18 Cylinders Isotta Fraschini (straight six-cylinder x3 )

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Peter Twissell

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HMEM Supporting Member
Even with compensated slave rod eye angles or the Edwards cylinder length adjustment, the stroke of each piston is slightly different, because the angle between the master rod and any given slave rod is wider at TDC ( when the pistons are furthest apart at the tops of the bores) and narrower at BDC (when the pistons are closest together at the bottoms of the bores).
It is possible to get equal compression ratios, by adjusting the length of each cylinder to match the combustion chamber volume to each stroke.
On my big radial, with 6:1 compression, it's not worth the bother.
For higher compression ratios the difference will be more significant.
Pete.

Foketry

Well-Known Member
HMEM Supporting Member
I've been meaning to work out some kind of simplified CAD tool to evaluate these sorts of engine kinematic issues anyways. (I've also done something similar in Excel but starts to get a bit complicated).

Can you provide me more dimensional details of the master rod assembly so it is fully defined? For example layout of the red cross (defines position of link rod bottom ends & crank throw rotation center etc.). Is the vertical rod dimension (pink) exact same as the left/right link rod length (blue). What is the crankshaft throw? What is angle of cylinders viewed from front?

View attachment 142535

The way I'm trying to solve is: make the master rod a fixed block element, insert into engine layout background, adjust alignment & angular constraints to some position of interest, then evaluate resultant position of link rod wristpin point which is indicative of TDC. We can add pistons in too, but but for purposes of CR evaluation, same thing. Example (with random guess measurements) shows TDC of right cylinder bank.

View attachment 142538 View attachment 142537

here is a drawing with more dimensions, I hope it will be useful for this analysis. Please ask if you need other sizes.
My opinion is that, the axes of the 3 connecting rods converge in a single center, TDC and BDC are the same for all 3 cylinders and therefore the compression ratio is the same. Probably the speeds of the pistons change, but all 3 arrive at the same height.

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petertha

Well-Known Member
HMEM Supporting Member
ok, here goes. let me know if I misunderstand any dimensions

definition of MR (master rod) & LR (link rod) and MR+LR assembly with LR aligned to MR center just to confirm your dimensions

layout of cylinders & crankpin throw radius

Now overlay MR & LR to crankpin, align MR to center cylinder which is TDC, take reference dimension

Position MR to find TDC of right cylinder. Here it is not quite straightforward. I initially assumed TDC occurs when LR is aligned to its cylinder, but I realized that is not the case because MR position is also constrained to center cylinder. There may be some unique geometric or mathematical equation solution but the way I approximate in CAD is iteration/convergence. So I specify crankshaft rotation angle, position MR & LR with their constraints, take resultant measurement like example (at 38-deg, throw = 79.8137mm). I vary CS angle across likely range in 1-deg steps. Resultant table & graph. TDC (maximum distance) occurs at peak of curve, somewhere around 79.83mm or 0.17mm less than 80mm reference TDC of center cylinder. So, not very much deviation in practical terms, but it is different! haha. Also note BDC of right/left cylinder is similarly different than center cylinder but I left that for now.

Let me know if you spot errors. First time I did this, always wanted to try. Maybe one of the spreadsheet guys can compare CAD results.

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Vixen

Well-Known Member
Hello Petertha,

Or in other works, ' The geometric errors are so small, they can safely be ignored in a low compression ratio (6:1) and low revving engine (less than 3000 RPM) such as the 18 cylinder Isotta Fraschini model'.

Mike

Peter Twissell

Well-Known Member
HMEM Supporting Member
Mike, yes - in this case the errors may be so small as to be insignificant, but I would leave that to Foketry to decide. Given his attention to detail in other areas, he may choose to correct for the slight differences in compression.

The princliple is sound - geometry of the master rod does alter stroke in other cylinders and it is worth going through the calculation in order to decide whether to ignore the difference or not.

petertha

Well-Known Member
HMEM Supporting Member
Or in other works, ' The geometric errors are so small, they can safely be ignored in a low compression ratio (6:1) and low revving engine (less than 3000 RPM) such as the 18 cylinder Isotta Fraschini model'.
Yes, I'm happy for Foketry the calculations turned out this way, resulting in no extra trimming/shimming work unless he wants to. The kinematics was a bit of morbid curiosity on my part. Now I have a new tool & some added understanding. I suspect the low CR deviation on this engine is a function of the narrower cylinder angles, tighter clustering of LR's & stroke length, either by design or the unique layout. The Edwards 5-cyl radial I mentioned has equal uncompensated LR spacing resulting in significant CR deviation requiring secondary modifications. It's MR-LR assembly is more spread out. Anyways, enough of this business, back to Fraschini 18 cylinders!

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Foketry

Well-Known Member
HMEM Supporting Member
Yes, I'm happy for Foketry the calculations turned out this way, resulting in no extra trimming/shimming work unless he wants to. The kinematics was a bit of morbid curiosity on my part. Now I have a new tool & some added understanding. I suspect the low CR deviation on this engine is a function of the narrower cylinder angles, tighter clustering of LR's & stroke length, either by design or the unique layout. The Edwards 5-cyl radial I mentioned has equal uncompensated LR spacing resulting in significant CR deviation requiring secondary modifications. It's MR-LR assembly is more spread out. Anyways, enough of this business, back to Fraschini 18 cylinders!

View attachment 142652
Your analysis is very interesting, in fact I hadn't considered the CR deviation. Basically there are no appreciable differences in the compression ratio, but I can take them into account when I drill the piston pin, 0.17 mm displaced per 12 cylinders compared to the central 6 cylinders. Thanks for the work done

Foketry

Well-Known Member
HMEM Supporting Member
Grinding to make 6 cylinders planar
As I said in this engine there are 3 heads, one head for 6 cylinders.
This means that the 6 cylinders of each bank must be at the same height, very precise and planar.
I mounted a small cup wheel on the 2.2Kw spindle, 20,000 rpm
The engine block with the 18 cylinders on the divider and tailstock, aligned on the crankshaft via a shaft

Basil

Well-Known Member
HMEM Supporting Member
This is going to be an amazing sounding engine!

Foketry

Well-Known Member
HMEM Supporting Member
All 18+1 pistons are finished (Aluminum 7075)

the radial groove on the piston crown is necessary because the spark plug protrudes horizontally into the cylinder

first step : turning on my little CNC lathe

second step : drilling

third step: drilling and reaming the wrist pin hole whit a jig

Fourth step: internal milling

fifth step: groove milling on the piston crown , a jig is required to orient the milling with respect to the wrist pin

after an ultrasonic cleaning the pistons are completed , 18+1

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ajoeiam

Well-Known Member
All 18+1 pistons are finished

Fourth step: internal millingView attachment 143012

fifth step: groove milling on the piston crown , a jig is required to orient the milling with respect to the wrist pin
View attachment 143013

after an ultrasonic cleaning the pistons are completed , 18+1

Curious - - - there is no obvious orientation for the 'internal milling'.

How did you keep a consistent setup to keep everything oriented?

Foketry

Well-Known Member
HMEM Supporting Member
Curious - - - there is no obvious orientation for the 'internal milling'.

How did you keep a consistent setup to keep everything oriented?
Yes, you are a keen observer , I put a long pin in the piston hole and with a 90 degree angle square I oriented the piston with reference to a plane of the vice , making sure that while I was screwing the ring nut the piston didn't rotate.

ajoeiam

Well-Known Member
Yes, you are a keen observer , I put a long pin in the piston hole and with a 90 degree angle square I oriented the piston with reference to a plane of the vice , making sure that while I was screwing the ring nut the piston didn't rotate.
Dunno if I'm a 'keen observer' (LOL) - - - - just had to fight a few of these kind of 'not simple' setups in the past and wondered what you had used.

Experience is a right royal female canine (to not offend any overly prissy ears) and she really doesn't care how much it matters or how much work it was to get to that point!

Always trying to learn and really know that I sure don't have enough years left to make all the mistakes never might all the projects - - - - LOL!!

Vietti

Well-Known Member
HMEM Supporting Member
I now hold the piston blank in a square 5C collet and do the internal machining and most importantly it's quick and easy to locate and drill the wrist pin hole properly. Pistons over 1" can be turned with a stub to fit the collet.

Foketry

Well-Known Member
HMEM Supporting Member
first running-in of the cylinders with pistons, rings, connecting rods and bushings, a couple of hours on the lathe at 120 rpm

Foketry

Well-Known Member
HMEM Supporting Member
Brass flywheel, This is an aeronautical engine but instead of a propeller on the crankshaft I thought of putting a flywheel to eliminate a safety problem and make the engine run more smoothly.
I made the project in 3D, printed the PLA mold and made the brass casting.
The flywheel is fixed to the crankshaft with a 5 degree cone. I inserted a steel bushing with a conical hole in the flywheel hub.
Weighs 2.2 kg

3D printing after painting and sanding

brass casting

flywheel after turning and sandblasting

I then fixed a second bushing with a one-way bearing to the steel bushing with the conical hole.
Through the one-way clutch I will start the motor with the cordless drill

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ajoeiam

Well-Known Member
Brass flywheel, This is an aeronautical engine but instead of a propeller on the crankshaft I thought of putting a flywheel to eliminate a safety problem and make the engine run more smoothly.
I made the project in 3D, printed the PLA mold and made the brass casting.
The flywheel is fixed to the crankshaft with a 5 degree cone. I inserted a steel bushing with a conical hole in the flywheel hub.
Weighs 2.2 kg

3D printing after painting and sanding

brass casting

flywheel after turning and sandblasting

View attachment 144042

I then fixed a second bushing with a one-way bearing to the steel bushing with the conical hole.
Through the one-way clutch I will start the motor with the cordless drill

View attachment 144043

View attachment 144045 View attachment 144046

Gorgeous looking work - - - sir!!!!

Curiosity question just in case anyone knows.
From the pics I've seen it was quite common for the spokes on a flywheel on a steam engine to not only be ellipsoidal in shape but also curved.
Is there any physical reason to do the curving? (i.e. for less noise, less air resistance etc)

Foketry

Well-Known Member
HMEM Supporting Member
Gorgeous looking work - - - sir!!!!

Curiosity question just in case anyone knows.
From the pics I've seen it was quite common for the spokes on a flywheel on a steam engine to not only be ellipsoidal in shape but also curved.
Is there any physical reason to do the curving? (i.e. for less noise, less air resistance etc)
One of the reasons I know about flywheel spokes is that if they are not thin and curved, when the cast iron cools it generates tensions that causing breakage or detachment.
In my case the brass didn't have this problem, I made the fins inclined so as to create a small movement of air for the cylinders.