This article was first published in 2003.
In this story I want to tell you about a brand new concept in road car intercooling.
It has the potential to be very efficient, cheap to put together, compact and can keep charge-air plumbing very short. It is also, AFAIK, unique in that no-one else has ever previously used the approach. The design is suitable for cars where the off-boost turbo outlet temperature is below 50 degrees C in nearly all weather conditions. That is, when driving down the highway, when driving in urban areas and when stopped in traffic, the temperature of the un-boosted air coming out of the turbo needs to be less than 50 degrees C. That's the case in many turbo cars - but not all of them. How much above 50 degrees the boosted air temp rises to doesn't really matter, because these temps will be reduced by the intercooler.
Intrigued? Well, hang on for the ride as we cover stuff that you haven't heard of since high school or university chemistry and physics!
But first, some background. Not many people realise it, but road car intercooling on turbo cars works in a way that's very different to the common perception. We said it in a previous article (the first part of our series on our Intelligent Intercooler Water Spray), but it's worth repeating here.
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?
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. 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 amplifier's 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, 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.
A Real-Life Example
I once had a high-boost Daihatsu Mira Turbo in which 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.
Let's run that point by you again: in a water/air intercooling system being used on a road car, the measured intake air temps are much the same whether the intercooler water pump is running or not. The point-and-squirt style of boost usage in a road car simply means that the heat gets dumped into the water/air heat exchanger (reducing the intake air temps over the turbo outlet temps) when the car is on boost, then gets slowly fed back into the intake airstream when the car is off-boost. The water/air heat exchanger knocks off the temp spikes that occur on-boost, at the cost of slightly elevating the off-boost temps.
So is it possible to build an intercooler - really, a heatsink - that has no external cooling? That is, its whole purpose in life is to absorb the on-boost heat and then feed it back into the intake air stream when the car is off-boost?
The limiting factor in such a 'closed-loop' heatsink intercooler design is the amount of heat it can absorb. In the small water/air core being used in the Mira Turbo, the thermal mass (the amount of heat able to be absorbed for a given temperature rise) was sufficient for all but the duration of boost used on long hills. And that was with only a few litres of water (and the copper tubes) for the heat storage.
Water has a very high thermal mass, or more technically, a high specific heat. Think of it like this: when you place a saucepan of water on the stove it takes the input of a lot of energy before the temperature of the water changes much. (Try heating that saucepan of water with a single candle!) In fact the specific heat of water is 4.18 kilojoules per kilogram per degree C. So, to raise the temp of 1 litre of water (1 litre of water has a mass of 1kg) by one degree C requires 4.18 kilojoules of energy. As I said, that's a lot.
In fact, as a comparison, have a look at some specific heat values of common materials:
So if you were using a solid block of aluminium as your heat storage mechanism, you'd need 4.4 times the mass of aluminium to get the same heat storage as water. (As you can see, specific heat doesn't have much to do with how good a conductor the material is.)
If you want to make a heatsink that's capable of absorbing lots of heat without increasing much in temperature, you could use lots of water. For example, if the heat of the boosted intake air could be conducted to - say - 20 litres of water, I'd bet that in a road car the intake air temp on boost would never get very high. But 20 litres of water is 20kg, and because water is a poor conductor of heat, you'd also need a really good heat exchange mechanism.
Hmmm, too big and heavy.
So is there a commonly available substance with a much higher specific heat than water? The short answer is 'no'.
Doing it Differently
These sorts of questions are also being covered extensively in an industry that has nothing to do with turbocharged cars. In solar house design, adding thermal mass is important because it knocks off the highs and lows of temperature extremes that are experienced by the occupants. For example, because water has a much higher specific heat than say concrete (see table above), some designers place storage containers of water within the house. This water gradually rises in temp on the hot days (keeping the house cooler) then feeds the heat back out as the temperature drops (keeping it warmer). In short, the water containers act as heatsinks, knocking off the peaks and troughs in the temp variations.
But the same problems apply to using large bodies of water in a house as they do to a turbo engine heatsink/intercooler application-lots of water is needed if you want to absorb lots of heat. So solar house designers are exploring a completely different way of storing that heat.
They are now using materials that absorb a lot of energy as they change in state.
Huh? That's it? Yes - now let's look at what that means. It is incredibly significant.
We've covered the idea that materials have a specific heat - the property of the material that determines how much heat it can absorb for a given temperature increase. But there's also another characteristic of materials, called the specific heat of fusion.
Let's have a detailed example. We'll start with a solid (rather than a liquid or a gas) which we'll call 'Performal'. We get a chunk of Performal and put it in a saucepan on the stove. We then place a thermometer probe in the Performal and turn the stove to 'high'. Every minute we take a temp reading, and as expected, the stuff starts getting warmer. It must have a pretty high specific heat, because even though the stove is set to high it warms up only slowly. In fact, the graph here shows the temp increase that we measured over the first ten minutes.
We're getting pretty bored with this experiment (what is this, a performance car magazine or a science class?!) and so when we jot down '50 degrees C' as the temp after 10 minutes we're thinking more of that night's cruise than anything else. And so when the next reading after another minute is also
50 degrees C, we get the uncomfortable feeling that we've stuffed up the readings. The stove is still running on high, pouring heat into the Performal, but a further minute later the temp of the Performal is still
50 degrees C!
What the hell is going on? Have the laws of physics and chemistry gone out the window? We're continuing to add heat - lots of it - but the substance isn't getting any hotter!?
What is happening is that the Performal is melting - it is changing from a solid to a liquid at 50 degrees. And when it undergoes that change in state, it can absorb lots of energy without altering in temperature. Instead of heating the material up, the energy from the stove is being used to separate the material's molecular bonds.
Until all of the Performal has changed from a solid to a liquid, its temperature will not change. That's what the above graph shows - and you can see that the temp is being held constant, even though we're continuing to pour in the heat energy from the stove. It's only when the Performal has completely melted that its temp will start to rise again - and then the rate of temp increase will be dependent on the specific heat of Performal in liquid form, which might be different to its specific heat in solid form.
So here's a graph that showed what happened as we heated the Performal. When it started to change state, it had the capability of absorbing a huge amount of energy without getting any hotter. And the amount of energy it can absorb during this change of state is called its specific heat of fusion.
Specific Heat - the quantity of heat required to raise the temperature of one gram of a substance by one degree Celsius.
Specific Heat of Fusion - the quantity of heat required to convert a substance from the solid to the liquid state with no temperature change.
Melting point - the temperature at which the substance changes from a solid to a liquid.
Back to Cars
So where does this leave us? Well, let's pack a conventional air/air intercooler core with Performal. We'll first heat the Performal up until it melts, then pour it through all the fins of the intercooler, filling them right up. After that, we'll place a water-tight jacket all round the core (so the Performal can't leak out when it melts) and then we'll insulate the assembly so that under-bonnet heat can't affect it. Finally, the new heatsink will be installed in the turbo-to-intake plumbing.
Remember, Performal has four different characteristics that interest us:
- Its specific heat as a solid
- Its melting point
- Its specific heat of fusion
- Its specific heat as a liquid
To remind you, Performal's melting point is 50 degrees C.
So, the car's cold and so you only gently drive it down the street on this 30-degree C day. But after half an hour of this gentle driving the temps have stabilised: at idle the temp of the air coming out of the turbo is 40 degrees C, and the temp of the Performal heatsink is also 40 degrees.
Then you put your foot down. The turbo spools up to 15 psi boost and the air coming out of the turbo rockets from 40 degrees to 80 degrees. But a lot of this heat is absorbed as the air passes through the aluminium and Performal honeycomb heatsink, so the intake air temp at the engine remains much cooler - say (with some heat exchange efficiency losses) it's 50 degrees C. After 5 seconds of full boost the heatsink has risen in temp to 45 degrees C, with the heat all being absorbed by the Performal's specific heat capability as a solid.
You get back off the throttle and go back to a cruise. The heat from the heatsink is now fed back into the airstream, which is now cooler than the heatsink. Effectively, the heatsink is being internally cooled.
But that five seconds of boost has whetted your appetite: this time you wind it right out through the first three gears at full boost. The temp of the heatsink rises: soon it has reached 50 degrees C and the Performal starts to change state, to melt. Its ability to absorb energy without rising higher in temp is now taking effect: despite a huge amount of heat being pumped into the heatsink, the outlet air temp stays just the same, at (say) a constant 55 degrees C. Again, when you get off the throttle and the air flowing through the heatsink is lower in temp than heatsink temp, the heat will be fed back into the intake airstream and the heatsink will cool. As it cools the Performal will start to solidify, until when it has all turned back into a solid, its temp will start to drop below 50.
The speed bug has bitten and you decide to go for a top speed run: on full boost continuously for a minute. The Performal then warms up, reaches melting point and holds the intake air temp steady. But it can only do this while the material is melting, and after 40 seconds it has all turned to a liquid. At this point, its temp will again start to rise, but even as a liquid it will be absorbing heat and so reducing the turbo outlet temp. Of course, when the police catch you and you are having a roadside discussion, that heat will be being fed back into the airstream: it is likely to stay warm for some time.
Ice/Water Fusion Intercooler
Drag racers who use a mixture of ice and water in a heat exchanger core are already using a fusion intercooler. The specific heat of fusion for ice (ie how much energy per kilogram is required to melt it) is 334 kJ/Kg. That's why ice/water systems are so effective - a lot of energy is required to melt the ice and the ice/water mix will stay at 0 degrees C until all of the ice is melted. Trouble is for a road car, the water doesn't turn back into ice when the car's back off boost.....
You can see now why some 2700 words ago I said that for the system to be effective, the off-boost intake air temp must be below 50 degrees. Otherwise, with the normal off-boost heat the Performal would be melted all of the time, and so its capability to absorb heat during its change of state would be gone. (That is, it would already be changed in state!)
That off-boost turbo outlet temp will be dependent on a number of things:
- The temp of the air being breathed by the turbo
- The heating of the compressor side of the turbo by the exhaust manifold and turbine
- The size and design of the turbo
- The temperature of the day
- The airflow through the engine bay
In areas of cold climate the off-boost turbo outlet temp is very unlikely to ever exceed 50 degrees C, however, when the ambient is 40 degrees C the air coming out of the turbo will often be above 50 even when off-boost.
I had intended fitting a Performal heatsink to my Nissan Maxima VG20DE turbo, however the transverse engine location (the turbo is heated by the radiator airflow), the very small turbo and the long intake path all conspired to give a turbo outlet temp of about 30 degrees above ambient! Since where I live the ambient seldom drops below 20, the Performal would be frequently already changed in state, even before boost hit. It's therefore not suitable for this car and climate, and it shows how important direct measurement of the actual temps really is.
(Of course, I could place a free-flowing but very small air/air intercooler in front of the Performal heatsink - this would cool the off-boost air sufficiently to keep the Performal as a solid, while the small air/air intercooler's performance on boost wouldn't matter - it would just need to flow enough air to not be a restriction. However, space constraints mean that such an approach on my car would be very difficult.)
The potential on the right car is huge: but what actually is bloody "Performal"? Time to let you into a secret - and some of you may have already guessed. "Performal" doesn't exist but a substance with very similar characteristics is commonly available. It is called paraffin wax, and is sold for use in making candles. Specifically, its typical characteristics are:
- Melting point: 52 degrees C
- Specific heat: 3.27 kJ per kg per degree C
- Specific heat of fusion: 210 kJ/kg
So to increase the temp of 1kg of the wax from 47 to 52 degrees takes 16.35kJ, but to push it past 52 degrees takes nearly 13 times as much energy. (Or, to risk causing confusion, you could dissipate in it a power of 14kW for 15 seconds to melt 1kg.)
Paraffin wax is non-toxic, doesn't explode (although it will catch fire if you expose it to a naked flame) and is easily handled. Special waxes designed specifically for this change-of-state heat storage purpose are also available with melting points in 10-degree C increments from 50 degrees to 100 degrees C, however their availability is obviously less than simple candle wax.
The easiest approach to making a fusion intercooler would be to obtain a small air/air intercooler core, which has adequate airflow for the application. The core could be sealed with sheet aluminium welded into place, much as the water/air design shown here was constructed. The assembly could then be heated to perhaps 60 degrees C in an oven and before having its core completely filled with molten wax - but with a small gap left for expansion. The resulting assembly would then need to be insulated from underbonnet heat, perhaps by being placed in a larger box and having the gaps filled with expanding foam applied with a spray can.
As I said above, the car that I had hoped to apply the technology to isn't suitable: its off-boost intake air temp is too high. However, in the past I have owned turbo cars where the off-boost intake air temp was only 5-10 degrees above ambient, and these would be suitable for the technique. Remember, this approach would not be appropriate for race cars or for cars spending long times on boost on a dyno; it would however be very suitable for turbo cars where packaging is very tight (or a rear- or mid-mounted engine is used) and where in normal road use the duration of boost is limited.
It's a fascinating concept, which as the URLs below show is now being used in other industries and applications. (To find more links do a web search under 'phase change materials heat storage' or similar.) Equipped with a wax or other phase change material that melted at (say) 65 degrees C, the fusion intercooler could even be used as a 'safety' heatsink, able to knock the top of peaks that only rarely occur, or occur for only a very short period.
Either as a main intercooler/heatsink, or as an additional safety device, it's certainly something with huge real-world potential.