<VV> Electric Fan - An Engineering Approach to What is Gained and Lost

[boB]

So why are Corvair owners chasing the elusive electric fan?  What is the point?

To me the point of an electric/hydraulic/anything-but-belt-driven fan is not fuel economy, nor belt flipping, nor even improved cooling, but *more available HP* right when we want it the most, at high rpm.

Anyone who runs at high rpm isn't concerned about fuel economy anyway, plus I never had belt problems once I got things sorted out on my '66 Monza, even up to 7000 rpm.  But I sure could have used more available HP!

By decoupling fan speed from engine speed we can maintain cooling *and* get more power to the wheels.  If the new fan provides the same *average* airflow over time as the original fan we should see the same average cylinder head temperature.  Yes, there will be temperature spikes but if they are very short in duration then the thermal mass of the engine should keep the temperature regulated well enough so no overheating is seen.  Kinda' like making a drag strip pass without the fan belt on, on the street it's difficult to use full power for more than 15 seconds without losing your license.

An electric fan runs at a constant speed and will provide sufficient cooling at some load, more cooling than the engine needs at lighter load, less cooling than the engine needs at any greater load.  For most street engines that midpoint load is probably a steady cruise of about 75 mph.  Of course this depends on where you drive; temperature, hills, traffic, speed limits are all factors.  I often drive I-95 to Florida, high temperatures, sustained speeds near 80, no hills to speak of.  My fan would be different from someone's who lives in the Rocky Mountains, and when I visit there I would have to carefully monitor head temperature and probably change my driving style to keep the engine from overheating.

The electric fan runs primarily from alternator power.  With an efficiency of about 80% for the alternator, and another 80% for the fan motor we get an overall energy transformation efficiency of about 64% *so* the belt driven fan is much more energy efficient from that approach.  Where the electric fan excels is when we can freewheel the alternator at high engine speeds and run the fan from the battery, freeing up available HP.  The battery charge/discharge efficiency reduces our overall efficiency maybe another 80% but this power comes from the average HP over all engine speeds, especially when we don't need more HP as in steady cruising or even deceleration.  But with all of these energy conversion losses don't expect any fuel economy gains.

This approach is not far different from that of a hybrid vehicle, use a small engine for cruising, assisted by batteries for peak demands.  In this case we're using engine power to provide average cooling, and the battery to provide cooling for peak demands.

Obviously this approach would require a much larger fan for racing than for street driving, where the race engine is putting out maximum power for most of the lap.  It's entirely possible that a large enough electric fan to keep a racing engine cool doesn't exist, at some point it is clear that the electric fan will *not* be able to keep up with the engine's cooling demands.  But for *some* applications the electric fan *will* provide enough airflow to keep the engine as cool as the original fan, with the added benefit of more available HP.  And for some Corvair owners that's worthwhile.

The question is: how well does your application fit the available airflow?!


boB