Engine breaking?

I'm no engineer (obviously) but just wanted to ask if this is right.
I noticed when I'm in Drive and braking my rpm goes down then up and then down then up.
It seems like it's down shifting while stopping.
I would have thought that this would waste more gas.
I'm just curious if anyone can explain why this is and why it would be more gas efficient? Or it's not gas efficient but help stop faster?

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The transmission downshifts while braking so that it's in the right gear when you accelerate again. Pretty standard.

Originally Posted by Taeksoo

I'm no engineer (obviously) but just wanted to ask if this is right.
I noticed when I'm in Drive and braking my rpm goes down then up and then down then up.
It seems like it's down shifting while stopping.
I would have thought that this would waste more gas.
I'm just curious if anyone can explain why this is and why it would be more gas efficient? Or it's not gas efficient but help stop faster?

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Most modern vehicles cut fuel when your foot is off the go pedal. So even though you see the rpm's moving up and down you should understand that most of that is caused by the wheels turning the engine not vice-versa. Bottom line is that it does not negatively impact fuel economy.

Downshifting helps reduce brake wear and as pointed out above puts your trans in the correct gear for moving out again.

Originally Posted by d geek

Most modern vehicles cut fuel when your foot is off the go pedal. So even though you see the rpm's moving up and down you should understand that most of that is caused by the wheels turning the engine not vice-versa. Bottom line is that it does not negatively impact fuel economy.

Downshifting helps reduce brake wear and as pointed out above puts your trans in the correct gear for moving out again.

Agree. I would add that, in this situation, the braking comes from the opposing force of the air (without fuel) being compressed by the pistons. Without fuel you have compression without detonation.

Originally Posted by d geek

Most modern vehicles cut fuel when your foot is off the go pedal. So even though you see the rpm's moving up and down you should understand that most of that is caused by the wheels turning the engine not vice-versa. Bottom line is that it does not negatively impact fuel economy.

Downshifting helps reduce brake wear and as pointed out above puts your trans in the correct gear for moving out again.

Alright that makes sense.
My last car was 12 years old before I got this so haven't notice this type of things.
Also, would this be bad for your transmission? I'm sure BMW has thought of everything and I'm sure it's ok but I'm just curious. My understanding is that if you have manual transmission and engine breaking would be bad on transmission. But then again this could be decade old fact.

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Originally Posted by Taeksoo

Alright that makes sense.
My last car was 12 years old before I got this so haven't notice this type of things.
Also, would this be bad for your transmission? I'm sure BMW has thought of everything and I'm sure it's ok but I'm just curious. My understanding is that if you have manual transmission and engine breaking would be bad on transmission. But then again this could be decade old fact.

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engine braking is not detrimental to transmission service life- either manual or auto.

Downshifting a manual will not wear out the clutch or other components if done properly.

Engine braking serves two purposes: to start off in the correct gear (not as likely since if you accelerate, it will downshift anyways) and most importantly, generate enough vacuum to apply braking pressure. You do need high RPMs to get good vacuum, but if RPMs are very low, you will get limited braking power. Notice your RPMs are not "true" engine braking and stays under 1500 rpm with each downshift. (in real engine braking, you will be hitting 2k+ with each downshift) Fuel is cut off under no load with RPMs are above a set threshold. The injectors will kick in under that threshold. You can see this by revving the engine and then, as the RPMs drop, you see a slight bounce 1k. Its not BMW specific, pretty much all cars do this nowadays.

Originally Posted by Taeksoo

I'm no engineer (obviously) but just wanted to ask if this is right.
I noticed when I'm in Drive and braking my rpm goes down then up and then down then up.
It seems like it's down shifting while stopping.
I would have thought that this would waste more gas.
I'm just curious if anyone can explain why this is and why it would be more gas efficient? Or it's not gas efficient but help stop faster?

Sent from my SCH-I535 using Bimmer App

I just came out of an Acura TL to a 328i xDrive and I also noticed the engine drag seemed greater than my Acura. My Acura almost seemed to coast in comparison (or disconnect from the engine) when I let up the gas, my Acura would certainly would coast further upon deceleration, however, upon further investigation, BMW has a Brake Energy Regeneration System (link below) that shows it as one of the Performance and Efficiency items on the 3-series features and specs page. I notice when you decelerate, it takes just a moment after you release the accelerator (about the same amount of time you wait for the auto stop/start to turn the engine off at a stop light) and then you will feel a slight perception of the engine downshifting, I believe that is the regeneration system taking the momentum energy and using it to charge your battery. I have found that I can utilize the drag to coast into a red traffic light with less braking and not irritating the person behind me. I am not real familiar with the system, but I had inquired about the deceleration and was told by one of the experts from BMW at the local summer Olympic Drive event last year that is what I felt.

http://www.bmwusa.com/Standard/Conte...and_specs.aspx

Thanks for all your input on this.
They do make sense. Like I said, I'm sure smart BMW engineer thought of everything in making an ultimate driving machine. It's nice to kind of understand some of them.

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