We frequently get questions about different aspects of LHI. I will be posting some of them here in the hopes that readers will find the info interesting or helpful.
Q: The actual energy storage is in the gas in the bladder of the hydraulic accumulator. Why not have a pneumatic hybrid? Way back in 1930, a diesel-pneumatic was more efficient than a diesel-electric, and air is cheap and easy to store.
A: The Hydraulic Hybrid in this configuration is essentially a pneumatic hybrid as mentioned. The energy is in fact stored by compressing air. The air that is compressed is nitrogen gas which is held in the accumulator bladder. The advantage of the hydraulic system is that you are pumping an incompressible fluid which allows you to achieve higher flow rates in a smaller package. A one hundred horsepower compressor that would allow you to compress and store enough energy to accelerate the vehicle from a stop would be half the size of the vehicle itself. Imagine an industrial air compressor. As I understand most pneumatic hybrid vehicles use a smaller compressor that runs continuously at a low power setting. The one hundred horsepower hydraulic pump occupies much less than a cubic foot of space and only weighs 45 pounds.