Forrest Edwards radial 5

[Glorfindel]

So many sub subject here

1) the conical shape of the piston and combustion chamber has no effect on CR as long as they match. The volume removed by the piston cone is equal to the volume added by the conical head.

2) There are small unaccounted volumes, such as the hole in the Glow Plug and its filament, the valves heads not seating flush with head.
It must be noted that the plug thread is o.25" and the Plug thread is 0.25" minus the gasket, not a big error there.

3) Everyone has his own preferred method to change the CR: Gasket Thickness, Changing the length of the Liner Lip, making a new piston. I may add my method which is to change the depth of the 0.060 bored section in the head. Its diameter and concentricity are not critical and is easy to re-chuck the head and bore a little deeper. YMMV

4) We have established that a Master Rod arrangement with equally spaced pins does not yield the same stroke on all pistons, is a fact of geometry. All depends from the interrelation of Stroke, Master Rod length, satellite rods length and position of the pins on the Master rod.
Someone has calculated that, for this specific design, the strokes are just about as big as normal machining tolerances. A perfectionist may move the location of the pins a few thousand in or out radially and overcome that concern. Anybody that built an engine has realized how little the piston moves by rotating the shaft a few degree back and forth at the TDC and BDC, is a fact that the cosine unction is very flat at the crests.

5) My biggest concern is none of the above, forgive me for restating, my concern is that according to my calculations (I have been wrong before in 1989, I though I made an error) the CR is already higher than specified and trimming the lip is not a solution.

I am attaching the spreadsheet where I did the calculations, if I made an error, please point it out. We all need to be settled on the CR.
The spreadsheet should be clear but if you have a question shoot on the thread or a PM.

Turns out I can not attach a spreadsheet so here is a TXT file, I tried to display the formulas used to go from place to place.

I checked your file and i saw that you have a squish of 0.080"??? That's a very big squish. I would guess about 0.040'' as a good starting point

 

[Glorfindel]

Please guys, post your compressed (cv) volume, the uncompressed volume (uv) and the squish you got from your calculation.

With lips at 0.150'' :

uv: 0.765ci
cv: 0.09ci
Squish:

CR: 765/0.09= 8.5

I didnt calculate the volumes, i asked the cad prog what was those volumes at tdc and btc directly in the assambly, so i cant see how my numbers can be wrong.

 

[tornitore45]

We are all talking about static or volumetric CR, just geometry neglecting leaks, and valves overlap.
There is no need to screw around with volumes, the cross section area does not change and get canceled out. CR = 1 + Stroke/ Gap the gap is the residual distance between piston and roof of the combustion chamber at TDC. The conical piston head has no effect.
THE VOLUME REMOVED BY THE PISTON CONE IS EXACTLY EQUAL TO THE VOLUME ADDED BY THE CONICAL CAVITY IN THE HEAD.

Think of cutting off the cone on top of the piston and gluing into the combustion chamber cavity, the residual volume is not changing.
Considering volumes does noting but add potential error sources in the calculations.

 

[tornitore45]

I appreciate all the theoretical disquisitions about combustion chamber and piston head shapes, BUT let's focus on the problem at hand: The Edwards Radial 5.

Has anyone noticed that the piston cone has the same Diameter and Angle as the Head cavity? They match to each other like hand and glove, there is no volume between EXCEPT for the gap or Deck Height like someone like to call it.
There is no Head Gasket in the design or the calculations, let's live that out.
We all understand the head gasket reduces the CR.

 

[tornitore45]

Glorfindel
If I use 0.15" for the liner lip and 1.925" for the distance between axis and crankcase face I obtain exactly your numbers CR: 765/0.09= 8.5

BUT the 1.925" dimension is not the one to be used here, that dimension defines the Oil Sump location.
Look carefully at the drawing, it is upside down. Cylinder #1 is at the bottom.
The big hole for the liner should tell, the small 0.25" hole at the bottom drains the oil in the sump (neglecting gravity). The 10/32 threaded hole next to the rear bearing should tell you about the drawing orientation when compared to the cut out view on sheet 2

The correct dimension to use is 1.86"

There is no point in debating math and method when we use different data.
Everybody here knows how to calculate a fkng CR, let's focus on reading the drawing correctly.

 

[tornitore45]

Petertha, you caught a couple of error, thanks.
I used the cosine instead of the tangent to calculate the cone height, my bad.
But I have not used that data in the CR calculation for reasons amply explained above.
I listed the Liner Length because I thought I may needed it but as you pointed out has no bearing on CR and I never used that data in the calculations.

If I could have posted the actual spreadsheet you would have been able to see the errors , the source of them and the fact that fortunately had no effect on CR accuracy.

 

[Glorfindel]

My model should be accurate and you're 100% sure of your calculation.

I will setup a couples of drawing to show my dimentions in case of an error on my part.

I actually hope that i made the 1.925 vs 1.86 error!!

We are doomed!! ;-)

Worst case is i will add shims if my CR is too high. I can cut anything at any thickness on our wire edm.

I'll double check anyway with your excell sheet.

Thx for taking the time to help me.

 

[tornitore45]

A 0.036" head gasket will fix the problem
If the engine is not yet built adding 0.036 to the liner lip or boring the head to 0.024 instead of 0.060 is a solution

 

[Glorfindel]

I dont want to hijack this thread, i will make a new one about CR.

 

[mu38&Bg#]

I just checked, because I found it odd that there would be an error for decades which nobody found. I get ~8.6:1 with a .15" lip.

 

[mu38&Bg#]

Tornitore, M in your calculation does not come into play.

 

[Glorfindel]

I just checked, because I found it odd that there would be an error for decades which nobody found. I get ~8.6:1 with a .15" lip.

I just did it "by hand" and i still get the same results.

 

[petertha]

Does this help shed light? I agree the 1.860 dimension is the one to be using because it pertains to the #1 cylinder. 1.925 does not factor into this aspect. Its an unfortunate choice of view projection because we are looking for #1 cylinder to be 'on top'. But you can see it has the liner hole 'on the bottom'.

In any event the 1.860" is only an intermediate dimension to attach the cylinder on top of & then the liner within resting on the lip and then the head on top of lip. Behind all this completely independently is master rod relative to crankpin throw & then piston superimposed onto MR. I'm guessing Glorfindel has these individual parts assembled within CAD using mates & determining resultant volumes at TDC & BDC. Do we have agreement on input dimensions?

 

[tornitore45]

Mea culpa, mea culpa, mea culpa. I am still trying to clean all the eggs splattered on my face.

As pointed out, the dimension of the 0.060 x 1.176 dia in the head has no bearing on the CR, I goofed.
For some insane reason I saw it as lowering the head and made a wrong inference.

My calculations agree 99.8% with Glorfindel. My plans show the dimension for the fins to be 1.415 versus 1.417. Nothing to stress about.

Once again I apologize for sending you on wild goose chase.

 

[Glorfindel]

Mea culpa, mea culpa, mea culpa. I am still trying to clean all the eggs splattered on my face.

As pointed out, the dimension of the 0.060 x 1.176 dia in the head has no bearing on the CR, I goofed.
For some insane reason I saw it as lowering the head and made a wrong inference.

My calculations agree 99.8% with Glorfindel. My plans show the dimension for the fins to be 1.415 versus 1.417. Nothing to stress about.

Once again I apologize for sending you on wild goose chase.

No problem, it made me double check everything ;-)

When i did the cylinder, i checked the lenght after the third one and i got 1.417", so i did all the remaining one at 1.417 ;-)

 

[tornitore45]

Let's change the subject
It was mentioned that the con rods tend to wear out.
I am surprised because AL 2024 is the usual recommended for tough parts like con rods and pistons and many commercial engines are not sleeved on the little end.

So I was looking at a possible Oilite bushing
https://www.mcmaster.com/6658k725
This is specified as 11o lbs of radial force and 120 RPM which is a fraction of the stress in the engine

Should one think of solid bronze, or forget the entire idea on the assumption that 2024 alloy is good enough.

Waddayoutink?

Something to consider
While the master crank pin actually revolves in the master rod big end, wrist pins and link rod big end never complete a turn but simply rock back and forth.

 

[petertha]

Almost every RC engine I have seen has bronze bushings run against hardened steel wrist pin or crank pin. Maybe more importantly, have some combination of angled holes or slits for lubrication. OTOH nobody has ever talked about master or link rod failure using straight aluminum of sufficient strength alloy. I think with radials of this scale the remaining wall thickness is already quite constrained. Not so much the piston end but bottom end to fit within the master rod slot & pin. Probably the best approach is like how Terry.M & others did their engines: Loctite a solid bronze rod in the aluminum, then drill/ream to final dimension so it can be made thin & vs. turning an annular ring & fitting that into the rod. I’m not sure what the lubricity & wear properties of high strength alloy aluminum is to bronze, but maybe beyond practical life expectancy?

 

[tornitore45]

[QUOTEAlmost every RC engine I have seen has bronze bushings run against hardened steel wrist pin or crank pin][/QUOTE]
I am old enough to have owned the smaller engine 0.15. 0.29 and 0.35 that you could buy (imported from USA and Japan) for the equivalent of a few dollars and those had no bushing. We ran them for hours every weekend from age 12 until girls started to become interesting, and I can't remember wearing out (the engine).
Modern High Tech commercial engine are targeted to a different clientele, guys that do not flinch to lay down a G for a top toy, they expect to outlast them by a couple of generations. Nothing wrong with that.
On the other hand, things are different. I am not building the engine to fly, I want to build a "good" engine to show with pride and when I am gone I want he fellow that open the engine not to say "He could have done better but choose to cut corners".
In conclusion I think I will light press a solid bronze pellet and drill and ream.
The bushing can not move axially once installed.

 

[petertha]

Sounds like you answered your own question. I don't think solid bronze link rods or master rod is practical. Depending on the alloy, it will be lower strength & significantly more weight which means increased rotational & reciprocating mass, more counterweight, stronger crankpin, bearings etc. For example SAE660 bronze has tensile of 20ksi, density 8.9 g/cc (other flavors increased strength & lower weight). By comparison 7075 aluminum is 73ksi, density 2.8 g/cc

 

[josodl1953]

I must remember you guys that this thread was about building an Edwards radial 5. However interesting your discussions are, maybe you should start a new thread in the department"General engine discussions".

Well,on wit the fabrication of pistons.
Some 30 years ago I was using Picco P90 engines in my racing boats but parts were hard to get. Especially ABC piston/liner sets were a problem. When the wrist pin securing circlips went their own way due to worn out grooves there was always damage to piston, liner, or both. Sometimes the liner was still in good condition but separate pistons were also not available.
Now the material for pistons was hard to get. These ABC systems ( Aluminium piston ,Brass liner, Chromium plated) required piston materials withe the same rate of expansion as the liner because of the tight fit ( piston rings were not used).
I found a big discarded diesel piston on the scrapyard of a diesel engineering company. The material of this piston prroved to be just the right stuff to make pistons from, which I have been doing till I quit model boat racing a few years ago.
There isn't much left of the piston but...

there was still enough to make pistons for the Edwards as they have a smaller bore. The material was harvested by slicing off sections of the donor piston . Drilling holes in a circle provided blanks for the Edwards pistons. A threaded hole in the middle provided a fixing point for a mandrel because there was insufficient length to clamp it in the chuck.

After fitting the blank to the mandrel it was necessary to machine the OD and the piston ring groove in one operation because it was impossible to refit the piston in exact the same position.