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      11-09-2020, 02:40 AM   #1
Xplorer1
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Query regarding Engine braking with Aisin transmission

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.
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      11-09-2020, 07:22 PM   #2
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I suspect the transmission might have become quite warm during the drive you described. If it had actually overheated, I'm sure there'd be a warning light.

To answer the question, yes it is possible but improbable to over-rev using engine braking on a t/q automatic transmission. The transmission ECU usually has various protections in place that should prevent over-revving in most circumstances. That said, if the descent conditions mean you need to use M1/M2 constantly, I'm thinking you perhaps should have used either the brakes more (shouldn't overheat too much at those low speeds) or perhaps engaged hill descent control. Both should enable mechanically sympathetic control of the vehicle
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      11-10-2020, 11:12 AM   #3
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Engine Braking

This was a useful feature in my previous car (Audi A3) and engine braking used to kick in when I hit downhill sections of the A470 near Brecon.

No need to switch to manual, it seemed to happen automatically.

Driving the same stretch on my X1, this feature appears to be absent, unfortunately.

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      11-10-2020, 03:58 PM   #4
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I drive up and back down Pikes Peak numerous times a year(I live closeby); in addition to lots of other mountain driving. I regularly make use of engine braking in order to save my brakes and maintain a safe speed, never noticed any adverse effects whatsoever so I'd think that you would be fine under your described conditions. Also I've never seen a situation where the car allowed me to go past the red line. It will always hold at the line or automatically shift up to save the transmission. It won't allow you to stall either.

All in all, I wouldn't worry about it. Maybe take care to check all your gauges frequently next time but I'd bet you didn't cause any harm or undue wear.
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      11-11-2020, 02:39 PM   #5
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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..
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      11-11-2020, 03:29 PM   #6
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Thanks everyone for your informative inputs. This forum has been really helpful in ironing out any concerns with the F48 X1. Just like you all have chipped in, I believe everything is alright but I will definitely be more careful and will use brakes more next time I'm going down hill.

BTW this is what I found on the i drive regarding manual mode. It says the gearbox will not upshift on manual mode as it reaches certain engine light speeds (which should be the redline) ONLY IF DSC is deactivated. In my case I was in Comfort mode with just the gearbox in Sport mode, which means incase I did reach redline, the gearbox would've upshifted automatically. So that is a pretty solid failsafe mechanism built-in is what I understand.

Quote:
Originally Posted by ttimbo View Post
That said, if the descent conditions mean you need to use M1/M2 constantly, I'm thinking you perhaps should have used either the brakes more (shouldn't overheat too much at those low speeds) or perhaps engaged hill descent control. Both should enable mechanically sympathetic control of the vehicle
Mine is an Sdrive 20d, does this variant of the X1 have hill descent control? If it does have it, will try to use it the next time.

Quote:
Originally Posted by Heathwood View Post
This was a useful feature in my previous car (Audi A3) and engine braking used to kick in when I hit downhill sections of the A470 near Brecon.

No need to switch to manual, it seemed to happen automatically.

Driving the same stretch on my X1, this feature appears to be absent, unfortunately.

Engine braking does kick in the X1 if you put the gearbox in Sports mode, but nowhere near what I could get on my manual transmission car. But on my Manual tranny car, I would have kept an eye on the revs too.

Quote:
Originally Posted by Shagohod View Post
I drive up and back down Pikes Peak numerous times a year(I live closeby); in addition to lots of other mountain driving. I regularly make use of engine braking in order to save my brakes and maintain a safe speed, never noticed any adverse effects whatsoever so I'd think that you would be fine under your described conditions. Also I've never seen a situation where the car allowed me to go past the red line. It will always hold at the line or automatically shift up to save the transmission. It won't allow you to stall either.

All in all, I wouldn't worry about it. Maybe take care to check all your gauges frequently next time but I'd bet you didn't cause any harm or undue wear.
Will keep an eye on the revs from now on Glad to know more about this as googling the same gives conflicting results.

Quote:
Originally Posted by MJE60 View Post
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.
Yes, the feel of engine braking is definitely very different from what is available in my old manual tranny vehicle. Going downhill, I would've preferred more powerful engine braking to stay in better control. Infact, the auto transmission feels easy when you go uphill, while manual tranny feels better with all that engine braking when you're coming down. That's an interesting conundrum.

Anyway thanks everyone. Really helpful even when it comes to a difficult topic for Google. I had read some comments online which said engine braking is bad for the torque converter etc.. Thanks to this forum, I can have proper answers to any query.
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      11-11-2020, 05:49 PM   #7
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I went down a long hill a few days ago for about 4 to 5km using engine braking in gear 2-4. I was monitoring my transmission oil temp the whole time. It barely went up one degree. So whatever you were smelling, might just be from the road.
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      11-17-2020, 04:57 PM   #8
Jack Watts
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brake pads are cheaper than transmissions...

There are enough nanny overrides to stop you from destroying your transmission but engine braking definitely raises fluid temps and puts more load on the system. Personally, I'd just use the brakes. I've yet to encounter a mountain pass where I'm getting brake fade. If were as snow-covered pass, that's different--but then, the speeds would be lower.
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