Quote:
Originally Posted by BillGrissom
An engine-driven clutch-fan system is more efficient than electric. That is because of energy losses in the alternator and electric motor. That is why longitudinal engine vehicles still use mechanical fans today (or at least thru early 2000's).
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The efficiency of of fan is the the ability of that fan to pull air through the radiator. Mechanically driven fans are attached to the engine which moves around on its mounts. To allow for this movement the shroud for a mechanical fan is much larger than the fan itself. That gap between the fan blade tips and the shroud creates lots of noise and turbulence but it doesn't help the fan pull air through the radiator. Electric fans are attached to the radiator so the distance between the fan and shroud is only a couple of mm. An electric fan might move less air but it can still provide better cooling than a mechanical fan because all of that air is moving through the radiator.
Another issue is the speed of the fan. An electric fan can run at its max speed while the engine is at idle. A mechanical fan will always max out at at the speed of the belt pulley its attached to. Not only is the fan's max speed limited by the engine speed, that fan is also spinning the other 99% of the time when its not needed, even a viscous clutch fan. Again wasted energy. A properly sized electric fan will always provide better cooling while consuming less energy than any mechanical fan, clutched or other wise. The only applications where mechanical fans are still used are in heavy trucks and earth moving equipment that need the fan cooling almost 100% of the time and sizing alternators and electric fans and alternators for the application is cost prohibitive. Many of these applications now use an electric clutch which can decouple of fan from the engine in order to save some power.