> 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."