Diesel hybrid, better or worse mpg?

Though the Elantra hybrid has a complex DCT, yet can match the Prius in fuel economy.

The fuel economy is mostly from using an engine that is efficient for most of a trip. Then maximizing the time spent at its efficient point. The other side provides power during the short times of the trip where the engine alone isn't enough; passing and climbing. Energy storage lets the engine run at more efficient loads when such loads aren't needed to propel the car, and then the storage allows the engine shut off when using it would be less efficient for stretches of road and stops.

We've settled on an electrical system for this, but hydraulic is also possible.

It's the rationalization, but isn't true in the case of vehicles in the same class.

 

What was the other part of the hybrid that they were designed to "be a part of a hybrid vehicle from the very start"?

Maybe you meant that there are some hybrid designs that were designed from the start to use diesels?

 

I made the post more readable.
English Lit wasn't covered in my Submarine Qualification card.
Rudolf Diesel's engine was one of those transformational and disruptive technologies that (some people say....) ultimately got him rubbed out
- OR NOT.
Opinions still vary, and the murder/suicide/accident/Operation Mincemeat theories all have lots of holes in them.

As far as 'boats'
The first operational Diesel Submarine in the US Navy was the USS F-1 (SS-20), which was commissioned in 1912. Since the hull designation wasn't revised to SS-1, this sorta implies that the first non-human powered submarines were not diesel - which is true, because they were NOT. We had a photo-finish tie with Germany who completed the U-19 class of subs in 1912–13 which had the first diesel engines installed in their boats.

There was a 'yuuge' burst of 'hybrid' innovation in the 19-teens because of Dubbaya Dubbaya One.
Diesel Electrics came into their own in the sequel.

We in the US have moved on from them.
The USN's LAST diesel boat (for now?) was USS Dolphin (AGSS-555) decommed in 2007.

 

Yes... It's clearly a loophole that probably won't last very long if more than one person makes use of it.

In fact, if log trucks were proposed today for the first time and had to go through a full environmental review they'd never be allowed on the roads. The amount of damage they cause have been grandfathered in and other similar sized vehicles that don't haul logs have far more severe restriction on what they're allowed to haul.