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Author Topic: Strimmer engine conversion with pics and stuff  (Read 24609 times)

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Offline Will

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Re: Strimmer engine conversion with pics and stuff
« Reply #40 on: August 08, 2005, 21:05:46 PM »
Well this looked like fun, so out came a well used Stihl FS86 (and no, I don't know where it came from or how it got under the work bench...) for some surgery. Carby appears to be second from the left in your line up. So far it's down to 1.6kg with the flywheel/magneto as supplied and the casings/pointless bits of plastic removed. Work would be quicker if I had anything more advanced than basic hand tools..  that said I think i've found a way to skim a suitable quantity off the bottom of the head. I am also slightly tempted to leave the pull start on to save needing a big starter..

Will
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Offline diablo

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Re: Strimmer engine conversion with pics and stuff
« Reply #41 on: August 11, 2005, 22:27:11 PM »
Well I'm back so will get back onto this project over the weekend.  Stihl - provided its been run on good quality oil it will be like new, they are VERY well made engines, much better than the poulan/weedeater one I'm converting to be honest.  I keep looking at my Stihl 450 brushcutter :P but at over £600 new I cannot bring myself to cut it up LOL....yet.....

For starting junk the pull start, its just extra weight to haul about in the sky and weight knackers performance.  Two things I've done.  First is to just wrap some rope around the crank and pull it, like a pull start but kind of detachable!  the second is to use my 12v starter (which wonlt turn over te 25cc engine this post is about) and just give it 24v instead.  Starts just fine now, and the starter does not get hot so long as you do not chew it over and over for 30 secs at a time.  I run my flight box on two 5ah 12v lead acid batteries anyway so I just swap the jump cables around to give it 24v to the panel instead of 12v.

Cheers,
Rich

never in the history of the sport [motorcycle racing] has there been an engine so splendid in its in

Offline Will

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Re: Strimmer engine conversion with pics and stuff
« Reply #42 on: August 12, 2005, 22:43:22 PM »
Well hopefully it'll still be a quality engine once i've finished with it... :-\

I've come to the conclusion the pull start has to go, as it's casing is the start of a fantastic radial mount. Starter is currently a 12v running off a 17Ah gell cell..  so we'll see if it's man enough!

Will
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Offline diablo

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Re: Strimmer engine conversion with pics and stuff
« Reply #43 on: August 12, 2005, 23:02:35 PM »
I could not resist some tinkering so it was back into the workshop tonight.  I thought I would firstly measure the port timings to see what I've got.  Like a 4 stroke you need to carefully set the inlet port and exhuast port opening times to get the best performance.  To compare it to a 4 stroke the port heights are equivalent to the camshaft timing, and the port widths are equivalent to the valve head diameters.  The similaritly is that generally you want the largest width possible, but the inlet and exhuast timings need to be balanced to achieve a good compriise between top and bottom end performance.

To start with you need a DTI and a an adaptor.  THe adaptor is simly a bit of 20mm dar stock with a M14x1.25 thread (undercut to seat it on a flat face) and a hole for the DTI.  It also needs a grub screw hole to nip the DTI in place.  Here is mine:



You simply screw it in the plug hole hand tight, put the DTI in so it goes about half scale at TDC and put your degree wheel on the crank.  I'm using a smallish wheel here as my proper 2ft diameter aluminium one is on another motor right now!



Next fix a bit of bent welding wire to make a pointer, set it about at TDC:

 

You then just set a zero measurement on the DTI at TDC, and rotate the engine 2mm each side of TDC and it should give the same degree measurement each side of TDC.  If not bend the welding wire a bit and keep checking until it does.  This is much more accurate than trying to guess TDC.  2mm each side equates to about 28 degrees on this size engine.


So you now remove the DTi and adaptor from the plug hole, remove the inlet manifold and exhaust and use a torch to shine down the exhuast and plug hole in trun to set the engine where the exhuast and transfer ports open and close.  Take the measurements off the degree wheel where the ports close, double it and this gives the total port open duration which is very useful.  

For this engine the measurements are:

Exhaust 134 deg
transfer ports (lower crankcase to cylinder) 81 deg
inlet port (timed by the piston, from the carb to the lower crankcase) 122 deg

For a guide in general two strokes for about 8-10k revs want to be about 105 to 125 deg for transfer, and 140 to 155 for the exhaust, although you can go to 180 but higher numbers work best on high revving engines and this isnt.  The inlet port 115-140.  Again every engine is different, and every tuner/manufacturer has their own ideas so there is no hard and fast rule.  The only thing for sure is that my engine with 81 deg of inlet is seriously restricted in this department.  I'll probably just do the transfer ports first as I'm sure they will make a huge difference by themselves.

So, I'll do the exhuast at 147, the transfers at 115, and the inlet port 130 degrees.  

I might be tempted to do all the BMEP (Brake mean effective pressure) and port-time-area calcs and really get into this, or I might do the old "suck it and see" method.  Depends if I'm feeling mathy one night.  The BMEP basically calculates the inlet, carankcase, combustion and exhuast flows and the idea is to balance them, the port-time-area uses the port heights, widths and open times at different engine RPM's to optimise them.  It also lets you angle the transfer ports to either oppose or point forward to give the best charge but its a bit tommy technical and for a plane motor I'm not sure if its a step to far to be bothered with.

Anway, just as a last bit I made a glow plug adpator to fit in the std plug hole and take an OS type F plug flush at the base so I can try the engine on glow sometime:

 

 

 


Cheers,
Rich
never in the history of the sport [motorcycle racing] has there been an engine so splendid in its in

Offline diablo

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Re: Strimmer engine conversion with pics and stuff
« Reply #44 on: August 14, 2005, 19:52:24 PM »
Well courtesy of the weekends weather (windy and rain here!) I've spent my time sticking the Pitts special together, and between bouts of epoxy drying I've sat down and done the theory for the engine.  I cannot put all the theory and info down here, it would be a book, but I'll put the main equations and how I've used them for this engine together with some graphs and tables.  There is only a few good books in existance for two stroke tuning, and even fewer websites.  I'll post a link later to the only decent website I've found, and also "borrowed" a some pics from.

Okay, lets get into this.  To recap, the engine I've got is a piston timed, 4 transfer port 1 exhuast port, 1 inlet port engine of 25.4cc.  Currently the measurements are (dims in cm reasons which become clear later):

inlet: 122 deg duration, port is 1.33 wide, 0.66 high.
front transfer ports 81 deg, 0.51c wide, 0.28 high
rear transfer ports 81 deg, 0.66 wide, 0.28 high
exhaust 134deg, 1.68 wide, 0.645 high

Now the measurements were not taken across the bore, they were done in the port normal direction.  These pics show what I mean:




This is the direction the gas flows into and out of the cylinder hence the measurements are done in this way.

Given this you now need to calculate the effective area of each port.  You make some assumptions here, the first being to multiply the hieght by the width and then by 0.9 to account for corner rads etc.  Doing this you get:

inlet: 0.784cm^2
front transfers: 0.257 cm^2
rear transfers: 0.333 cm^2
exhaust: 0.975 cm^2

Now the piston moves up and down so the net effect of this is the top of each port spends more time open than the bottom.  What you need to do is calculate the area of the port exposed at the mid point of its angular duration.  For instance the transfer ports have a duration of 81 degs, or 40.5 each side of BDC.   Half of this is then 20.25 degrees so you would draw it out by hand and work out the area of the port exposed at this half angle.  The port area you get is called the "mean port open area".  IE the average amount of port exposed during the engine cycle.  You can do a reasonably accurate estimation though and in this case it will be enough as I'm not doing a highly strung tune here, more of an optimisation.  From past work I've done comparing actual data you can multiply the actual port areas in the list above by 0.8 and this is a good approximation. 

So we now get the mean port open area:

inlet: 0.627cm^2
front transfers: 0.206 cm^2
rear transfers: 0.267 cm^2
exhaust: 0.780 cm^2

We now need to calculate the amount of time each port spends open per revolution.  This is a function of engine speed and port duration.  The equation is as follows:

T = ( 60 / N ) x ( Z / 360 ) or T = Z / ( N x 6)

where T is the open time, N is the crank speed in rpm, Z is the port open duration.

I'm aiming for optimum at 8000rpm so working this out for each port:

inlet: 0.00254 sec
front transfers: 0.00169 sec
rear transfers: 0.00169 sec
exhaust: 0.00279 sec

We can now calculate the port-time-area, one of the two important factors to determine an engines potential.

The PTA = open time x mean port area / displacement

Doing this gives

inlet: 0.000063 sec-cm^2 / cm^3
transfers: 0.000031 (note - areas for the front and rear transfers are added together to give one value)
exhaust: 0.000086

Nearly there now.  The last set of numbers we need is the port angle area calculation.  This is:

PAA = open angle * mean area / displacement

inlet: 3.01
transfers: 1.50
exhaust: 4.11

This tells all the story of this engine.  The PTA and PAA give the character, performance, balance etc of how the engine will perform.  In effect the inlet value is teh air in, the transfers are the air moving in the engine, and exhuast is obviously the air out.  Common sense says these should be similar values, and as you can see they are WAY out.  As I suspected in my earlier post the transfers are creating a massive restriction.  Here is a graph to show possible values you can use for a two stroke engine, you will notice the values change with the RPM you are aiming for which is why 2 strokes have power bands.



In the 80's a very clever bloke called Jennings did a load of work on 2 strokes and came up with the following guide info:
Transfer port time area boundaries of .00008 to .00010 sec-cm^2/cm^3.
Exhaust port time area boundaries .00014 to .00015 sec-cm^2/cm^3.
Piston port engine the intake time area .00014 to .00016 sec-cm^2/cm^3

To remind we have at the moment:
inlet: 0.000063 (2.5 times too low)
transfers: 0.000031 (5 times too low)
exhaust: 0.000086 (2 times too low)

And from the graph at 8000rpm we should have ideal figures for PTA of:
transfers 4 to 8
exhaust 6 to 9
inlet (from Jennings data) 6.7 to 7.7

To remind we have at the moment:
inlet 3.01 ( over 2 times too low)
transfers 1.5 (4 times too low)
exhaust 4.11 ( 2 times too low)

If you've stayed with me this long you will have seen by calculation this engine is serisouly restricted in all ways, but especially the transfers.  What you do next is put new figures into the equations to give the correct PTA and PAA values, then simply cut the ports to suit these figures.  I'll stop this post here before it runs over limit and start a new post on the new calcs... stay tuned!
never in the history of the sport [motorcycle racing] has there been an engine so splendid in its in

Offline diablo

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Re: Strimmer engine conversion with pics and stuff
« Reply #45 on: August 14, 2005, 20:06:45 PM »
okay, we know what we've got, and what we want, so I set up a spreadsheet and pluged numbers in for port angles and dims until the correct figures came out.  I based these new figures on my guesses in an earlier post just to see how close they were.  Not far off it turned out!

So with the old values:
inlet: 122 deg duration, port is 1.33 wide, 0.66 high.
front transfer ports 81 deg, 0.51c wide, 0.28 high
rear transfer ports 81 deg, 0.66 wide, 0.28 high
exhaust 134deg, 1.68 wide, 0.645 high

change these to:
inlet: 135 duration (I'd guessed 130), 2.0 wide, 0.9 high
front transfers: 120 deg (I'd guessed 115), 0.6 wide, 0.6 high
rear transfers: 120 deg (I'd guessed 115), 0.8 wide, 0.6 high
exhaust: 150 deg (I'd guessed 147), 1.95 wide, 0.88 high

The end result of these new figures is

PTA:
inlet: 0.000143 (was 0.000063)
transfers: 0.000119 (was 0.000031)
exhuast: 0.000152 (was 0.000086)

PAA:
inlet: 6.88 (was 3.01)
transfers: 5.71 (was 1.50)
exhuast: 7.29 (was 4.11)

So its out with the tools and report the engine to the new figures and see if I produces a nicely tuned, flatish torque curve perfect plane engine, or a highly strung 20k rpm , 500rpm wide powerband screamer.  If you compare the figures above with Jennings' chart and the graph its a middle of the road spec so it SHOULD (putting my money where my mouth is here) produce a very nice motor.   Also my guesses work out from the theory numbers so I'm pretty confident.  Heres hoping!

Just a note, is anyone reading this and finding it useful......?  I'f so I'll carry on and maybe add more detail, if not I'll get me coat.....

Cheers,
Rich





never in the history of the sport [motorcycle racing] has there been an engine so splendid in its in

Offline Evil Homer

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Re: Strimmer engine conversion with pics and stuff
« Reply #46 on: August 14, 2005, 21:48:20 PM »
Hell yeah!, carry on think i need to find a skip this is getting very interesting  :af
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Offline Dragon Wings

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Re: Strimmer engine conversion with pics and stuff
« Reply #47 on: August 14, 2005, 22:41:31 PM »
Yup, me too :af

How much would you charge to fettle another (Ryobi 31cc) strimmer motor Rich? :ev

Offline diablo

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Re: Strimmer engine conversion with pics and stuff
« Reply #48 on: August 15, 2005, 07:34:17 AM »
hehe Dragon, you want to wait and see how this turns out before asking stuff like that ;D ;D  It wouldn't be the first engine I've turned into a paperweight by playing with it too much :D

I'll hopefully get it ported this week and have it running again next weekend, then only the exhaust left :)
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Offline diablo

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Re: Strimmer engine conversion with pics and stuff
« Reply #49 on: August 15, 2005, 21:35:08 PM »
tonight I marked out the bore ready to attack it tomorrow.  Given the port timings and widths I've decided on you put the barrel on the engine with the piston ring removed.  Then turn the engine to the specified degrees of each port and run a sharp scribe to scratch the bore and using the top of the piston as a guide.  This gives the top of the new port.  Then scribe the sides to the correct width and you get it to look like this exhaust port (hope you can see the scribe mark above and to both sides of the port).  Note I'm altering the shape from essentially elliptical to square so the port closes in a controlled and measurable fashion instead of a rounded top.  It will have a VERy slight curve to give better ring wear but no where near the std port.



Hopefully tomorrrow night will be an "after" pic :)

Cheers,
Rich
never in the history of the sport [motorcycle racing] has there been an engine so splendid in its in

Offline Maxpoly

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Re: Strimmer engine conversion with pics and stuff
« Reply #50 on: August 16, 2005, 20:23:36 PM »
Good luck with the cutting  :af Looking on with interest  :)
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Offline diablo

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Re: Strimmer engine conversion with pics and stuff
« Reply #51 on: August 16, 2005, 21:25:40 PM »
Well I've uncovered myself from a pile of aluminium dust armed with a lighter engine than before ;D

See the pic above of the exhaust port, well here it is now (the port at the top):



and a couple of other views:





I did have problems with the inlet port though, I could not get it wide enough to match my calcs without risking the port overlapping the ring end with obvious consequences.  To get round this I made it as wide as I dare, then headed back to the excel spreadsheet to recalc using the max width and adjusting the port timing and height to get the required numbers.  I did this but then the piston skirt (which controls the open and closing of the port) was still in the way.  Only one thing to do, cut the piston skirt with a recess to allow the port to stay open longer.  The piston was a complete circle at the base, but now it looks like this:



End result is correct PTA and PAA values.

So, all cleaned up and assembled then I though there is no way I can run this with the slightly modified std exhaust as it would just kill any gains.  The std exhuast is 3 plates and has a kind of double back design, so I just took off the outer cover using large tools and violence to free it up.  Once I've test run the engine I'll be designing and making a tuned pipe so this will literally only get run once with this exhuast:



Fingers crossed for decent weather tomorrow evening and I'll fire her up :)  It feels nice and has a good *pop* sound when turning over by hand which is a good sign given it was more like a wet fart before I attacked it.

Cheers,
Rich
never in the history of the sport [motorcycle racing] has there been an engine so splendid in its in

Offline diablo

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Re: Strimmer engine conversion with pics and stuff
« Reply #52 on: August 17, 2005, 12:55:09 PM »
5pm tonight is set for the first test of the new porting, and provided all goes well it only leaves the exhuast to sort out.

Two stoke exhuasts make or break an engine and you can totally transform its character using only the exhuast.  You can turn a dead duck into a nice revving clean carbutating motor, or equally kill a really good engine.  Basically a two stroke will run on a straight bit of tube but where the gases exit the tube it creates a backpressure wave.  This wave goes back down the pipe and can either have the effect shoving unburnt charge back into the cylinder to help, or if it arrives at the wrong time it sucks the charge out!  Early racing two strokes used straight pipes and you simply cut the pipe to length to suit whatever character you wanted.  You can slash cut the end of the pipe to reduce the pressure wave, but its still there.  Then someone put a megaphone end on the pipe and this amde them better, until a German bloke working for MZ added a converging cone after the megaphone, creating what we know now as the expansion chamber.  The rest as they say is history.

To understand pressure waves and stuff more have a look at the animated files here:

http://www.stanford.edu/group/evpilot/Reasearch/Grouppages/01group/jon/dirt%20bike/tips/twostroke.htm

So onto the design for my engine.  Using some flows and formulas you can calc a good starting point for the design.

1. header.  Should be between the same area and half as big again as the port in the engine.  For me this gives dia 15mm.  Length should be from 5 times to 12 times the dia, the longer the pipe the wider but gentler the power band.  So 150mm for me.

2, divergent cone.  5-15 deg with the steeper angle giving narrower sharper power band.  Exit area about 6 times the inlet area.  I chose 8 degrees so this gives inlet dia 15mm, exit 37.5mm, length 80mm at the 8 deg.

3. parallel section.  longer gives wider smoother power.  I chose 50mm.

4. convergent cone.  usually about double the angle of the divergent cone, steeper angles give sharper power band.  Inlet dia is the parallel section, so 37.5mm, angle is 18 deg, and end dia is as follows.  Length 44mm in this case.

5. Exit.  This is the pipe that ends the system.  Should be 60% the dia of the inlet to the system, so I want dia 9mm and the length should be anywhere from 2 to 12 times the dia.  Generally start off long and cut it down to fine tune the pipe.

Obviously none of the above silence anything.  To silence it you either put a perforated cone (large dia to the engine, small to the pipe exit) in the belly section, and/or double skin the exit pipe which you make form perforated pipe and jam pack the double skin with wadding.

The main snag for the above design is it a optimiesed one, and for this it ends up 432mm long which is a lot of pipe to hide in a cowl.  I will get the motor mounted in the plane and see what I can do but it may end up with a couple of double back sections and maybe scale it a bit to shorten it for practical reasons.  Best I go find some thin sheet and get fabricating.

Cheers,
Rich
never in the history of the sport [motorcycle racing] has there been an engine so splendid in its in

Offline diablo

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Re: Strimmer engine conversion with pics and stuff
« Reply #53 on: August 17, 2005, 17:54:05 PM »
If you look back at page 2 of this thread when I first ran the engine it had pretty poor throttle response and managed to drag itself to 6750rpm.  After the mods detailed (and with the hacked up std exhaust) I just had it running and it gave.............

8500 :)

It pulls from 1900 tickover to full revs sweetly and you can bang the throttle flat out at any revs and it picks up very cleanly, something that is vital for a plane and something the engine would not do as std.  I've very pleased and its only had 2 mins running, it will get maybe another 1-200 rpm after half an hour to bed it all in a bit better.  And then there is the exhuast, so being an optimist it might touch 9000rpm once its all finally finished.  I'll try a glow plug in it for the next set of running too to see if I can hand start it, and by all acccounts glow will give it another 100 or so revs too.

Still more to come out of this old girl :af

Cheers,
Rich
never in the history of the sport [motorcycle racing] has there been an engine so splendid in its in

Offline Evil Homer

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Re: Strimmer engine conversion with pics and stuff
« Reply #54 on: August 17, 2005, 21:54:37 PM »
Well done, so when are you going to go into business
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Offline diablo

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Re: Strimmer engine conversion with pics and stuff
« Reply #55 on: August 21, 2005, 20:41:38 PM »
well it flew today, give me half an hour to write a review and it will appear under "Cermark Pitts review" in general chat!

Cheers,
Rich
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Re: Strimmer engine conversion with pics and stuff
« Reply #56 on: August 22, 2005, 12:53:59 PM »
I'll see if I get time to put up a few pics of the engine in the plane with no cowl on, and how I bodged together an exhuast to get it flying this weekend.  Next job is a decent one and I'll start it this week, but exhausts take a lot of messing about to get the bends, pipes etc all in the right place to hide it in the cowl.  I'll keep updating though.

One other trick I tried this weekend was to put in my glow adaptor and try and run it on the glow plug.  Well it sluttered and coughed a few times but would not keep itself going so I just stuck the spark plug back in and went flying.  I'll play about with the glow thing a bit more some other time, its bottom of the list right nowl  Also I'm trying to get hold of another engine the same so I can do a serious rework on it.  I don't want to push this engine as its the only one I've got and its good!

Cheers,
Rich
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Re: Strimmer engine conversion with pics and stuff
« Reply #57 on: August 23, 2005, 14:57:30 PM »
http://www.freedom-motors.com/

If you read through the site , they are going to be producing a 27cc Wankel engine for leaf blowers etc .
I wants me one of them !

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Re: Strimmer engine conversion with pics and stuff
« Reply #58 on: August 23, 2005, 21:31:26 PM »
Wankels, I'm sure a company already does them for models....

I took some pics tonight of the engine installation:

Carb sside showing bellmouth and throttle linkage.  The bellcrank was made from a servo horn running on a plain bearing and a thru bolt onto the casting lug on the engine.  Yellow pipes are fuel, the clear one is from the carb diaphragm and piped into the fuz to stop speed affecting mixture.



A view of the underside.  The black pipe is a quickly made exhuast to get it flying, it took 300 revs off but I'm in the process of making a new pipe for better performance.



the exhaust side



and some other views






Cheers,
Rich
never in the history of the sport [motorcycle racing] has there been an engine so splendid in its in

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Re: Strimmer engine conversion with pics and stuff
« Reply #59 on: August 24, 2005, 07:34:11 AM »
Quote
Wankels, I'm sure a company already does them for models....


I have the little OS Wankel ( they used to make a 30cc version ) but you might be thinking of the Erickson "Migrating Combustion Chamber" .

http://www.ericksonmotors.com/

Looks good but expensive . Certainly not as appealing as your reworked strimmer !

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Re: Strimmer engine conversion with pics and stuff
« Reply #60 on: August 24, 2005, 21:27:29 PM »
New exhuast time tonight so I started tinkering.  First is a Pitts style made to fit the plane and have the twin exit pipes underneath.   I've also started a tuned expansion chamber version but I'm reluctant to cut the cowl up to fit it so it might only see test work :)

First a pile of bits of pipe:



Now mount the flange plate (what a cool word, the best in Engineering) onto the engine:



Now I tacked the two exit pipes to the collector chamber:



and tacked the header pipe to the flange:



But its getting late for cutting and welding so I just left it at that for tonight, but here it is lead in roughly the right place so you can see how it will end up across the back of the head:




I'm going to make a removable end plate in the chamber so I can add a perforated baffle if required, it depends how noisy it is with an empty can.  Noisy I suspect :ev

Cheers,
Rich
never in the history of the sport [motorcycle racing] has there been an engine so splendid in its in

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Re: Strimmer engine conversion with pics and stuff
« Reply #61 on: August 24, 2005, 21:56:47 PM »
Great stuff Rich  :af

I had a Moki 135 once that had a similar arrangement for the silencer with the two exit pipes. I was told that by bunging one of them up I'd get more power, which turned out to be true to the tune of about 400rpm  :o Would the same principle apply to your engine being petrol?
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Re: Strimmer engine conversion with pics and stuff
« Reply #62 on: August 24, 2005, 23:15:05 PM »
Quite possibly Tac, but by getting more revs you got more top end power, but I would bet it took power off the bottom end.  Thats the thing with 2 stroke exhuasts, there is no free money/power with them.  I'll try it with this once its done but of course I would guess blocking the pipe nearest the cyl head will give more revs, but blocking the pipe further away will give more low end torque due to the standing waves set up and the different effective pipe lengths.  You might see I've cut the pipe exits at an angle to reduce the wave created, it all goes to smoothing the power out buy may not nessisarily give the highest revs possible.

Anyway, time will tell, I'll do the pipe block test and post up the results :af

Cheers,
Rich
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Re: Strimmer engine conversion with pics and stuff
« Reply #63 on: August 25, 2005, 08:05:17 AM »
Rich, I had no idea you would get different results by blocking different pipes and having the pipes cut at an angle  ??? I must admit i didn't notice the bottom end change but thats probably due to the Moki carb being pretty good? Not sure!
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Re: Strimmer engine conversion with pics and stuff
« Reply #64 on: August 25, 2005, 08:39:49 AM »
What you need is a servo contolled bung mixed in with throttle , the best of both worlds !

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Re: Strimmer engine conversion with pics and stuff
« Reply #65 on: August 25, 2005, 08:44:31 AM »
I seem to recall that some RC hydroplane racers use a servo to lengthen/shorten the pipe for much the same reason.

Dave S

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Re: Strimmer engine conversion with pics and stuff
« Reply #66 on: August 25, 2005, 09:24:07 AM »
A few high performance production cars (like 911's) use variable length inlet trumpets for the same reasons as its easier to alter a cold bit of metal than a hot one :o  Doing it on the throttle does not work too well as you can have the engine flat out at low revs, you need either a servo slow so it has a 1 or 2 sec delay to allow the motor to spool up, or a rev linked servo.

Cheers
Rich

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Re: Strimmer engine conversion with pics and stuff
« Reply #67 on: August 25, 2005, 09:40:10 AM »
My MX-6 uses what they call a VRIS (variable resonance induction system) in which the length of the inlet manifold is varied by actuators depending on speed and load of the engine. This gives a great flat torque curve with tons of bottom end torque spreading right up through the rev range.

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Re: Strimmer engine conversion with pics and stuff
« Reply #68 on: August 25, 2005, 21:51:27 PM »
More exhuast work.  First I brazed the bits I tacked up yesterday.  Why braze...?  Well Mig on thin wall pipe makes it brittle on the edge of the welds and it will crack eventually.  Brazing is a much more gentle heat so anneals rather than hardens the metal, and the braze itself allows a certain amount of flex to prevent fatigue cracks. 

 

So now I put the bits on the plane and used bits of wood to put it all in the right place

 

So then tacked it and removed the wood

 

As there is quite a bit of pipe hung off the side of the engine I also added a small tab to support the far end of the silencer which bolts to a hole in the fin I drilled in the head

 

So the silencer ready for final brazing and end caps tomorrow

 

I also did a quick fit check to make sure it all fits and exits the cowl where I wanted it to, and all seems well:



Cheers,
Rich
never in the history of the sport [motorcycle racing] has there been an engine so splendid in its in

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Re: Strimmer engine conversion with pics and stuff
« Reply #69 on: August 25, 2005, 22:56:33 PM »
Hi Rich,
I did read your post earlier about the exhaust system but I think it went a little over my head ::)... anyway, I was just wondering how the system you're showing here was designed? I don't see any cones... just cylinder volumes and lengths ???. S'cuse my ignorance btw.
Cheers!
Steve

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Re: Strimmer engine conversion with pics and stuff
« Reply #70 on: August 25, 2005, 23:38:21 PM »
Please Sir, i also have a question  ;D

I saw a video recently of a petrol powered Extra and it had 2 exhaust outlets much like yours sticking downwards out of the cowl. But they were different lengths so what would that do?
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Re: Strimmer engine conversion with pics and stuff
« Reply #71 on: August 26, 2005, 07:36:07 AM »
Dragon Wings, this system was not so cuch designed as "made to fit inside the cowl" in a traditional Pitts style that a lot of two stroke planes use.  its a long way off the ideal that a I calculated a few posts up but has the same bits, just in different places!  The header pipe is as it should be, just a bit too short, the 40mm dia pipe across the back of the head creates the expansion chamber, and the exit pipes are as they should be, but the dia is probably too large.  The convergent cone will be created by changing the inlet end of the 40mm pipe, it will not be cut flat, it will have a shape to it for reflecting waves which you'll see tonight once I've amde it.  The divergent cone will be made using a conical perforated baffle inside the 40mm pipe, again pics to follow.  This is not an idea layout, pipe, or design, but I hope it will be "pretty good" but I hope to test it this afternoon and find out!

Tac, The general reason for cutting two exit pipes at different lengths is to give two different standing wave tunes lengths, so spreading the power out!  I'm hoping the 80mm seperation of the two exits on the 40mm pipe will do this for me, if not I can easily just cut one shorter as I left them a bit too long on purpose.  Again, I might have guessed right and not need to ;D

Cheers,
Rich
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Re: Strimmer engine conversion with pics and stuff
« Reply #72 on: August 26, 2005, 18:10:21 PM »
exhuast finished and tested!

First the brazed up pipe was checked to see it fitted





You'll notice the end is open, this is for a bolt on cover so I can put a baffle in or out to see the difference, bafle and end cap here:



The baffle is conical to get the alrgest possible surface area, this in effect creates a bit of sound deadening, and acts as a convergent cone to help the standing waves form, the angle is very gentle to spread the power without making it too peaky.

All fitted together it looks like this:



I then painted it with some black exhaust spray and here is how it looks, and with the cowl on too:





So out to the test garden.  First off I had been using my 12v RC Power cheapo £15 starter on 24v which worked well but finally it died.  So it was time to either invest £70odd ona  dynatron, which are okay, or dig through my garage and build a "proper" starter.  A nice 24v 2.5kw 100+amp motor should do the job so I made an adaptor for the front, robbed the switch from the old starter and came up with this monster:



It spins the engine like it was not there even on 12v :ev  On 24v it would start the space shuttle ;D ;D

So time to annoy the neighbours again.  First I tried it with no baffle.  Remeber it did 8500 on the opened up std and 8200 with the std ex with long exit pipe.  It ran at 8400 with no baffle, 200rpm gain, and only just a tad noisier, surpirising really given its an open pipe.  Now I fitted the baffle and it ran 8500 so my only conclusion is that the baffle is indeed helping the waves and stuff.  It also throttled better with the baffle in.  As a last bit remember I have the carb bellmouth pointed straight out fo the side.  I found by holding my hand in a cup shape behind hte bellmouth gave another 200rpm, it was singing at 8700 and sounding good. 

I also tried blocking each exit pipe in turn using my calibrated finger and found it robbed 400rpm off.  Two pipes it is then.

So next job is a 90 degree turn for the bellmouth.  I just wish I had access to a TIG so I could make the pipe in ally, now that would be tasty.

Thats all for now, its all back togeher hoping for good weather and lots of flying for the weekend!

Cheers,
Rich
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Re: Strimmer engine conversion with pics and stuff
« Reply #73 on: August 26, 2005, 20:47:45 PM »
Quote
It spins the engine like it was not there even on 12v :ev On 24v it would start the space shuttle  ;D ;D

ROFL  ;D Wish I was as enterprising as you Rich  :af
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Re: Strimmer engine conversion with pics and stuff
« Reply #74 on: August 26, 2005, 21:58:19 PM »
Quote
ROFL   Wish I was as enterprising as you Rich 

its not enterprising, its Yorkshire tendancies to make rather than buy, and access to a large scrap heap you need ;D ;D ;D
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Re: Strimmer engine conversion with pics and stuff
« Reply #75 on: August 27, 2005, 17:57:45 PM »
Went out with it flying today, but it dead sticked second flight of the day and flatly refused to restart.  I checked all the fuel system and that was fine but could not find a spark.  I could not see anything obviously wrong so packed up and came home.  When I took the cowl off I noticed the coil was moving a lot and there was tell tale marks round the flywheel where it had been rubbing the coil.  On further inspection the coil mounting part of the casting had all but broken off the crankcase :'( :'(

Now with hindsight its probably my fault.  The day I finished assembling the plane I dropped it dead square upside down on a concrete floor from 3 feet up.  The cowl bent down and hit the flywheel leaving a flywheel sized hole in the top of the cowl.  I thought I got away with it buy when I turned the flywheel to the area that hit the ground typically the opposite side had a mark where it had in turn hit the coil, and presumably this cracked the crankcase and 7 flights later the fatigue broke it off, loosing the spark and hence the dead stick.

With 2 metal brackets to go across the crack and provide strong support I think it will live again, but I'll replace the crankcase as soon as I get another one.  Bugger, I'm due to be packing it next Friday for a 10 day flying holiday in France and today was the last check flights.  Mind you, at least I've got a week to fix it rather than it fail on the 2nd flgiht of a holiday!

Exhuast worked and sounded good though but I'm worried its a bit noisy even with the baffle in!


Ordered an APC 16x8 prop today though so I can benchmark it properly as everyone seems to use APC's.
Cheers,
Rich
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Re: Strimmer engine conversion with pics and stuff
« Reply #76 on: August 28, 2005, 06:52:54 AM »
I'm wondering if this APC superiority is really true ?  I was using APC 17 x 8N props on my ASP 180 FS, I 've just replaced it with a Just Engines wooden prop and got a lot more thrust and better throttle response. And the wooden ones are £10 cheaper !
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Re: Strimmer engine conversion with pics and stuff
« Reply #77 on: August 28, 2005, 06:58:57 AM »
He didn't say they were superior, only that they represented a standard. I think one of the reasons they are so widely used is that they are available in such a range of sizes, many other makes offer less choice of diameter/pitch combinations.

Dave

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Re: Strimmer engine conversion with pics and stuff
« Reply #78 on: August 28, 2005, 08:27:20 AM »
Well ragarding APC, I've always found them okay with a wide range, and it seems just about everyone uses APC's as benchmark props hence I've ordered one for me, I've heard stories of engines changing 500rpm on the same dia/pitch props just from different manufacturers.  The 16x8 I've been using was on an engine I bought ages ago and all it had was "Super" written in a diamond, "made in Germany" and the size, maybe a rebadged graupner or something but I've no idea if its a super hot one, or the worst ever made.  At least with APC you know they are pretty good.  I don't use wood props as it has been known for me to do the odd nose over :-[ and APC's survive where as a wood one would be in the bin. 

Anyway, I'll let you know when it arrives!

PS.. how do you measure thrust, do you tie the plane to something and use a fishing scales type thing?  Just curious.

Cheers,
Rich
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Re: Strimmer engine conversion with pics and stuff
« Reply #79 on: August 28, 2005, 19:35:15 PM »
I always regarded APC as the best, I was just surprised when I found I had an extra 1/2 lb of thrust when going to what I regarded as an inferior prop.  I use a digital tensile gauge 'borrowed' from work.

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