Inherent balance & the Otto cycle engine
Mea Culpa if I took the 'previous' thread referred to astray, but out of that came a good discussion.
A lot of engine machine shops are full of buncum and codswallop so the inline 4 flywheel dowel discussion is welcomed. These sweet Lampredi SOHC engine likes to spin and they like to buzz like a wasp nest too. As this thread wandered a little already [I feel appropriately diverging] into a discussion about how imbalanced the 4 cylinder really is.
My list of balance from best to worst [ not all permutations included], as a 'enlightened shade tree mechanic' is a little general but none the less hopefully useful when trying to deal with a machinist that wants to do what they are capable of [or most likely what they are used to] and not what the relative truth is about what you need with your Fiat SOHC. I see where Paul is coming from and having noted Bernice’s previous comments regarding how the harmonic probably adds to the weakness of the joint between flywheel and crank can we explore that a little?
Notes;
Inherently balanced engines have motions that cancel each other out.
Primary modes are vibrations at the first harmonic (at the engine speed/frequency), imbalances caused by variations in the component weights.
Secondary modes are 2nd harmonic (occuring at twice the engine speed) vibrations caused by non sinusodal motion of the pistons amongst other things.
Not engines all are the same.
The fabulous vee twelve is IMHO the smallest vee configuration with near perfect inherent state of balance, due to having two inline sixes joined at a common crank, the V-12 is naturally balanced regardless of its V angle. So count in the Flat 12 for inherent balance as its a boxer as well. Plus when the fabulous vee twelve is spinning 'on song' with velocity stack'd carb's they sound glorious and are good for the soul, what's not to love - price and they a huge.
The common vee-eight engine with crankshaft phase angles of fi = 0, 90, 270, 180° has an unbalanced primary moment as does the inline four from which it is derived, their bank angle allows ease of assembly on the production-line tooling. Notice I didn't say cross-plane, 90° V8, different beast.
The vee-six engine with 0, 240, 120° crankshaft has an unbalanced primary and secondary moment as does the three-cylinder inline from which it is derived. I had made a comment about the vee six which IMHO its 'true' form needs a 120° vee angle for proper balance. Bastardry by going to 60° vee angle to reduce packaging width on 6-throw even firing crank. Or using 90° vee angles for ease of engine building enjoyed by production-line 90° vee eights. 90° vee sixes will run rough due to uneven firing unless the crankshaft is redesigned to shift (or splay) the two conrods on each pin by 30°. Homage should go to Buicks mid-Seventies idea with its ingenious “split-journal” unit, essentially a 90° V8 with two cylinders lopped off but achieved 120-degree firing despite its V angle.
The straight six has excellent idle characteristics with one cylinder on one end of the crank firing followed by a cylinder at the other end combined with the overlapping torque generation at every 120° of crankshaft rotation it minimises the rocking motion in the horizontal plane (ie in line with the crankshaft). So has superb plane balance of reciprocating and rotating mass, with the addition of the perfect phase balances they have a reputation for smooth power delivery.
The straight four would like to fire at every 180 degrees of crankshaft rotation (720/4=180). Having firing events that occur in equal increments, for balance. But due to the geometry of the crankshaft and rods within the engine, fours shake in both the horizontal and vertical planes. Fours normally do not have overlapping power stroke, so tend to vibrate the engine back and forth rotationally on X-axis, Bernice explained how this motion tries to shrug the flywheel off. It may seem solid but metals can really be elastic, these imbalanced forces combined with sudden rotational forces can shear bolts.
I tagged
this page I stumbled upon a while back as it did a better job than I could at explaining straight fours imbalance when I was bumbling around the Web looking at building a 2.6 litre flat type 4 for my bay window VW Kombi. It was unrelated at the time as it’s an I4 discussion rather than boxer about the large capacity Granada engine but went into a little depth about issues with larger in line 4's and why balance shafts became popular as engine capacities grew. No doctoral jargon but something that is easily understood by most of the guys I have pointed at it. Might help Jeff with his young fellas understanding of their latest’s acquisition. As well dispelling the modern myth that Mitsubishi invented the balance shaft. They didn't, goes back to Frederick Lanchester’s cars in 1911 apparently. So the old adage "want a new idea - read an old book" comes to mind. That our SOHC doesn't have a balance shaft probably comes from its inception as a small capacity roughly 1 Litre engine, as you grow to 2 Litres and beyond the unwelcome harmonics become a bigger problem with I4 motors.
Despite
flat-fours having a problem common to all four-cylinder otto four-stroke cycle engines in that the power strokes do not overlap. The flat-four fires at 180-degree intervals, and its V angle is 180 degrees, maths which leads to a balance of firing forces. The flat-four, in fact, balances all three of the different types of forces. Plus I love the "flat four burble" of the old Kombi, sadly Subaru engineered that sound out.
To go a little further OT before I come back again I'd like to show how the old VW boxers did things and how pinning is a no brainer in that marquee. I hope to show why a flywheel is mated to an engine described in one solution does not fit reasoning in all engine permutations. If you come across a engineering shop that says 'Nnnarrrr' we don't think bla bla bla, you have a bit of an understanding that you should take your motor and your wallet elsewhere. Most AirHeads don't pin their flywheels because of the high rev happy nature of the old aircooled engine but because the way they are fixed to the crank is rubbish.
Give what Paul said
Quote
"If you pin the two together, the dowel pin only carries the amount of torque above what the frictional force is carrying but very importantly prevents any movement that would loosen the bolts and lessen the frictional force."
Relate this to how inferior the clamping force is in a type one VW engine where the standard 4 dowels manage to stop it from shearing and spinning on the end of the crank. Is this similar to the way safety wire stops the cap head screws walking out of the CV joints?
VW's type One and Four engines flywheels are held on the crank with ONE "Gland Nut' ( sililar to the way we do the crank pulley) and four dowel pins [8mm]. Despite the flat 4 boxer being more 'balanced' than our SOHC for anything of a serious upgrade the 'insurance' was to double that count to 8 pins. The factory VW fixture is pretty lame as it relies on just one big 'nut' [a holow bolt in reality], because they engineered that design to a price and a low specific HP, low end torque and not high revving HP. It’s common to drill the crank and flywheel for pins, you can jump on eBay and find 'puck' guide templates with accompanying pins easily from a number of brands that have been in the VW game for a long time. It’s not something I would do at home as the quality of the mill and interference fit of the pins is important, I've seen pins elongate out of a crank with what I surmised as poor fitting pins.
I still own ~140 HP VW engines that have sustained years of work with such small 'land' between the pins. Would I drill the Fiat SOHC crank for four pins rather that the current two, good question I have no data or experience to answer that. But I'd be interested to hear some thoughts. Two is all I did as that is what Fiat drilled into the crankshaft, so I just went with their 'intention'. Some luminaries in the board stated that pinning is not just a good idea but desirable and I was trying to shove ~200 HP thru a kevlar friction plate. I knew that my VW experinces showed there was indeed benifit so for the small outlay why not follow thru on the flywheel as the crank was already ready. I guess it was an intention that never implemented as I have never seen a factory SOHC with pins in the flywheel so am flummoxed as to why they only went 1/2 way. Or were the two holes in the crank for something else in the manafacturing proccess. First world problem I know but I have always wondered what they did that for.
So like Ulix if you came across and engine builder that says
"Don't need it if properly clamped" and "if there is enough force to snap 6 bolts, what is that pin going to do?"
look around the shop an look at the types of machines he is working with and on. Not all engines are the same despite being Otto 4 cycle combustion engines. Inability to calculate the co-ordinates of a PCD drilling is a dead giveaway.
The Lampredi SOHC is a buzzy little engine.
Oil in the mix;
V12, straight 6, boxer engines. The only common 'completely balanced' engines [nothing is truly completely balanced] there are but the point is often moot as hard as one might try and blueprint and component balance an engine once you stick oil in it it goes a little out the window. Try youtube etc to see if you can visualise what happens to oil flying around inside and engine at high Revs it is a random mess and oil clings to the crank in different places with no real dicernable pattern. Hence dry sumps do more than just ensure adequate oil supply, keeping it off reciprocating masses helps too.
In all a fascinating subject that has an interesting Wiki page
https://en.wikipedia.org/wiki/Engine_balance
Quote
"the common types of four cylinder engine. Normal inline-4 configuration[note 5] has very little rocking couples that often results in smooth middle rpm range, but the secondary imbalance, which is undesirable for high rpm, is large due to two pistons always moving together. Rotational vibration on the X axis, which is often felt during idling, tend to be large because, in addition to the non- overlapping power stroke inherent in engines with 4 or fewer cylinders, the height imbalance from connecting rods centre of gravity swinging left and right[note 2] is amplified due to two connecting rods moving together. Intake and exhaust pulse on ordinary inline-four engines have equal 360° spacing between the front-most and the rear-most cylinders, as well as between the middle two cylinders. So an equal-length (longer-branch) four-into-one exhaust manifold, or two 'Y' pipes each merging exhaust flows from #1 and #4 cylinders, as well as #2 and #3 cylinders are required for evenly spaced exhaust pulse. Older twin-carburetor setup often had each carb throat feeding the front two and the rear two cylinders, resulting in uneven 180°-540°-180°-540° intake pulse on each throat. Modern inline-four engines normally have four equal-length runners to a plenum (which is fed by a throttle at 180° evenly distributed frequency), or four individual throttles (at 720° equal spacing on each throttle)."
I certainly hope that I don't come across as stupid and cocksure. I have been oft told I was intelligent but I remember one of my fallen elders saying.
"Self promotion is no recommendation"
Thanks for the discussion, I might learn something.
Best regards
Sandy
