earlwb

I don't think it really applies as much with model engines. Usually a short stroke engine has less piston speeds or acceleration inertial stresses than what you would see with a long stroke engine. But model engines being small already don't quite have the stresses one sees in large full size engines.

The vintage engines all tended to be long stroke designs many years ago. Not too many were short stroke engines back then. But as time passed more and more engines came out with shorter strokes to allow for more RPMs. so at the time the accepted practice was to have a long stroke engine and some engine designs had really long strokes too.

Now a long stroke engine can still out perform a short stroke engine. The intake ports, bypass ports, and exhaust ports typically have a longer duration in how long they are open as compared to a short stroke engine. Thus they can breathe a little better and develop more power. You see this with the classic .60 or 10cc pattern plane engines. The pattern plane long stroke engine usually out performed its short stroke bretheren. also a long stroke engine tends to develop more torque than the short stroke engine does.

The short stroke engines of today, tend to be overbored engines trying to max out the displacement in a smaller crankcase. For example taking a .45 engine and boring it out to make it a .55 engine. These overbored engines usually wind up where the cylinder sleeve now impinges and restricts the air/fuel flow through the passages into the combustion chamber, thus becoming flow restricted. But if the engine had a pretty large crankcase and crankshaft design to start with, it is less of a problem, than you see with a engine design where the crankcase is a little smaller.

I would think that it looks like having a stroke being a little larger than the piston bore is probably getting close to optimum. The model diesel engines tend to behave differently and breathe differently than the glow or gasoline engines do. But not by much though. Maybe a 5 to 10 percent difference between them.

Mr Cox

The length of the rod should not affect the piston acceleration, that is set by the stroke.
I think as short rod length as you can fit is the way to go. On high performance engines they take this to the extreme with different cutouts on the piston skirt and/or crank and back-plate.
On the Rex .12 for instance, you cannot remove the back-plate unless the piston is near TDC, due to the fact that the piston runs in a slot in the backplate around BDC.

123Cat

Yes just read that rod length affectes( shorter rod increases acc) piston acceleration and speed away from TDC,,,,,,, but mainly short rods produce higher piston side thrust, causing lost power, through friction

Ive got a very short rod at 1.5 x stroke

I can make a rod up near 1.8 / 1.9 x stroke , but I need to space the cyl up about 7mm to do it , I may if I get time

123Cat

Another thing on the SS is that the piston is really unsupported below the ports

its loose and nearly drops out

its not a good area of the engine
I cant fix it short of doing another liner and no way , its a complex liner

123Cat

I had a look at the Cyclon F2A engine and it has a very long rod at over 2 x stroke , so I will see if he does his diesels the same way , he's Russian and Russians know everything ,,,

first in space

123Cat

Saw an article comparing a Nascar engine to a F1 engine in 2006 the Cup eng does 10,000 RPM the F1 engine does 19,000 RPM, the rod to stroke ratio is 1.9/ 2.56 respectively

2.56 is a long rod

a 2.56 rod in a 2 stroke would mean you have to lift your cyl up about 9mm say in my 09, that would mean a lot more crankcase volume , is that a good thing in a small diesel ?

By the way the cup engine had a rod from a steel called 300M ( Modified 4340 , vacuum melted ), and the F1 engine had high grade titanium , I think its about 40% the weight of steel

Anyway I dont feel like running a short conrod anymore after looking at all the latest in models and high revving car engines , they all have long conrods ,

To space the cyl upwards on my silver swallow is hardish , but I may give it a try ,

and also the cylinder is not located very well in respect of the bottom end so that is another thing ,

I could check to see if it is square with the crank axis

Recycled Flyer

Perhaps you have taken the 'lightening' quest that bit too far.

123Cat

Getting back to this project , yes Im taking the lightening too far , I will use a 2.5mm piston pin instead of the usual Silver Swallow 4mm , that should be right on the limit ,

I will just test my rubber mounts and get back to building this interesting motor,

my last 3 cranks lasted an average of 6 controline flights before big end failures


Diesel is hard on big end ?????

Motorboy

In generally answer is seldom or not, the diesel engine will last long if right use and handling the engine and the fuel is Fatty compared with fuel made for glowplug engine.

Depending on which material was usen in Silver Swallow under production. I am unknown with the material in the Silver Swallow..
And how much is percentage of oil in fuel.

1. I will replaced with a duraluminium (Dur = Hard) as material. If you has acess to a lathe or milling machine, you can make a new con-rod.
2. Drill the oil hole or small saw cut under and above ends of con-rod to improve lubrication between con-rod and crankpin/wristpin. Important! Never drill oilhole where the crankpin ans wristpin has contact with con-rod since there is a hydrodynamic oilpressure under use. If there is oil hole on wrong place, the hydrodynamic oilpressure will disappear and give more abration between con-rod and crankpin and wristpin. see pic...

earlwb

I sorta like where they drill the oil hole on the top facing the intake hole in the crankshaft. That way it gets more fresh oil coming in more cool and hits the crankshaft on the way into the crankcase. This example is from a old Irvine .61 engine. But the oil hole could be either way, top of bottom as both methods work. I am not that fond of the narrow slit or cut in the rod. I see that as a point for failure as it weakens the rod more, but that would be with high RPMs or higher stress situations where it would be a issue.

Motorboy

This hole is not right placed as you showed the pic of drilled oil hole in con-rod and see this link why the idea to drill a hole there is pressure zone is a bad idea: http://www.tribology-abc.com/abc/tower.htm

I had never seen the engine been damaged by slitted recess for oil in the bottom of connecting rod. I has some model engines with slitted recess in connecting rod, in fact there is not much load in running condition.

See the pic..

In the engine who has a oil hole placed on the top of con-rod has own oil pump to give continous oil pressure whole time as you find in the automobile car engine to example..
The purpose with the oil hole placed on the top of con-rod to lubricate the pressure zone between the cylinder and piston before the piston goes down during the combustion period.

Edit: forgotten to write more..

123Cat

Thanks for the Technique , those Russians are smart , first dog in space

Recycled Flyer

A trick that I have seen on a few diesel conversion and Team Race diesels is to simply file a 'star' shape or cross on the big end eye of the rod closest to the crank web, this allows four points for the oil to get out once it has entered, and it gives a full flow through from the star points to the back of the rod.

It OK providing an oil entry point with a hole or a slit but its more important that it has a really good exit point and a 'star' shape, much like the much vaulted crank web grinding seen on RC cars, will act as an oil slinger flicking away what is coming out of the bearing.

In talks with some racing guys here in Oz, they reckon that the star alone is far better than any oil hole or slit with the rods lasing longer.
(I found it hard to get my head around it but the proof is in the pudding!)

Cheers.

123Cat

Thanks for the conrod tip , Im doing a little machining when I can , I like the Silver Swallow its a good subject for a rebuild, boy the casting is bubbly and has cracks and faults when I cleaned it

crank 50% done

piston 50% done


Cylinder honed


rod

cant make up my mind whether to use a longer rod and space the cylinder upwards , ,,,, just dont know yet

By the way its a flight engine only , it will get half a tank run in maximum , so I will set it up on the loose side

Recycled Flyer

123Cat,
I see that you are in Canberra Australia so the very best advice that I can ever give you concerning model diesel construction would be to contact Geoff Potter in Sydney - he has been building engines from scratch since he was a lad, has a full machine workshop, chroming bath, makes his own carbon props and his son was an ex world champ Team Racer (did I mention that he is experienced?)

If you want sage advice then go over to the Barton forum and private message him (far be it for me to hand out phone numbers in public).
Ask him what you ask here and just listen to how the conversation evolves into common sense solutions tattooed with "Ah Ha" moments!

Anything 'can' be done but is it worth it? That's the real question here.

Cheers.

123Cat

Thanks I might talk to them at a Diesel meet , I will post some pictures of the parts before my son crashes them he's learning controline takeoffs , real dangerous for the model

iskandar taib

My Feedback: (5)

And also, interestingly, the current dominance in Pylon Racing of the "new" long-stroke Nelsons and Jetts.

Iskandar

Recycled Flyer

At the rpm those fliers use its no wonder they need the extra port opening duration that a longer stroke gives!

123Cat

I will have another go, soon  , would not run , parts too rough , 

123Cat

It seems to run OK with the longer stroke than the bore I will fly and see how it goes , I will go up from an 8x4 to an 8x5 , the shaft is loose about 1.5 mm play at the end of a 9x6 prop , and its leaking out of the comp screw a little , Ive given it 5 minutes on a 9x6 , I will put it on the plane , starts OK
Ive got a lump of copper in the counterweight, as big as I can get, in and the piston is a lot lighter than stock , the stock piston is quite thick

rod is a lot thicker as I bent my first one as its only 2011 free machining aluminium so I made this one big ,

its out to 1.75 cc up from 1,5 cc 's
Also I made the inlet in the crank look like the inlet in my OS 15 , about the same timing roughly , and the piston is about 2mm longer with a smaller wrist pin

Wrist pin is 3mm down from 3.5 , piston is 0.7 thick down from 2mm , big end the same at 4mm , it got a reduced bore taper as the first try was too steep , its surging a lot , and getting hot , I dont want it to burn down , its fitted farily tight and needs a blow torch to heat it before starting , once its hot its OK , Ive burned a few fuel lines that way and had one fire , smothered with a big rag
I will give it another 5 minutes on the small prop before I fly, as Im good at cooking them before I get to the handle

123Cat

Some pictures of the build

123Cat

I went from the 9x6 to a 9x4 before flying it , , and got a bad vibe and overheat , it seems a lot more power but erratic ,the head blurs

Im in a bad resonance

I will fit an 8x6 and see if it comes out of it

123Cat

An 8x6 and in the plane would not start cold has to be warm , vibration not bad , a buzz comes down the lines a little ,,,, more power than the stock motor ,,,,,, thank goodness considering all the mods , shaft is quite loose, rod holding up OK , its only 2011 free machining aluminium , no bushes , I had at least 8 flights, more like 12 , at the highest power I could get no sign of burndown even when set real lean , Im not pushing the compression , just till it stops burping, Ive got 20% castor in

123Cat

Had another 6 or so flights and its good on an 8x6 Master

I would like to put another couple of weights into the crank web but its running too good to take apart


I spaced the cooling fins up a couple of mm' s, you can see the spacer just above the exhaust collector, as the contra piston was hitting the inside of the cooling fins , because of the extra 2mm stroke

It should last good because that cylinder was quite hard

In the normal Diesel fashion there was good blood on the circle as it barked one of my knuckles , cant show you as I lost the darn camera for about 3 weeks so far

Its quite powerful , I think its a copy of the Webra Mark 1 or Mach 1 ,

It looks like a real Diesel and I have to back the comp off to start it , not like my other 2 diesels which I can just leave the compression set and start ( I wonder why that is ?)

I wouldnt mind doing a rebuild on a 2.5 Silver Swallow if I could find a burned out one

123Cat

Thanks to Fiery

I have 2 new suspects , both 1.49's, who knows how they will end up I would like to do one with a conrod to stroke ratio of about 1.9
I have the cylinder spacer half made

I will buy a new camera and post a picture, I dont really feel like running the standard piston,,,, as its as thick as the Great wall of China , very robust