Johanssons Jet-Cart

[Johansson]

Update on the gocart engine problem:

I disassembled the engine today to see what the problem was since it was so hard to start the last time I tried, but couldn´t find anything that was broken.

My guess is that the throttle valve is shit, it is a cheap propane needle valve that has become very unreliable lately. I can turn it around 360 degrees without any increase in throttle and then when I turn it 30 more it opens fully. That along with cold weather could have been the problem, the bearings are fine so I won´t blame them.

The flame tube needs some attention though, I can tell by the colour that the flame starts over 10cm below the injector reaching almost down to the turbine scroll. Not good. I will try to close most of the holes above the injector since it is possible that the air entering there pushes the partially liquid propane down the tube and open up the primary holes below the injector instead. I will also close one row of the dillution holes and double the area of the ones left to get deeper penetration *smile* of the air entering there to avoid hot gasses in the centre of the flame tube reaching down to the turbine.

When I am at it I will smoothen the turbine scroll inlet to get a better gas flow without sharp edges that can increase turbulence and back pressure. A better TIT gauge will be mounted in the turbine scroll since the cheap hand held one I used before was very inaccurate.

//Anders

[Johansson]

Yesterday I ordered a new brake cylinder for the cart, hopefully it will arrive some day next week so I can make the mounts on the frame. I have also done some more thinking about the flame tube problem.

The weather when I last ran it and got the bad combustion was pretty cold, my guess is that because I run the engine on liquid propane without any kind of pre-heating it might not vapourise enough in the fuel lines if the outside temp is too cold. I cannot think of anything else since it ran just fine during the summer months, so I will leave the flame tube as it is and make a simple heat-exchanger by turning a length of copper tube around the exhaust pipe so the propane gets fully vapourized before it enters the combustor.

At the end of the summer I will have to put the cart out for sale, if I could I would keep it and rebuild it for liquid fuel but I am about to move in with my girlfriend 200 km away from here and I don´t think that I can find space for something bigger than the pocketbike there... :-(

//Anders

[Johansson]

Update on the jetcart:

A vapour coil is fitted to the jetpipe to preheat the liquid propane and hopefully solve the combustion problem I´ve experienced in cold weather, the cart is assembled again and the only thing left to do is some wiring and to fit the brake.

Since it is much more fun to work on the bike I have not much time left to spend on the cart, but I will try to get it on the road this fall...

//Anders

[Johansson]

...and yes, I will fit a FOD shield to the intake! ;-)

//Anders

[Zippiot]

Pretty :)

Think it'll be faster than the pocketbike?

[Johansson]

Thanks!

I measured a thrust of 16 kg with afterburner on the cart, and the bike engine is theoretically capable of almost 13 kg "dry" thrust. The bike weighs much less and has less rolling resistance so it should outperform the cart pretty easy.

I am getting a bit nervous about driving the cart at speeds of 100km/h though, it is severely under-steered due to the heavy engine at the back so I won´t be able to do any quick manouvers with it...

//Anders

[Zippiot]

Well there always is the ability to add a shell around the cart, amke it more aerodynamic and provide the driver (you) with some protection during an accident...

How back heavy is it, any idea the front/rear distribution?

[Johansson]

Nahh, won´t bother since I will try to sell it as soon as I have driven it...

I had to move the battery from a position beside the seat to the front just to keep the cart from tipping over when I am not on it, the long afterburner pipe is the biggest cause of this. If it wasn´t for the stiff rear axle the cart would be very easy to steer, but now it tries to go straight forward all the time.

I didn´t build the cart to get mileage on it anyway, it was more like a pretty test bench for the engine...

//Anders

[Johansson]

I assembled the cart yesterday and did a couple of runs to test the propane preheat coil and the afterburner, and both works great. No visual flames through the turbine and instant ignition of the AB even thought I removed the flameholder last winter.

The EGT settles at 670C at a P2 of 1 bar and rises to 710C with the afterburner lit, but since my tachometer didn´t work I chose not to push it any further for safety reasons.

The tachometer is a handheld laser type that needs a reflective tape fitted to the compressor, but the tape comes off above 50.000 rpm so I will have to try something else. Perhaps if I paint all blades but one black and the one left white it can work, I tried that with a black marker pen but I guess that I need to use better paint to get a stronger signal...

//Anders

[hagent]

Hi Johanssons,

Can you buff one spot on the compressor wheel to make it shinny?

Or is the compressor material not good for this?

Cheers,

[Johansson]

Hmm, it could work but I don´t think that removing even the slightest bit of material from a blade would be safe to do.

I will buy some of those small buckets of paint used for scale models and try to get a thin layer on each blade on a scrapped comp wheel I have at home and see how it works when spun with an electric drill.

//Anders

[Johansson]

Finally!

Yesterday I got both of my thumbs out of my lame *ss and started to fit the brake to the cart. I finished it today so all that is left to do is to get the air out of the system and hit the road. It WILL be road tested this summer, the only problem is to find a suitable road for the test. I have a long and straight road just outside my garage but I don´t really like the idea of going through the center of the village at >100km/h in traffic...

//Anders

[Johansson]

After some head-scratching I found out that the brake cylinder was broken, no matter how much I tried to get the air out of the system it wouldn´t work. I have also stolen one ignition coil from the cart and fitted it to the bike, so I guess that the test run will have to wait until next summer if I can manage to find a safe place to drive it.

Anyway, the engine works like it should and produces enough thrust to get the cart up to a decent speed so I concider this project to be finished. I find it more fun to build things than too ride them anyway, so if I could find a serious buyer I would sell the cart to free up some garage space. At the moment I have to climb over one project to get to another...

//Anders

[Johansson]

A new idea came to me today, what if I would fit the engine to a stand and try to make a pulse-combustor for it? With the magnitude of know-how on this forum I could get some good suggestions on how to design the valveless engines for the combustor.

I will consider this project seriously during this fall, any comments on it is most welcome!

//Anders

[milisavljevic]

Hello Anders --

I, for one, would be very happy to help you with your proposed project! It would be an interesting
and (ugly word alert) *educational* exercise to develop a new pulsed combustor to power your GT.

It would be even *better* if the end result could still be mounted to your jet-kart. Not impossible...

Please let me know if you are interested.

Best regards,
M.