I've noticed on a few flights with HOP! on the Embraer E-170 that it is as if they start the second engine during the taxi to the runway. Is the E-170 capable of single engine taxi, or is it just some other system winding up? I have never heard of single engine taxi on any other types of jet, nor have I ever experienced it on other E-jet variants. Is it variant specific?
Nope, it's used on basically every type. Off the top of my head, Southwest is known to do this with their 737s, which was attributed to fuel savings (though I believe there's some controversy of whether or not it works and if it contributes to premature engine wear).
I can't speak to the prevalence of this behavior, but it's not particularly rare.
Yup, I fly Southwest regularly and it isn't unusual for either a late start or an early shutdown of one engine before takeoff/after landing. I would say it happens perhaps 20% of the time.
Where I fly, it is SOP to taxi on one engine when possible, on A320 series and E190/175 aircraft.
There are a lot of reasons why single engine taxying is not feasible, so it is not 100%. The last statistics I recall reading indicated about 60% single engine taxi from gate to runway for take-off and about 85% on taxi in after landing.
That's quite interesting. I haven't experienced it on any type or airline apart from the HOP! E-170s. Even ATR discourages single engine taxi these days in its standard procedures.
And if you're going to shut down an engine to save fuel make sure it's NOT the one that's powering your only working hydraulic pump like this DC-9 pilot, oops.