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Drifter 04-09-2022 06:53 AM

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

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:

On modern engines, carbon monoxide emissions are higher stopping the engine & restarting rather than idling for 5 minutes:

On modern engines, NOx emissions are higher stopping & starting compared to idling 5 minutes:

Stopping the engine for 5+ minutes results in fewer hydrocarbon emissions than idling for all engines:

CO emissions:

NOx emissions:

HC emissions:

As a reminder of the emission tiers:
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%.

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.

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