balance question 1

[Redcobra2401]

let me start off by saying that, I am no engineer. never went to college just a trucker with a lot of free time in the sleeper and an obsession with anything mechanical.

I am currently designing a 905cui 90-degree v16 that is a flat head/L-head, my current crank design won't accommodate a standard firing order, like that in the Cadillac v16 of back when, I'm wondering if it can be balanced. Would a simple harmonic balancer suffice or if some counterweights on the crank would help with the vibrations? the rpm limit I'm shooting for is around 3500 rpm and projected compression ratio is between 6.5:1 and 7.5:1.

there are 3 possible firing orders I have come up with, this is really difficult to attempt to wrap my head around and I've spent the good part of 5 hours getting a migraine from just getting this far with it. I'm trying to have to keep from scraping the whole crank shaft and starting over, the whole reason for designing it the way i did was to make it easy to machine not requiring counterweights for the primary balance.

1,2,7,8,13,14,5,6,15,16,9,10,3,4,11,12 -

1,2,9,10,3,4,11,12,15,16,7,8,13,14,5,6 -

1,9,3,11,15,7,13,5,2,10,4,12,16,8,14,6 -

attached are snap shots of the crank and the current assembly. any and all help is welcome especially a solution.

 

[Lou Scannon]

https://www.amazon.com/Kinematics-Dynamics-Machines-George-Martin/dp/1577662504

 

[GregLocock]

While you can design a crankshaft without counterweights it means you'll rapidly run into torsional and bending resonance problems. This is especially likely in the case of long cranks like that of a V16. As to firing orders, that rather depends on what numbering convention you are using.

 

[Lou Scannon]

The V16s I'm familiar with have firing orders like two inline 8s offset by 45 degrees. Inline 8s are setup with a 4 cylinder arrangement of the centre 4 cylinders, and the outer 4 cylinders set up like a 4 cylinder with a gap in the middle, offset at 90 degrees to the centre 4. If there are other arrangements of V16 or L8 cylinder engines, I'd be interested to learn about them.

 

[Redcobra2401]

https://www.amazon.com/Kinematics-Dynamics-Machines-George-Martin/dp/1577662504

awesome! i briefly tried to find material along these lines to read up on but had no luck. you know of any other texts that would help me with this engine in the near future ?

 

[Lou Scannon]

That was the text I had in uni 40 years ago. I guess physics hasn't changed much in the meantime.

 

[GregLocock]

So on production engines the counterweighting is calculated to minimise peak loads in the bearings. The firing order is set by concerns about minimising the variation in VE of each cylinder, and also a desire to minimise the excitation of the torsional and bending resonances of the crank, and possibly block. NVH also gets a look in sometimes. I did briefly look at a long crankshaft V12 with a high specific power, we found the crankshaft resonance problems so intractable that we split it into two and proposed a centre power take off. The project was abandoned soon after.

 

[gruntguru]

The crankshaft in your posted image has 4 pistons at TDC every 90 degrees of rotation. This will only permit simultaneous firing with a pair of cylinders firing together. If you wand regular individual firing you will need to put a 45 deg twist halfway along the crank.

 

[BrianPetersen]

If all you're considering is the first-order vibration, a 90-degree V-twin with the right crank counterweights can achieve it. It follows that they can be stacked end-to-end while maintaining each "V-twin" balanced. (A conventional V8 is, conceptually, four 90-degree V-twins stacked end to end.) Some cancellation of counterweights between adjacent "V-twins" is possible (and normal).

A 90-degree V4 with single crankpins (usual arrangement) has an uneven firing pattern. Examples - Honda RC30, Honda VFR, late model Ducati V4 models. There's a few different ways of phasing the two "V-twins" which result in different firing patterns. Ducati's 70-degree phasing between the two crankpins has resulted in a rather dominant layout in both MotoGP and World Superbike ...
A 90-degree V6 with single crankpins can have balanced first-order vibration but will be uneven-firing. Example: the original GM/Buick 231 cubic inch (3.8 litre) "odd-fire" V6. Newer designs have offset crankpins to get an even firing order ... and balance shafts.
A 90-degree V8 with single crankpins is well-balanced and even-firing. Second-order vibration is best with a cross-plane (90-degree interval) crankshaft. This is a very common layout.

It follows that your V16 should be, conceptually, two conventional cross-plane 90-degree V8s stacked end-to-end with an offset between them that is some odd multiple of 45 degrees. Or maybe a conventional 90-degree V8 in the middle, with the other one cut in half and each half added to the outer ends with that odd multiple of 45-degree offset built in at those interface points.

Crankshaft torsional vibration will be a nightmare.

 

[GregLocock]

Due to our desire to run our I6 at high revs without a complete redesign we decided on exactly that approach - a 12 counterweight crank- so the engine was effectively 6 single cylinders in a row, which reduced the bending and torsion in the cast iron crank. The improvement in NVH was detectable but not really value for money - the new crank was 2 kg heavier and was of course cost reduced out 2 years later. It turns out the 8 counterweight crank actually survived durability. The 12 has a good rep with the enthusiasts.

 

[TugboatEng]

Yamaha runs the V4 firing order on an inline crank.

The GM 2-stroke diesels ran crossplane crankshafts in their inline engines.

All of these engines run very smoothly despite the reciprocating mass being unbalanced. They also have counterbalance shafts.