Well yes.. 40% on F1 cars.
But it's great nonetheless;
I've seen the mechanical storage system that was originally developed for Renault F1
(well the engineer said "a french formula one car maker" I can't think of any other)
basically it was box, the size of a shoe box, inside an electro magnetic gyroscope.
as soon as you break the energy is transfered to the gyroscope which starts turning..
up to 90k rpm.
And if you want to accelerate it can return that energy (about 100 HP worth of momentum)
But they got scared away because of that rpm.
Porsche has a similar system (much bigger, much lower rpm..same idea)
In all their hybrid race cars
The Porsche 918 hybrid for example..
887 HP, one 4.8l V8 engine with 601HP and two motors with 210kW (286HP)
the disc modul/motor provides about 115kW and a AC motor with 95kW
max torque 1280 Nm
fuel consumption 3.3L/100km (71 mpg)
as a comparision the 4.8l v8 of a cayenne needs 18L/100km (13 mpg)
So it's possible yes, the pricetag is what's making it rather dramatic (800,000 Euros)
Also, range of the electric motors is 30km (not noteworthy)
if you don't hit the brakes really hard every now and then you cannot charge the
gyro.
it will not work very well under your regular street condition, but that's great for a racecar.
The only steam ERS I know is that of a BMW (I think they call it turbosteamer)
on a 1.8l straight four engine it can recover 80% of the exhaust heat and produce about 10kW 20Nm...
resulting in 15% better fuel efficiency.
But a combined set of systems? never seen it,
but good to know,
if you can, please try and find the report,
if it's english we could read ourselves
'sid