One of our most popular series ever has been the Intelligent
Intercooler Water Spray, which starts at Intelligent Intercooler Water Spray - Part 1. The brilliance of that
design was that, as the name suggests, the module intelligently controls when an
intercooler water spray functions. That saves a huge amount of water (meaning
much less tank refilling) and also improves the cooling efficiency.
Yep, you really can have a colder intercooler without having to
spend a lot of time or money.
But, talking about money, the original Intelligent Intercooler
Spray Controller cost AUD$114 unboxed or AUD$239 when installed in a box. Well,
if you can put together a simple electronics board, you can now have a controller
that’s just as good as the original and will cost you only about AUD$50! That’s
a kick-arse price for a kick-arse controller...
But let’s take a look at the background to intercoolers and
intercooler sprays first....
Intercooler Functioning
It seems straightforward enough. An intercooler acts as an
air/air radiator for the intake air, cooling it after the compression of the
turbo has caused it to get hot. The compressed air passes through the
intercooler, losing its heat to the alloy fins and tubes that form the
intercooler core. This heat is immediately dissipated to the outside air that’s
being forced through it by the forward movement of the car. (We’ll get to
water/air systems in a moment.)
The trouble with this analysis is that – for a road car – it is
not entirely correct. Huh? So what actually happens, then?
I’ve watched turbo engine intake air temperatures every day for
over 15 years. All have been displayed on digital gauges permanently stuck to
the dash of the many different turbo road cars that I have owned - a Commodore
VL turbo, Daihatsu Mira turbo, Subaru Liberty (Legacy), C210 RB20DET Skyline,
R32 Nissan Skyline GT-R, Audi S4, Nissan Maxima V6 turbo and a turbocharged
Prius. This list includes cars with boost pressures of up to 21 psi (the Mira),
air/air intercoolers (GT-R, VL, S4) and water/air systems (Mira, Liberty, C210).
And – irrelevantly – the list also includes turbo three, four, five and six
cylinders and an electric motor! You might say that I’ve watched intake air
temperature gauges on turbo road cars for more than half a million kilometres.
So what?
The reason for this build-up is that what follows is likely to
be seen as incorrect by many people. For example, someone who measures intake
air temps while running a turbo intercooled car for a power pull on a dyno, or
who drives it around the block, or who sits back and simply theorises, is almost
certain to think that what follows is wrong. But, it isn’t.
Heat Sinks
In road cars, intercoolers act far more often as heat
sinks rather than as radiators. Instead of thinking of an intercooler
as being like the engine coolant radiator at the front of the car, it’s far
better to think of it as being like a heatsink inside a big sound system power
amplifier. If an electric fan cools the amplifier heatsink, you’re even closer
to the mark.
In a sound system amp, the output power spikes are always much
higher than the average power - for example, big output spikes are caused by the
beat of a bass drum. Each time there’s an output power spike, extra heat is
generated by the output transistors and dumped into the heatsink. But because
the heatsink has a large thermal mass (it can absorb lots of heat with only a
slight temperature rise) the actual working temperature of the transistors
doesn’t increase much. And because the fan’s hard at work blowing air over the
heatsink, this inputted heat is then gradually transferred to the atmosphere,
stopping the heatsink temp from continuously rising.
Importantly, because the power spike is just that (a spike, not
a continuous high output signal), the heat that’s just been dumped into the
heatsink is dissipated to the air over a relatively long period. This means
that the heatsink does not have to get rid of the heat at the same rate at which
it is being absorbed.
Now, take the case of a turbo road car. Most of the time in a
turbo road car there’s no boost occurring. In fact, even when you’re driving
hard – say through the hills on a big fang – by the time you take into account
braking times, gear-change times, trailing throttle and so on, the
‘on-full-boost’ time is still likely to be less than fifty percent. In normal
highway or urban driving, the ‘on-full-boost’ time is likely to be something
less than 5 per cent!
So the intercooler temperature (note: not the intake air
temp, but the temp of the intercooler itself) is fairly close to ambient most of
the time. You put your boot into it for a typical quick spurt, and the
temperature of the air coming out of the turbo compressor rockets from (say) 40
degrees C to 100 degrees C. However, after it’s passed through the intercooler,
this air temp has dropped to (say) 55 degrees. Where’s all the heat gone?
Traditionalists would say that it’s been transferred to the atmosphere through
the intercooler (and some of it will have done just that) but for the most part,
it’s been put into the heatsink that’s the intercooler. The temperature of the
alloy fins and tubes and end tanks will have risen a bit, because the heat’s
been stored in it. Just like in the amplifier heat sink. Then, over the next
minute or so of no boost, that heat will be transferred from the intercooler
heatsink to both the outside air - and also to the intake air going into the
engine.
Real Life Stuff
All getting a bit complicated? OK, let’s take a real-life
example. In South Australia (where I once lived) there’s a good, four lane road
that climbs a very large hill (for locals – it’s Willunga Hill). Many less
powerful cars struggle at full throttle to crest the top of the hill at 110 –
120 km/h. Others can manage only 80 or 90 km/h. My Skyline GT-R could top the
hill at about 200 km/h, with full throttle and full boost being used for perhaps
the prior 30 seconds.
(Only 30 seconds? Another point often forgotten in this debate
is: how long can you hold full throttle in a turbo road car? Answer: in the real
world, not very long!)
Using a quick-response K-Type thermocouple working with a
high-speed digital LED dash meter, I could watch intake air temp, measured to
one decimal place. From the bottom to top of the hill, the intake air temp
never rose by more than 2 degrees C, and in some cases, actually fell a
very small amount! However, after the top of the hill had been reached and the
throttle was lifted, the intake air temp would then typically rise by 5
or even 10 degrees. Why? The stored heat was being dumped back into the engine’s
inlet air as well as to the atmosphere.
In my Audi S4 (again equipped with a K-Type thermocouple intake
air temp display), the smaller intercooler meant that once over the top of the
same hill, the intake air temp rose by a greater degree – an increase as high as
20 degrees C in fact.
Another example. In my high-boost Mira Turbo I ran a water/air
intercooling system. The water/air heat exchanger comprised a highly modified
ex-boat multi-tube copper heat exchanger, with a few litres of water in it. An
electric pump circulated the water through a separate front-mounted cooling
core. Intake air temp was measured using a thermistor and a dedicated LCD
fast-response meter.
In normal point-and-squirt urban driving, the intake air
temp remained the same with the intercooler pump switched either on or off! Why? Because when the car was on boost, the heat was being dumped into the
copper-tube-and-water heatsink, and when the car was off-boost, this heat was
fed back into the (now cooler) intake air flow. Of course, if I was climbing a
long hill (ie on boost for perhaps more than 15 seconds) the pump needed to be
operating to give the lowest intake air temps. But even in that tiny car, 15
seconds of constant full boost would achieve over 160 km/h from a
standstill...
The latter shows why water/air intercooling in road cars is so
successful – but why most race cars use air/air intercooling. Water has a very
high thermal mass, so easily absorbing the temp spikes caused by a road car’s
on/off boost driving. However, race-style boost (say on full boost for 70 per
cent of the time) means that the system has to start working far more as a
real-time heat transfer mechanism – which is best done by very large air/air
intercoolers.
The key point is that typical road car air/air and
water/air intercooling systems act as heat sinks during boost periods at least
as much as they act as heat transfer mechanisms.
In
a way the point being made in this article is obvious. When you’re testing a
car’s intercooler, it’s common to occasionally stop the test and feel the
temperature of the core. If it’s hot you know it isn’t working very well. And
that’s because you automatically realise that it is primarily acting as a
heatsink! If it was just a radiator, the hotter it was, the better it would
exchange heat with the ambient air.....
Water Sprays
So, that’s a pretty big prelude to the topic of intercooler
water sprays, isn’t it? But if you’ve been following along, you’ll see that
having a spray that switches on only when the engine’s on boost is not very
helpful. Why? Because there will be plenty of times when the intercooler heat
sink is quite cool, so there’s simply no advantage in spraying water on it.
The only time the spray needs to operate is when the
intercooler itself (not the intake air temp!) is hot and the
engine is under moderate/high load. Furthermore, the longer the spray has run,
the longer it should keep running after the load or temperature drops below the
trigger point. (Why? Because it’s likely that the amount of heat stored in the
intercooler will then be greater.)
Having a spray controller that takes into account the above
points automatically allows for:
-
Days where it’s raining (and so the intercooler is already
being sprayed with water!)
-
Short bursts of boost (where the intercooler won’t have risen
much in temperature)
-
Hard driving where there’s lots of full throttle work mixed
with zero throttle (the spray will keep running during gear changes and quick
throttle lifts for corners)
-
How effective the spray actually is at cooling (weather with
low relative humidity will allow the spray to work more effectively, so it
doesn’t have to be on so long)
-
Variations in road speed and so airflow (eg you’re stuck behind
a truck slowly climbing a long hill)
In short, the New Intelligent Intercooler Water Spray
Controller does all of this – and also has a coloured LED bargraph showing
actual measured intercooler temp. The latter makes it much easier to set the
system up. Intrigued? Next week we’ll cover the new system.
Summary of Intercooler Water Spray Control
Approaches
|
Control Technique |
Advantages |
Disadvantages |
|
Boost Pressure Switch |
|
-
Relatively late switch on
-
Switches off on gear changes, even when driving hard
-
Switches off on trailing throttle, even when driving hard
-
Operates even when intercooler is cold
-
May be on continuously during high speed cruise
-
Wasteful of water |
|
Throttle Position Switch |
-
Simple, cheap
-
Early switch-on |
-
Switches off on gear changes, even when driving hard
-
Switches off on trailing throttle, even when driving hard
-
Operates even when intercooler is cold
-
Wasteful of water |
|
Intake Air Temperature Switch |
|
|
|
Boost or Throttle Position switch plus series intake
air Temperature Switch |
|
-
Switches off on gear changes, even when driving hard
-
Switches off on trailing throttle, even when driving hard
-
Depending on system dynamics, can be wasteful of
water |
|
Adjustable boost or throttle position input plus
adjustable intercooler core temperature plus inbuilt off
delay |
-
Operates only when intercooler core is hot
-
Early switch-on
-
Runs through gearchanges and short throttle lifts
-
Runs longer when intercooler has copped a lot of heat
dump |
|
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