So, that means it's speculation time. InsideEVs claims to have spoken to an unnamed source who says that the replacement engine will be the new 2.0-liter turbo. This source says this engine has been tested on the Volt's Delta II compact vehicle platform, which is also used in the Buick Verano and many others. A larger engine does not necessarily imply worse fuel economy – see, for example, the jump from the 1.5-liter engine in the second-generation Toyota Prius that got 46 combined miles per gallon to the 1.8-liter in the third-gen that gets 50 – so we're not discounting the rumor outright. InsideEVs goes on to suggest that the 2.0T will also find its way into the Cadillac ELR (the production version of the Converj concept from 2009) that uses Volt technology, whenever that car arrives. To feed the speculators, here are some other stories about Volt engine possibilities from the archives.
Our former colleague Sam Abuelsamid is skeptical of the 2.0 turbo story, though, writing that whatever engine replaces the 1.4, it will "almost certainly will not be a 2.0-liter turbocharged four as used in numerous GM vehicles including the Buick Regal GS and Verano T." He continues:
A 200+ hp turbo 2.0-liter makes absolutely no sense when you only have 161 hp of electrical power. The 2.0-liter is larger, heavier and thirstier than the 1.4 and makes way more power than can be used by the electric drive system.
The 2.0T would only make sense as part of a parallel plug-in hybrid drivetrain in some other vehicle. My guess (and I have no inside information on this, pure educated speculation) would be that they would opt for the new small engine family being co-developed with SAIC which starts at 1.0-liter. Those engines could be in production by next year and would be a much better fit.