The hike in fuel consumption was attributed to an increase
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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The particular flat-gate valve chamber



