Radiator Fan Intercooler System
There are a few automotive cooling myths. Probably the worse and most popular
myth is that removing a thermostat will make water flow too fast to cool. In
reality, unless flow becomes grossly turbulent through significant areas
of the system, the more flow the better the cooling becomes. I can’t imagine an
automotive system ever coming close to that point, with the sole exception of
grossly excess water pump speed causing cavitation and reducing flow.
Cooling System Basics
Typical USA water cooled engines flow like this:
Cool water is drawn from the radiator outlet tank (A) into water
The pump (B) pushes cool water into and though the block
towards the rear (C). Strategically placed “steam holes” allow some water to
flow past bores up into the head. These holes keep water moving around the
cylinders. The small gasket holes promote even bore warming and prevent trapped
steam and air pockets. This is where most heat is added to the water.
The cylinder head (C) returns water to the thermostat housing
(D). The cylinder head is the least critical for temperature changes. This is
the last place where heat is moved into the water.
The thermostat (D) closes when below the rated temperature.
This restricts flow to a small bypass hole. Greatly reduced water flow rate
water temperature to rise. There must still be some water movement. The water movement,
even with the thermostat closed, must be enough to prevent hot spots and
promote evenly warmed cylinder bores and heads.
thermostat opens and provides full water flow at a predetermined
temperature. Thermal lag in system stabilizes engine temperature around the thermostat
temperature opening threshold.
Water from the thermostat flows into the radiator’s hot tank
(E). This is normally a high spot on the radiator. This is normally a high spot
because hot water is less dense. The warmer water rises while cooler water sinks.
This is called “convection”.
Water from the radiator inlet tank (E) is forced through many
small hollow tubes. The tubes have fins attached outside to increase
air contact area. If you examine the construction you will see the air surface
contact is many times larger than the water contact area. This is because water
has a much higher heat capacity than air. Water even has a higher heat capacity
than metals, water’s heat capacity being several times than that of aluminum
(pound for pound)!
Air flowing through radiator fins is intentionally made
turbulent. The turbulence helps the air scrub heat from the metal radiator fins. The air absorbs heat from the water,
cooling the water.
Water flow is restricted to increase temperature, water flow
is increased to reduce temperature. All flow controlled cooling systems work this way.Thermostats
reduce flow below the target, and progressively open wider near or above target.
The radiator cold tank always has to be below
target temperature or water will not be able to cool the engine to target. The
ideal way to control fans is by monitoring the radiator cold tank. Early cars
had no fan control, but in the 70’s thermal clutch fans became popular. Thermal
clutch fans would spin slowly, pulling a radiator temperature air sample across
the clutch. When that radiator sample reached a known target, usually 10-40F
below the thermostat, the clutch would fully engage. Modern sensors and
electronics allow direct monitoring of the cold tank and variable fan speed
control through PWM (pulse width modulated) electric fans.
doesn’t mean controlling the fan from cold tank temperature is the only
way, it is just most reliable and accurate method. It is
also possible to monitor engine outlet temperature. Engine outlet temperature,
especially if data includes engine operating conditions or fuel consumption, can
be used for fan control. For example with a 200F
engine water outlet exceeds the 200F target, a fan system can be started. This
is perfectly fine for normal light or steady load systems such as small
passenger vehicles with smaller engines, because they don’t have extremely high
BTU thermal spikes. A smaller capacity system also has little thermal response
Heavy duty systems, such as trucks or high horsepower vehicles,
have a great deal of water in the system. They also generally have a long water
path in the engine, and a great deal of thermal mass. By the time an exit
temperature fan controller responds and starts the cooling fan, the engine can
spike over temperature before increased airflow can cool the cold tank
adequately. For this reason, it is better to watch the cold tank in high power
or large thermal capacity systems.
Heat energy can be defined by British Thermal Units over a
period of one hour. Air absorbs heat at a rate of:
this translates to CFM = BTUhr/ΔT
where ΔT is temperature delta (temperature change)
If your engine is producing 100,000 BTU/hr of heat, the inlet
air is 90F, and you need the water to be at 140F, you have a temperature rise of
50F. You would need 100,000/50 = 2000 CFM of air
minimum. This formula assumes
the radiator is perfect.
After reaching the cold tank (A), the
water repeats the cycle. Water flow in gallons per minute for a given heat exchange rate has to be
In order to properly cool the
engine and maintain stable temperature, inlet water from the radiator has to
be much cooler than the engine target temperature. Then, and only then, will
the thermostat set temperature by restricting flow. Let’s assume we allow the engine to
be 30 degrees more than the radiator cold tank water, or with 90F inlet air
and 140F cold tank the engine outlet would be 170F.
100,000 BTU/hr engine heat example, we need at least 100,000/500*30 =
100,000/15,000 = 6.7 GPM.
This assumes the engine water system is
perfect without hot spots. In reality we likely need 2-4 times that flow to
assure all parts of the engine water system are safe.
BTU/hr engine heat with 90F inlet air requires more than 2000 CFM of
radiator airflow and more than 6.7 GPM of water flow. It cannot cool enough
to be thermally stable with less than either of these two flow rates.
Gasoline is about
114,000 BTU/hr per gallon. If a vehicle going 60 MPH uses 2.5 gallons an
hour, that is about 23 MPG. 2.6 gallons an a hour is about 250,000 BTU/hr.
If the engine puts 1/3 of that fuel energy into water heat (the rest of
thermal energy going out the exhaust, radiating to air directly from the
engine, and moving the car) we are at this cooling system BTU level demand. The
more fuel consumed over a time period, the more thermal energy generated and
the more water (and air) must flow to maintain a stable temperature.
|Normally, a limited water volume constantly recirculates from the engine
water outlet (D) to the water pump inlet (B). The recirculation ensures
the engine warms evenly, and the thermostat sees a constant water supply at the
engine temperature. This circulation from the engine outlet at thermostat is
almost always necessary. This small circulation prevents formation of steam
pockets or hot spots, or delayed T-stat opening.
Pressure and Flow
Horsepower are almost meaningless ways to compare or
select a radiator, fan, or pump! Pumps, radiators, and fans may be specified
that way, but the number does not mean much by itself. Flow without a pressure
curve is just as bad. As usually happens,
meaningful information is too complex or unfamiliar for most consumers. Since
the good information would be confusing, manufacturers move outside the
meaningful circle to a fringe specification that the consumer is comfortable
with….even if the information is next to useless.
Looking at CFM of a fan or pump, we find graphs like this:
The black curves are flow rate or volume, while the red curve
is pressure. Notice the maximum pressure (red line) stays about the same over a
wide range of flow rates, from zero (dead headed) to around 450 CFM. After 450
CFM, pressure rapidly drops off. This fan flows around 950 CFM of air, but at zero differential pressure.
In the middle of an empty room, without any restrictions, the fan would move around 900 cubic
feet of air per minute. Most automotive fans and pumps are, unfortunately,
advertised by the non-restricted operation. While this makes the numbers look
larger, pumps and fans with lower non-restricted flow ratings can move more
volume against pressure than higher open flow-rated fans or pumps.
If we put
a radiator at the fan outlet, and if that radiator required 1 inch of pressure
differential to flow 800 CFM air and properly cool, this fan would only produce
about .9 inch static pressure at 650 CFM. The fan would fall short, even though it is a 950
CFM fan and could produce 1.17 inches of static pressure at 400 CFM. The CFM (or GPM for a pump) and pressure at that flow have to match the system, and neither
is close to the published ratings.
If we placed a restriction at the fan inlet, making the fan a
“puller”, the fan would be even more sensitive to restrictions. When restricted,
almost every fan or pump works better when pushing into the restriction.
I measured three fans (at exactly 12 volts) pushing and pulling into a stock Ford
Mustang GT radiator. The shroud or plenum, as a proper design should, covered the entire
radiator area. The shroud was also about 4 inches deep, to even the air pressure
and flow across the plenum area. DH is dead head pressure, pressure is inches of
water differential. More water differential across a radiator will directly
translate to more air volume, while less pressure differential will translate to
less air volume (CFM), through the Mustang radiator:
I discovered measured fan flow does not agree with
manufacturer’s published or advertised CFM flow. Because of this we can buy a
fan advertised as being “bigger” and wind up with a fan that is
actually smaller! For example, in actual real world performance a low-cost single speed Hayden 3700 flows almost identical air
through an OEM Ford radiator’s restriction as Derale’s fan in high speed mode, even though
Hayden’s advertised CFM is just over half Derale’s advertised CFM! We
also see, when restriction is high, all fans work significantly
better as “pushers”. They may not be better as pushers when
backpressure is low, but at high back pressures they were better.
These tests were made with the fans spinning in the optimum direction for blade
The inch water column differential (pressure) was measured
this way. Notice it is similar to using a volt meter. I was measuring pressure
across a flow resistance to determine flow rate:
Simple electrical equivalent:
For both the radiator and the fan, just as with a water pump
and radiator, we must know the back pressure and the flow rate at that back
pressure. Flow by itself, just like pressure by itself, is meaningless.
Pushing or Pulling
A fan works by pressure differential. Atmosphere pressure
pushes air into the inlet because the blade creates a low pressure area. The
blade compresses the air on the outlet side, increasing pressure above
atmosphere. This pressure differential causes air to flow.
At very low pressure differentials it doesn’t matter much
if a fan pushes or pulls. In other words, if the radiator system does not
“block” or choke airflow with backpressure, a pusher or puller will work equally
well with a fan blade optimized for the service type. Pushing does require a
slightly different blade design than pulling. As a matter of fact, severe
restrictions require a centrifugal fan that cups and slings air out into a
scroll housing. Think about a supercharger or turbo fan, or your furnace blower.
Those are the extremes of working against back pressure, or pushing air through
At low restriction, pusher or puller work equally well
**if the fan is properly designed for the system**. You may have noticed that
earlier, where I made fan measurements.
More Detailed Look
There are some special things going on in the cooling system.
The block has holes directly along the head. These holes, even
the small holes, are very important for the following reasons:
1.) The upper areas of the cylinder walls, along with the cylinder
head around each combustion chamber and exhaust port, are subjected to very high
temperatures. At the least, this results in some areas running hotter than other
areas. At the worse, localized heat causes some very hot areas.
2.) Trapped air, steam, or lack of flow can cause
localized boiling inside the engine. The localized boiling forms a gas
pocket of steam, preventing water from cooling that area. This further
overheats the area where steam collects, overheating some small areas even
though the overall engine is not overheated.
The solution to this is the addition of small “steam holes”
between the block and head. The block typically has dozens of these small steam
holes, along with larger water water passages, through the head gasket
into the cylinder head. The gasket holes are often sized to place the largest
passages furthest from the water pump, with small steam holes strategically
located in places where steam or air might become trapped. The function of the multiple small holes from block to head is to prevent
stagnant areas or steam pockets. The
larger passages, mostly concentrated at the rear, ensure adequate water flows to cool the
entire block and
Almost all systems have a way to bypass a small flow of water
around the thermostat. The thermostat bypass ensures a limited water volume
circulates even when the thermostat is closed. If the bypass wasn’t there, water
would sit stagnant. This would cause very uneven water temperatures as the
engine warmed up, likely resulting in steam forming long before the thermostat
area reached opening temperature.
The opening temperature is roughly 10-20% below the
atmospheric pressure boiling point of water. This sets the operating temperature
of the engine coolant, which varies roughly around the thermostat temperature
with a properly function cooling system. Water leaving the thermostat and
flowing into the radiator inlet tank (E), on a fully-warmed engine with properly
functioning cooling system, is roughly around the thermostat’s temperature.
As water passes through the radiator, heat is transferred from
water into the air. This cools the water.
Old data I have from an SAE paper shows a belt-driven water pump has
to flow at least 3 gallons/minute for every 10 hp (steady power)
to adequately remove heat from an engine.
Contrary to myths and outright falsehoods, lack of a
thermostat, or too cold of a thermostat, cannot cause an overheated engine. As
water flow increases, temperatures throughout the system become more even. The
ambient temperature, radiator design, airflow velocity through the exchanger,
and water velocity through the radiator tubes determine the total system heat
dissipation. As radiator air or water flow rates increase, more heat is pulled
from the water system.
The graph below shows water flow rate (GPM) vs. heat transfer
from water (BTU) for various water flow rates with a small radiator at maximum rated airflow.
This is a radiator we use to cool electronics. Electronics, like cars, can be
anything from a steady thermal load or heat dissipation rate over a long period
of time, to a varying dissipation with very high peaks and long recovery times.
the faster the water, the more heat is pulled from the system (until the
radiator limits flow). All radiators behave this way:
Pressure is not a factor in cooling, with the exception
water’s boiling point increases about 3 degrees F for every 1 psi pressure
increase. Pressure caps increase the coolant’s boiling point. Pressure does not
increase cooling rate, but pressure does allow hotter temperatures without boiling.
During operating times when
external airflow or convection does not allow adequate water-to-air heat exchange in the
radiator, the fan’s role is to force air through the radiator. Open flow is not
important. The important parameter is flow through the radiator’s restriction.
This is measured as a pressure differential between the radiator inlet side, and
the radiator outlet side. Despite pressure differential between inlet and outlet
being the determining flow factor, most fans are specified by open air flow!
There is a good electrical analogy for airflow:
My Cooling System
This is my home made fan shroud.
An ideal shroud would cover all of the radiator fin area,
forcing air through all of the cooling fins. Additionally, the fan shroud should
be as deep as possible. A deep shroud, especially deeper away from the outer
edges, makes pressure more uniform across the surface of the radiator. To limit
having to weld pieces together, I made my fan shroud as deep as possible just
with box and pan brake bending.
My deep shroud, because of limited water pump clearance, required an inset
fan. The inset moves the fan away from the water pump. This was an unfortunate
compromise over ideal.
For physical reasons, I elected to use an electric water
pump. I used the
Meziere 300 series electric water pumps WP311S. This pump supposedly
flows 55 GPM.
Pump and Fan
I chose 160-170F as a
target engine temperature. This is because I drive limited street miles, and I
want ~160F at the starting line in a race. I prefer a stable repeatable
temperature that keeps my engine well below boiling at the end of a hard high
For a long term street car, closer to 200F is more ideal.
This is because emissions are reduced, less moisture collects in the crankcase,
and cylinder bore more fully expands. The engine may produce less power, but the
engine oil collects less moisture and the cylinder bores wear less.
Sensors and Sample Points
we can get away with less than idea sensor locations, each sensor has an ideal
Water Pump Velocity
ideal sample point for pump speed is at the thermostat housing. This makes
sense, because that is where the thermostat is normally located. It does no good
to use a restrictor or thermostat when using electronic pump control. The
thermostat housing should be left open.
When well below target engine
temperature, my water pump turns at slow speed. This lazily pumps water through
the engine and radiator. The slow circulation promotes even block and head
warming and allows the temperature sensor to get an accurate engine water
As the outlet water temperature approaches engine
water target temperature, my pump controller speeds up. Just over target
temperature, the pump runs full speed. There is some system hysteresis
caused by the ~10 degree slow to fast pump speed range. The engine tends to
hover very stable right in that range, with the pump just pumping enough water
to keep it there.
My water pump sensor is located here. The RTV is
just to prevent wire fatigue from constant vibration flexing:
Air Velocity or Fan Speed
The ideal place to sample fan speed is the cold tank of the radiator, or the
radiator outlet hose. The water in the cold tank must always be at least 20-30F
below engine target. If the water is too warm, the pump will run unnecessarily
fast in order to keep the engine at target or worse might not be able to
maintain target. The engine might even overheat if the cold tank is too hot.
If the cold tank is too cold, the engine might stay too cold. This will slow
the pump, and the water might move so slow the engine heats unevenly.
my system the ideal cold tank temperature turned out to be about 20F below
target on the street, and 30-40F below target at the track. I compromised my
settings to stay around 30F below target.
I sample my cold tank water in
the cold tank itself with a thermistor installed in the unused transmission
cooler bung. My electric fan runs full on at 140F cold tank.
If I sampled at the radiator inlet, or worse at the thermostat, my fan would run
too long and too often. I would risk uneven engine heating, and the system would
waste significant battery/alternator power.
Cold tank sensor. Again the RTV is only to reduce chances of damage from
Water sensor adapter
Brass rod. This was barbed
(angle cut) to lock into Teflon insert and to improve water seal
Rod was pressed interference fit into Teflon plug
Assembly before pressing Teflon plug and sensor into
Finished sensor adaptor assembly without
Final assembly with sensor
End of text for now.