Rev hang is a term describing an engine revving decreasing slowly
Rev hang is used on purpose to curb NOx emissions
Your driving style could be making it worse
Manual transmissions are great, do not complain
I wouldn’t say that I hug trees, but I do like them an awful lot. I like shade, nuts, fruits, and leaf piles in the fall for my son. More broadly, I like the environment and do not want to cause harm if I can avoid it. If that means I have to let the engine revs drop a little slowly on a manual transmission equipped VW Golf, or most any modern manual car out there, then so be it. I’m not going to complain and neither should you.
The phenomenon is known as rev hang. Rev hang is a delay in engines revs decreasing when you lift off the throttle. It occurs on most modern internal combustion engines, but is more noticeable on manual transmission equipped cars because you are in complete control of shifting. And it’s annoying. The engine revs staying elevated makes quick upshifts less smooth, and downshifts clumsy.
But rev hang also plays an important role in keeping internal combustion engine’s emissions in check. And it all has to do with the stoichiometric ratio of air-to-fuel of 14.7:1. Douglas Skorupski, Volkswagen of America, Inc.'s powertrain strategy manager explains it this way.
“As you pull your foot off the throttle, what we are trying to prevent is a big hunk of air getting into the chamber without any fuel. If air gets in that way, when you do reapply throttle, it creates a lean condition (a higher than 14.7:1 air-to-fuel ratio), which generates nitrous oxides or NOx. By delaying the fuel cut-off in the cylinder, we get some fuel in there with the air to keep the mixture correct and avoid the lean condition and prevent the NOx from occurring.”
In literal terms, engineers delay fuel cut-off until several milliseconds after you completely lift your foot off of the throttle. Doing so adds enough fuel to keep the air from throwing off the chemical balance of clean combustion. And while it’s true that catalytic converters can reduce how much NOx leaves the tailpipe, we’re still much better off producing as little as possible in the first place. Turns out NOx is gnarly stuff.
Wolfram Wendt, the Manager of Quality Analysis at Volkswagen, has applied the method of rev hang for years to mitigate NOx in several VW engines. And his team is constantly refining the process in an effort to make it less intrusive. Engineers don’t like the way rev hang feels any more than we do.
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