could be easily determined with some experimentation.   My breif experment wth the B position was that it did offer some extra braking when you let off the acc... although it was no where near that of a lower gear such as 2nd on an auto or dropping down a gear on a manual.   Now the effects of this would be highly dependant on the compression generated by the ICE and since the car was brand new, I would think that effect would become more pronounced as the rings seated and compression improved. Also, I did not get to try out the B position at freeway speed, only on surface streets (although I did have it up to 60 mph at one point   At low speeds the B position would help your MPG by applying extra EMF braking, at higher speeds where the compression of the ICE is used for braking your MPG may suffer some because energy that could have gone to the batteries is wasted as heat in the ICE.
#45 of 110 Coasting is coasting by tsotsi May 05, 2005 (6:23 pm)
To me, coasting means that the car is not being slowed down or sped up by the engine or motors. While coasting the battery may be getting recharged by the ICE, but not by loss of forward speed . . . since there isn't any (or much). Either (or both) selection of "B" or pressing the brake pedal sends a signal to the computers that the driver wants the system to slow the car and use the system for regeneration. It seems that since at high speed the ICE also comes into play to slow the car in the "B" position, it is probably better to use it for economy at city speeds. At highway speeds, descending a mountain, it mainly saves the brakes. For maximum economy you want maximum regeneration when stopping so, since while coasting the car is being slowed by rolling and wind resistance instead of regeneration, it is actually better to use the brake pedal or "B" than to coast.   That is what I am getting when I put all of the comments together. Yes? No? Replies to this message: skyfish400h (May 06, 2005 8:28 am)
#46 of 110 Re: Coasting is coasting [tsotsi] by skyfish400h May 06, 2005 (8:28 am)
Replying to: tsotsi (May 05, 2005 6:23 pm) Coasting in the 400h or any car for that matter involves some slowing due to the engine(s). In the case of the 400h the EMF engines help slow the car and use that to charge the batts, and (depending on the D/B thing) the ICE engine uses compression to slow the car.   The big difference with the 400h is that it should not be using any fuel for this, just the the compression of the air in the cylinders since the engine does not need to be "running" to be used as a brake.   To experience the difference between real coasting and simply letting off the acc peddel, try slippnig the gear selector into N... now that's coasting.   But don't try it on a down grade
#47 of 110 Battery Status (vdoc) by cyclone4 May 11, 2005 (2:55 pm)
The Hybrid Overview Manual says that you will hardly ever see the battery status fully charged (green) unless you are driving down a mountain road. This makes sense since the battery is constantly being chaged while driving down a long mountain road. The typical status is "blue".
#48 of 110 Short trips (headless, etc.) by cyclone4 May 15, 2005 (8:09 pm)
Short trips definitely give lousy gas mileage. I had the average up to 35+ when I was testing it around the city on a few occasions more than a week ago. However, my wife making a lot of tiny trips has since then, brought the average down to about 32mpg. This warm-up period really eats up the gas. I am convinced that as long as one does not have to make these very short trips this vehicle will do even better than the EPA estimate on city driving. But the reality of it is that some of us cannot avoid these short trips. Headless, I hope you read my response to you 4-5 days ago concerning my driving like an old lady and that's why I was getting this awesome gas mileage. Happy driving with the RX400h everyone!!!
#49 of 110 Battery "consumption"... by wwest May 23, 2005 (10:09 am)
hixon:   Your calculation method works just fine for averaging, but what if I manage to drive the vehicle for, say, 20 miles on batteries alone? For instance one of the posts was quoting fuel economy for a short 51 mile trip.   I'm really curious about all of this "special handling" driving techniques being espoused since our 2003 Prius doesn't seem to need any of that sort in order to render the stellar 42MPH overall that it has.   A "V8" powered RX that must be driven carefully, with "kidd" gloves, is that what the buying public really wants? Replies to this message: dylan hixon (May 23, 2005 11:53 am)
#50 of 110 Battery "consumption"... by wwest by dylan hixon May 23, 2005 (11:53 am)
Replying to: wwest (May 23, 2005 10:09 am) Good luck getting 20 miles on batteries alone at any speed, let alone highway speeds, that post was a steady 55mph. The car wont go much above 15mph on electric only unless you have a very full battery, and accelerate very slowly (or go down hill). I fear that this is part of the performance otiented tuning philosophy Lexus has.   I will redo my calculation with the actual battery capacity if I can find it, but it sounds like I am a factor of two high. That would agree with my experience that the battery charge depletes pretty rapidly in electric only city driving. And in the future, I will correct for net change in battery charge when I test, altough the effect will be minimal.   Techniques like early gentle braking, avoiding jackrabbit starts, and use of cruise control are not special handling, they are basic common sense, and easy to do. The beauty of any hybrid system, like your Prius, is that it performs quite well when driven normally, and even better when only modest care is taken. If you want the big numbers though, you have to work on your skills and techniques.

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