How to calculate ...

Fast Eddie · Mar 9, 2016

Can any of you nerds out there point me in the direction of a simple on line calculator for calculating the volume of a dish / dome? I want to play with it to ascertain effect of a dished piston on compression ratio in a big Commando engine.

I am basically interested in finding out if a 1007 motor could be built to run a tight squish band and still have a reasonable CR...

Deets55 · Mar 9, 2016

fill it with liquid and measure volume?

o0norton0o · Mar 9, 2016

https://en.wikipedia.org/wiki/Spherical_cap

it's all there...

Nater_Potater · Mar 9, 2016

Fast Eddie said:
Can any of you nerds out there point me in the direction of a simple on line calculator for calculating the volume of a dish / dome? I want to play with it to ascertain effect of a dished piston on compression ratio in a big Commando engine.

I am basically interested in finding out if a 1007 motor could be built to run a tight squish band and still have a reasonable CR...

Try this - http://www.monolithic.org/calculators/dome-calculator Use the "Sphere Calcs" and enter the diameter and height/depth of the dome/dish. You can ignore the "Stem Wall" entry, as this is an architectural calculator.

Nathan

Fast Eddie · Mar 9, 2016

Deets55 said:
fill it with liquid and measure volume?

That would be challenging... the pistons only exist in my mind!

The idea here is to estimate a pistons design, I'd probably then just get simply 'dummy' pistons made to accurately measure, before committing to a final design.

It is purely a 'virtual exercise' at present though.

Fast Eddie · Mar 9, 2016

Thanks for the online pointer guys. Trouble is, calculating it myself as per the wiki page gets a different result to the monolithic calculator!

Which reminds me of my school days.

Anyone care to tell me the volume of a 70mm diameter, 5mm deep dish...?

comnoz · Mar 9, 2016

Fast Eddie said:
Can any of you nerds out there point me in the direction of a simple on line calculator for calculating the volume of a dish / dome? I want to play with it to ascertain effect of a dished piston on compression ratio in a big Commando engine.

I am basically interested in finding out if a 1007 motor could be built to run a tight squish band and still have a reasonable CR...

Sure they can. Just put a little dish in the middle if you want to run pump gas. Jim

Fast Eddie · Mar 9, 2016

comnoz said:
Fast Eddie said:

Can any of you nerds out there point me in the direction of a simple on line calculator for calculating the volume of a dish / dome? I want to play with it to ascertain effect of a dished piston on compression ratio in a big Commando engine.

I am basically interested in finding out if a 1007 motor could be built to run a tight squish band and still have a reasonable CR...

Click to expand...

Sure they can. Just put a little dish in the middle if you want to run pump gas. Jim

That's just teasing me Jim!

What's a realistic dish size and CR arrangement dya think?

Deets55 · Mar 9, 2016

Fast Eddie said:
Deets55 said:

fill it with liquid and measure volume?

Click to expand...

That would be challenging... the pistons only exist in my mind!

The idea here is to estimate a pistons design, I'd probably then just get simply 'dummy' pistons made to accurately measure, before committing to a final design.

It is purely a 'virtual exercise' at present though.

oops!

Use a hypothetical liquid? :shock:

comnoz · Mar 9, 2016

Fast Eddie said:
comnoz said:

Fast Eddie said:

Can any of you nerds out there point me in the direction of a simple on line calculator for calculating the volume of a dish / dome? I want to play with it to ascertain effect of a dished piston on compression ratio in a big Commando engine.

I am basically interested in finding out if a 1007 motor could be built to run a tight squish band and still have a reasonable CR...

Click to expand...

Sure they can. Just put a little dish in the middle if you want to run pump gas. Jim

Click to expand...

That's just teasing me Jim!

What's a realistic dish size and CR arrangement dya think?

Well for that I would have to sit down and think. But I do know it's easily do-able. Jim

comnoz · Mar 9, 2016

So I usually figure the average big valve head is going to have 35cc above the squish band.

If you run steel rods and full skirt pistons in aluminum barrels you can run the squish down to about .5mm so add 2.6cc there for an 81.4 mm diameter bore in the head.

That will get you 14.39 with a flat top piston.

11.07 with a 50 cc chamber
10.15 with a 55cc chamber
and 9.39 with a 60 cc chamber

so 11.5 with a 12.4 cc cup
10.15 with a 17.4 cc cup
and 9.39 with a 22.4 cc cup

If the cup is 65mm in diameter then it will be 3.33 cc per mm.

so a 6.7mm cup will give you 9.39
and a 5.22mm cup will get you 10.15

If the cup were made the shape of the chamber [which it should be] then it would be considerably shallower.

If my frazzled brain is thinking right. Jim

Fast Eddie · Mar 9, 2016

Thanks Jim, that definitely demonstrates its easily doable.

Your calculations are based on a clear cylindrical cup shape I see. My thinking is that a smooth curved dish shape would be better for flame travel, hence my desire to try and calculate dish volume. I'll play around with that one.

(BTW I get a 50cc chamber volume to be 11.07 CR?!)

comnoz · Mar 9, 2016

Fast Eddie said:
Thanks Jim, that definitely demonstrates its easily doable.

Your calculations are based on a clear cylindrical cup shape I see. My thinking is that a smooth curved dish shape would be better for flame travel, hence my desire to try and calculate dish volume. I'll play around with that one.

(BTW I get a 50cc chamber volume to be 11.07 CR?!)

I must have fat fingered that one.

I would use a dish that was the shape of the combustion chamber at the ID of the squish band. So basically round with sloped sides and offset like the chamber.

acotrel · Mar 9, 2016

'
I am basically interested in finding out if a 1007 motor could be built to run a tight squish band and still have a reasonable CR...'

You probably need to start by defining a target comp.ratio. Find the combustion chamber volume you need to achieve it at TDC. . Then calculate backwards from the difference in volume created by segments of two spheres of slightly different centres and radius to get the shape of the piston crown.. I think the variation in volume as you move up a sphere is sinusoidal - you need the algorithm.

johnm · Mar 10, 2016

It would be a bit of work but you could also do this graphically.

Make a contour "map" drawing of the top of your piston with the height measured in mm from a selected datum.

Then make another contour "map" of the combustion chamber at the same scale with respect to the same datum.

Overlay the two diagrams and make a map of the thicknesses between the two surfaces.

Then calculate the volume of the thickness diagram. You can do this by gridding up the diagram and then adding up the volumes of each grid cell.

The advantage of this method is you can make any shape you like and do not have to conform to simple geometric shapes to suit the arithmetic.

This is how we used to work out oil field volumes back in the old days. Before mapping programs did it all for us.

o0norton0o · Mar 10, 2016

Eddy, the parameters you quoted of 5mm height and 70mm across produces roughly 9.686 cc per hemisphere, so a commando with those dimensions would have a volume minimally starting at 19.372 cc's before any other manipulation of squish band, deck height, valve relief volume/ piston dome, etc is factored in. It would seem from Jim's comment about how much volume @ tdc produces what Compression ratios that the dimensions of 5mm height and 70mm hemisphere could yield a rediculously high CR. I would bet that there's a sweet spot where you get a good usable volume to produce the CR you want and a hemi head shape that flows well. (as in that bathtub shaped hemi that someone posted recently)

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