Well, the preheating is hard to notice any difference with or without, but, for slow driving you reduce the risk of tar by keeping the temps up.
The WK gasifier is a proof, it can idle forever, and even power much smaller things.
To keep temps up, air preheating is among the easiest and most effective ways.
But with a hot-filter the gas needs to have enough heat left, to keep the filter warm.
Well, im maybe not the right person to talk about this, as i use practically no air preheating, but i compensate that with a pretty heavy right-foot.
Well, i sometimes stops beside the road and talk to friends, and have idled up to 15 mins with no noticeable tar production (but Extreme hesitation when taking of)
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Yes, since I can drive for several hours in the forest at about 1500rpm, and mostly have no major problems with hesitation when I get to major roads, I think my generator is the smallest size, but it is pretty well insulated.
However, you can see that the output temperature changes a lot and quickly depending on the load.
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You will not learn anything new from my opinion about a good gasifier:
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preheating of fresh air is important for several reasons, firstly to preheat the air well, secondly to cool metals exposed to high temperature and create insulation, thirdly to cool the outgoing gases from the hot zone. A certain surface area is required for heat transfer, here we take into account the fins inside the WK hot zone and the external heat exchanger, which I designed in terms of a horizontal grid, well, I think this is enough surface area, since the gas temperature rarely exceeds 100°C, I should add. that the gas also cools on the outer wall of the gasifier as it rises towards the outlet.
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the diameter of the hot zone must be large enough (at least 30 cm) to be smoothly filled with charcoal, which is created higher up in a wide area (nozzles next to the wall of the funnel), and at the bottom near the restriction opening, it is good to have a small air supply for burning fine of charcoal and long-term operation of the engine at low load (tar cannot pass)
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at low load, we still have to maintain high temperatures in the gasifier, and this requires energy. This energy is obtained by burning wood, and at the same time water vapor is produced, which is too much to be converted into hydrogen in the hot zone, and for this we need effective cooling of the condensation zoneâŠ
Effective condensation zone, forced cooling (slow driving, work on site), wide area (no fuel bridging) and heat exchange - air preheating, this is the key to success - clean powerful gas without tar
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Thanks Tony.
Now Iâm not a premium member so I donât know what a WK looks like.
My fire pipe is 2mm thick, and I havenât seen any faults with this yet, after 3-4 years, I think the ash protects this.
Preheating is certainly good, what I donât understand is when the preheating is taken from the outside of the fire tube, which has to cool down the hearth.
I also canât have preheating when I have a hot filter and cyclone, I need the heat in the filter.
slow and stationary driving I know is a problem, because I drive with the iller in the forest and have no speed wind and stand still and load.
So it will probably be difficult to achieve on a stationary generator.
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Jan,
You should be on the premium side, it is a good investment. Ask Chris S. "âhow-toâ again. 
The WK gasifier harvests heat energy for the primary air at several âopportunityâ spots. The most obvious is in the âHeat Exchangerâ away from the hearth. Less visible is a heat exchanger in the engine exhaust path. Invisible is what happens inside the gasifier, which takes some excess heat from the hearth and puts it to work. All of these (and many more design features), working together helps a WK to have such a wide operating range. Yes, a traditional âImbertâ has no pre-heating, and a swinging check-valve to save the char-bed. Also an Imbert is more critical in every way. Just my opionion, for what it is worth. 
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I think the SMP generator is a little different to a normal imbert.
Swedish Machine Testing conducted tests on both tractors and trucks during the 1960s and around the oil crisis in the 1970s, and made many changes and improvements.
They also did tests with wood chips and peat as fuel, but above all a much better filter than what had been used before.
I could make a heat exchanger on the exhaust pipe, but Iâve tried it before on the outlet pipe from the generator and didnât notice any difference.
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The Imbert gasifier is quite well designed and also has some preheating of fresh air, as the hot gas that rises up first âheatsâ the air pipes, and then the upper part of the gasifier, where it helps with pyrolysis and wood drying, unfortunately this gasifier does not have a condensation zone . The fresh air is thus first heated by the exit gases, and then in the double wall of the hot zone, if the lower part of the gasifier is insulated, the exchange is quite effective and the surface of the pipe is almost sufficient to heat the air to a high temperature, I should note that even with a larger load, the air flow is relatively small, so the air has time to heat up well.
I âstoleâ this picture from Goran, let me add that Goran eliminated Imbertâs shortcomings.
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I can show a picture of an SMP, I believe it is the latest version.
They have preheating on the incoming air in pipes that are outside the fire pipe in the outgoing gas.
They have a perforated inner jacket in the wood store, which improves condensation.
They have a loose ring in a cone to be able to change size more easily and for the ash to insulate between the hearth tube and the choke, and that the cast that imbert had did not hold or was deformed.
They have replaceable nozzles to be able to change for better adaptation to the engine.
They have a partition between the wood container and the generator, to facilitate service, and remove debris in the condensate chute.
They have a grate that goes back and forth to avoid a pile of ash remaining in the middle like on a rotatable grate.
I think thatâs all I remember, and if its god or bad, i dont now.
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@Woodrunner
Göran, how much larger diameter do I need to have on the outer jacket than on the fire tube? Is 3-4 cm enough?
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Hi Jan, do you mean between outer housing and air jacket, or between air jacket and firetube?
In both cases 3-4 cm would be fine (on Smp)
I have 2 +2 cm on the chevy.
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I have about 5mm on the Volvo gasifier 
That is firetube to air-jacket
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I had to look how you had done, is that enough surface to not choke the air into the unit?
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I think so. Itâs equivalent to about a 4" pipe. But I had to put some extra effort into having it centered.
About those 2cm for outgoing gas. I think that would be plenty for the gas itself, but Iâm not sure what the velosity would do to the char under the grate. It might want to get airborne. I think it depends a lot on the geometry around and under the grate and of course cleanout intervalls.
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Yes, i measured on the chevy, the space for outgoing gas is about 15mm, and it draws all char and ash to the filter.
Im going to put in a âspacer ringâ to stop it from emptying the reduction on heavy load.
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Ok, looking at the picture of SMP, it looks like they had about 5 cm, because they usually have 10 cm to the grate from the fire tube.
Is it perhaps better to try to go up to 3-5cm, should retain the heat in the fire tube better as well.
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Some questions: I want a shut-off before the filter on the gas, the pipe into the filter is 73mm, the shut-off is 60mm, will there be too much restriction so far from the unit?
Found an expansion vessel 1.25mm thick, stainless, would it be possible to have it for the outer jacket?
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I would say OK. You may lose half a hp at full power, but Iâm sure you will never notice the difference.
Thatâs the kind of material Iâm using a lot. Perfect for the outer jacket (gasifier housing). Not as good for heat transfer (air jacket).
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Thanks for the answer JO, then I can continue looking for things.
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One more question before I put in a new closure, I am closing the incoming air completely, can the moisture move away to the filter then?
This morning when I was going to light the fire, I had gas left in the hopper, so there was a pop and the ash flew out of the fan.
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