Forrest Edwards radial 5

[Mechanicboy]

While increasing the CR is easy, reducing it is not. It may need to move the wrist pin (new piston) or cutting the cone on the top.
At this point I like to hear from anyone that has built and run a glow version of this engine.

Compression ratio is determined by volume of combustion camber + volume of cylinder volume.

Thick cylinder head gasket = lower compression ratio and vice versa. Use syringe with fluid to measure the volume via the spark-/glow plug hole in the combustion camber with piston on TDC and calculate the compression ratio. The important: All piston stroke and bore must be same in all the cylinders then the correct compression ratio will be easy to adjust with cylinder gasket in various thickness due the volume of combustion camber is never 100% exact in all the cylinder heads when you are calculating the compression ratio and selecting the correct thikness of cylinder head gasket to get same measure of compression ratio in all the cylinders.

 

[petertha]

I haven't worked out the volumetrics myself, but just confirming - are you saying you calculate minimum 12.8 on the MR by using the full liner lip thickness (0.175") semi recessed in the head where it shows 0.060"? If 12.8 is correct, that does seem suspiciously high. The open hole portion below the glow plug will add a bit of head volume & possibly same with flat facet valves but would depend on how they are seated. The best way to know in advance of building is CAD model / calculation which I could do.

 

[Mechanicboy]

See at the assembled drawings of cylinder and cylinder head, there is not cylinder head gasket who means the cylinder liner must be adjusted in height of cylinder on lathe then the engine get correct compression ratio. The alternative is the cylinder is lowered and adjust with thickness of cylinder head gasket and get correct compression ratio.

 

[Glorfindel]

To have 8.5:1, the lips must be .150"

Other cylinders are within 0.002" of strokes vs the main rod.

 

[josodl1953]

It is interesting to see how discussions develop on a subject. Even more surprising is the fact that I came across the C/R and timing issue myself in the past days. With most of the parts complete, I made a test fitting with master and link rods and found a lower C/R with the link rod cylinders compared with the master rod cylinder. I wonder what would be the effect of altering the position of the link rod holes in the master rod, bearing in mind that this is a proven design and that 7 and 9-cylinder radials also have equally spaced link rod pins .I am also considering to adjust the C/R by adapting the height of the piston rather than trimming the upper end of the liner. I always adapt the part that is easiest to make to the part that is more complicated , and in this case the easy part ( for me) is the piston.

I'd like to see your comments.

Jos

 

[Glorfindel]

Here is how i did it.

In my cad soft (CatiaV5) i extracted surfaces of cyl, piston top and head dome.

I generated a volume for tdc and bdc.

You see the volume at bottom left, so i adjuated the lips of the sleeve to get exactly 8.5:1. I did it on the master rod.


The other pic is the "trace" of a poi t on top of each pistons for 1 cycle, so it guve me the exact stroke of each pistons.

As you can see, they are wihtin 0.002", so no big deal.

Hope this helped.

 

[petertha]

Trimming the thickness of the upper lip of the cylinder liner means the head would now sit at a lower position relative to the piston TDC position and therefore would increase CR. This has the identical effect as removing head gaskets. The cylinder is not lowered and there is no other change to the piston / rod throw geometry. I always assumed the Edwards lip thickness was sized to yield a conservative CR let’s say 7.0 for example. Now the builder’s job is to trim the appropriate thickness of each liner lip to achieve a constant number across all cylinders 8.5 for example. If someone wants 8.2 or 9.4 that’s fine, it’s the same methodology. The issue that conflicts with this assumption is that you are calculating 12.x MINIMUM and it can only increase by trimming. So that’s why I am asking how you derived that number because it seems too high.

The other thing to recognize over & above this is – because he chose to phase the master rod / link rod holes equal 72-deg, that inherently results in unequal CR across cylinders purely by motion geometry of the link rods. So he is ‘fixing’ 2 issues simultaneously, achieving the desired CR to some target value AND also making them equal across cylinders. By contrast in other radials, the master rod has angular positions on +/- either side of 72-deg depending on the cylinder number so geometrically the CR’s are equal. There are 2 ways to skin the cat.

 

[petertha]

To have 8.5:1, the lips must be .150"
.

I didn't see your CAD computation until after my post #67. This is exactly what I was wondering. So you are computing essentially what I was suggesting - that 0.175" lip corresponds to some lower CR, say 7.X And by trimming to 0.150" you get CR = 8.5. And the suggested CR of 12.x does not make sense, some error somewhere?

 

[Glorfindel]

I didn't see your CAD computation until after my post #67. This is exactly what I was wondering. So you are computing essentially what I was suggesting - that 0.175" lip corresponds to some lower CR, say 7.X And by trimming to 0.150" you get CR = 8.5. And the suggested CR of 12.x does not make sense, some error somewhere?

0.175 = 7.25:1

On my drawings, it say 8.5:1

There is only 0.002" difference in strokes between cylinders, i dont think that will change CR that munch one from another.

 

[petertha]

Yup, your plans show the exact same as snip I posted in #62. I believe your CAD derived numbers & resultant CR's looks right. If we want to get into even deeper detail, we factor valve facets & any hole volume beneath the glow plug. That's why it this is harder to do with paper & pencil. Maybe Edwards intended the liner lip trim to be final step using syringe & liquid.

Years ago I had a spreadsheet that calculated CR's by inputting master & link rod geometry. I seem to recall about 8% variation in CR from highest to lowest cylinder but possibly that was a different 5-cyl design. CR does vary between cyl #1, 2 & 3 with equal (72-deg) phasing. Did you extend each to TDC to measure that .002" or just one of them? Maybe the spreadsheet had a bug because 0.002 / 0.150 is only about 1.3%difference on basis of bore diameter, so I agree its negligible. Good discussion! Thanks for the validation.

 

[Glorfindel]

If you look at my third pics, i traced every pistons srtoke. They are all within 0.002" from the master rod.

I'll simulate what 0.002" mean in our ratio tomorrow.

 

[petertha]

Ah, I see that now. #1 is 0.966 (2 x crank pin radius, check). You displaced each piston to extremity along liner axis. #3 and #4 are 0.968, so maximum difference is 0.002"
#2 and #5 are 0.967" so 0.001" difference. Looks good to me.

Kind of interesting - similar displacement 5-cylinder radial engine I am building (Ohrndorff) has compensated master rod. I checked the link rod pin angles against a published graphical method used to equalize TDC position for same CR & they match. As you can see the resultant angles are quite different +/- nominal 72-deg. But I never have went back to determine the actual CR effect if they were at 72-deg. It does vary by combination of stroke & link rod dimensions, just assumed it was more. Like I said it was based on spreadsheet, but something tells me something was a bit fishy about how the bottom of stroke was determined for each cylinder.

 

[Glorfindel]

I'm happy that i saw that thread, i was not aware that we can have different strokes vs angle.

And sorry if i jumped in without introducing myself, i will do it in the right place.

 

[tornitore45]

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.

 

[tornitore45]

Alert, I may be wrong but it looks like there is a confusion about the dimension to use in calculating the CR.

On sheet 5 of the plans, the crankcase is shown with the sump on top and cylinder #1 at the bottom. You will notice the 10-32 threaded hole for the Idles Shaft position orient the drawing view.

The dimension to be used in calculating the CR is the 1.860, NOT the 1.925.
Using the 1.925 the CR turns out to be 7.6. If that was the case then by changing the lip to 0.15 would set the CR = 8.9 much closer to the spec. But is wrong.

In reality reducing the lip to 0.15 would raise the CR to 18.1

 

[Glorfindel]

Alert, I may be wrong but it looks like there is a confusion about the dimension to use in calculating the CR.

On sheet 5 of the plans, the crankcase is shown with the sump on top and cylinder #1 at the bottom. You will notice the 10-32 threaded hole for the Idles Shaft position orient the drawing view.

The dimension to be used in calculating the CR is the 1.860, NOT the 1.925.
Using the 1.925 the CR turns out to be 7.6. If that was the case then by changing the lip to 0.15 would set the CR = 8.9 much closer to the spec. But is wrong.

In reality reducing the lip to 0.15 would raise the CR to 18.1

I will check your excell sheet tomorrow.

My model is acurate and it doenst make mistakes calculating the volumes.

There is something wrong. I will double check everything tomorrow.

 

[petertha]

Sorry I can’t quite follow your calculations.

To compute CR you require two volumes. Vc or clearance volume or combustion chamber ‘squish volume’ that occurs above the piston when at TDC. And Vd or displaced volume of the piston when at BDC, which is controlled by bore area times the stroke length. Then CR = (Vd + Vc) / Vc. What are those two respective volumes on your calculation?

I don’t quite see where you might be determining the cone volume element for either the piston or the head, but maybe something happening behind the scenes? I would be looking for something like Vol = PI * R^2 * H / 3 (where R = bore/2 and H = height of cone). For example if the bore is 0.945” and angle is 20 deg, I get H = 0.172” H = tan(20) * R vs your cone height = .162???

I can’t quite determine what is what in your calculation but you refer to liner length. That doesn’t factor into anything, only the stroke and in this configuration the lip thickness which is the head rests on & influences Vc

There is no prerequisite that the piston crown shape must ‘match’ the head chamber like cone within a cone. Obviously they can’t mechanically interfere with one another but many piston/chamber permutations are seen between flat, domed, non-symmetrical lobes. I think these features come into play to control burn front & and intake/exhaust flow, but that’s a separate discussion. Two engines can have identical CR and Vc but completely different piston crown & combustion chamber shapes.

 

[Glorfindel]

.

 

[petertha]

Sometimes different terminology is used for static CR & I am guilty too. Here is a pictorial with their corresponding wording.
https://www.rbracing-rsr.com/compstaticcalc.html

 

[Glorfindel]

Sometimes different terminology is used for static CR & I am guilty too. Here is a pictorial with their corresponding wording.
https://www.rbracing-rsr.com/compstaticcalc.html

Not sure anymore. My cad prog already add vc+ vb when the piston is at BDC.

I will simply check squish to see if im good.

I dont have my 3d model at home.


it would be nice if someone who have already built the engine would give us some info.