THE VLB ENGINE COOLING SYSTEM

Talk about heat pipe cooling technology to someone who knows little or nothing about it and they will probably be impressed. Tell them how you're going to put it on an IC engine and they will follow the logic of it easily enough. But do the same to someone who knows "all about" heat pipe technology and they will probably laugh.

I get uncomfortable with people who laugh at my technology, not for the obvious reasons, but because it is so difficult to recover from the situation. If I explain it to them, they tend to back off a bit and take the view that what I'm doing isn't what I claimed. With respect to the VLB cooling system, they say, "That's not what I call heat pipe technology". As if I had said it was!

Air cooling of vehicle engines has been around, and has worked well enough, for many years. A friend of mine had a VW with a hot-rod engine, capable of 130 MPH on the flat – It was air cooled, and it didn't melt.

The VLB engine is air-cooled, but at maximum engine speed, and on a hot day, the thermal conductivity of aluminium is such, that the temperature gradient required to keep the combustion chamber within a reasonable working range, isn't always going to be there. Making the cooling fins bigger doesn't help much, because the problem is in the combustion chamber. What is required is an improvement to the thermal conductivity of the fins.

The chamber wall and the first few millimetres of aluminium define the critical area. (As yet, I've not come across a case of an engine's fins melting off.) So what is required is something to assist the heat-flow close to the cylinder. What better than water? Hollow fins, filled with water! Well, not exactly. The problem with filling them with water is that above 100 degrees centigrade, the pressure could become a serious problem. (I wouldn't want to be standing close if one of the fins ruptured.) So how about just enough water that if it all turned to steam, it would be safe? Or better still, how about a woven aluminium wick, plus a drop of water, and fins large enough to ensure that at their extremities, even on the hottest of days, they will never reach 100 degrees?

The fin assembly is a pre-fabricated unit, which can be easily and quickly replaced. The cooling system does not require a fan, even in heavy stop-start traffic situations.

So, what do we have here? A closed circuit water-cooled engine, an air-cooled engine with a water jacket, a low-tech heat pipe cooling system, or what?

What we actually have is a gas path for the heat to travel along. The transport of heat away from the cylinder walls is driven by the temperature gradient in exactly the same way as through the aluminium, but because the gas molecules themselves are mobile, it is now a mechanical transfer, which does not rely entirely upon static molecule to molecule gradients, but rather upon the entropic differentials of the gas molecules. Which is the same heat transfer mechanism that is employed by a heat pipe. So what should I call it, if not a heat pipe?

The physical geometry of the fins ensures that gravity locates the liquid water at the chamber end, so it doesn't rely upon the wick in the same way as regular heat pipes. (A woven aluminium wick wouldn't work too well anyway – I expect you spotted that.) In fact the wick is something of an anomaly.

Early experiments were carried out, not on an engine, but using somewhat more defined apparatus, where all of the parameters could be controlled and monitored. The apparatus was driven to destruction several times, both due to excess heating and to insufficient air-flow cooling. The wick was found to extend the operational range, without (materially) adversely affecting normal operation. In truth, the wick was the result of inspiration, rather than scientific deliberation.

I had better add that catastrophic failure of the apparatus took place well beyond the operational range of a VLB engine.

John Allen