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Hydromechanical CVTs for Hydraulic Hybrid Applications

The Folsom 50/100hp CityCar CVT.

Folsom Technologies has developed a line of hydromechanical continuously variable transmissions (CVT) that are configurable either as a standalone CVT or as the CVT component of a hydraulic hybrid system.

At the New York International Auto Show, the company displayed its 50/100hp CityCar CVT. This unit is designed to go into passenger vehicles of up to 100hp but, because of its compact size and weight, it is also suitable for application in small city cars of 50 hp. It is currently fitted to a 50hp city car and is being used for controls development and technology demonstration.

The company is also developing the second-generation of a 350hp full-size rear wheel drive (RWD) passenger vehicle unit with co-funding from NYSERDA.

Instead of using commercial hydraulic units, Folsom designed its own hydrostatic pump/motor, focusing not just on efficiency but also on size—the transmissions are designed to be a drop-in alternatives for existing transmissions—as well as cost.

For a hydraulic hybrid application, the CVT needs to be paired with a set of accumulators and flow valves. For passenger car applications, Folsom sees its transmissions enabling mild, parallel hybrid functions: regenerative braking, launch assists, acceleration assist and engine starting (for a start-stop function), in addition to normal CVT operation.

Although the transmissions could support an “all-hydraulic mode” for several miles, Folsom says, the accumulator size would be too big to make this a practical mode of operation for passenger vehicles.

The control logic for the different applications is currently being developed by a Tier 1 supplier, according to the company.

We predict significant fuel savings due to the combination of the CVT effect coupled with hydraulic braking energy regeneration. The fuel savings are obviously dependant upon vehicle applications. For operating duty cycles that have frequent start/stops (such as NYC taxis, school buses, garbage haulers, etc.) the fuel savings can be greater than 50%. A Chevrolet Silverado has been fitted with one of our transmissions and operated in CVT-only mode. Shortly the necessary accumulators and control valves will be added to this vehicle and testing of the complete hydraulic hybrid system should commence in about 3 months.

—Martin Hughes, Folsom Technologies



This really puts the C in CVT. The 3 cyclinder Geo Metro G10 engine put out about 50 hp and could get 50 mpg. Coupled with this it might rival some hybrids.


A small hydraulic hybrid like this would be good for postal delivery vehicles.

C Harget

How much do the accumulators weigh? These are basically 5,000 PSI Nitrogen tanks with some oil in them right?

I wonder how much they offload the engine during launch assist and acceleration assist? You'd have to do some funky programming of the gas pedal to decide on the fly how much to pull from the pressure in the accumulator vs engine RPM's. What if the accumulator pressure is depleted from acceleration on hills or something? It would be unsafe to have an unpredictable response when the driver hits the gas pedal.

Rafael Seidl

Their website is long on marketing blurb and short on specifics. For example, they claim the efficiency of their transmission is always higher than that of belt and traction CVTs but they provide no data to back that up. They don't even describe if the engine power is always routed through the hydraulic pump/motor assembly (serial hybrid) or if there is a parallel, purely mechanical path (compound hybrid) for freeway travel.

The Chevy Silverado mentioned in the article presumably featured the original 300hp design, not this new 50-100hp derivative.

So far, hydraulic hybrids have only been considered for specialty trucks and SUVs, because the size and weight of the accumulators makes them hard to fit in a passenger vehicle. Some, like the Merc A- and B-Class feature a sandwich floor that could accomodate them. They are already used for the CNG tanks in that variant.

Another hurdle is the way CARB skewed the formulation of its ZEV mandate toward electric hybrids.


All you see here, and mostly elsewhere on this site, is another interim solution. This one is a CVT coupled to an ICE. Various forms of this arrangement will be around for some time until the final solution is derived: The Plug in total battery-driven EV. I may wait for a long time; but, I'm willing to drive my old Volvo until the future shows up.


Technology is never "done." Auto tech hasn't advanced this fast for many decades, if ever. That's one of the things that makes the buying decicion more interesting, and complicated.

I want a hybrid now, but plug-ins are coming soon, IMO. Do I buy now or wait a bit? Lad, like you I'm leaning towards waiting a little longer.


If you can't wait, then lease. If you choose to hold out untill
the first model year of the high mileage PHEV, you might be
somewhat dissapointed as more models and choices will follow
shortly thereafter. And quite probably, the early adopters of
this technology will have have to suffer through any
initial roll out glitches, that are inherent with unproven large
scale manufacturing. Choice is a powerful thing for consumers
when the options are few. I will be in the lineup for any
high mileage PHEV 40, that is in its second model year, that
has Consumer Reports thumbs up. 2011/2012 Prius? I can almost
taste it, although impatiently. I will pay the premium.

Michael McMillan

a 2.5 gallon accumulator at 3000 psi is 55 pounds, at 5000 psi it is 90 pounds.
I got the numbers here:


How would this gearbox compare with the 7sp auto that you can get in Mercedes Benz models? And Audi has an 8sp.
I am inclined to believe that the folks at Tipronic aren't too concerned about job security until you can come up with some specific numbers (liters/km and rate of accelaration)

youssef ghazal

im engineering student in the miena unvercity in automotive department in BS.C year and i need the new automatic transmission in my project (oprataion-disign-control) and thank yuo very match.

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