Geartrains are great for longevity but can pose problems for noise. Automotive manufacturers keep that in high consideration these days. Quiet engines, especially diesels are a priority for many applications. Using a belt is a simple solution.
Actually a much cheaper solution. I don't think noise is a consideration with a diesel engine in this instance. Timing belts became popular in the 70's and 80's because single and double row chains would "grow" as they got to operating temperature. This became an issue for emissions compliance. Timing belts offered more consistent valve timing regardless of temperature, until they broke. Current triplex type chains and variable valve timing makes this a non-issue, but are more expensive to manufacture.
I don't think anyone in this thread mentioned timing chains, as you posted. fburgtx mentioned gears, and I posted "geartrains". Not timing chains. Big difference.
I guarantee that noise is always a concern, especially for a passenger vehicle. Even industrial engines. Spark ignited and especially compression ignited.
Ok, not going to argue semantics here, but technically, "geartrains" refer to components that transmit rotating torque, as in a manual or automatic gearbox. Belts are not generally used in automotive applications for that purpose. The alternative to a belt on a crankshaft driven component, whether it be a valvetrain or an oil pump, would be a gear drive, direct drive, or a chain drive. So, me referring to a timing chain drive, instead of just a chain drive was not accurate. Still no question in my mind though, that the use of a belt in this case is cost driven, or a concession to a packaging problem, rather than anything else.
