I recently came across an instructive case study. I think that you will
find it interesting and important. In the quest for improved fuel
consumption, the manufacturer of an engine-powered, mobile hydraulic
machine replaced the fixed-displacement pump powering the machine’s
attachment with a variable-displacement unit.
The ground drive on the machine already featured a variable piston-pump
(hydrostatic transmission), and so upgrading the attachment’s hydraulic
circuit to a more efficient configuration was considered logical
progression by the machine’s design engineers.
When this modification was tested check valves and globe valves, the engineers were shocked to find
that fuel consumption had actually increased by 12 to 15 percent! Upon
analysis, the hike in fuel consumption was attributed to an increase in
oil viscosity brought about by a drop in operating oil temperature of 30
degrees Celsius. In other words, the “thicker” oil had resulted in
extra drag on the hydrostatic transmission powering the ground drive,
causing the machine to use more fuel.

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