Phase-change material for down-sizing cooling system

[Tesla]

Originally Posted by Air-Hybrid

If you insulated the engine bay completely (you'd need to manufacture new cowling to pipe the air exiting the rad) the header and exhaust heat would quickly raise the temps around the engine and then the coolant system would need to deal with quite a considerable additional thermal load. And even if the car was driven gently the extra need to radiate heat via the coolant through the radiator would not do anything but bad things for efficiency, and may still boil the coolant.

Entirely agree, but wouldn't it be easier to seal off the entire engine bay area, and just add and appropriate cooling air exhaust at the rear bottom of the engine bay to open and release heat, as well as allow airflow through the radiator, than it would be to add another complete cooling system as a heat sink.
Eg when warming up all vents closed, engine warms up very quickly, once warm, vents open as much as required, then when stopped and shut down, all vents close and retain engine heat, when re started and running vents open as much as required.

[Air-Hybrid]

Originally Posted by Tesla

Entirely agree, but wouldn't it be easier to seal off the entire engine bay area, and just add and appropriate cooling air exhaust at the rear bottom of the engine bay to open and release heat, as well as allow airflow through the radiator, than it would be to add another complete cooling system as a heat sink.
Eg when warming up all vents closed, engine warms up very quickly, once warm, vents open as much as required, then when stopped and shut down, all vents close and retain engine heat, when re started and running vents open as much as required.

I see your point about working with what you've got rather than adding further components to the cooling circuit (as I suggest), but a few things occur to me:

• As well as having to fabricate an airtight space under the bonnet, you need to fab at least two actuated vents (air-in & air-out).
• Especially when stationary or going slow, your cooling air will need larger (and perhaps extra) electric fans to force enough air through. This is because you've strangled the free air flow in the engine bay.
• The manifold (header) and top of the exhaust would need to be thermally separate from your controlled ducted airflow (no idea how you'd do that). If not, because normally the header relies on radiating some portion of the hundreds of degrees it will reach into the free air of the engine bay. Either way, this large additional heat dissipation needs to be factored into your alternative design.

The thing is there's nothing like the amount of thermal capacity, even in all the air 'contained' in an engine bay, as can be held in a few kilos of thermal salts (PCMs). And I'm not convinced your alternative 'plumbing' would even be as simple.

[Tesla]

Not the air I'm talking about keeping hot, it's 200kg of engine, steel, alloy & fluids etc.how many kilos of salt and plumbing will you need to match that thermal mass?
Insulate & seal the bonnet, stop the heat venting out the top, the firewall is usually pretty well insulated to the cabin anyway, insulate the side guards, the bottom doesn't so much need to be insulated as properly sealed.
When driving the rear bottom vent is opened to let air out, and the normal radiator opening is left open to let air in, as much as required by the conditions, when parked, close lower vent and front grille area, no need for additional fans.
Once you stop convection, the heat stays in, and any added insulation minimises the conductive losses.
I think if you pull out some numbers on it you will find you will need to fill the entire trunk with atleast 50kg of salt, with all the associated equipment, just to get engine from ambient cold to 40C.
Remember the salt can't give up all it's heat, it can only equalise the engine temp.
Let's say you have 50kg salt at 100c and it stores twice the heat of steel etc.
You have 200kg of engine at 10C, best you are going to get is 55C across everything, but remember as the two bodies get closer in temp, the rate of heat transfer slows down, so you will get it to 40C fairly quickly, but to get that last 15C might take half an hour of pumping coolant through.
If you can keep the engine hot enough, say 40C by the end of a working day, then you just, open your vents, get in and drive away.
If I had to deal with the extreme cold issues, that's what I would look at first.
Edit: Just thought, has anyone just tried using one of those cheap car covers to see what difference in heat loss from engine bay area is, just trapping a layer of air may actually be quite effective for a few hours?

[Air-Hybrid]

quote: "I think if you pull out some numbers on it you will find ....
Let's say you have 50kg salt at 100c and it stores twice the heat of steel etc."


PCM (salt hydrates / eutectics) - storage density: 93 kWh/m3
Water (as just sensible heat) - storage density: ~8 kWh/m3

And Iron fairs much worse than Water by weight (water is actually very good)
Water has a Specific Heat of 4190 J/kg.degC
Cast Iron has a Specific Heat of 540 J/kg.degC


Remember the great thing with PCM salts, etc is that they can tap the large amount of energy needed for phase change (The latent heat).

So from above 200kg of iron holds about the heat of 25.7kg of water.
So we'll be generous and say a biggish engine block with water has a heat capacity of equivalent to 30kg of water. And the PCM can hold over eleven times as much heat once molten.
Unless I'm mistaken we find that just 2.6 kilos of our pcm can hold the equivalent heat of your entire engine.


Also, the PCM salt can give up very near to its entire heat as it solidifies/freezes (as most of the thermal capacity is the latent heat) providing a salt is chosen that melts just below the ideal coolant temp.

So whether you're trying to keep coolant heat stored for fast warm up later on or overnight (as you're idea is focused on), or you're trying to cut aerodynamic overheads required by a large radiator and opening (as is my starting point), phase-change materials are the way to go.

[Tesla]

Ok, I'll pay you that,
I hadn't gone back to look at the actual numbers.
But I still think the complexities & return are still going to take it down the path of diminishing returns.
Unless the salvaged heat is somehow converted to useable energy, then the overwhelming issue is to still remove the heat generated during normal operation as it is currently a waste product for ICE's, this will still mean radiator & cooling system cannot be downsized to any great degree.

[Air-Hybrid]

Originally Posted by Tesla

Unless the salvaged heat is somehow converted to useable energy, then the overwhelming issue is to still remove the heat generated during normal operation as it is currently a waste product for ICE's, this will still mean radiator & cooling system cannot be downsized to any great degree.

Read:
Engine Cooling System with a Heat Load Averaging Capability
to see why it can.

Note that:
"
- Under typical driving conditions, an engine generates only about 30% of available power for 90% of the time.
- The remaining 10% generally [near peak output] is used for accelerating or climbing steep inclines.
"

So with a sympathetic driver at the wheel (who understands that he will be 'borrowing' heat capacity for an overtake, etc against driving with a light foot later) I would estimate that an 80% reduction in rad-size/grill-opening would not be unrealistic for anywhere but the most hot, arid environments.