Johansson´s Pocketbike

Johansson · Post by **Johansson** » Wed Jan 17, 2007 7:25 pm

Thanks for the feedback. I know that this way of testing the valve is a bit crude, but it is just done to get the throttle response roughly sorted out to avoid a "jumpy" throttle range while driving it.

So it is possible to make the needle flat at the tip you say? I will have to get more brass so I can make a couple of needles and test different designs once I get back home from work...

//Anders

Ash Powers · Post by **Ash Powers** » Wed Jan 17, 2007 11:32 pm

Johansson,

Keep in mind that in order to double the flow of fuel at the nozzle, you have to quadrouple the pressure (it is a square function). 3X the flow requires 9X the pressure, etc etc. Although your valve may seem to produce unusable results in the sense of the highly exponential pressure characteristic, it really may not be that far off at all when it comes to pushing fluid through your nozzle in a linear fashion to throttle position.

It would be best to perform your test at given lever positions for a set duration of time for each test when flowing a liquid through a nozzle (preferably not something flamable for testing purposes) and measuring the flowrate. You may very well find that your flowrate is pretty linear to lever position given the pressure response curve of your graph.

Modifying the valve to produce a linear pressure rise to throttle position will make your engine's throttle curve very sensitive at low throttle and laggy for the rest of the throw - you need an exponential pressure curve to have a linear fuel delivery rate.

racketmotorman · Post by **racketmotorman** » Thu Jan 18, 2007 4:09 am

Hi Anders
LOL...don't be too fussy about getting a nice linear throttle .
On the bike I went to great lengths to build a "perfect" throttle .
On the kart I was lazy , so it was very basic .
Both proved perfectly usable throttles because we tend to be either idling or giving it full bore .
Unlike a piston engine where we need a "good throttle" otherwise the vehicle is hard to control , turbine vehicles need relatively huge amounts of throttle to get going , so theres not the need for those "fine" adjustments like we have on an IC engine .
As long as you can get a decent idle , then a reasonable throttle travel before full bore is reached , she'll be right :-))

Cheers
John

Johansson · Post by **Johansson** » Thu Jan 18, 2007 5:10 pm

leo and Ash: You guys have convinced me, I will do a flow rate test tomorrow to find out how it behaves before anything else. Thanks for saving me much trouble while testing!

Racket: Point taken. I guess that I have a hard time to get my head around the fact that a turbine throttles very different from a piston engine, since the power output shaft wonÂ´t be mechanically coupled to the "drive shaft" any sudden throttle movements wonÂ´t make the bike jump forward like it would with a piston engine...

//Anders

Johansson · Post by **Johansson** » Fri Jan 19, 2007 1:59 pm

Here is the flow test chart along with some pics of the needle modification and the test rig. I had to make another tip since the old one produced a bad throttle range, did a quick test yesterday when I got home from work.

The sudden drop in fuel flow below 2.5 cm is caused by a non-return valve sitting before the nozzle that closes the fuel line if the pressure gets too low. I think that the curve is acceptable as it is now, it ainÂ´t exactly straight but straight enough to work as a throttle...

//Anders

Johansson · Post by **Johansson** » Fri Jan 19, 2007 3:45 pm

The flat flow section on the chart is caused by the spray nozzle, it is quite strange actually:

At 2.5 cm a small jet of fuel is "peeing" out of the nozzle, but when the throttle is set to 3 cm the jet disappears and the fuel starts to drip from the nozzle. The dripping continues up to 3 cm where it suddenly stops and a nice spray cone developes.

This probably happens because the nozzle was never intended to be used at such low fuel pressures, but I cannot figure out why it is dripping instead of peeing at those low pressures. I tried both a new 2GPH nozzle and an old 0.5GPH nozzle with similar results...

//Anders

Ash Powers · Post by **Ash Powers** » Fri Jan 19, 2007 4:28 pm

Vaporizers!, FTW!

Johansson · Post by **Johansson** » Fri Jan 19, 2007 9:41 pm

Johansson · Post by **Johansson** » Sat Jan 20, 2007 1:31 pm

The last two days I have been working on a new oil tank, the old plastic one can only handle idle P2 pressures so a better one is needed once the pressure starts to rise. I have also mounted the throttle valve to the frame and made some T-connections for the fuel hoses.

//Anders

Ash Powers · Post by **Ash Powers** » Sat Jan 20, 2007 7:38 pm

Are you using vaporizer tubes in your combustor?

With vaporizers, a "dribbling" nozzle wouldn't dump liquid fuel to collect and pool in the CC. It will also improve combustion.

"FTW": F*$K The World.

Johansson · Post by **Johansson** » Sat Jan 20, 2007 9:32 pm

Aha, I was a bit confused by your comment you see...

I use four J-type vapourizers in the flame tube, on page 10 there are some pics of the combustor where they can be seen. The dribbling wonÂ´t cause any problems with combustion, but the throttle response in the "dribbling zone" is much flatter than with a similar increase in throttle stroke at higher pressures when a spray cone has developed. Not much of an issue, just an observation...

It will be interesting to see how well the main fuel system works since I wasnÂ´t able to test it with the original throttle valve that failed, I will have to be careful though not to overspeed the steel bearings I use at the moment.

//Anders

Johansson · Post by **Johansson** » Fri Feb 02, 2007 5:08 pm

I added the long version of the last run on Youtube in case anyone is interested. At the end of the clip the battery runs out so the revs drops until I shut her down...

http://www.youtube.com/watch?v=gbJvrC9Gejo

I havenÂ´t had time to finish the throttle yet since the turbo-bike has my attention at the moment...

//Anders

Ash Powers · Post by **Ash Powers** » Fri Feb 02, 2007 5:16 pm

Man, the startup of your pocketbike's turbine is one of the most impressive I've seen. "pushbutton - flip switch - THUD - screeeeeaaaaaaaaaaaammmmmmmmmmm!" Hella nice! =)

Johansson · Post by **Johansson** » Wed Feb 21, 2007 8:27 pm

Thanks!

IÂ´ve bought the throttle wire now, so this weekend I will try to finish the throttle and make the final touches to the bike before the next test run. I am very exited to see how the vapourisation will work!

//Anders

Johansson · Post by **Johansson** » Thu Feb 22, 2007 7:12 pm

I just noticed that some site had linked the youtube video so I checked it out, it was a thread in a pocketbike-forum with a seven pages long discussion about the bike...

Enjoy! ;-)

http://www.pocketbikeplanet.com/129-4st ... video.html

//Anders