17,5 ccm 4 stroke single cylinder engine

[bluejets]

My intention is to first try to let the engine run on a glowplug (i have bought the OS 4 stroke glow plug's)
If it works then i want to try let it run on a spark plug.
Stef

Bought some 1/4-32 sparkplugs that supposedly came from China as some others were as much as Aus$40.00 each plus about the same for postage which I thought was a bit over the top.
Especially since I'm building a V8 along with another that a mate of mine is building.
Anyhow, these were around $12.00 a piece and we tried one first up just to see if they would stand up ok.
Ran all day in an atkinson engine and thought that was good enough so bought quite a few more.(30 plus)
They are the same length below the copper gasket as an OS 4 stroke plug so you will have no problem there.
What may be a concern is, if the engine is currently on fuel/oil mix and usual for this to be around 20% oil, then you may get oil up problems with the little spark plugs.
Glow plug is no problem.

 

[petertha]

Nice work Stef. What alloy are the valves made from?
On another note, I was curious. Did you buy your plans recently? I noticed CAD+Modelltechnik Jung is no longer showing their plans pages or price list, just a dead link. I sent him an email but no response.

 

[stef110]

I bought the plans like 1,5 month ago then the website also didn't work every day. Once it worked i ordered it and nicely received the drawings a few days later.
Im already looking for new plans now but also cant get on the website.

@bluejets i was looking at these spark plugs and ignition systems https://www.justengines.co.uk/acatalog/Ignition_Systems_for_Glow_Engines.html

stef

 

[bluejets]

@bluejets i was looking at these spark plugs and ignition systems https://www.justengines.co.uk/acatalog/Ignition_Systems_for_Glow_Engines.html

stef

Plugs here look similar.
For ignition systems, I have been experimenting with the CDI systems as used on the "pocket rocket" motorcycles.
These are the little 50cc type.

Coil cost around Aus$6.00 and is approx 40mm long and 30mm diameter. I modify the leadout where it screws into the coil body by using a hv cable (3mm diameter)
Note that these coils will not work as kettering style ignition.

The "black box" of which there are a few different types, cost is around Aus$7.00.
Only drawback is these units are not built to run from a hall effect but rather an inductor coil.
For this I extract the coil from a miniature relay ( maybe $2.00) and slightly modify for it to work as a pickup.

Tests so far have been good.
I once did a complete write up of the details and offered it to a model engineering magazine but no one was interested it seemed.

 

[stef110]

This evening i machined the piston, actually a fun an easy to machine part. Started on the lathe turning the outside diameter to nice fit in the cylinder. Then drilled and reamed the hole where the lock pin will be fitted to connect the piston arm. After that I cnc milled the inside out. Using the already drilled hole to line it up along my x axis on the mill. After that finding the center of the piston and hit the start button.

stef

 

[bluejets]

Normal practice to machine pistons with a couple of different sizes to allow for expansion.
Actual difference in dimensions are not massive but it might save a lock-up.
One size above the ring, another around the gudgeon area and yet another towards the skirt.

 

[Cogsy]

I think that's normal practice for full-size automotive but I've never heard of it being performed at model sizes. The expansion rates are just too small to make a difference.

 

[bluejets]

Perhaps but it was the way the old blokes did it way back when and I don't get any lock-ups.
Little 25cc 4 cylinder I built was done this way and it turns at 10,000rpm.

 

[mu38&Bg#]

I think that's normal practice for full-size automotive but I've never heard of it being performed at model sizes. The expansion rates are just too small to make a difference.

Or the given clearance on plans is already large enough that it doesn't matter.

 

[Cogsy]

I roughly did the math just to satisfy myself. For a 1 inch aluminium piston, heating it from room temperature to just shy of its melting point would increase its diameter by around 0.4 mm or 15 thousandths of an inch. Of course the bore will also increase in size as well, but not by as much (roughly half for cast iron). As the volume of material increases with full size components, thermal expansion becomes more of an issue but it just doesn't come in to play at our scale. I guess that's why lapped pistons work for us without seizing.

 

[petertha]

Example 4-stroke RC engine with CI ringed, aluminum piston.

There are some detailed studies of piston temperature profiles of model 2S & 4S engines online. This isn't the one I'm thinking of but shows some generic heat profiles. Kind of supports what most engine tuners know is the top 20% crown area is where most of the make or break 'action' is. LOL. It might be just as dangerous to apply some overall expansion factor to piston when the temperature profile itself is varying from top to bottom. Averages don't really mean anything because there is no time for conduction. Its more about maximum & the critical ring & groove area. 2S is even more complex, but now we are digressing.
https://www.researchgate.net/publication/273301165_ANALYSIS_OF_PISTON_OF_TWO_STROKE_ENGINE

Back to engine building - I have a question for steff110. On that particular Jung engine, is the ring split, opened to a specified gap & heat treated? Or some other procedure to get the finished ring? I seem to recall some other (Jung) discussion where it was machined oversize & gapped. I cant recall if that was Jung's prescribed method or that particular builder's method.

 

[Cogsy]

Example 4-stroke RC engine with CI ringed, aluminum piston.

There are some detailed studies of piston temperature profiles of model 2S & 4S engines online. This isn't the one I'm thinking of but shows some generic heat profiles. Kind of supports what most engine tuners know is the top 20% crown area is where most of the make or break 'action' is. LOL. It might be just as dangerous to apply some overall expansion factor to piston when the temperature profile itself is varying from top to bottom. Averages don't really mean anything because there is no time for conduction. Its more about maximum & the critical ring & groove area. 2S is even more complex, but now we are digressing.
https://www.researchgate.net/publication/273301165_ANALYSIS_OF_PISTON_OF_TWO_STROKE_ENGINE

The expansion factor is not an 'average' but a physical property of the material called the Thermal Expansion Coefficient and is listed in the paper you linked to. Its units are simply expansion in metres per degree increase in temperature (Kelvin or Celsius). It can be applied to a linear dimension as-is, or squared to generate the increase in area, or cubed to calculate volume increase. In the case of pistons, we are really only concerned with the thing binding in the bore so we can apply the linear calculation. How 'deep' into the piston this temperature extends is irrelevant. The point of using the (almost) melting temperature of the piston was to illustrate how little difference temperature actually makes at this scale. Drawing from the paper you linked, the actual temperature of any part of the piston cannot exceed 60% of its melting point or structural changes in the alloy result, so the actual expansion will be less than 60% of the maximum I calculated.

In larger scales, this small amount of expansion per inch quickly adds up into something which must be taken into consideration. For example, the train tracks between my university and the city centre grow in length by over 14 metres with a 10 degree increase in ambient temperature (this was an exercise during my undergrad, I don't normally monitor train tracks) and every year (it seems) we have major train derailments across the middle of the country during summer from buckled tracks due to thermal expansion.

 

[petertha]

Yes I'm aware of what thermal expansion factor is. Poor wording on my part. It was more in response to post 26 suggesting sizes at specific piston locales (crown/wrist pin/skirt). I'm saying that when you have an object with a temperature gradation such as a piston, then thermal expansion growth will be proportionate to the temperature at that locale times the metal's expansion coefficient. So if an object is 600C at the top & 50C at the bottom, the top will proportionately larger than the bottom if this top-to-bottom temperature profile holds for consecutive cycles. There isn't time for the temperature to average itself by conduction.

You could probably engineer a tapered piston shape so that it would yield a more constant annular gap at operating temperature gradation. But the subject was avoiding thermal seize conditions, so sizing based on the hottest crown area, as we are both saying, ignores the bottom (which by default results in a larger annular gap at the skirt than the crown if the piston were machined typically cylindrical). This is a CI ringed piston so I suspect a few additional factors are also at play over & above this. Anyways, I think the engine designer has this figured out. Back to the build thread.

 

[bluejets]

Don't forget allowance for lubrication.
Talking expansion rates is all well and good but without sufficient oil, back to square 1.

To give a quick perhaps seemingly unrelated part, OS used to make pylon engines with a big end clearance of 3 thou. Rods were coming out the sides of engines everywhere.
We uped the clearance eventually to 6 thou and no more problems.
This is of course on methanol and 20% castor( castrol M)

 

[stef110]

Its have been an frustrating week. Last weekend i made the piston ring and connection rod and i was able to do a first test run. But nothing happens... I had some old fuel so i didn't trust it and ordered some fresh fuel.
Also no result. What i figured out is that my valves where not sealing properly. sometimes there was even fuel pushed out of the carburetor.

Yesterday the engine after re cutting the cylinder head valve seats give some puffs but then complete disaster strikes. On of the valves came lose and val in to the cylinder when i was trying to start the engine. result a bend valve luckily the piston and cylinder head where fine.

What im planned to do now is make some brass valve cage's and press fit these in he existing cylinder head. And then cut the seat with the famous tool a lot of you people use. I don't want to give up yet on the engine but its a pity so far.

Will keep you guys posted on further steps.


Here are some pictures off the last parts i made before the testing.

 

[petertha]

Keep us posted.
Did you make the original valve cage out of bronze & now switching to brass? I think Jung specifies 'red brass' but if I understand the translation correctly, it means a bronze alloy in North America terms. Just curious if the modification has more to do with re-cutting the seat geometry or you are now thinking brass would be better?

What about the new piston ring? Did it get damaged or you ware wanting to improve?

 

[bluejets]

Not a good idea to heat treat a ring in that manner.
Ring to be treated should have the wedge fitted and then the assembly sandwiched between two "much larger volume" steel plates. This is heated for around 5 minutes at cherry red and the whole assembly left to cool.
This ensures correct heat treating of the ring.
While you are at it, I tend to make a few extra rings at the one time as it is not uncommon to unexpectedly break one when fitting to the piston.

Ring is then fitted to another "fixture" to bring back to "round" by turning the last couple of thou to bring to diameter.
Reason being, the wedging action pulls the ring out-of-round.
Then ring is fitted to the bore using the piston to squarely push it in, and measure ring gap maybe 2 thou.
Ring gap opened up using a small flat file.

Valves I always turn both the valve seat AND the valve face without moving the cross slide ensuring the angle cut is exactly the same.
I always use cages with the above method and have never had to "seat" a valve yet.
( bronze NOT brass)

In fact I will go as far as to say that in my experience, any attempt to do re-seating will end in a worse fit.
There are exceptions and it is just plain luck.

Circlip fell off eh....??? I did mention that before.