https://www.thedrive.com/the-war-zone/3 ... -has-flown
Celera 500L spotted flying with a twin prop chaser plane.
I don't see any exhaust ports.. but I think they're running two of these diesel V12 which would spin at a really high RPM but produce high power. So you gear them to reduce the RPM of the prop (so you don't tear the prop apart and send the tips of it sonic) but you conserve the power through the gear.
When I was in college we played around with this concept. You would take two RC racing car/truck engines which typically have a lot more power than aircraft RC engines (for equal displacement size) but spin at really high RPMs. You'd take two of them and gear them down to one so that you could spin a large diameter prop with a lot of power but low enough RPMs that you didn't tear it apart. Its simple but really quite effective.
As for the fuel efficiency discussed. You have to consider that making comparisons with a car on the ground with totally different environment than an aircraft at 65,000 feet.