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This article was first published in 2008.
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For forced aspirated cars, intercooling is one of
the most vital considerations. In fact, after ensuring that you get plenty of
boost over as wide an engine load range as possible, and there’s the right fuel
and (in petrol engines) ignition timing, intercooling is the key to making
power. That applies whether you’re talking an Impreza WRX, Falcon XR6 Turbo – or
a diesel passenger car.
But in other respects, diesels are different.
Simply, the intercooling approach that works on a high performance petrol engine
car does not necessarily work on a diesel turbo. Let’s take a look.
Air/Air and Water/Air
Intercooling approaches can be divided into two
basic types – air/air intercooling and water/air intercooling.
Air/air intercoolers are the most common type of
intercooler, both in factory forced induction cars and aftermarket. They are
technically simple, rugged and reliable. An air/air intercooler consists of a
tube and fin - or bar and plate - radiator.
The induction air passes through thin rectangular
cross-section tubes that are stacked on top of the other. Often inside the tubes
are fins that are designed to create turbulence and so improve heat exchange.
Between the tubes are more fins, usually bent in a zig-zag formation.
Invariably, air/air intercoolers are constructed
from aluminium. The induction air flows through the many tubes. The air is then
exposed to a very large surface area of conductive aluminium that absorbs and
transfers the heat through the thickness of metal.
Outside air - driven through the core by the
forward motion of the car - takes this heat away, transferring it from the
intake air to the atmosphere.
In the absence of any other description, an
‘intercooler’ is an air/air design.
A water/air intercooler uses a compact heat
exchanger located under the bonnet and normally placed in-line with the
compressor-to-intake-manifold path. The heat is transferred to water which is
then pumped through a dedicated front-mounted radiator cooled by the airflow
generated by the car's movement.
A water/air intercooler system consists of these
major parts: the heat exchanger, radiator, pump, control system, and
plumbing.
An air/air design is cheap (these days, with the
advent of eBay cores, very cheap) and simple. In many applications a
large core can be used and the only real difficulty is getting plumbing to and
from the core. However, in some cars that can be hard indeed!
A water/air intercooler has some distinct cooling
advantages on road cars. Water has a much higher specific heat value than air.
The 'specific heat value' figure shows how much energy a substance can absorb
for each degree temp it rises by. A substance good at absorbing energy has a
high specific heat value, while one that gets hot quickly has a low specific
heat. Something with a high specific heat value can obviously absorb (and then
later get rid of) lots of energy - good for cooling down the air.
A water/air design has the potential to have a
much bigger heat sink effect (more on this in a moment) and so can cope with
temperature spikes very well. Getting the small diameter water hoses to the
front-mounted radiator is usually a lot easier than getting the large diameter
tubes associated with an air/air intercooler to the front of the car. However, a
water/air system is more complex, more expensive and often heavier than an
air/air design.
Heat Sinking
In most petrol car applications, intercoolers work
more as heat sinks than as radiators. That is, they seldom – if ever – shed the
heat at the same rate as it is being absorbed.
So 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.
Audio output power spikes generate heat that's dumped into the amplifier's
heatsink to then dissipate 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 petrol engine turbo road cars there's no boost occurring. In fact,
even when you're driving hard, 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,
importantly, also to the intake air going into the engine. (Since the engine's
now off boost, that heating of the intake air is of no consequence.)
However, there’s a very important point to note.
This sequence of events applies only to cars where turbo boost events are
relatively rare.
Diesels
Turbo diesel road cars are on boost far more than
an equivalent petrol engine car. In fact, because the off-boost airflow of a
diesel doesn’t vary with load, there’s potentially enough exhaust flow to keep a
turbo boosting even when the driver has their foot off the accelerator on the
over-run! And a diesel turbo that runs 2 or 3 psi boost in normal highway cruise
is common.
There are two reasons for this differing behaviour
– a diesel doesn’t use a throttle and so the airflow generated by the turbo is
not artificially reduced; and because diesels are intrinsically less powerful
for their capacity than petrol engines, to make up for that power deficit, the
turbo is usually sized to work harder more of the time.
In addition to being on boost a far higher
proportion of driving time, diesels generally use higher boost figures than
petrol engine cars. And irrespective of whether the car is under load or not,
the air temperature exiting the turbo depends on how much it has been
compressed, that is, its boost pressure. Higher equals hotter.
This combination of factors – higher boost used
more often – results in different intercooler demands. In short, a diesel
intercooler has to work far more at ‘real time’ removal of heat rather than
acting as a heatsink.
So what implications does this have?
In a diesel, heat soak becomes a huge problem.
Take an intercooler that’s mounted under the bonnet, as many factory diesel
intercoolers are. The car is driven and then parked. The underbonnet environment
then gets hot – much hotter than when the car was running and there was airflow
through the engine bay, and (separately) through the intercooler. In fact, on a
hot day, it’s not hard for a heat-soaked intercooler to be at 70 degrees C.
The car is started and driven off. Because it’s a
diesel, straightaway it’s likely to be on boost – and plenty of it. So air is
exiting the turbo at (say) 100 degrees C and then passing through an intercooler
that’s at (say) 70 degrees C. As can be seen by the relativity of these figures,
the intercooler will still drop the temp of the induction air, but only by a
small amount. The result is hot induction air entering the engine, resulting in
poorer power, fuel economy and throttle response.
Now here’s where it starts getting really
interesting. In most cars, an intercooler with a high heat sink capability is a
good thing – it allows the intercooler to knock off the temp spikes caused by
short-term high boost events. But if the heat-soaked intercooler has a high
heat-sink capability, it will stay hot for longer.
So for example, an underbonnet water/air design,
which has a high thermal mass (water has a very high ability to absorb heat, or
to put this another way, stay hot once it is hot!) will have poor effectiveness
as an intercooler for quite some time after a hot car has been re-started.
That’s just the same as for a water/air
intercooler under the bonnet of a petrol turbo car. Measured intake air temps on
a factory water/air intercooled Subaru Liberty RS shows that intake air temps
remain high for some time after a hot car has been restarted. But in the petrol
turbo car, boost is used less and is lower in level, so allowing time for the
water/air heatsink to drop in temp after driving off.
Real Time Heat Removal
Because boost is used far more often in a diesel,
the intercooler has to be designed to be effective at real-time heat transfer to
the atmosphere, rather than storing the big spikes for later, slower
dissipation. This means the intercooler efficiency must be higher – much higher
than in a petrol turbo car. And because a turbo diesel can generate high boost
pressures at low road speeds, forced air cooling (eg by fans) is needed far more
often than it is in a petrol engine car.
At a given boost pressure, a diesel has equal
airflow at one-tenth full load, half full load, and full load. In other words,
diesel airflows are on average considerably higher than are experienced in a
petrol engine car of the same capacity. This has major implications for
intercooler sizing. If the intercooler causes a pressure drop (ie has a flow
restriction), not only will top-end power suffer but, through the increased
pumping losses, cruise fuel economy may also be affected.
Water
and Diesels
Because
of their small cylinder clearance volumes (ie high compression ratios), diesels
are very susceptible to engine damage if they ingest water. This has
implications for water/air intercooler designs – any internal water leak could
quickly prove catastrophic.
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Sunmmary
So where does all this lead us for diesels?
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An underbonnet intercooler should not have a high
heat-sink capability. That is, its thermal mass should be low. This has
significance for water/air systems that place a large volume of water (eg 1 or 2
litres) in the heat exchanger. It also implies that air/air intercoolers should
be physically light.
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Intercoolers should be sized and designed to allow
large amounts of heat to be removed real-time. For example, air/air cores (and
for water/air system, the water radiator) should be located so that lots of
outside airflow passes through them. In other words, the old
plonking-of-a-big-core-in-front-of-the-radiator without paying any attention to
actual flows (ie difference in ambient pressures before/after the core) may not
be very successful. (Many high performance petrol engine cars get away with this
approach because a 10 second burst of boost might have 120 seconds to get rid of
the absorbed heat...)
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Intercoolers should have a very high internal
airflow capacity. For example, for the same intercooler size, an air/air
intercooler with a small number of long tubes will have a greater flow
restriction than an intercooler with a large number of shorter tubes.
Conclusion
It’s easy to look just at the power figure of a
turbo diesel and assume that little intercooling capacity is needed. In fact,
view the intercoolers of some factory turbo diesels and you could certainly
think that! However, because more boost is used much more often, intercooling a
diesel turbo requires much better design than an equivalently sized petrol turbo
engine.
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