# Low Speed City Use



## BobBNY (Sep 2, 2011)

I keep reading "The other reasons may include driving short distances at mild speeds" about CBU on BMW diesels. Not sure I understand. Can some one explain?

What I don't understand is with the 8 speed transmission even at highway speeds the car is running at 1500-1800 rpm. In city/suburban rush hour traffic of 9-15 mph you cycle up and down from idle to 2500 rpm. I have a 10 mile commute in Bay area that has 3-5 miles at 50-70mph and the balance of stop and go and low speed. I run the vehicle hard at times and have good highway driving on the weekends. Is this bad? I would think the car is designed for this.

BB


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## Ruggernaut (Apr 25, 2014)

It's hard to open this forum without seeing a thread regarding the CBU issue. 

My concerns are much like yours. I know it wants to be driven like its stolen, but most of the time I don't. Lots of short trips mixed with longer weekend runs. I had a play time miata that I would use Seafoam on, to fight carbon. Why can't that be used on our Ds? Are these CBU issues going to be part of life with the 2.0 engine or not? Will there be a better chemical solution found?


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## KeithS (Dec 30, 2001)

BobBNY said:


> I keep reading "The other reasons may include driving short distances at mild speeds" about CBU on BMW diesels. Not sure I understand. Can some one explain?
> 
> What I don't understand is with the 8 speed transmission even at highway speeds the car is running at 1500-1800 rpm. In city/suburban rush hour traffic of 9-15 mph you cycle up and down from idle to 2500 rpm. I have a 10 mile commute in Bay area that has 3-5 miles at 50-70mph and the balance of stop and go and low speed. I run the vehicle hard at times and have good highway driving on the weekends. Is this bad? I would think the car is designed for this.
> 
> BB


This is the PERFECT driving conditions for any of the electric cars out there including the i3.


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## Michael47 (May 9, 2014)

Law of physics: it takes less energy to move a car down the road at 35 mph than it does at 65 mph. It also takes less energy to accelerate gently to 35 than it does to floor it up to 65. It not only takes less energy, meaning it burns less fuel, it also takes less air moved through the intake.

CBU is a function of soot and oil vapor being deposited, and that deposition is subject to being blown into the engine at high RPMs, but not so much at low RPMs. Result being that gentle city driving might create less soot and deposit less oil, but more of it sticks.

At least that's the theory. All we really have to go on is that there seem to be more reports of CBU among the gentle city crowd than among the drive-it-like-it's-stolen or out-on-the-highway crowds.

All that being said, it should also be noted that BMW diesels are not the only ones with this issue. VWs have it too, Jeeps, you name it. The simple fact is that the combination of routing crankcase fumes into the intake and adding exhaust as well makes for CBU. The original reason for the exhaust gas was to lower peak temperatures to fight NOx emissions. Personally, I'd rather add a bunch more DEF than have to gouge out the intake later. But nobody asked me.


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## henrycyao (Oct 23, 2012)

Michael47 said:


> Law of physics: it takes less energy to move a car down the road at 35 mph than it does at 65 mph. It also takes less energy to accelerate gently to 35 than it does to floor it up to 65. It not only takes less energy, meaning it burns less fuel, it also takes less air moved through the intake.
> 
> CBU is a function of soot and oil vapor being deposited, and that deposition is subject to being blown into the engine at high RPMs, but not so much at low RPMs. Result being that gentle city driving might create less soot and deposit less oil, but more of it sticks.
> 
> ...


There is more than just a gentle drive cycles. What is very important is to allow your diesel to go through clean cycle by running it for a while. I would argue it does not matter the speed but your RPM must be above idle such that the engine is hot enough to burn off soot and be long enough to finish the job.

A city drive cycle is actually harsher on the engine assuming you don't spend a significant time idling. It runs from idle to x rpm and comes back to 0 again. Heat generation is actually higher because of it. That is one of the reason why you have poorer fuel economy in the city. Where as in freeway, your engine may be generating as low as 5 to 10 hp to keep the momentum going. That is a key reason why freeway travel is actually better for your engine. Your power generation is very low. That means you have less heat to deal with and good amount of air to cool the engine. From my perspective, less power = less fuel = less soot + time to burn off soot = low CBU.

As for drive it like you stole it, that is highly questionable. People believe that idea because they want to drive hard on their BMW and not feel guilty about it. The only benefit of that is bring your engine up to proper temperature quicker. Since soot get burn off at hot, you get that benefit. However, you also generating a lot of soot. It is a wash. Nobody really can drive it like you stole it on public road. I have yet to read anybody got a engine shutdown warning due to overheat cause by their driving style. You don't get that until you go to the track. We never drive it that hard. Think about it. 0-60 only last 7 seconds. 0-100 may be 12 s or so. That is very short amount of time.


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## henrycyao (Oct 23, 2012)

BobBNY said:


> I keep reading "The other reasons may include driving short distances at mild speeds" about CBU on BMW diesels. Not sure I understand. Can some one explain?
> 
> What I don't understand is with the 8 speed transmission even at highway speeds the car is running at 1500-1800 rpm. In city/suburban rush hour traffic of 9-15 mph you cycle up and down from idle to 2500 rpm. I have a 10 mile commute in Bay area that has 3-5 miles at 50-70mph and the balance of stop and go and low speed. I run the vehicle hard at times and have good highway driving on the weekends. Is this bad? I would think the car is designed for this.
> 
> BB


As for you, I recommend engine conditioning cycle at least once every tank. That is when our diesel burn off the soot in the DPF in the regen cycle. Basically, you need to take a long road trip with at least 30 miles of your engine away from idle with at least 30 minutes on the road. That is what I do.

Note, don't do full acceleration before your engine warmed up. That is one sure way to shorten your engine life. That advice is applicable to both diesel and gas engines. It takes 5 miles or so to warm up my diesel. Newer engine may be faster and depending on how they design the engine. Watch your coolant temperature and you will learn when to be really gentle on your engine.


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## MotoWPK (Oct 5, 2012)

henrycyao said:


> From my perspective, less power = less fuel = less soot + time to burn off soot = low CBU.
> 
> As for drive it like you stole it, that is highly questionable. People believe that idea because they want to drive hard on their BMW and not feel guilty about it. The only benefit of that is bring your engine up to proper temperature quicker. Since soot get burn off at hot, you get that benefit.


This seems to express a common mistaken perception regarding heat and CBU. To burn off soot takes several hundred degree F minimum. Indeed, fuel is added to the exhaust to provide sufficient heat in the DPF to burn off the soot during regeneration where temperatures may exceed 1000 degrees F. The temperature in the inlet tract is hundreds of degree below this and well below that which will burn off soot.


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## sirbikes (Aug 17, 2012)

I think the reason they say to drive it like you stole it, including my bmw head service advisor recommending me to get the rpm needle past the 12 o'clock mark, is because at high loads (>2500 rpm and >480Nm) there is no egr going on. You can see the map on pg 32 of the BMW technical training manual (image attached).


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## henrycyao (Oct 23, 2012)

sirbikes said:


> I think the reason they say to drive it like you stole it, including my bmw head service advisor recommending me to get the rpm needle past the 12 o'clock mark, is because at high loads (>2500 rpm and >480Nm) there is no egr going on. You can see the map on pg 32 of the BMW technical training manual (image attached).


You get >480 Nm only under full throttle applications. Basically, you need a huge drag in order not accelerate. As soon as you stop accelerate, the torque leaves that region. That is very small amount of time.

I don't even think you hit x5 or 335d limit with air dynamics. You have to defeat the electronic nanny first.

Don't do full throttle before engine warm up!


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## henrycyao (Oct 23, 2012)

Hmm, I wonder if I should buy a remote obd ii so I can measure torque and horse power. Does any one know if that information is available in x5 obd ii port and what is a good one to get.


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## Hoooper (Jun 17, 2013)

henrycyao said:


> Hmm, I wonder if I should buy a remote obd ii so I can measure torque and horse power. Does any one know if that information is available in x5 obd ii port and what is a good one to get.


The DDE uses a computed torque value which can be read and while it's not 100% accurate it should be about as close as a dyno anyway. From there you can easily calculate the hp number.


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## MotoWPK (Oct 5, 2012)

sirbikes said:


> I think the reason they say to drive it like you stole it, including my bmw head service advisor recommending me to get the rpm needle past the 12 o'clock mark, is because at high loads (>2500 rpm and >480Nm) there is no egr going on. You can see the map on pg 32 of the BMW technical training manual (image attached).


But...on the street this will only represent a very small percentage of operation since you simply cannot maintain these high levels of output. A few seconds of relatively high output accelerating and you're back to low load with EGR.


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## Hoooper (Jun 17, 2013)

MotoWPK said:


> But...on the street this will only represent a very small percentage of operation since you simply cannot maintain these high levels of output. A few seconds of relatively high output accelerating and you're back to low load with EGR.


Yes thats why it doesnt work in that sense. For the 335d, drive it like you stole it means get the car impounded since it wont get CBU while its sitting in the impound lot or in your garage when you dont have a license anymore.


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## F32Fleet (Jul 14, 2010)

BobBNY said:


> I keep reading "The other reasons may include driving short distances at mild speeds" about CBU on BMW diesels. Not sure I understand. Can some one explain?
> 
> What I don't understand is with the 8 speed transmission even at highway speeds the car is running at 1500-1800 rpm. In city/suburban rush hour traffic of 9-15 mph you cycle up and down from idle to 2500 rpm. I have a 10 mile commute in Bay area that has 3-5 miles at 50-70mph and the balance of stop and go and low speed. I run the vehicle hard at times and have good highway driving on the weekends. Is this bad? I would think the car is designed for this.
> 
> BB


IMO the issue is with load. The 335d is a light car relative to the amount of TQ the M57 can deliver. The suggestion is that you get more blow-by when you're puttering around at low loads. More blow-by = more oily vapors getting routed back into the intake. These vapors cool inside the walls of the intake/cylinder head/valves and capture soot passing by.

At highway speeds (80+) wind resistance adds additional load to the engine. So hopefully you're getting less blow by and therefore less oily vapors.

The X5d is a lot heavier vehicle.


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## Jimmy68 (Jan 19, 2015)

Guys - From my research, here's the connection between light load and carbon build up:

Diesels put out a lot of NOx and they normal operate with a lot of excess oxygen (relative to gas engines) except when the engines are heavily loaded. How do you reduce NOx? - recirculate exhaust gas (EGR) that has most of the oxygen gas already removed through combustion. No excess oxygen = greatly reduced NOx emissions. The problem is that all that EGR circulates a lot of carbon through the intake manifold.

If you operate the engine under heavy load, then it needs more oxygen and less EGR, ergo less carbon build-up.

Or stating it another way, generally the 335d has too big of an engine that is lightly loaded most of the time and therefore is running a lot of exhaust through the engine, causing the build up.


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## DaveN007 (Oct 4, 2013)

KeithS said:


> This is the PERFECT driving conditions for any of the electric cars out there including the i3.


I could not agree more.

:thumbup:


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## Hoooper (Jun 17, 2013)

F32Fleet said:


> The X5d is a lot heavier vehicle.


The x5d has a secondary EGR system that is filtered. That is its primary advantage from a CBU standpoint.


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## ChasR (Aug 29, 2012)

Ruggernaut said:


> It's hard to open this forum without seeing a thread regarding the CBU issue.
> 
> My concerns are much like yours. I know it wants to be driven like its stolen, but most of the time I don't. Lots of short trips mixed with longer weekend runs. I had a play time miata that I would use Seafoam on, to fight carbon. Why can't that be used on our Ds? Are these CBU issues going to be part of life with the 2.0 engine or not? Will there be a better chemical solution found?


The problem with chemical solutions to carbon buildup in direct injected vehicles is that the buildup is in the intake tract and none of the chemical is directly applied to it, but is injected directly into the cylinder. Only if the chemical solution reduces the soot produced will it have an effect. If a cetane improver reduces soot, it may have a beneficial effect. If the chemical keeps the injectors cleaner and thus reduces soot, that may have an effect


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## txagbmw (Apr 15, 2013)

henrycyao said:


> You get >480 Nm only under full throttle applications. Basically, you need a huge drag in order not accelerate. As soon as you stop accelerate, the torque leaves that region. That is very small amount of time.
> 
> I don't even think you hit x5 or 335d limit with air dynamics. You have to defeat the electronic nanny first.
> 
> ...


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## BobBNY (Sep 2, 2011)

Thanks all for the info. Makes more sense now. Everything is a compromise isn't it?

As a follow-up I always let the engine warm up before hard driving. My day is not all stop and go. Appointments during the day and weekend trips get the car moving 70-80 mph at least 30 min minimum. Certainly not going to worry about this.

BB


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