Yeah, it really seems like this would be most useful for pushing a massive weight from a stop. They hydraulic systems are heavy and the power output is, from what I've seen, fairly short lived. Not really ideal for marketing to city/highway drivers of small vehicles.
According to the article the stored energy is used for short bursts of acceleration, not for steady state driving. I imagine diesel engines are incredibly inefficient when trying to accelerate a massive load from stop, and that problem is what this technology addresses. This technology isn't useful for highways, a primary use case for plug-in hybrids, so I don't know if one could make any statement about a hydraulic system being applicable for that use case based on this article.
The underlying assumption being that the amount of energy stored by the compressed gas, or whatever the hydraulics is actually compressing, will never match the energy storage of a battery. Perhaps the fact that neither stores an adequate amount of energy makes the comparison moot, in my mind.
Specifically, there's no reason to assume that you can't have a plug-in "hydraulic" hybrid. You simply have an electric motor compressing whatever gas, while the car is parked at home. Much like electric plug-in hybrids change the battery while parked.
Some time ago, I read an article about flywheel-energized buses in Zurich. Same idea. Spin up the big flywheel on top, then use its energy to help move it along.
I have heard of using pressurized gas as a propulsion mechanism, but this is the first that I have heard of using it as an energy recovery mechanism. It would seem to make sense, although obviously it depends on the efficiency of the storage and reutilization of the energy. Does anyone know what the comparative energy density between this and lithium batteries would be?
I am not sure these should be called hydraulic hybrids, are they not "compressive" or "mechanical" hybrids? or even "pneumatic" and "fluid power" hybrids?
I like the idea of compressed gas hybrids because they could easily be designed to be safer than chemical batteries and the conversion losses should be equal if not better. Although gas at high pressure could present a hazard it can be placed in sacrificial casings which prevent explosive decompression.
The date of 2016, that Peugot propose for first production, seems quite a long time given our collective knowledge of both hybrid and compressive technologies, especially as work has been on going in various places since at least 2008.