Rhenic

Rhenic t1_iuty56f wrote

Even to that sounds counterintuitive, it's not impossible!

First off; It's about particulate emissions, not carbon. Incomplete/inefficient combustion produces much more particulate (think of a fire smoking badly when it's not getting enough oxygen, the smoke consists of particulate).

Engines tend to produce much more particulate as well when not running without a proper load, at an efficient RPM, just like a badly burning fire.

Then the devices that are tasked with catching and nullifying that particulate (like the catalytic converter), can also potentially rely on sufficient pressure (and thus RPM/load), to function properly.

And then to finish that; A modern car uses about 0.5l/hour to idle. A modern diesel would use about 0.75l to 1.0l of fuel to drive 20km (which would take 15 minutes at 80km/h, against the 30 minutes of idling).

So with about 50% to 100% more fuel used, but burned much less efficiently (because no load, inefficient RPM, no turbo pressure); It's not at all unlikely for a car to emit more particulate emissions in 30minutes of idling, than in 15 minutes of driving at an efficient speed.

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