Secondary engine balance
Secondary engine balance
(OP)
Hi! I can't understand, why seconadary forces are 2x per revolution? why from 0deg to 90deg secondary force curve goes down, and from 90deg to 180deg curve goes up? can someone explain?
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RE: Secondary engine balance
https://en.wikipedia.org/wiki/Scotch_yoke#mediavie...
However, it is not just at second order, the fourier series for that waveform has components 1,2,4,6,8, etc
A scotch yoke has a pure sinusoidal displacement graph, hence has only a first order component.
Cheers
Greg Locock
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RE: Secondary engine balance
Do that on an inline four, and when the crank throws are at 90 degrees, the pistons are all below the halfway point. When the crank throws are straight up and down, two pistons are at top, two are at bottom. Figure out where the centerofgravity is in both cases. There's your answer for the traditional inlinefour secondaryimbalance buzzy verticalshake vibration at higher revs.
And that's not the only thing going on in an inlinefour. When the pistons are all TDC/BDC, their kinetic energy is zero. When things are at 90 degrees, their total kinetic energy is at a maximum, and neglecting combustion/compression forces, that kinetic energy came from the crankshaft. Result ... instantaneous crank rotation speed is lower at the 90degree point than at TDC/BDC, neglecting combustion forces. That happens twice per revolution ... and if you again look at conservation of momentum, the irregular rotation speed of the crankshaft also translates to an irregular rotation speed of the cylinder block. The average rotation speed of the cylinder block is obviously zero, but the instantaneous might not be. More buzzing.
And speaking of that combustion force ... that also happens twice per revolution on an inlinefour, and it's phased differently from the inertial effect and the magnitude depends on where your foot is on the accelerator pedal.
There's a lot going on at twice crank rotation speed in an inlinefour.
RE: Secondary engine balance
And all the beatiful mathematics that comes with it will be lost if/when we move away from reciprocating engines as prime movers. 100ish years of steam, then 100ish years of internal combustion.
 Steve
RE: Secondary engine balance
RE: Secondary engine balance
RE: Secondary engine balance
A single with balance shafts that counteract the firstorder imbalance will still have the secondary imbalance for exactly the same reason that an inlinefour does.
Remember that these balance factors are a sum of ALL of the higherorder frequencies; see the link in Greg's post above. Just because you have a big firstorder imbalance that dominates the situation doesn't mean the rest of them don't exist as well!
RE: Secondary engine balance
The crank must tug on The pair at TDC tomake them "hurry" away from TDC due to the stroke/rod length relationship. The crank doesn't have to tug very hard on the pair at BDC because they tend to "dwell" at BDC. At each pair's TDC the crank is being yanked upwards. It is almose like there is extra piston assembly spinning uncounterweighted at 2x crank speed
RE: Secondary engine balance
Combining and phasing sets of pistons to cancel these harmonics is part of engine balancing.
 Steve
RE: Secondary engine balance
RE: Secondary engine balance
Think of it this way. The primary "sine wave" is the main upanddown motion. The secondary (twicefrequency) "sine wave" is the component introduced by the connecting rod going sidetoside. If you look ONLY at that connecting rod sidetoside motion and neglect the primary upanddown for the moment, the piston will be highest at 0/180 and lowest at 90/270.
It's not quite that simple, because that secondary motion is not purely sinusoidal which means there are higherorder frequencies as well.
RE: Secondary engine balance
Engineering is the art of creating things you need, from things you can get.
RE: Secondary engine balance
 Steve
RE: Secondary engine balance
RE: Secondary engine balance
RE: Secondary engine balance
You need to understand Fourier series. If you have an engineering degree and they didn't teach Fourier series, you should find another University and start again.
Any signal (but especially repeating ones) can be broken down into a series of simple sine waves which "add up" to form an exact copy of that signal. Each sine wave is twice the frequency of the previous.
The secondary force is not an "extra" force being applied. The piston motion (and therefore the force) is not a pure sinewave due to crank/rod/slider geometry. It is "close" to a sinewave at crank frequency so that sinewave is the starting point (fundamental) of the Fourier series. Most of the deviation from this pure sine wave is corrected by adding the first harmonic (double frequency) component (for the reasons given by other posters), but additional frequencies (at much lower amplitudes) are also present. The first harmonic is the "secondary balance" we are discussing.
Engineering is the art of creating things you need, from things you can get.
RE: Secondary engine balance
RE: Secondary engine balance
Engineering is the art of creating things you need, from things you can get.
RE: Secondary engine balance
This is the way I like to think of the terms of a Fourier series. Dynamically, as rotating complex vectors. The artist here has switched the normal convention of Xreal, Yimag (or rotated them), but the basic concept of independence and superposition (addition) of is shown very well. Real signal in the time domain is the projection of the summation on the real axis.
 Steve
RE: Secondary engine balance
" But why these secondary forces going up, when piston is or going at BDC? "
The crank is saying, " come here, piston ", speaking by proxy thru the connecting rod.
http://s3.amazonaws.com/rapgenius/rottweiler2.jpg
Force is related to acceleration. Velocity can be +, minus, or zero and acceleration can still exist. I'm accelerating at 1 g and I'm just sitting in my chair.
RE: Secondary engine balance
"
Should be "the frequency of each sine wave is a multiple of the fundamental frequency"
Engineering is the art of creating things you need, from things you can get.