Diesel longer lasting engine?
 

I am not much of an engine technician, but I'll bet some folks out here are. Diesels have a reputation for lasting a long time. Will the X5 diesel have a longer life than the regular gas engines?

As I rack the miles up on my E70, I am pondering if the next one will be the diesel or not. My biggest concern with the E70 is maintenance costs in the future (and the associated hassle).

Granted most of the X5 issues are not with the engine itself, but I'm curious to more about diesel engine maintenance and issues compared to the normal gas engines.

Im no techinicial either, but the word is they last longer, They tend to be built a lot stronger because diesels run very high compression rations. There is not an ignition system that needs lots of regular maintenance. Also diesel fuel itself acts as a lubricant so the cylinder walls and piston rings wear much slower than they do in a gas engine.

As gercajr said, diesels tend to last longer for the reasons he stated. However, the X5 diesel has two turbochargers, which adds more complexity and temperature and speed-stressed parts and bearings, e.g., while a typical normally-aspirated Diesel might last longer/be more reliable than a typical normally-aspirated gasoline engine, I'm not sure a dual-turbocharged Diesel will last longer/be more reliable than a normally-aspirated gasoline engine.

Turbocharging is a neat trick, but it does not help reliability.

Engine technician here. Expanding on what has already been posted...

The only reason that diesels have a reputation for long life (durability, not the same as reliability) is because traditionally those diesel engine blocks were built very strong to deal with the high peak cylinder pressures required for the compression-ignition cycle. If two identical engine blocks are available as diesel and spark-ignited versions, the spark-ignited version lasts much longer, all other things being equal (but they usually aren't actually equal). The parts that last longer are the cylinder bores, and the crankshaft. A good example would be the spark-ignited natural gas industrial engines built by Caterpillar, which started out as diesels, and which are so under-stressed that they run for a very long time between overhauls, even longer than their diesel equivalents.

An automotive-duty diesel will not last longer, as it is typically designed to the same service or duty cycle as the gasoline equivalent. Reference the VW diesel Rabbit as an example. Old Mercedes diesel taxis lasted a long time (built like boat anchors) but then so did the gasoline equivalents (also built like boat anchors) BMW is using a different block design for diesels than gasoline engines, built to withstand the high cylinder pressures, but expecting it to run longer than the gasoline equivalent is misguided. It shouldn't last any less time. Now, since this is all theoretical, does it really matter? You are very unlikely to trade in either model because the cylinder bores are worn, the electrics will sideline you long before that. Even better, wait and see how the higher diesel torque helps the automatic transmissions last longer, lol.

Turbocharging isn't a determining factor in the wear equation, power output is. You can get that power through NA or turbo means, but I wouldn't expect turbocharging to have any impact on durability.

Now consider reliability, a totally different concept. This isn't about how long it lasts, but about how likely you are to have a problem that interupts your trip. Diesels have no ignition system (a plus) but they are less tolerant of fuel variances (a negative). Modern ignition systems cause far fewer problems than ignition systems did 10 or 20 or more years ago. Overall, with modern direct injection fuel systems on gasoline vehicles, and other equalizing factors like turbos being installed on both versions, I don't think there is any significant difference in reliability.

You also mention maintenance costs. I wouldn't expect any significant difference. Both have fuel filters, air filters, and injectors that may require maintenance. Spark plugs are a non-issue these days. A bigger issue may be the skill level/familiarity of the technicians working on the newest generation of diesels in the first few years until they become more mainstream.

Thanks JCL. Excellent response.

> The parts that last longer are the cylinder bores, and the crankshaft. A good example would be the spark-ignited natural gas industrial engines built by Caterpillar, which started out as diesels, and which are so under-stressed that they run for a very long time between overhauls, even longer than their diesel equivalents.

Much of the cylinder bore longer wear can be attributed to the lubricating qualities of diesel "oil," as it used to be called, vs. gasoline. The reason idling a diesel is not considered as harmful to an engine as idling a gasoline engine, has much to do with the lubricating quality of diesel fuel vs. the cylinder-wall washing characteristics of gasoline (although this difference is not as great as it used to be, now that gasoline engines use precise fuel-inject vs. older carbs). I would also add that natural gas engines tend to have a much longer life than gasoline engines, simply due cleaner burning and a much lower rate of deposit build-up, as compared to gasoline. In the 1960's my Father worked for the local natural gas utility company and the had several trucks with their gasoline engines converted to Propane. These propane converted trucks long outlived their identical gasoline-fueled brothers and, when a valve job was done on one of them at 250,000 miles, the mechanic was amazed at the lack of deposits.


As for maintenance costs, with the addition of a couple of turbochargers and a urea injection system, complete with pumps and heaters to prevent Winter freezing, I suspect the long-term cost of an X5 Diesel for maintenance and repair will be higher than a naturally-aspirated gasoline engine. Most turbocharged diesel engines will also require oil changes more frequently than NA gasoline engines, adding to maintenance costs.

And I would disagree on turbochagring not having an impact on durability, but it ends-up being an exercise in semantics. Turbochargers do stress engines by getting more HP out of a lower displacement than NA engines, in general. This means higher BMEP's, temperatures, complexity, heat stress on oil flowing through tubro bearings, etc. And the turbocharger(s) itself is an added component and complexity which can cause reliability problems (no turbocharger, no turbocharger problems). Now, if the engine is designed from the start for turbocharging, then the designers will make things more robust to compensate for this and one might end-up with the same or better reliability, i.e., one can design around the inherent lower reliability of a turbocharged engine and perhaps end-up with something even more reliable than a cheaper NA engine with the same HP... but it does take more money to get the same HP and reliability in a smaller, more fuel-efficient package via turbocharging.

I would note that a gasoline X5 will not refuse to run if it thinks it is out of urea solution, which does give one extra point of reliability to the gasoline X5... just one less thing to potentially go wrong.

P.S. Here's an interesting GMRC paper about reliability of Caterpillar turbocharged natural gas engines:

"Mitigating Turbocharger Reliability Issues on Caterpillar G3600 Series Engines

William Couch–El Paso Western Pipelines/Mitchell Opat–Universal Turbo/David Krenek– Caterpillar, Inc.
Since the introduction of the natural gas Caterpillar 3600 Series Engines in the nineties, operators of these engines with the first generation VTC turbochargers have been plagued with chronic turbocharger failures resulting from deposit buildup (fouling) on the turbine blades. This paper will briefly review and update the following: the casual factors surrounding turbocharger fouling; update population information of VTC and TPS configuration of G3600 engines; the statistics regarding the number and types of VTC turbochargers affected by the fouling buildup; discuss the costs involved in the repair/replacement of both ailing and failed turbochargers; the economics of repairing or replacing turbochargers prior to catastrophic failure; review methods of detecting eminent failure, thus allowing the turbochargers to be removed from service before a catastrophic failure. The following topics will also be reviewed and discussed in detail: the root cause of the deposit buildup on the turbine parts and review possible methods of preventing catastrophic failures of the turbochargers; methods of preventing the deposit buildup on the turbine parts with ethylene glycol and water injection and specialized blade coatings; the economics and benefits of replacing the first generation VTC turbochargers with the more fouling resistant second generation TPS turbochargers; the economics and benefits of replacing the ESS panel with Adem III panel."

That paper seems to be about the turbochargers used on the first series of G3600 engines in the '90s. Given that turbochargers are a scheduled changeout item (25,000 hours IIRC), I would think moving to the newer part number would be a straightforward decision. I am a little removed from that area now. I was involved in the first ever customer installation of a G3616, a 3 mW genset we installed at a power station in the Canadian Rockies in 1991. It was pretty reliable, and I suspect it is still running (it would be well over 100,000 hours now). EDIT: I went and looked up the paper. It took me back, and I remember the part numbers, the Asea Brown Boveri (ABB) VTC254 turbos. They were huge.

I agree that diesel has lubricating qualities, more so on #2 fuel oil than other grades. A significant issue we used to have with winter fuel that was blended with #1 and #2 was the reduction in lubricating qualitities (the designers called it lubricity) and resultant failures, particularly the hydraulic unit injectors. The pre-cup diesels that used to lubricate the cylinder walls are long gone now, due to emissions rules. With modern direct injection systems, whether diesel or gasoline, I don't see much impact of the fuel washing down cylinder walls. I could be wrong.

I think if we are comparing compression ignition engines to spark ignition engines (gasoline, CNG, propane, etc) then other variables should be equal. That would suggest that we should compare turbocharged engines to turbocharged engines, and NA engines to NA engines, whichever cycle the engine is operating on. With BMW's move to turbos on new models, it is likely a moot point, as NA engines will become less common.

Interesting discussion. Always up for an engine thread, it makes a nice change from the threads about which 22" all season tire to buy or what % tint, lol.

> That would suggest that we should compare turbocharged engines to turbocharged engines, and NA engines to NA engines, whichever cycle the engine is operating on.

I agree completely.

But if one is talking about the X5, you have to go with what's available, so the X5 comparison is a NA gasoline engine vs. a twin-turbo diesel with urea injection.

However, I suspect that when the 2010 X5 model comes out, the base gasoline engine in the X5 may very well be a twin-turbo version. BMW then will raise the base price a bit to cover the increased cost, and the premium differential to get a diesel over the base engine will come down to a more reasonable level, e.g., BMW has this all planned-out when they put the rather high price for the diesel in-place earlier this year.

engine durability
 

The original question was will a diesel engine last longer?? All of the replies have been excellent and well thought out. My thoughts on the original question..if the owner is going to keep the car for 100,000 plus miles comparing the turbo diesel and the naturally aspirated gas that the turbo will go before the gas engine will as far as maintanence. Anything with more moving parts will finally give up first and the turbo whether gas or diesel will go first. As far as reliability the electronics will go first while the engines will be a lot more trouble free. just my opinion.

I'm a happy new '09 X5 35d owner (5 weeks and 1,230 mls ago)! I'm also a long-time VW Turbo-Diesel owner with 100s of thousands of miles on those engines.

Properly maintained, VW's turbo-diesels will handily go 300,000+ miles without breakage of any kind, including the turbo. It's a matter of keeping the engine clean, with good synthetic oil, belts replaced, etc. I have 140k on one now and it drives like the day I bought it brand new. I've only ever replaced the wearable items/fluids.

I REALLY hope my X5 will mimic the engine longevity of the VW engines.

Some folks have reported VW diesel reliability issues, and more times than not, those complaints can be traced to some (or several) wearable items not being properly replaced or parts not being properly cleaned (injection systems gumming up, etc.).

Hope this perspective helps.