[Isaac Zachary]

As I posted elsewhere, there was a time when city trucks (garbage, buses, mail, etc.) were being hybridized in some places. Then they all seemed to have up and disappeared. Supposedly the interest has been replaced with interest in all-electrict BEV's.

With the long haul big rigs BEV technology does not look promising and there's not much reason to make them hybrids either, except maybe in Colorado. Being able to brake and absorb energy as you travel down steep mountain passes would be a wonderful addition to any vehicle that frequently drives over steep mountain passes. But unless you do a sizeable amount braking, a hybrid system becomes just added weight.

Edit:

BEV and hybrid technology aside (kind of), there's only so much more that could be done to improve the efficiency of big rig engines. One would be to further reduce friction. That could be done by using fewer cylinders. But the straight 6 is great because it's the one of the few engines that has both a perfect primary and secondary balance great for longevity (and comfort). There has been some talk of a redesigned 4 cylinder that has reduced (or eliminated) an inline-4's secondary imbalance problems which could make it a better candidate for a large efficient engine with great longevity.

Another design would be opposing pistons. A single cylinder two piston opposed piston engine would be ideal in some ways but more complicated in others. But if we want a 60% or better efficient engine in a road going vehicle that may be the only way to go.

Sizing also is important. Close to full load is where most engines are the most efficient. But how do you size an engine to run within a small window of load and RPM? This is where a hybrid could help. If it has enough energy to help the truck accelerate or climb any sort of grade and then efficiently absorb as much as possible whenever possible (including excess from the engine on small descending inclines), then the engine could be sized much closer to an ideal size for average power needs.