View Single Post
      11-11-2020, 02:39 PM   #5
MJE60
Captain
MJE60's Avatar
England
483
Rep
733
Posts

Drives: BMW G01 xDrive M Sport
Join Date: Jan 2019
Location: Dorset, UK

iTrader: (0)

Quote:
Originally Posted by Xplorer1 View Post
I have a silly doubt about engine braking which Google doesn't answer. From whatever I've read until now, especially on our auto transmission equipped cars, engine braking is a safe method to use while going downhill along with brakes to slow down the car. In a manual transmission vehicle downshifting to a very low gear may cause the engine to redline but in an automatic transmission, the transmission simply wouldn't allow such a risky downshift.

But, can engine braking cause redline/ overrev/ harm the transmission even on an automatic transmission vehicle? What if I'm staying in a gear like M1, & I take my foot off the brakes on a somewhat steep decline, can it cause the engine to overrev/redline/cause any harm to transmission or engine as the cars rolls down the road and reaches a speed that's near to max speed in that gear/near redline. Or will the automatic gearbox upshift automatically as the revs near the redline?

Reason why I'm asking it is this. Yesterday me & my friends were traveling down a hilly road and I was making good use of engine braking on our F48 X1 by shifting to M1 or M2 along with brakes to slow down the vehicle. I was really tired and the road had enough potholes so it was more like drive (S mode on gearbox), then M1/M2 in slopes, brakes when noticing a pothole. In some stretches where road was decent and there was enough slope, I decided to rely only on engine braking and the revs where somewhere between 3-4.5k for multiple periods of 30 seconds or more. In between the journey we got a slight burning smell inside the cabin, which I'm almost 80 percent sure came from outside environment. It lasted for a minute max and was gone. Air recirculation was switched ON but AFAIK even then the car switches to external air on a fixed interval. Now I've seen how inexperienced drivers can fry the clutch on hilly roads, and this caused me to overanalyse what I was doing too.

I believe I hadn't taken the engine anywhere near the redline while engine braking in a particular gear, but we had the AC in full blast & I'm paranoid whether I actually missed noticing what revs my car was at due to this and took the car to high revs while relying too much on engine braking. There were no warnings that came up, and the car drives absolutely normal too. Even if I took it to high revs using engine braking, it wouldn't have lasted for more than 30 seconds as the road was filled with potholes (that's where I was focusing more on) so I would've slowed down with the brakes before hitting them.

But is this a loophole w.r.t to engine braking on autotransmission equipped cars? Can a good slope take the car in M1 to its redline, and if it does, what would really happen? I'm someone who used to believe engine braking on A/T is almost idiot proof, so never used to give it too much care until that slight smell got me overthinking. Incase something like this happens, will the transmission throw in a transmission temperature warning? Anyway I will take more care from now on while using engine braking. Hope I haven't caused any damage to my car too. I would like to hear from experts about this.
Agree with ttimbo, namely it might be possible but improbable to over-Rev. The double clutch system (on Sdrive) and gear box are protected with a huge amount of electronics, the auto box will change gear before allowing redline to be exceeded.

Also, note that unless you have your foot on the accelerator/gas pedal there will be no/little fuel in the system (modern fuel emissions and economy setting and fuel injection), any engine braking being from friction and resistance (unlike the old days when engine braking included actual resistance from engine firing) and is the main reason why engine braking does not really work effectively in modern road car engines and why you should ideally use brakes as primary mechanism to control speed.

Nothing in your description makes me believe you did anything to cause/risk damage.

Last edited by MJE60; 11-11-2020 at 02:45 PM..