B Mode Questions

Is it OK to shift from D to B (or vice-versa) when accelerating? I use B regularly on a 7% 4-mile grade, as well as a 2-miler that varies 14% - 22%. After rounding the last switchback turn at the bottom of the hill, its time to get right back to highway speed again. My routine at that point is to shift to D with no pressure on accelerator (or brake), and it seems to take about 3-4 seconds for the engine braking to cease and the ICE to quiet down (in B mode it sounds like its turning 5000 on the way down) where it feels like its "coasting" again. I let it have that 3-4 seconds of "recovery" time before adding throttle. But it would be a smoother transition if I could add throttle out of the last turn and accelerate up toward highway speed in B, then shift to D without having to take foot off accelerator first (which results in engine braking). So, would it be permissible to shift between B and D while on the accelerator, with ICE generating power? Will that cause PSD or engine damage? Or is the shift to D simply a computer remap that disables the engine braking upon accelerator release? Thanks.

 

You can safely shift from B to D, or D to B, at any accelerator level. Just shift and go when you are ready. The computer won't let you hurt the engine.

Tom

 

Thanks Tom. Sure appreciate all the expertise available on this site. Almost always find the answer within a few minutes.

 

Recently i visited hill station, here one of the track is very steep and rough, on uphill it was ok but on downhill i engaged b mode but still speed was way fast due to steep, my question is how to control speed on such steep hills, i miss first gear of manual in these hills, please help

 

can you pull to the side and use the brakes to creep down?

 

Depending on the typical driving speeds on the road and the amount of traffic, the solution may be as simple as to drive more slowly.

There is a limit on how much power can be dissipated by the engine braking. From this post it is around 11 to 14 kilowatts for a Gen 3.

The power you are getting from gravity is simply the product of the mass of the loaded car times the rate of descent. You can get the rate of descent from your indicated speed and the steepness of the grade, but without doing a bunch of math, the key point is it is strict proportion. If you take the descent at half the speed, that's half the power.

If you take the hill at a speed where gravity is giving you more than 14 kW, the engine braking will be inadequate, and you will pick up speed. If you start at a lower speed where gravity is giving you less power than that, the engine braking will be enough to hold your speed or even slow you down. (And somewhere between those two speeds will be one where the power from gravity exactly equals what the engine can dissipate, and the engine braking will be just barely able to hold you at that speed.)

The ways this can fail are: (1) if the hill is so very steep that the required speed would be too slow to make full use of engine braking, or (2) if there is much traffic, and the required speed would be slower than everybody else wants to go.

In cases like that, just let the engine braking do what it can, and snub the excess speed off with the friction brakes as often as needed.

 

How will i come to know that i am on regen breaking or ob fiction breaking during my pedal press so i can make sure to press pedal without overheat my brakes

 

Posting the same question in about 5 threads, makes it difficult for you and responders.

 

Other drivers around you in non-hybrid cars have to use their brakes, and they just do it. Some of them will overheat their brakes, but there are ways to be careful about that.

It is better to avoid 'riding' the brakes. Instead, occasionally 'snub' the speed down. Snub it down to below your target speed, release the brakes, allowing them to cool for a while, snub again when the speed exceeds your target.

It may not be obvious that helps (after all, you have to apply the brakes harder when you snub), but there was a study published for commercial vehicles that showed an advantage for snubbing. It was discussed here some years ago and I posted that link then; I don't have it at hand right now.

Naturally, if you smell hot brakes, find a turnout, stop, and let them cool.

Again, remember that the power from gravity is proportional to your speed. Just as taking the descent more slowly reduces the demand on engine braking, taking the descent more slowly reduces the demand on your friction brakes.

When driving in mountains, I often stay in D gear and set the cruise control at the lowest settable speed (25 MPH, in the US). That gives a stronger braking effect than B gives, when the go pedal is released, but does not apply the friction brakes. Of course, that technique is no good if the descent calls for a speed below the setting.

Cruise control, of course, gets canceled by using the brake pedal, so if you end up combining regen/engine braking via cruise and snubbing with the friction brakes, you have to remember to resume cruise after each snub.