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IanHB.

Energy Recovery Vs Harsh Braking

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My new G30 520d MHT has the makings of a very smooth and comfortable car. However whilst it works smoothly at higher speeds the MHT energy recovery system engaging at lower speeds results in severe grab or snatch like forced engine breaking in a manual gearbox car might be. When combined with normal breaking as I attempt to bring the car to a gradual halt results in an difficult to control combination of energy recovery and breaking creating a rather uncomfortable and occasionally dangerous low speed manoeuvre.

Does anyone have similar experience and / or solution. (Sorry if there is an existing thread, I couldn’t see one)

The car is a July 2020 build registered demonstrator in March 2021.

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Take it back and moan like hell!

I had issues with the erratic brake pedal height, the MHT system not working, poor brake feel.

Sytner/BMW started with "no fault found". On the 3rd time they had it in they fitted a new electric gear box pump (it has 2, electric and mechanical) Then claimed it was just a software update they was waiting for. 10 months after they first took the car in I rejected the vehicle under the Consumer rights act. Sytner offered a full refund, I'd had the car a year and done 10k miles. Only at this point did BMW Germany tell them to change the master cylinder and brake servo.

 

Since the new parts it has been like driving a different car. It feels like they have fitted performance brakes and the MHT system is as smooth as anything. The car is now lovely to drive.

 

Your dealer can only do what BMW tell them. BMW don't want to agree there is a problem with their MHT system because this might involve a recall. 

With a bit of research I have found that there have been similar braking issues with the 530e in America

 

My car was register Feb 2020 and I bought it "new" with 18 miles on the clock in August 2020

 

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Thanks for your comments. I had read some of your thread but wasn’t sure it aligned with my problem. Reading again, there may be similarities.

I do I ntend to go back to dealer but hoped to gather some other similar examples before making an approach, your experience may help.

Thanks.

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First of all, I think Stressed's car is kind of an exception as it is some component fault. Below is my explanation to majority of pre-LCI 520d MHT car.

 

Let's start with how the system works. Basically, the 48v system is connnected to the engine crankshaft via belt (same as air-con). The 48v motor/generator can, based on plotted data via BimmerLink, generate maximum of about +/- 35 Nm of torque depending on whether it's acting as a motor or a generator. Then, the torque varies depending on the engine speed (rpm) as well as the battery charge state. When in generator mode, lower engine speed *and* lower battery charge level will give more negative torque. On the other hand, the higher engine speed *or* higher battery charge level will give less negative torque. In motor mode, above are still correct but the torque is now positive instead of negative.  

 

Now, let's just look at the scenario when you decelerate and the 48v system is doing energy recovery. To make everything simpler, let's assume the generator always generate -35 Nm of torque at the moment. When you lift off the acelerator, the generator generates -35 Nm of torque. You can consider this torque as, as what you've described, engine brake but the amount is much larger than engine brake. This torque is then enlarged by the gearbox further. The lower gear you are in, the more the gearbox enlarge this negative torque. Now, let's image you drive a manual car and drop the gear from 2nd to 1st when you decelerate without any rpm compensation and release the clutch very fast. This is where the Harsh Brake comes from.

 

Now, the issue is worse than above scenario as the negative torque from the generator can vary based on rpm, battery charge state and whether the car is actually recovering energy or not. In above scenario, you can smooth out the gear change by e.g rpm compensation, slowly release clutch, or adjust the brake paddle force when drop gear. However, for auto, MHT car, you don't know when/how the car will drop gear, how the rpm and battery charge state will affect the negative torque, it is really hard to modulate the brake paddle to compensate the variation in stop force. For example, first time when you come to stop, the battery is fully charged so that there is no energy recovery taking place. So, all the stop force is from the brake system. Thus, you need to brake harder. Then, the next time you try to stop, the battery may be empty due to MHT motor assisting aceleration. If you still brake using the same force as before, the deceleration will be harsher than before. Finally, there are two, I think, most uncomfortable and sometime dangerous scenarios. First one is when you come to stop, the battery becomes fully charged (a threshold) half way through the deceleration especially when the car is in 2nd or 4th gear and the car is dropping gears. In this case, the energy recovery go away when the battery charge level reaches the threshold. As the speed is low so I'm quite close to the front car. Also, the gear is low so the torque enlarge effect is more so I actually apply less brake so that the combined (brake+energy recovery) stop force is enough to stop the car. But as the energy recovery go away (it is now quite a big force considering the gear ratio enlargement), all a sudden, the stop force is not enough and you need to immediately apply for brake (my wife says she actually feels the car is accelerating), which make the stop not smooth. The second scenario is when you are in 1st gear and the car's speed is around the threshold of having or not having energy recovery. In this case, sometime the 48v will kick in to have negative torque to slow the car. sometime it won't have anything at all to slow the car. It's hard to perdict so that hard to control.

 

Overall, this is what I observed and thought about the mht 520d after about 1.5 year of driving. My solution so far is to either let the 48v do the early braking (energy recovery) without any brake input. Or, I will brake harder than I used to so that the brake force contribute most part of the stop force and the variation in the remaining force from the 48v is much less noticable.

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What you are saying about how the car works is 100% correct. The reason for my "take it back and moan" statement is BMW are constantly updating the software. Over the year of issues I has 4 different software versions on my car, each giving a different feel to the brakes and MHT system. I also test drove at least 3 other MHT bmws as either Loan cars or back to back test cars.

The software version I have now with the new master cylinder and servo give the best brake pedal feel form harsh braking and soft gentle stops. The MHT energy recovery is smooth and not intrusive not matter what the speed.

 

You have to remember that there are so many different bits of technology that effects brake and MHT performance. For example the emergency brake assist when it senses possible danger actually pre arms the ABS system to enable the best emergency braking. The brake servo is vacuum and electronic to ensure it works if the engine is on or off depending on the MHT system.

If the software doesn't have all systems working smoothly together you get bad brake feel and can get harsh MHT recovery.

Sytner even change the software on one of their cars when testing mine and it gave a different brake and MHT feel.

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39 minutes ago, Stressed said:

If the software doesn't have all systems working smoothly together you get bad brake feel and can get harsh MHT recovery.

That is totally true. However, there is a pre-assumption that all components are working as within the designed tolerence. You mentioned that you had your master cylinder and brake servo changed eventually with some sw update. Then, the brake system back to normal. My thought here is that some components in your car were already faulty and were working outside the tolerence. Thus, regardless of which version of sw is programmed, the brake system was still working abnormally. 

 

On the other hand, I totally agree with you on how the SW may change how the car feels nowadays (I did my PhD in real time system area and worked with many people from automotive industry. I even had a chance to have a seminar with Simon Furst, BMW's lead on FuSa and autonomous driving). In those scenarios described in my previous long post, if the MHT SW (part of engine ECU SW) can fine tune the torque output from the BSG (motor/generator of the 48v system) so that when you decelerate, the total stop force can maintain a constant as long as your brake padel input is constant, then the stop will become very smooth without my right foot compensating the torque variation from the BSG. In other word, your brake padel will now request the amount of overall stop force from all sub-systems instead of just requesting certain amount of stop force from the mechanical brake system. However, this is obviously not achieved yet. Thus, all MHT, full hybrid, or EV I've driven so far have some strange brake feeling that I don't like. Difference is just some car's brake is very strange and somes are less strange (due to sw calibration). This is also why I got my z4 this year instead of waiting for LCI next year (toy car so not hurry and can wait) as I know the LCI will have 48v MHT and I will likely to dislike the brake feel.

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Thanks q96169we for your very detailed if a little technical commentary, I will read it all again to make sure I understand it.

 

As always Stressed the bigger problem is often getting the garage to agree there is a problem. I have spoken to a very receptive service manager this morning who agreed that my description of the problem certainly sounded like something was wrong and agreed to see the car middle of next week.

 

However, based on something I read in another thread, I tried this morning setting the car through iDrive into "Individual Sport Mode." I selected sport mode but then deselected, within Sport Mode, individually back to Comfort Mode on Steering, Engine and Transmission. I have only managed a few miles with this set-up so I'll need a few days to test and compare but the difference seems to be quite noticeable, smoother and more refined and if the Energy Flow meter is to be believed much quicker to charge the battery. As I said a few days testing might suggest a software solution to improve the MHT in Standard Comfort Mode.

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1 hour ago, IanHB. said:

based on something I read in another thread, I tried this morning setting the car through iDrive into "Individual Sport Mode." I selected sport mode but then deselected, within Sport Mode, individually back to Comfort Mode on Steering, Engine and Transmission.

If I understand correctly, what you said here is to select sport individual mode then configure the sport individual mode to be comfort for engine, steering, and transmission. Basically, what this does is to make the car still run in comfort mode but disable the 48v auto star/stop function (with MHT, no button to switch it off). In other word, the car will not coast when you are not press accelerator and will always recover the energy until the battery is full. This is how I always use my car and it sort of makes the car behaviour a little more predictable as I rule out the coasting function, and all brake servo boost is from mechanical vacuum pump (as the engine never switch off when the car is running). However, the overall scenario of how the 48v MHT system varies the torque output so that the total stop force is varied even brake padel input is constant (as my long post described) is still correct. If you want further explanation about the scenarios I mentioned in the long post, feel free to discuss further.

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Just had a brief run out in the car and you have described just about exactly how the car is performing. Your first comment which mentions reduced drag when the battery is full is beginning to make sense too.

I now plan to run alternately in both modes over the next few days before taking the car to the garage when I will be able to demonstrate and highlight the problem to best effect.

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An update after a visit this afternoon to my local dealership:

 

After explaining the issues to the workshop foreman who seemed to clearly understand the matter my car was taken, there and then, for some initial diagnostic testing which, surprise surprise, came back with no fault found. However the car was also taken for a test drive and the foreman was not happy with the aggressive nature of the MHT “breaking” after the footbrake pedal was released. It is now proposed to make a back to back comparison with an identical car as the aggressive MHT breaking on my car had not been experienced before. The dealer also proposed to raise the issue with BMW.

 

I mentioned the brake issues described by Stressed and while I wasn’t sure these were relevant the dealer thought them worth exploring however the diagnostics would not allow these to be checked as they sat within the computer structure below the element of diagnostics that had had “no fault” so access was denied. The dealer was not happy with this and proposed to explore later if necessary.

 

Overall my concerns were well received and remain a work in progress for now, with the identical car comparison as soon as the other car is available.

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Glad the dealer has listened to your concerns.

The dealer has a slight problem. When they plug in your car they see diagnostic information. Unfortunately they only see the info BMW actually want them to see. Bmw actually sees the full information form your car. This means any MHT issues don't show up on the dealer's system.

Sounds a bit dodgy, but BMW don't want to highlight issues that could involve a recall !!!

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While not saying that exactly they did seem to be aware of that kind of problem and warned that a solution may take some time if it ultimately comes from Germany, as it might.

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I thought an update might be of interest.

Having driven another 520d MHT the dealer requested my car be brought in for more testing and discussion with BMW. While booking the car in for 1st November I told the service desk about a single time that the brake pedal had dropped to the floor while driving in traffic but recovered without issue or repeat.

Having heard this the dealer insisted my car came in immediately and arranged a BMW Assistance replacement car. The workshop started exploring the fault again last Monday. However the fault is not yet resolved and I am told at present lies with BMW in UK or Germany.

By complete chance the replacement hire car is an 18 month old 520d M Sport MHT with 18000 miles. This car handles the MHT much better, whilst the MHT can be felt it is never intrusive on my driving.

 

Hopefully some progress in the coming week!

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Strange! I also had the pedal go all the way to the floor whilst stationary in traffic!

Since the new servo and master cylinder it's been like driving a different car!!!

Sent you a PM

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On 10/30/2021 at 8:01 PM, IanHB. said:

I told the service desk about a single time that the brake pedal had dropped to the floor while driving in traffic but recovered without issue or repeat.

This indeed sounds alarming and BMW should start changing hardware instead of just software as Stressed's car. Although I don't like the brake feeling of MHT car (as you said, the MHT can be felt), technically, I can have reasonable explanation of what is happening and why it is normal so that I can accept that the car is working as design (but the design is not optimal in terms of brake feeling). However, I have never had, even once, the brake padel dropped to the floor. This is absolutely wrong and no reasonable technical explanation to say the car is working as design. Maybe follow what Stressed has done to have his servo replaced by BMW under warranty is the best option now.

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Update:

 

Got my car back just over a week ago, BMW instructed the dealer to replace the starter motor/generator.

 

What an improvement, this was obviously a major part of the problem.

 

Still getting used to the "new" drive, the MHT system is much less intrusive although I am still aware of it at times. I'll give it a while longer to perhaps adjust my driving style to suit.

 

There does still seem to be an issue with braking. Prior to today, possibly once, I was aware, as mentioned before, of the brake pedal falling away during braking in traffic. No drama but noticeable. Today it happened again on approach to traffic lights with a car in front which stopped quickly. I had to change from a controlled coast to stop with a sharper braking action and the pedal dropped but did stop safely. I was only able to re-create this once . I need to be able to demonstrate to the dealer before it will be taken as a fault that they can see and act on.

 

I'll update again in due course.

 

 

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7 minutes ago, IanHB. said:

Today it happened again on approach to traffic lights with a car in front which stopped quickly.

Just wondering, whether the emergency brake assistant kick in (red car symbol on the middle of the dash where you can configure to display the map on iDrive 7, plus some urgent beeping)?

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Hmm 

Actually the red car symbol did appear in the head up display more than once today which did surprise me but I cant quite link the two issues at the moment.

 

I plan to do a little testing when my wife isn't there to complain about the driving.

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13 minutes ago, IanHB. said:

Update:

 

Got my car back just over a week ago, BMW instructed the dealer to replace the starter motor/generator.

 

What an improvement, this was obviously a major part of the problem.

 

Still getting used to the "new" drive, the MHT system is much less intrusive although I am still aware of it at times. I'll give it a while longer to perhaps adjust my driving style to suit.

 

There does still seem to be an issue with braking. Prior to today, possibly once, I was aware, as mentioned before, of the brake pedal falling away during braking in traffic. No drama but noticeable. Today it happened again on approach to traffic lights with a car in front which stopped quickly. I had to change from a controlled coast to stop with a sharper braking action and the pedal dropped but did stop safely. I was only able to re-create this once . I need to be able to demonstrate to the dealer before it will be taken as a fault that they can see and act on.

 

I'll update again in due course.

 

 

May I suggest it's NOT your responsibility to demonstrate the fault. It is their job to check and test. They did keep telling me no fault found. This is probably where my case will be useful!

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I agree completely, however life will be that much simpler if I can recreate the fault  for the benefit of the dealer who I believe will then be more disposed to pushing BMW.
 

Remember the dealers computer system had already not found any fault.

 

I’ll update again with more progress.

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1 hour ago, IanHB. said:

I agree completely, however life will be that much simpler if I can recreate the fault  for the benefit of the dealer who I believe will then be more disposed to pushing BMW.
 

Remember the dealers computer system had already not found any fault.

 

I’ll update again with more progress.

Please be aware that the dealer's computer system does not tell the whole story. Dealers are only allowed to see what BMW want them to.

Because the dealer is NOT BMW they only get a "filtered" version of the data that BMW Germany see.

 

I gave Sytner a detailed list of dates and times the fault occurred. They could not see these occurrences on their data but BMW did let slip that they could see when it happened.

The technician did confirm to me that BMW Germany get far more information than dealers are allowed to see!

 

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16 hours ago, IanHB. said:

Actually the red car symbol did appear in the head up display more than once today which did surprise me but I cant quite link the two issues at the moment.

When the red car symbol appeared, the brake system will either do some pre-tense to the brake system so that it requires less driver input (both force and distance) to apply more brake force to the wheel, or the car will simply brake for you under emergency condition. Depending on the timing of the brake system decide to do (or stop doing) anything and your brake padel input, the padel may feel differently sometime. To rule out the possibility of the emergency brake assistant system doing anything, you can either try to configure it response latest or disable the system by long pressing the green circle buttom next to the emergency light until it turns red or orange. Just to say that it's your own responsibility to make sure driving safely.

 

16 hours ago, Stressed said:

May I suggest it's NOT your responsibility to demonstrate the fault. It is their job to check and test. They did keep telling me no fault found. This is probably where my case will be useful!

I would say it's personal perference. You are not required to demonstrate exactly for them to fix the fault if there is any. However, if you are able to tell them exactly how to trigger the fault, then they will have no execuse to say "everything is fine, no fault found on the car". I'm an engineer in semi-conductor industry and I've done both design and verification task so far. While I'm a designer, I really appreciate the verification engineer can tell me exactly how to trigger the bug in my design so that I can focus on how to fix it instead of how to replicate the bug. While I'm doing verification, I really enjoy spending half day to fully analysis the bug and trace the root cause to it. A lot can be learnt during the process. One of the master technicians at my local dealer (Sytner Sheffield) knows a bit of my background and worked on the car with me several times. What he described is that it is so much easier for them to let BMW authorise the warranty repair as no matter what BMW wants them to try, they can simply follow the exact flow to trigger the issue then push back to BMW instead of spending several hours trying to see any fault if they have the time. Most of the time, as long as the computer told them everything is fine and they are not able to replicate the issue in an hour or less, they will simply tell the customer everything is fine and keep driving to see whether it appears again. Anyway, as said, it's personal perference and not everyone enjoys the debug process.

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Thanks for your comment on the Intelligent Safety System, it is a worthwhile prompt for me to read in a little more detail the Drivers Guide Book. I will review your comments and the Guide along with any possible adjustment I might make to the settings.

 

I agree with your comments on fault finding, thanks again.

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