2 stroke thread (with occasional F1 relevance!)

[Tommy Cookers]

.. Why design otherwise (an even number of cylinders in a row) when at design stage one can already achieve less vibration and provide for more even pressure pulses in the exhaust ....

firing intervals are nothing to do with engine vibration ie vibration transmitted to the vehicle
they are to do with torsional vibration within the crankshaft - this isn't transmitted to the vehicle

different pistons in a radial must travel different distances, angles etc - so can never prevent all engine vibration
yes eg each cylinder capacity etc is different to its neighbours
regardless of whether cylinder quantity is even or odd
(here the 10 cylinder Smith Static is praised for its lack of vibration)
https://www.airwar1.org.uk/american%20s ... engine.htm

the benefits to the crankshaft designer of odd numbers are less with a larger quantity of cylinders
and eg crankshaft design deficiencies obtained in Armstrong Siddeley 3x7 and 4x7 engines
crankshaft expert Tresilian was brought in - he designed a 4x6 engine (the 1941 Wolfhound) - it was given to RR
(his F1 BRM engine had 4 main bearings and 1 counterweight - till BRM redesigned it and increased its bearing losses)

the V and the radial were expected to merge into a multi-bank multi-row ohc type of engine
the Hydra the Chieftain and the Wolfhound were part of this movement
only the Hydra crankshaft design was deficient (because it had no centre bearing)

computer-savvy people can now armchair-design radials and see their inherent geometrical discrepancies .....
inherent regardless of cylinder count
the vibration consequences of these discrepancies of course reduced by 2 row engines having 180 deg crankshafts

[J.A.W.]

Yeah, its like those Curtiss-Wright design-engineers had a good look at the Napier Lion with its triple 4-banks,
& figured they could 'improve it' by a 'mirror image' section, cut across, & rotated to form a hex-12 (6 x 2)..

The complexities thereby introduced served to demonstrate the folly of taking on recip' dynamics in X/Star form..

[saviour stivala]

A 4-T radial with an odd number of cylinders per row design have more consistent every-other-piston firing order, providing for smoother operation. Less vibration, more even pressure pulses of the exhaust gas. A 4-T radial with an even number of cylinders per-row an equally timed firing cycle is not feasible. Why design otherwise when one can help suppress vibrations at the design stage?.

[Tommy Cookers]

A 4-T radial with an odd number of cylinders per row design have more consistent every-other-piston firing order, providing for smoother operation. Less vibration, more even pressure pulses of the exhaust gas. A 4-T radial with an even number of cylinders per-row an equally timed firing cycle is not feasible. Why design otherwise when one can help suppress vibrations at the design stage?.

Charles Fayette Taylor's book 'the Internal Combustion Engine in Theory and Practice' volume 2 says .....
on p 278 of chapter 8 'Engine Balance and Vibration' .....

'the only serious imbalance in a conventional single-row radial engine is a second-order force which can be represented by a constant vector leading the crank by 180 degrees at top centre of the master-rod cylinder and revolving at twice crank speed in the direction of crank rotation'

[saviour stivala]

‘’The only serious imbalance in a conventional single-row radial engine’’ Can turn out to be very serious indeed, in fact can be greatly amplified by virtue of all cylinders in the row concentrating/contributing their power forces to the winding and un-winding of the crankshaft on just one crank-pin. In fact one of the biggest ever flop, some say a scandal, happened in British aviation in 1917 and it involved the design of a single-row 9-clinder radial engine – ABC motors ltd – Dragonfly. This total flop/scandal happened because the designer managed to design its crankshaft with a resonance exactly in the operating range of the engine. In radial engine design the imbalance will increase with greater number of cylinders in a row, and will increase more the bigger the cylinders are, and will increase even more with each added row of cylinders. A tween-row radial will double the imbalance to that of a single-row, this contrary to some believing that the second-row will even-out the imbalance of the other row.

[coaster]

I remember the Rotec engine was balanced like a single cylinder motor, master / slave rods and a pistons were all considered one mass on the big end journal.
The appearance had the likeness of a large weed whacker crankshaft.
I was not divulged the balance factor.

[Tommy Cookers]

.. the Rotec engine was balanced like a single cylinder motor, master / slave rods and all pistons were all considered one mass on the big end journal.
I was not divulged the balance factor.

notionally it would be 50% of that one mass
though there's different ways of calculating the mass
eg 'Improved Formula for Computing Counterweights of .... Radial engines' by Coppens Trans SAE 29, 101 (1934)
(the Rotec isn't really balanced like a single - crankshaft counterweighting can't give a single full 1st order balance)


and interestingly ...... according to 'Theory of Machines' by W G Green pub. Blackie
for radials with coaxial crankpins - (and of course full 1st order balance via crankshaft counterweight)
3 cylinders is unbalanced in 2nd and 3rd orders of vibration etc
5 cylinders is balanced in 2nd order but unbalanced in 3rd order
7, 9 etc cylinders is balanced in 2nd and 3rd orders but unbalanced in all or some higher orders
4 cylinders is balanced in 2nd and 3rd orders
6, 8, 10 etc cylinders is balanced in all orders

so (in these ways) eg 4 cylinders is better than 5 and 6 cylinders is better than 7
our poster Manolis Pattakos so designed a 2 row 8 cylinder 4 stroke radial engine (and 4 cylinder 2 stroke) see c. P60

[saviour stivala]

The master-rod construction of a radial engine poses a special set of problems, as none of the knuckle-pins move in a circular path, and no single knuckle-pin has exactly the same path as any other, so each knuckle-pin is set at a slightly different distance from the crankpin centre. (see my post on the Bistella 500cc 2-stroke ''carful desgine of the link-pins location on the master-rod''). The counterweight of a radial engine can only balance the average of all inertial forces arising from the variations in piston and articulated rod motion. When any piston is at T.D.C, the counterweight is too heavy. And when that same piston is at B.D.C, the counterweight is too light. This unbalance process occurs twice for each revolution of the crank. A vector that rotates at twice crank speed. The above problems of a radial engine are tamed pendulum/bifilar type of damper/s inserted on the lower end of the counterweights. The Rotec brothers uses a two-piece crankshaft (the two-piece being clamped together by a bolt) on both their 7-cylinder R2800 and the 9-cylinder R3600 radials. The crank front piece (propeller side) contains the crank-pin, on which the rear piece counterweight is bolted/clamped. This two-piece crank system allows the use of one piece master-rod, and so eliminate the use of a bolted-on main bearing cap. Three pendulum/bifilar type of dampers are used on each of the Rotec crank counterweights.

[Tommy Cookers]

.. When any piston is at T.D.C, the counterweight is too heavy. And when that same piston is at B.D.C, the counterweight is too light. This unbalance process occurs twice for each revolution of the crank. A vector that rotates at twice crank speed.
The above problems of a radial engine are tamed pendulum/bifilar type of damper/s inserted on the lower end of the counterweights.

the counterweight is also 'wrong' when any piston is at mid-stroke

the unbalance 'problems' are not 'tamed' by the dampers
the dampers 'tame' the torsional vibrations of the crankshaft - so giving it a long running life
torsional vibrations exist regardless of balance or unbalance

the Rotec is the same as other radial engines - even if its publicity material isn't

[Slo Poke]

What was the idea/reason of the twin engined Dornier bomber planes, during WW2, of having one engine reving faster than the other? The one person I know that can remember them still maintains the sound of a Dornier was very distinctive.

[coaster]

Saviour, you mention

Three pendulum/bifilar type of dampers are used on each of the Rotec crank counterweights.

I distinctly remember the dowel pins were a neat fit for the counterweights, they held fast and did not allow any rattle.
The Lycoming 6 cylinder were a loose fit to allow this rattle movement, clunk about minutely on a hardened steel bush bored slightly oversized.
There were 3 on the lycoming, there is nice exploded view assembly on youtube of 7 cylinder, pretty sure its 1 counterweight.

[saviour stivala]

Saviour, you mention

Three pendulum/bifilar type of dampers are used on each of the Rotec crank counterweights.

I distinctly remember the dowel pins were a neat fit for the counterweights, they held fast and did not allow any rattle.
The Lycoming 6 cylinder were a loose fit to allow this rattle movement, clunk about minutely on a hardened steel bush bored slightly oversized.
There were 3 on the lycoming, there is nice exploded view assembly on youtube of 7 cylinder, pretty sure its 1 counterweight.

If you mean ‘’pretty sure it’s a ‘one’ counterweight’’ for the Rotec, no. both Rotec 7 & 9 cylinder radials have two counterweights, one on their crank-webs, with each counterweight having three inserted pendulum/bifilar type dampers.

[coaster]

I saw the 2010 spec in production, something must have changed, i remembered a wash ground ellipse maybe 220 od and maybe 16mm thick fixed in place by 2 x 14?16? stepped pins.
I dont recall those 3 round holes shown in these videos;
https://youtu.be/eCR1P5w2Oik
https://youtu.be/w6PgUx6LZVM
I concur with your interpretation Saviour.

[saviour stivala]

Cheers Coaster.

[J.A.W.]

2023 iteration of Rotax/BRP/Ski-Doo 850 snowmobile, 180hp with (altitude compensation) turbo.

https://sledmagazine.com/ski-doo-pushes ... c-turbo-r/