Engine off hypermiling & tailpipe emissions
 

While lurking, I came across a couple of ~10 year old threads expressing concern that certain engine-off hypermiling techniques might actually increase emissions:

https://ecomodder.com/forum/showthre...ute-14571.html

https://ecomodder.com/forum/showthre...ing-20338.html

With engine start-stop technology becoming commonplace, I was pretty sure someone must have done a scientific study by now so I did a little digging and came across two.

This one measured Cold, Hot, and Warm start emissions of vehicles of various ages (and emissions standards). While engine off was generally better, it did find that certain emissions were lower when idling for a full 5 minutes rather than shutting off & restarting. Counter intuitively, newer cars were more prone to this:
https://www.weber.edu/wsuimages/ncas...nal_Report.pdf

On modern engines, carbon monoxide emissions are higher stopping the engine & restarting rather than idling for 5 minutes:
https://ecomodder.com/forum/attachme...1&d=1649500534

On modern engines, NOx emissions are higher stopping & starting compared to idling 5 minutes:
https://ecomodder.com/forum/attachme...1&d=1649500705

Stopping the engine for 5+ minutes results in fewer hydrocarbon emissions than idling for all engines:
https://ecomodder.com/forum/attachme...1&d=1649500799

CO emissions:
https://ecomodder.com/forum/attachme...1&d=1649500896

NOx emissions:
https://ecomodder.com/forum/attachme...1&d=1649500969

HC emissions:
https://ecomodder.com/forum/attachme...1&d=1649501025

As a reminder of the emission tiers:
Tier____Age
Pre-0 __1980 and older
0 _____1981-1993
1 _____1994-2000
NLEV __2001-2003
2 _____2004-2016

This study of a modern euro-4 standard scooter in real-world city traffic found that actual emissions of CO and HC were greater by 57% and 4%, respectively for the scooter with the start-stop system enabled, while the emission of NOx decreased by 10%. The application of the start-stop system contributed to the reduction of the fuel consumption/CO2 emission by approx. 12%.
https://www.sciencedirect.com/scienc...61920920308683



As many others noted in the prior threads, these are just tailpipe emissions. At this point, the total emissions from extracting, refining, and transporting the oil are much higher than those from tailpipe so it is probably "less bad" to maximize mpg rather than burning more fuel to minimize parts-per-million tailpipe emissions. And even though tier 2 engines showed higher CO & NOx emissions when stopping & restarting, those emissions were still lower than idling older engines.

There are cars with V engine layouts (2 cylinder banks, 2 cats) that don't actually shut off the valves when deactivating a bank of cylinders, thus cooling down the cats. They alternate banks to keep both cats warm enough. It seems like as long as the cats are above light off temperature emissions are negligible.