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
Gate Valve cooler and ran the test again. This returned fuel consumption to the
original level, but no significant improvement was seen.

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