Still, an accumulator tank with enough volume to launch a heavy truck up to speed is relatively bulky. The Tonka concept's two tanks, housed between the frame rails, are each slightly less than a foot in diameter and about 2 feet long. They're reinforced with carbon fiber to withstand pressures that can reach 5,000 psi, and lined with a material that absorbs the heat generated when the gas compresses. The pressure in such tanks is high, so safety testing has been thorough -- similar to what storage tanks for compressed natural gas receive. The hydraulic motor itself, mounted midships alongside the tank, is compact, roughly the size of a standard differential.
With the tank charged, the system nearly duplicates the power of the primary diesel, adding another 600 lb.-ft. of torque to launch the truck from a standing stop. This is enough to let it drag race competitively with a BMW roadster, and also means that the engine doesn't need to kick in until the vehicle is rolling at 20 to 30 mph, when it is operating at a more efficient rpm range. Brake life also benefits -- with the system providing most of the stopping force, brake linings last three to five times as long.