In a message dated 3/12/99 12:23:25 PM Eastern Standard Time, LBPB1@aol.com
writes:
> I work with a friend on racing projects every now and then and he
> informed me that two ways to collect wasted HP were to one, remove the
> standard fan and replace with an electric one (about $60.00 at disount
auto)
> and two, set the alternator up with an on-off switch. According to him both
> these things are worth about 12 hp each. Its almost like free power! Some
> people spend hundreds or thousands of bucks to get an extra 24 horses.
I'm always amazed at how much list traffic these two topics generate. With
both of the above statements, there is a little bit of truth, a little bit of
malarkey, and a lot of misunderstanding in between. If I may, without sounding
like an egotistical SOB, I'd like to take a stab at clarifying these two
statements.
Let's talk about the alternator first. The most powerful alternator fitted to
the TR6 by the factory was rated at 45 amps. At a design base output voltage
of 14.6 volts, this is equal to 14.6 X 45 = 657 watts. Given that 1 HP equals
approximately 742 watts, the output of the alternator is equal to 0.89 HP.
Assuming an alternator efficiency of 50%, the alternator, at full load, will
consume 1.78 HP from the engine. This is just from the electrical load only,
and doesn't take into account any frictional or other losses just from
spinning the alternator. The purely mechanical losses would, I think, not be
much more than the losses from a simple idler pulley, and would be very small.
Safe to say, I think, that the total load on the engine would be less that 2
HP.
That's with the alternator fully loaded. In any type of competitive situation,
it would be more likely that nothing would be loaded on the alternator other
than the ignition and the gauges. In this case, the alternator would be seeing
a load in the range of 5 - 6 amps. At 6 amps, the HP drain on the engine would
be 2 (inefficiency factor) X 6 X 14.6 = 175 watts, or approximately 0.24 HP --
not really enough to get excited about.
Assuming, though, that you are looking for every bit of HP gain you can get,
and even 0.24 HP is worth while, let's take a look at the other side of the
coin, so to speak. The alternator is regulated to produce 14.6 volts. A good
battery, with a full charge, puts out 12.6 volts. Right out of the starting
gate, your ignition system is seeing a 14% reduction in power. If you ignition
system is capable of providing 114% of the required energy, you will be OK,
but if it's only capable of providing 100% of the energy requirements, you
will be operating with only 86% of the needed ignition energy. It's difficult
to say what effect that will have on power output, as so much depends on the
engine itself, but for sure it isn't going to be good.
And that's right out of the starting gate. If you're running in cutthroat
competition at the drag strip, where the total full power run is only a few
seconds, that might not be a problem (assuming your ignition system can handle
the reduced voltage). If you were to be running at the 24hrs of Le Mans, you
would not last the duration unless you swapped your battery out for a fresh
one at each pit stop. For normal street use, it just doesn't seem like a good
idea.
Now, let's talk about electrical vs mechanical fans. First off, we have to
make some assumptions:
1) The fan blades (and fan installation details -- shrouding, etc) used on
both the mechanical and the electrical fans are identical. We have to make
this assumption so we are comparing apples to apples, rather than apples to
doughnuts. If the fans are not identical, the differences in fan efficiencies
would confuse the issue. If this is not true, we could always swap blades (and
redesign the installation) so that it is true.
2) Given the above, we have to assume that the blades on both fans require the
same power and move the same amount of air if they are turning at the same
speed.
3) The alternator is 50% efficient. This may not be the correct value, but it
will do for the analysis, especially since we don't know the actual value.
This value can be adjusted as desired, skewing the end results up or down
accordingly.
4) The electrical motor driving the fan is also 50% efficient. It takes twice
as much electrical energy in as the mechanical energy out. The same arguments
as in 3) also apply here.
Now, there are some facts to be considered:
1) An electrical fan is pretty much a single speed device. Speed may change
somewhat as car speed, and air speed through the radiator, changes, but the
change should be small.
2) The mechanical fan speed is always a constant percentage of the engine
speed. Since the fan is driven by the crankshaft on a TR6, the percentage is
100% - the fan turns at engine speed. On most American cars, the fan turns a
bit faster than the crank.
3) Power required to spin the fan blades increases exponentially (I think that
is right - correct me if I'm wrong) as fan speed increase.
4) At higher car speeds, air flow through the radiator from vehicle velocity
is sufficient to cool the car without a fan.
5) At low speed, and especially at idle, the fan is the only real source of
air flow through the radiator, and is mandatory.
6) Any fan produces more air flow at higher speeds than at low speeds.
Considering all the above, we can draw some conclusions:
1) With the engine turning at an RPM such that the mechanical fan speed is the
same as the electrical fan speed, it will take four times as much power from
the engine to operate the electrical fan than it will to operate the
mechanical fan -- a 50% loss through the alternator and another 50% loss
through the fan motor. (I can't be certain at all, but I would guess that this
speed is somewhere in the range of engine speed needed for driving in stop and
go traffic, as this is probably the condition for the most stringent cooling
demands) How much power are we talking about? If the electrical fan draws 9
amps (a reasonable number, based on the fans available), the HP drain on the
engine will be 0.35HP, and the equivalent HP drain for the mechanical fan at
this speed will be 0.35/4 = 0.09.
2) At very high speed (and corresponding high engine speed), the mechanical
fan may draw more power than an electrical fan; however, because neither is
required, the electrical fan will be off, so the mechanical fan will draw
significantly more power.
3) Even at lower speeds, the electrical fan can be shut off when not needed;
the mechanical fan cannot (excluding some of the clutch type, or the "flex
blade" types -- these are not "turned off," but the power requirements to
drive them are greatly reduced at times). Any time the electrical fan is off,
it will, of course, draw less power then a mechanical fan which is still
running.
4) At idle, the mechanical fan speed may not produce enough air flow to
provide sufficient cooling.
There are too many unknowns to make a definitive conclusion, but in general,
from a horsepower standpoint, it might be concluded that an electrical fan is
beneficial if you do a lot of driving at higher speeds, while a mechanical fan
might be better if you do mostly low speed driving. If you spend a lot of time
idling, the electric fan may again be the better choice -- not because of
power or economy concerns, but because of better cooling.
Real world numbers may vary from above, but not by an order of magnitude.
Power gains from an electrical fan on a TR6 will not be as great as on an
American car, because the fan turns at a lower speed for a given RPM. I've
seen claims of horsepower gains as much as 20 HP from switching to an
electrical fan. These claims may be true on a high performance, high RPM
engine, using a very powerful fan, but I don't think we will see any where
near these numbers in a TR6.
Correct me if I'm wrong (yeah, right, like I have to ask!).
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://members.aol.com/danmas/
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74
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