I didn't have time to elaborate, but I chose not to mention technology on purpose. Unless some inconceivable technological leap happens which makes energy cheaper than $15 oil or coal, technology won't be enough to save the planet.
Right now, if one country limits the amount of oil it uses, that just makes oil cheaper for another country which hasn't set limits. 50 years from now, Saudi Arabia could be still burning its last drops of oil to produce electricity, while the rest of the world has moved on to.
So yes, you're correct that improving internal combustions engines might not help at all, but that seems to be true of any other technology, too.
The only thing that will save out planet is our ability to organize ourselves globally to drastically cut emissions.
Side note: don't take the downvotes here personally. HN is a place where quality discussions happen, and a lot of folks would prefer to simply downvote something they don't agree with rather than clutter up the whole thread with responses. :)