“Engine was to run two up one down firing not 120 degrees, easier to balance.”
With “two up one down” (crankpins at 0, 180 and 0 degrees):
the two cylinders fire together (big bang), the other alone,
while the free inertia moment of the conventional triple is eliminated (so you don’t need any longer an external balance shaft), the arrangement creates a heavy unbalanced inertia force that needs a pair of counter-rotating balance shafts to be cancelled out (say, as in the big single cylinders).
To correct this problem, you should double the weight of the central piston and of the central connecting rod; this will give a balancing similar to the inertia balancing of the conventional 4-stroke even-firing straight-four (those used in most cars).
To exploit the need for a much heavier central piston and cure the uneven power pulses, you can increase the central cylinder bore 1.4 times (1.4*1.4=2, i.e. from 80mm to 112mm): this way you have the balancing of the 4-stroke even-firing straight-four, you have also equal power pulses at equal time intervals, i.e. the 2-stroke triple becomes quite similar (in vibration quality and power delivery) to the 4-stroke even-firing straight-four.
It has been done (more or less) by Illmore in their "5-stroke" three-cylinder engine:
An alternative solution is to use the “T” crankshaft of Triumph.
Quote from https://www.cycleworld.com/story/bikes/ ... explained/
(written by Kevin Cameron
- Triumph’s New T-Plane Firing Order Explained
Do the math: 180 plus 270 and 270 add up to 720.
This Tiger 900’s “T-Plane” crankshaft places crankpins one and three 180 degrees apart, with the number-two crankpin 90 degrees from the others, defining a “T.”
. . .
But in the case of this Triumph, engineers increased the firing interval on two cylinders to 270 degrees by closing up the third interval to 180—not a big change, 12-1/2 percent. Yet without testing the result ourselves, who can say how much this change improves “feel”?
Ah, but doesn’t such a change require a balance shaft?
Even-firing inline-triples already need a balance shaft to cancel what would otherwise be a substantial side-to-side rocking motion, so the only change needed is to reangle the eccentric weights on the already-existing shaft.
End of quote.
The last paragraph of Kevin Cameron is wrong.
Because if you just reangle the eccentric weights on the already-existing shaft, you can’t have the same vibration-free quality as you have in the even firing triple (that with the crankpins at 0, 120 and 240 degrees); the architecture of the latter cancels out all inertia forces while the architecture of the first leaves unbalanced heavy inertia forces of first order and of second order.
You can think of it differently: the T-plane triple is actually your twin (with their cylinder axes at double distance) plus a central cylinder at 90 degrees phase difference. The first order balance shaft can cancel out the inertia moment of the pair of the external cylinders (leaving only a second order unbalanced inertia force) but it can't do much with the central piston.
the even firing triple (crankpins 120 degrees apart from each other) is, by far, the best solution, especially for a lightweight vehicle like an autogyro.
But if you run it without a counter-rotating balance shaft, its vibrations would be more than those in your unbalanced twin (which was like day and night after adding the balance shaft).
In simple words: your best choice for your autogyro is the even-firing triple with an external balance shaft.
Having said that,
when it comes to extra lightweight vehicles (say like the Portable Flyer) a better solution appears the Opposed Piston engine: simpler, more lightweight, more fuel efficient (elimination of cylinder heads, lower thermal loss etc) and top as regards its vibration-free-quality (allowing the direct support of the engine on pilot's body
The single cylinder 635cc PatOP Diesel (no balance-webs at all, not even on the crankshaft) stands free on the desk:
This single cylinder OPRE Tilting that stands free on the floor needs not external balance shafts. A fast “static balancing” was applied (if it was a correct dynamic balancing, it would stay completely immovable on the floor).