# Poor fuel economy / mileage?



## sk1216 (May 26, 2010)

*Is D worth it?*

If the greater percentage of ones driving is city > highway (say 70/30 or 60/40) is it worth getting the diesel?


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## lalitkanteti (Nov 15, 2009)

With current offers definitely yes (Eco credit and tax credit). Irrespective of your drive, mileage will be more than 335i. And mostly in all states octane 93 is pretty close to diesel and in some cases higher.
Without those discounts you will have to take into account how long you will keep and how comfortable are you with HPFP failures in i and xi.


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## Snipe656 (Oct 22, 2009)

It depends entirely on why you buy one engine over the other. Down here supreme gas is usually a little less than diesel, sometimes the same and very rarely is it more. So if mpg and money spent on fuel is your main reason for selection then I'd say the diesel is a better option no matter how our driving is. Sounds like in a number of places diesel is a lot less than supreme, which would just help with the appeal, if mpg and cost of fuel is the primary motivator.


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## Evilsizer (Apr 4, 2008)

driving a 335i loaner with the cruise set at 65mph on the highway, i was seeing 30.8MPG. that one way trip was about 30-45min drive...


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## Snipe656 (Oct 22, 2009)

I always look at average mpg for a tank. Because even though I routinely see mid to high 30s in my car for stretches of road, I never get in the 30s for a tank due to all of the other driving.


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## anE934fun (May 10, 2008)

sk1216 said:


> If the greater percentage of ones driving is city > highway (say 70/30 or 60/40) is it worth getting the diesel?


City mileage with the 335i E93 that went lemon law was 17 - 18. City mileage with the 335d is 23 - 24 depending on how hard I push on the Go pedal when I accelerate from the light. In my neck of the woods, Chevron diesel is running about 5 cents per gallon cheaper than Chevron Premium (I filled up today with Diesel at $3.29/gallon; yesterday, I filled one of my gassers and the price was $3.35/gallon). The combination of lower price/gallon and better efficiency (higher mpgs) from diesel combined with the incentive BMW was providing (Eco Credit) made the diesel decision a no-brainer.


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## sno_duc (Sep 3, 2008)

I would not worry to much about mileage until 20k -30k miles, diesels take awhile to break-in.

I have added an EGT and boost gage to both of my diesels (93 cummins / 00 VW tdi). The turbo cool down comes from the fact that the EGT (and the hot side of the turbo) routinely get over 1200*f and even the best synthetic oils will coke above 700*- 800*f. So I let the EGT drop below 400*f before shutdown (doesn't take long, idling down our 100 yard long driveway does it). I don't know why diesel OEMs don't include EGT / boost gages as standard equipment. I also avoid more than 600*- 700*f or 5 psi boost until the engine is warmed up.

The other big factor in mileage is the right foot, my wife always gets 5-6 mpg more than I do. ( I tend to drive it like I stole it :angel: )


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## Snipe656 (Oct 22, 2009)

My Mercedes has 200k miles and gets the same mileage as when new. My truck has 150k miles and up until 20k miles ago it got the same since new and now gets a little worse but I think something is wrong right now. My chevy before that had around 250k miles and got the same mpg it's entire life. I do not buy into the whole "they get better mileage after X amount of miles" since it just does not happen for me.


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## TXPearl (Apr 16, 2010)

sk1216 said:


> If the greater percentage of ones driving is city > highway (say 70/30 or 60/40) is it worth getting the diesel?


This is not as clear with the introduction of the N55 engine in the 335i for the 2011 MY. Here are the EPA mileage specs for the vehicles in question:

2010 335i - 17/26
2011 335i - 19/28
2011 335d - 23/36 (same as prior MYs)

Prior to 2011, the diesel had a 35% improvement over the 335i (23 vs. 17) in the city. For MY 2011, that has dropped to 21% (23 vs. 19).

The diesel seems to really shine on the highway and provides a 29% improvement over the 2011 335i.

Regardless of EPA specs and potential fuel savings, I think (with all that low end torque) that the diesel drives better in everyday city driving. Therefore, I still think the D is worth it, no matter what your driving habits.


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## bballfreak (Jul 27, 2010)

Can we start a thread or something what everyone is getting with their 335d for mpg, city and hwy.
As my mpg dropped by 6-8, where I used to get consistently get 36+ mpg at 70mph, set on cruise
going on i-10 from San antonio to Houston and back. I go once to twice a month.
So, am comparing driving the same way, on the same road, with a/c on, stereo etc.
I think my mpg dropped after all the updates, don't know if this is a myth or what? Have 
41K miles on my 2009 335d.


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## Flyingman (Sep 13, 2009)

bball,

Take a few seconds and input your fuel data in Fuelly.com. You can just SMS it from your phone. Works great and after several years of data you can trend it. That is how I figured out that my summer MPG was lower than winter because of A/C use.

The traffic is even less in summer with kids out of school, and my MPG still is worse.

I was getting a bit worried the other day until I hit a nice streatch of open highway, no traffic, and got a solid 36mpg at 80mph, so right where I expect it to be.

Traffic is really a beaach on the MPG.


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## robster10 (Oct 8, 2012)

*fuel economy???*

First of all how are you driving your car? I'm getting 800-900 km & have even got 1000km on a tank with fuel left over. I don't baby the car either. That must translate to 500-600 miles on your scale. You can't drive a diesel like a gasser. You have to get up to speed quick and then ease of the turbo. Remember all the torque is down low in the rpm's. If you keep bumping the rpm's higher in the rev range & turbo is spooling up you are using more fuel. Remember these are torque monsters not horsepower kings. Try a different driving habbit, it should help your mileage.:thumbup: As well I never use cruise control. I roll up on the throttle as I come up to a hill or go to pass someone, where in cruise control the car will drop down a gear & bump up rpm. Remember the torque is the key on the highway drive. 
It just putters along at high speeds where gassers need to be pushed.


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## bballfreak (Jul 27, 2010)

The problem at hand is I used to get 36+ mpg all the time for the first 2.5 to 3 years but then in the fourth year it dropped off to 30 mpg for hwy,
under same road conditions etc. So, why did it drop down so much and the dealer cannot find anything wrong with the car and no codes either.
BMW will respond more appropriately if more 335d owners complained to corporate. I think my sharp drop was after the updates. Car has been
well maintained and has only 42K. 85% of miles are hwy miles. The sad part is I don't drive like a maniac, the car is on cruise at 70mph giving me
like 36+ on the hwy, now I get 30 or 30.6 all the time.


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## txagbmw (Apr 15, 2013)

Is the X5 that much heavier or inefficient than a d sedan. Appears from reading many posts the X5 does +- 10mpg less than a sedan.
Around here diesel has been a little under premium unleaded. Have no idea what the weight is on either vehicle.

Have also read where people with X5 gas is not that far off from a diesel mpg


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## ChasR (Aug 29, 2012)

bballfreak,
I can't complain to BMW since I have nothing to complain about. Great mileage, and fantastic power and torque (thanks Evolve). Updates haven't made any difference in mpg either. @67.8K miles, 33.06 mpg average over the last 30K miles and that's about to start it's annual rise as the AC runs less.

@Robert,
I use the cruise control religiously and I can't remember the car ever dropping out of 6th gear on Interstate standard roads around the SE US. It certainly won't in San Antonio.


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## BB_cuda (Nov 8, 2011)

anE934fun said:


> City mileage with the 335i E93 that went lemon law was 17 - 18. City mileage with the 335d is 23 - 24 depending on how hard I push on the Go pedal when I accelerate from the light. In my neck of the woods, Chevron diesel is running about 5 cents per gallon cheaper than Chevron Premium (I filled up today with Diesel at $3.29/gallon; yesterday, I filled one of my gassers and the price was $3.35/gallon). The combination of lower price/gallon and better efficiency (higher mpgs) from diesel combined with the incentive BMW was providing (Eco Credit) made the diesel decision a no-brainer.


Is that a typo? $3.29/gallon in San Francisco for diesel. I've never heard of diesel cheaper in California as compared to Texas. I paid $3.55/gallon last fill up. AnE934fun shows S.F. as his location. Maybe he/she was away from home.


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## Snipe656 (Oct 22, 2009)

BB_cuda said:


> Is that a typo? $3.29/gallon in San Francisco for diesel. I've never heard of diesel cheaper in California as compared to Texas. I paid $3.55/gallon last fill up. AnE934fun shows S.F. as his location. Maybe he/she was away from home.


You are quoting a post from July 2010. In July 2010 I was paying $2.739 to $2,799 for diesel, I did nine fillups that month so a few at different price points. I do not log fuel fillups in my gassers so not sure what gasoline was costing at that time. Diesel here though has been almost always more than 93 octane gas for me since late 2004. There have been some dips where it was less and those dips even sometimes lasted months but for the most part it has been a more expensive fuel. Prior to that point it was for the most part around the cost of 87 octane gas. So for me up until late 2004 it was cheaper(fuel expenses wise ) for me to drive my 2003 F250 Powerstroke that back then averaged around 21mpg than my old 2000 Accord EX which had a 4 cylinder that ran on 87 octane gas and averaged just a little better mpg than the F250 did.


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## BB_cuda (Nov 8, 2011)

hahaha, laughing ay myself. thanks snipe. I was thinking there is no way.


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## robster10 (Oct 8, 2012)

bballfreak said:


> The problem at hand is I used to get 36+ mpg all the time for the first 2.5 to 3 years but then in the fourth year it dropped off to 30 mpg for hwy,
> under same road conditions etc. So, why did it drop down so much and the dealer cannot find anything wrong with the car and no codes either.
> BMW will respond more appropriately if more 335d owners complained to corporate. I think my sharp drop was after the updates. Car has been
> well maintained and has only 42K. 85% of miles are hwy miles. The sad part is I don't drive like a maniac, the car is on cruise at 70mph giving me
> like 36+ on the hwy, now I get 30 or 30.6 all the time.


Have you opened your D up on the highway lately? I mean for a good couple of miles. Wide open +120mph. I remember others have also found when mileage began to drop they opened it up (Italian tune up!). After that they noticed increased mileage again, seems to clean out the crap in the intake. I don't condone high speeds on public roads city, highways during busy traffic times, but late at night when things are quiet out in far away places, well.....:thumbup:


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## bballfreak (Jul 27, 2010)

yeah, did that with a can of BG244 for diesel. That is what dealer asked me to do


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## robster10 (Oct 8, 2012)

I've heard of some ECU tunes that can shut off our egr valves. This alone sounds like a problem solver for our cars because of the buildup in the intake tract. If the egr valve is shut down no buildup should occur ever again.


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## bballfreak (Jul 27, 2010)

Really, I assume EGR relates to emissions.
Would be interesting to see if this can be done


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## Flyingman (Sep 13, 2009)

bball,

If you are this anal about your mpg, load up your data in Fuelly.com.

Once you can really see what is going on visually, you just might find the answer to your problem and that most likely you got no problem!:thumbup:


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## robster10 (Oct 8, 2012)

bballfreak said:


> Really, I assume EGR relates to emissions.
> Would be interesting to see if this can be done


That's right, it takes gaseous vapours from the exaust and directs them through the egr valve into the intake to be burned up in the combustion process. As well vapours from the crankcase are recycled this way as well to be burned up. Problem is it builds up (carbon) and oily residue that block up considerably in the intake port, as stated by some on this forum. 
I know JB tuning have a catch can for the crankcase pcv to collect the oily deposits but I haven't heard back if they have one for the 335d.


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## Pierre Louis (Oct 23, 2011)

robster10 said:


> That's right, it takes gaseous vapours from the exaust and directs them through the egr valve into the intake to be burned up in the combustion process. As well vapours from the crankcase are recycled this way as well to be burned up. Problem is it builds up (carbon) and oily residue that block up considerably in the intake port, as stated by some on this forum.
> I know JB tuning have a catch can for the crankcase pcv to collect the oily deposits but I haven't heard back if they have one for the 335d.


EGR as well as crankcase ventilation have been around a lot longer than oily deposits and carbon buildup. Blaming EGR is easy to believe but to me is not "obvious" nor has it been proven. In fact, the exhaust gas tends to be pretty clean (ULEV, or "clean diesel" remember), and it is used for its thermal and oxygen altering properties to aid in improving combustion.

Carbon buildup is more likely a result of other factors such as incomplete combustion perhaps in the newer direct injection designs, since even earlier direct injection diesels didn't seem to have much of a problem with this (although it was reported, and mostly in engines that were not put through enough heavy duty use). Obviously gasser engines also have a similar problem, so the jury is not back yet as to the cause or the solution. I agree there might be another factor that involves crankcase ventilation, but incomplete combustion and blow-by the rings also sounds plausible to me. I am not a engineer, but I can understand technical jargon as well as scientific evidence when I read it.

Making statements that are easy to believe and commonly thought by some to be true but not legitimate doesn't help anyone. I don't like the entire idea of recycling possible crud back into the engine, so can understand how people might think, but would like more solid information as to where the carbon actually comes from.


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## robster10 (Oct 8, 2012)

Pierre Louis said:


> EGR as well as crankcase ventilation have been around a lot longer than oily deposits and carbon buildup. Blaming EGR is easy to believe but to me is not "obvious" nor has it been proven. In fact, the exhaust gas tends to be pretty clean (ULEV, or "clean diesel" remember), and it is used for its thermal and oxygen altering properties to aid in improving combustion.
> 
> Carbon buildup is more likely a result of other factors such as incomplete combustion perhaps in the newer direct injection designs, since even earlier direct injection diesels didn't seem to have much of a problem with this (although it was reported, and mostly in engines that were not put through enough heavy duty use). Obviously gasser engines also have a similar problem, so the jury is not back yet as to the cause or the solution. I agree there might be another factor that involves crankcase ventilation, but incomplete combustion and blow-by the rings also sounds plausible to me. I am not a engineer, but I can understand technical jargon as well as scientific evidence when I read it.
> 
> Making statements that are easy to believe and commonly thought by some to be true but not legitimate doesn't help anyone. I don't like the entire idea of recycling possible crud back into the engine, so can understand how people might think, but would like more solid information as to where the carbon actually comes from.


The exaust gas that you say is "clean" is after it has gone through the filtering process DPF with the urea added. The problem is the egr takes exaust gasses before it goes through this process. Just look at where the plumbing lines are placed in this sytem. As well it's more complicated than others, we have a high press, & low press egr, as well as a heater for crankcase vapour. Now when you take a hot vapour, add it into the intake tract & force cooler air (ie intercooled/turbo) there most likely will be a build up small amounts of sediment. liquid(solid)flashes to gas back to a solid if it cools. Now over time this just grows until blockage can occur. Like you stated heavy use. If this motor is not pushed hard once in a while the crud won't get cleaned out.


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## Pierre Louis (Oct 23, 2011)

Unfortunately, carbon deposits are common enough in direct injection gasoline engines that don't have urea injection, but there seems to be a bit more literature on this for gassers: Here is an article that quotes a mechanic who believes it is the EGR/crankcase ventilation system design that matters:


> All modern gasoline engines return some crankcase and exhaust gases back through the intake manifold in order to help control emissions, but, according to Chick, some exhaust-gas recirculation designs are "dirtier" than others. Some, he said, are less-effective at preventing the passage of tiny bits of oil, carbon and other particulates that eventually get baked onto the intake ports and valves.


but according to an engineer at GM, we don't know exactly which molecules the buildup is coming from:


> Buczynsky says the exact cause of engine deposits is not known. Gasoline contains over 200 different molecules, and so far, no one has isolated a particular brew that is more likely to cause engine deposits. GM and other manufacturers, however, noticed there were regional concentrations of fuel-system warranty claims and traced the problem to the fuel.


Chevron has published the only industry specific general review of diesel fuel and mentions what we all know:


> The cetane number (see page 4) of the fuel defines its ignition quality. It is believed that fuels meeting the ASTM D 975 Standard Specification for Diesel Fuel Oils minimum cetane number requirement of 40 provide adequate performance in modern diesel engines. The minimum cetane number in Europe is 51. (See Chapter 5 - Diesel Fuel and Biodiesel Fuel Specifications and Test Methods.) Some researchers claim that a number of modern engines can benefit from a higher cetane number when starting in very cold climates. Smoothness of operation, misfire, smoke emissions, noise, and ease of starting are all dependent on the ignition quality of the fuel. At temperatures below freezing, starting aids may be necessary regardless of the cetane number.


So far, there is no "top tier" for diesel brands, and cetane is the only variable that BMW states in its recommendations, so we are left with not much to go on, pun intended, as far as what best to do, as to whether or what the best cetane booster to use has not been determined or officially recommended by BMW, but...


> A Cetane rating of 51 or higher is recommended for use in BMW Advanced Diesel Vehicle.


As fas as I know, carbon buildup does occur in Europe where cetane is higher.


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## Pierre Louis (Oct 23, 2011)

Here is an article that implies lower cetane decreases particulate emissions in direct injection diesel engines: Effects of fuel cetane number and aromatics on combustion process and emissions of a direct-injection diesel engine


> Abstract
> This study investigated the effects of fuel properties on combustion characteristics and emissions such as NOx. THC, smoke and particulate in a direct-injection diesel engine. Cetane number and aromatic content of fuels were varied independently. The results showed that reducing cetane number resulted in the increase of NOx and the decrease of particulate at high load. The aromatic content had little effect on combustion characteristics. However, increasing aromatic content for high cetane number fuel resulted in high NOx and particulate emissions. For low cetane number fuel, increasing aromatic content produced high THC emission at retarded injection timing. In the case of high injection pressure, fuel properties showed little effect on particulate emissions.


Here, the higher cetane was associated with higher torque and power: Effect of fuel cetane number and injection pressure on a DI Diesel engine performance and emissions


> Abstract
> In this experimental study, the effects of different fuel cetane numbers (CNs) and fuel injection pressures on a Diesel engine emission and on the performance were investigated. For this purpose, the fuels with 46, 51, 54.5 and 61.5 CN were tested in a four cycle, four cylinder DI Diesel engine. Measurements were conducted for each of the injection pressures 100, 150, 200 and 250 bar. Keeping 150 bar injection pressure and the other parameters constant, the changes of engine performance for the different CNs were also tested at full load condition.
> 
> The results showed that NOx, SO2 and CO emissions are reduced about 15% and 5%, respectively, when the fuel CN is increased for the standard injection pressure, but the smoke value is increased dramatically when the injection pressure is reduced to 100 bar. In contrast with the lower pressure, NOx is increased and smoke is decreased when the injection pressure is increased to 250 bar. Increases in engine torque by 5% and power output by 4% were observed at the maximum torque speed of 2500 min***8722;1 when the CN is increased from 46 to 54.5. However, when increasing CN above 54.5, no significant increases in engine performance were observed.


Again, if you want more smoke, use cetane improver: The effect of fuel cetane improver on diesel pollutant emissions


> Abstract
> A base fuel having a cetane number of 40.2 was split into nine batches. To eight of these batches, varying quantities of ignition improver were added, resulting in cetane numbers up to 62. The base fuel and each of the blends with ignition improver were used in a co-operative fuel research (CFR) diesel engine, and the exhaust emissions of NOx unburnt hydrocarbons (UHC) and smoke were measured, to ascertain the effect of varying cetane number on emissions when the basic chemical structure and physical properties of the fuel were almost unaltered. The results showed that the exhaust NOx progressively decreased with increasing cetane number, due to the reduction in ignition delay and amount of premixed fuel burnt. Similarly, the UHC decreased. However, the smoke increased due to the reduction in the amount of relatively smoke-free premixed fuel burnt.


So theoretically if you want to use the maximum output from the 335d as it is designed for 51 cetane, maybe a cetane improver can help. But if you want to limit the soot, don't.


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## F32Fleet (Jul 14, 2010)

Pierre Louis said:


> Here is an article that implies lower cetane decreases particulate emissions in direct injection diesel engines: Effects of fuel cetane number and aromatics on combustion process and emissions of a direct-injection diesel engineHere, the higher cetane was associated with higher torque and power: Effect of fuel cetane number and injection pressure on a DI Diesel engine performance and emissionsAgain, if you want more smoke, use cetane improver: The effect of fuel cetane improver on diesel pollutant emissionsSo theoretically if you want to use the maximum output from the 335d as it is designed for 51 cetane, maybe a cetane improver can help. But if you want to limit the soot, don't.


EGR is closed on start up so the "extra soot" isn't a factor besides we're talking negligible amounts to begin with. I've read studies which show this increase in PM lasts for a period of about 5-10 seconds after start.

Note: The study you cited experienced the increased soot with lower injection pressures (100 bar).


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## Pierre Louis (Oct 23, 2011)

BMWTurboDzl said:


> EGR is closed on start up so the "extra soot" isn't a factor besides we're talking negligible amounts to begin with. I've read studies which show this increase in PM lasts for a period of about 5-10 seconds after start.
> 
> Note: The study you cited experienced the increased soot with lower injection pressures (100 bar).


Yes, but using cetane as an independent variable is not to be assumed as most do to be easily interpreted. That was the point. The electronics matter the most, not whether we put cetane improver in, which may make things worse for all we know as far as carbon deposits. It seems like it would improve the conversion of fuel into usable power in engines designed specifically for higher cetane but with an increase in soot formation when compared to lower cetane fuel.

The higher pressures did decrease the soot but with the same cetane fuel used in comparison. Increasing the cetane increased the soot when pressure was held constant.


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## Pierre Louis (Oct 23, 2011)

Its an odd conversation that the 335d engine was designed for 51 cetane fuel as it is a European style engine but its current pollution control systems, as well as its coated fuel pump components, were designed for North American diesel fuel. Its impossible to accurately guess what effects the different parameters have on reliability issues without true field testing and evidence from real world data, especially when some problems such as carbon buildup are not exactly well understood.


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## F32Fleet (Jul 14, 2010)

Pierre Louis said:


> Yes, but using cetane as an independent variable is not to be assumed as most do to be easily interpreted. That was the point. The electronics matter the most, not whether we put cetane improver in, which may make things worse for all we know as far as carbon deposits. It seems like it would improve the conversion of fuel into usable power in engines designed specifically for higher cetane but with an increase in soot formation when compared to lower cetane fuel.


I disagree, it's fairly obvious that a cetane improver will not make things worse. Help prevent? Probably very little (at idle).


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## F32Fleet (Jul 14, 2010)

Pierre Louis said:


> Its an odd conversation that the 335d engine was designed for 51 cetane fuel as it is a European style engine but its current pollution control systems, as well as its coated fuel pump components, were designed for North American diesel fuel. Its impossible to accurately guess what effects the different parameters have on reliability issues without true field testing and evidence from real world data, especially when some problems such as carbon buildup are not exactly well understood.


US version does have some design differences but the literature states there were made mainly for acoustic reasons.


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## Pierre Louis (Oct 23, 2011)

BMWTurboDzl said:


> I disagree, it's fairly obvious that a cetane improver will not make things worse. Help prevent? Probably very little (at idle).


Well, I guess you can write to the authors of the study where cetane was used as a variable and they found more soot was created with higher cetane. Must have been a mistake.



> US version does have some design differences but the literature states there were made mainly for acoustic reasons.


Acoustic reasons? Really! Europe doesn't have the stricter emissions we have had on diesel since 2009 until 2014 I believe. But I guess the sound through the exhaust makes for noise pollution too!


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## Pierre Louis (Oct 23, 2011)

A similar finding occurred with higher octane fuel where the extra benzene rings etc. created more polluting emissions even though engines designed for premium ran better in other ways.


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## F32Fleet (Jul 14, 2010)

Pierre Louis said:


> Well, I guess you can write to the authors of the study where cetane was used as a variable and they found more soot was created with higher cetane. Must have been a mistake.
> 
> Acoustic reasons? Really! Europe doesn't have the stricter emissions we have had on diesel since 2009 until 2014 I believe. But I guess the sound through the exhaust makes for noise pollution too!


The study is valid but it must be taken in context.

Yes, acoustic reasons. The offset pin at the piston and reinforcement at the bottom of the engine were made for acoustic reasons.

IIRC we did receive a heated intake and our swirl flap actuator is different.

Oh and btw..in the UK at least you could buy the car equipped with DPF.


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## robster10 (Oct 8, 2012)

BMWTurboDzl said:


> EGR is closed on start up so the "extra soot" isn't a factor besides we're talking negligible amounts to begin with. I've read studies which show this increase in PM lasts for a period of about 5-10 seconds after start.
> 
> Note: The study you cited experienced the increased soot with lower injection pressures (100 bar).


I can't remember where on this forum but there was a recall on faulty egr valves. Now if this could be linked to owners who have had build up of intakes or not I can't tell since haven't been able to track down no's. So the computer takes care of these controls, but if there is in fact a fault in egr units then maybe this could be a factor in auto's with carbon/sludge build up. Egr valves could open earlier than desired or even be open always. So theoretically the engineering is a good thing in our cars but when the system is not functioning properly then voila carbon/sludge buildup can occur. These negligible amounts will build up over time, since were talking about auto's (don't quote me on this) that had over 25-30k miles.


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## Pierre Louis (Oct 23, 2011)

BMWTurboDzl said:


> The study is valid but it must be taken in context.
> 
> Yes, acoustic reasons. The offset pin at the piston and reinforcement at the bottom of the engine were made for acoustic reasons.
> 
> ...


Here is another article: SOOT CHARACTERISATION IN DIESEL ENGINES USING LASER-INDUCED INCANDESCENCE


> Effects of in-cylinder pressure, fuel injection pressure and cetane number on soot formation and characteristics were observed. High injection pressure, cetane number and in-cylinder pressure caused a reduction of soot particle size and volume fraction but an increase of the soot particle density.


Another paper talks about the combustion process

Chevron's rather comprehensive Diesel Fuel Technical Review mentions


> Cetane Number
> Increasing the cetane number improves fuel combustion, reduces white smoke on startup, and tends to reduce NOx and PM emissions. NOx seems to be reduced in all engines, while PM reductions are engine-dependent. These cetane number effects also tend to be non-linear in the sense that increasing the cetane number produces the greatest benefit when starting with a relatively low cetane number fuel.


So Chevron says there is no soot increase associated with cetane, but that decreases in PM are engine dependent.


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