A quick google search tells me pretty varied results on exhaust temperatures (anywhere between 500F and 2000F). Anyone here know a real live accurate number? Depending on how high it is (for this application, the higher the better) a stirling engine of
this size may be an idea. The guy who makes that video makes a bunch of really cool DIY alternative energy generation videos, btw. Check out his youtube channel.
I'm thinking a beefy stirling engine with a heatsink affixed directly to the bottom of the manifold or as high up on the pipe as possible (wherever there is space enough) could constantly power an alternator, and actually run faster the faster you go. Would be particularly effective on long highway trips, and would only take the initial investment to get the stirling engine - everything else is cheap and easy to cobble together. BTW, here's a
video demonstrating the ballpark of the temperature on the business end of that engine generated by a big fresnel lens. Roughly 1000F. Doesn't necessarily need to be that hot, but like I said the hotter the better.