Johansson´s Pocketbike

[Johansson]

A bit more work is done to the engine, the shaft tunnel is turned down to aproximately the right size and the plate mounting jig is finished. Next up is welding the flanges, but that will have to wait to next weekend. I can start imagine how it will turn out now, fun fun fun... ;-)

//Anders

[Johansson]

I spent a couple of hours today in the workshop and got some work done on the compressor plate, only thing left is to turn the bearing seat and take a couple of 0.1:s of the total thickness.

//Anders

[racketmotorman]

Hi Anders

Are you going to stress relieve the welded parts before/after/during machining ??

I found with my bikes freepower tunnel assembly that I had to silversolder the joints to get a perfectly oiltight seal despite all the flanges and tubes being press fits with only tack welds for security

The bits are looking good :-))

Cheers
John

[Johansson]

Hi John!

Never thought of that, but I don´t really think it would be necessary. At least not on the compressor side. Since the material is so thick and the flanges are welded 10cm at the time on both sides the internal stress should be moderate.

I guess that I will find out if the seals leak the hard way, if that is the case I can try a thin layer of high-temp liquid gasket. My main concern right now is to get her ready for the first startup, leaks and small fires will be fixed later... *LOL*

Thanks!

//Anders

[Fricke]

Looking good...

Did You heat the assembly after You welded it? If not then You´ll have some problems with it warping when it gets hot... It shouldn´t be that big of a problem with the compressor side... But do it to the turbine section...


/F

[Johansson]

Will do, I know a place where I can borrow a gas welding rig to heat the piece after I welded it.

The turbine plate will be done last since I then can make adjustments to the drawings if the assembled turbo mesurements differ from what they were intended to be. Of course they won´t, but just in case... ;-)

//Anders

[Fricke]

It´s better to use a propane torch instead of an oxy/ace torch.

You can put the flange/assembly in your oven at max temp for several hours before, and the add some extra heat to it... Ideal is just above the max working temp of the engine...

[Johansson]

We have recently installed a coal-burning forge in the workshop in which I can heat the piece, then a propane torch can be used to take it to the desired temperature. 800C should be enough.

What kind of stainless is best for the flame tube and vapour tubes by the way?

I intend to build the flametube similar to Johns gocart FT with a thicker removable lid to which the vapour tubes will be welded. It will have the shape of a shallow cone with a flat top, the hole for the idle spray nozzle will be in the center of the flat top and the surrounding tubes will be welded at the edge of that plate with the hook exit pointing upwards at the cone, so the fuel spray from the idle nozzle would hit and heat the entire elbow. I tried to draw it in Paint but it just kept looking like crap no matter how hard I tried...

//Anders

[Fricke]

Heat resistant stainless... I don´t remember the name for it offhand... But any stainless goes... The duration of running of the engine is not that big so it will hold for many years... =)

*Added list of grades*

• Inconel 600
  Inconel 601
  Incoloy 800/800 HT
  Nimonic 75
  2337
  2361
  2368
  Titan grade 7 & grade 11

[Johansson]

Ok, there is a lot of sheet SS laying around in the workshop and no way of telling what kind of SS it is, but if any kind would do I can save a couple of bucks by using it for the combustor. If I would order SS from my local metal shop I would have to pay for an entire sheet, and that would be expensive since I only need a very small piece of it.

I am thinking of building D-sticks for the combustor, the added turbulence should give a better mixing of fuel and air inside the vapour tube but would also restrict the air flow through it and therefore flow less primary air through the tubes.

//Anders

[Fricke]

Use ordinary tubes... No need for D-tubes...

Are You going to use Nozzles to inject fuel to the tubes?

[Johansson]

Not sure yet, my idea right now is to make 5 separate syringe needles pointing into the tubes at an angle. They will be connected to eachother on top of the combustor plate by banjo fittings. I guess that they will act similar to a spray nozzle because of the size of the hole in it and the fuel pressure, but I haven´t really thought of that yet. I am saving that to when I start testing the combustor...

//Anders

[Fricke]

Do You have Kamps book? There is some equations for resizing the vapo. tubes and number of them based on different compressor sizes...

One big problem with that design of fuelmanifold is that it´s really tricky to get the exact same flow thru each syringe needle...

[Johansson]

Yup, I know that uneven fuel distribution can be a problem. But if I make two fuel inlets on opposite sides of the ring of spray needles and keep the hole area small so the fuel pressure can be as high as possible the fuel distribution should be roughly the same to all injectors. It will also help that I will use a standing combustor, if it was tilted gravity would mess things up...

I do have the book and remember to have seen a page about resizing different parts of the engine, I will use those calculations as a rough guideline when I start designing the combustor. Since it will be made so different parts of it can be exchanged there is room for improvements without having to rebuild the entire combustor, the flame tube for example will be separated from the cone part connecting to the turbine housing so it will be easy to try a different hole pattern if that would be necessary. The lid holding the vapour tubes will also be removeable.

In order to keep the assembled flame tube from moving around in the combustor I intend to put a push-spring around the idle nozzle holder to keep a gentle pressure on the flame tube, not too much but enough to keep it from jumping around. I know that the flame tube walls will probably be red hot while running so too much pressure could make the tube collaps...

I am still not sure of what to use for the throttle valve, it cannot be made by simply adding a bypass line with a needle valve to the fuel line since it has to be completely shut when it is not activated. I would really prefer to buy one instead of making one myself since it would steal much time which I don´t have right now, at most I can spend 20-30 hours per month if I cut down on sleeping so I don´t want to spend any precious time on making something that can just be bought (and would probably work better).

//Anders

[larry cottrill]

Yup, I know that uneven fuel distribution can be a problem. But if I make two fuel inlets on opposite sides of the ring of spray needles and keep the hole area small so the fuel pressure can be as high as possible the fuel distribution should be roughly the same to all injectors. It will also help that I will use a standing combustor, if it was tilted gravity would mess things up...

Johansson -

You are over-rating the power of gravity. In the case of a pressurized fuel ring, it will be essentially meaningless. If your fuel were the density of water, a 1-foot diameter (305 mm !!!) vertically-oriented ring would have less than 0.5 PSI more pressure at the bottom than at the top due to the weight of fuel in the ring. Sorry, I don't know the metric equivalent for the pressure - but one foot depth of water is 1/33 of an atmosphere.

L Cottrill