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.
The machine employed a two-section, combination heat exchanger for both
hydraulic oil and engine coolant (which was sized for the original,
fixed-displacement pump) with a hydraulic fan drive. The fan drive was
thermostatically controlled based on engine coolant temperature.
The drawback with this arrangement is, due to the engine being
thermostatically controlled and the hydraulic system not, air flow
through the combined heat exchanger depends entirely on engine
temperature. This meant that the reduction in heat load from replacing
the fixed pump with a variable-displacement unit resulted in a
significant reduction in hydraulic oil temperature (normally a good
thing!).
The engineers blocked off most of the hydraulic oil section of the
cooler and ran the test again. This returned fuel consumption to the
original level, but no significant improvement was seen.
It was concluded that the modification tested could result in a small
cost saving with respect to a reduction in size of the oil cooler. But
with fuel consumption being more important than any modest saving in
cooling capacity, the idea of paying more for a pump that resulted in
the oil being kept at a lower operating temperature – but increased fuel
consumption – was irreconcilable to the machine’s engineers

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