F1 engine RPM

[xpensive]

xpensive,

Recip mass inertia forces are a function of instantaneous accelerations, not velocities. F=Ma right?

Terry

I think you mentioned this fact the other day Terry, but thanks anyway. In that context, accelleration is actually max-piston-speed over time-to-zero.

[riff_raff]

xpensive,

In mathematical terms I believe it's dv/dt. No?

[xpensive]

Quite obviously so. But simplified, max piston speed is a pretty good place to start in order to estimate accelleration,
when speed always have to come to zero twice per revolution.

[F1_eng]

Xpensive, piston acceleration is not linear.

Terry[/quote]

"In that context, accelleration is actually max-piston-speed over time-to-zero."

You simply can't use this method for calculating, it is too wide a time gap to average, you are going to lose all the acceleration detail.
The "time to zero" method is heavily flawed and results obtained from it would almost always be misleading.

Build a piston displacement model, can be done in 5 minutes in excel.

[xpensive]

Xpensive, piston acceleration is not linear.

Terry

"In that context, accelleration is actually max-piston-speed over time-to-zero."

You simply can't use this method for calculating, it is too wide a time gap to average, you are going to lose all the acceleration detail.
The "time to zero" method is heavily flawed and results obtained from it would almost always be misleading.

Build a piston displacement model, can be done in 5 minutes in excel.[/quote]

Oboy! If you could please spare us five minutes of you precious time, it would be to immense benefit to all of us?

[xpensive]

Something wrong with the quoting here, sorry about that. However, for F1_Eng;

Oboy! If you could please spare us five minutes of you precious time, it would be to immense benefit to all of us?

[bazanaius]

well your piston speed is going to vary sinusoidally right? So your acceleration will too... so if you know the engine rpm and various measurements you can work out your A and your w from your Asinwt... differentiate and set to zero to get your maximum...
Don't even need excel.

B

[xpensive]

well your piston speed is going to vary sinusoidally right? So your acceleration will too... so if you know the engine rpm and various measurements you can work out your A and your w from your Asinwt... differentiate and set to zero to get your maximum...
Don't even need excel.

B

So why don't you enlighten us with a lazy-dog formula on this

[xxChrisxx]

well your piston speed is going to vary sinusoidally right? So your acceleration will too... so if you know the engine rpm and various measurements you can work out your A and your w from your Asinwt... differentiate and set to zero to get your maximum...
Don't even need excel.

B

That would be true if it acutally were sinusoidal, but it isn't. Unless you have an infinitely long conrod... Depending on accuracy needed this may or may not be an issue.

A numerical model is easy to set up in excel or matlab.

EDIT: I fail at uploading formula, so here is a pic.


A as a function of crankshaft rotation.
Where Z = R(crank radius) / L (conrod length)

[xpensive]

Xcellent, a scolar at last! Thanks Chris4x! If phi is zero at TDC, that should also be the maximum a, or what?

[xxChrisxx]

Xcellent, a scolar at last! Thanks Chris4x! If phi is zero at TDC, that should also be the maximum a, or what?

Yeah max force will be at TDC, this will act in the opposite direction to the gas pressure force.

You'll get maximums at TDC and BDC, depending on the length of the conrod these will alter. I think with longer rods the TDC value is higher due to less 'dwell time'.

[bazanaius]

Fair shout - I perhaps simplified the geometry a little

[malbeare]

i think that a short rod will produce a rapid change in direction at TDC and a pronounced dwell at BDC. as the conrod lengthens the the effects tend to even out until you get equel sinosoidal effect with an infinite long rod.

[xxChrisxx]

i think that a short rod will produce a rapid change in direction at TDC and a pronounced dwell at BDC. as the conrod lengthens the the effects tend to even out until you get equel sinosoidal effect with an infinite long rod.

Spot on bud.. I think. I'm a bit drunkk now (aplogies but is it friday night ) so i'll ckech in the morn.
As Z tends to zero, ie as conrod tends to infinite the motions becomes sinusolda. PIC!



EDIT: I've just noticed that the labels on that graph are the wrong way round. The 1/Z = 6 is the blue line. The 1/Z= 4.5 is the red.

[xpensive]

Teriffic Chris4x! Numericals for that curve please?