Check out the wikipedia link it spells out all of the physics on the deal. It can be confusing but it really does work.
Calculations
Calculation of the required max. power output for the diesel engine, rough estimation:
(1) Check the max. powerpoint, i.e. the point where pressure times flow reach the max. value.
(2) Ediesel = (PmaxｷQtot)?#951;.
Qtot = calculate with the theoretical pump flow for the consumers not including leakages at max. power point.
Pmax = actual pump pressure at max. power point.
Note: η is the total efficiency = (output mechanical power input mechanical power). For rough estimations, η = 0.75. Add 10-20% (depends on the application) to this power value.
(3) Calculate the required pumpdisplacement from required max. sum of flow for the consumers in worst case and the dieselengine rpm in this point. The max. flow can differ from the flow used for calculation of the diesel engine power. Pump volumetric efficiency average, piston pumps: ηvol= 0.93.
Pumpdisplacement Vpump= Qtot ndiesel 0.93.
(4) Calculation of prel. cooler capacity: Heat dissipation from hydraulic oil tanks, valves, pipes and hydraulic components is less than a few percent in standard mobile equipment and the cooler capacity must include some margins. Minimum cooler capacity, Ecooler = 0.25Ediesel
At least 25% of the input power must be dissipated by the cooler when peak power is utilized for long periods. In normal case however, the peak power is used for only short periods, thus the actual cooler capacity required might be considerably less. The oil volume in the hydraulic tank is also acting as a heat accumulator when peak power is used. The system efficiency is very much dependent on the type of hydraulic work tool equipment, the hydraulic pumps and motors used and power input to the hydraulics may vary a lot. Each circuit must be evaluated and the load cycle estimated. New or modified systems must always be tested in practical work, covering all possible load cycles. An easy way of measuring the actual average powerloss in the system is to equip the machine with a test cooler and measure the oiltemperature at cooler inlet, oiltemperature at cooler outlet and the oilflow through the cooler, when the machine is in normal operating mode. From these figures the test cooler powerdissipation can be calculated and this is equal to the powerloss when temperatures are stabilized. From this test the actual required cooler can be calculated to reach specified oiltemperature in the oiltank. One problem can be to assemble the measuring equipment inline, especially the oilflow meter.
In the simplest terms if you apply force T to a lower RPM horsepower goes up.
