TDC Tool (Prezoom)

[rgh0]

Only true for simple harmonic motion, connecting rod angle changes it.

Jon the Chief

As Jon undoubtedly knows a "Scotch yoke" assembly generates true sinusoidal motion. A crank and connecting rod assembly generates a modified motion which approximates sinusoidal motion. The bigger the rod to stroke ratio the closer to sinusoidal motion the piston adopts. At a theoretical infinite rod length you have true sinusoidal motion. However when you are close to TDC or BDC ( within a few degrees) the piston motion is in practice equivalent to sinusoidal regardless of rod ratio as the rod is in practice vertical at these points with no effective rod angle

Interesting mathematics if you do the analysis but not really relevant to the issue.

To get TDC accurate you need to measure either side of TDC where you have a decent vertical movement for a small angular movement and seek the centre point between the two measurements assuming no slop in the various bearings on direction reversal !

cheers
Rohan

[john.p.clegg]

"To get TDC accurate you need to measure either side of TDC where you have a decent vertical movement for a small angular movement and seek the centre point between the two measurements assuming no slop in the various bearings on direction reversal !

cheers
Rohan "


Which takes us back to page 1

It's O.K. taking TDC from the piston movement through the plug hole ( very little movement over quite a few degrees) but much more accurate taking it from somewhere like halfway down the stroke ( equidistant BTDC,ATDC) with a timimg disc and finding the mean (TDC)...

John

[Craven]

Hi,
Generally accepted as a method of find TDC but in theory doesn?t the gudgeon pin off come into play in absolute terms. Here are some numbers I worked out for a motorcycle engine many years ago, these show why you need to move away from TDC to take your measurements.

Ron.
Deg mm BTDC
1 ------ 2.91
2 ------ 2.93
4 ------ 3.02
6 ------ 3.17
8 ------ 3.39
10 3.66
12 3.99
14 4.39
16 4.84
18 5.35
20 5.92
22 6.54
24 7.22
26 7.96
28 8.75
30 9.60
32 10.50
34 11.45
36 12.45
38 13.50
40 14.60
42 15.74
44 16.93
46 18.17
48 19.44
50 20.76

[billwill]

BILLWILL: TDC is at the top of a sine wave with a magnitude equal to the stroke of the crankshaft.

Only true for simple harmonic motion, connecting rod angle changes it.

Jon the Chief

Hmmm... Must be very close to sinusoidal near TDC and BDC, because the conrod is vertical at those points

[oldchieft]

BILLWILL: TDC is at the top of a sine wave with a magnitude equal to the stroke of the crankshaft.

Only true for simple harmonic motion, connecting rod angle changes it.

Jon the Chief

Hmmm... Must be very close to sinusoidal near TDC and BDC, because the conrod is vertical at those points

Not true, the change of angle is most near TDC and BDC, in theory the offset of gudgeon pin that reduces piston slap would also be a factor, but for practical purposes it make no difference.

By the way, from a college class 40 years ago.

Q, Where do you find a scotch yoke?

A, In a scotch egg!

Jon the Chief

[holywood3645]

Im sorry I started this Rob

[billwill]

Im sorry I started this Rob

Never be sorry for starting a topic which educates others (especially when it proves me wrong)

I got interested in this so I mentally drew a diagram:

Piston Trigonometry

PistonPositionDiagram.png (8.48 KiB) Viewed 992 times

And then did the calculations. Being lazy I used an Excel Spreadsheet, so here it is.
You too can use it to do the calculations for any piston and crank. Just fill in the stroke of the crankshaft and the conrod size and it will do the rest for you:
Excel 2007 version
http://www.datahighways.net/support/downloads/PistonPositionCalculation.xlsx
Excel 2003 versiom
http://www.datahighways.net/support/downloads/PistonPositionCalculation.xls

Incidentally I used this to discover the error of my ways and found my initial sinosoidal assumption would be about 10 thou wrong by 12 degrees from TDC.

<later> I made a slight change to the doc so that the diagram would not obscure the figures in LibreOffice, so if you go the earlier version downnload it again.

<later still> There is one minor layout error on the spread-sheet, the distance shown as h in the diagram (above) is column labelled [g] on the spreadsheet.

[john.p.clegg]

Why won't my excel open this...

John

[billwill]

Why won't my excel open this...

John

It was done with Excel from Office365, & says it is an Excel 2007 Workbook.

I've added an Excel 2003 version in the above message, try that one.

[billwill]

I've added an Excel 2003 version in the above message, try that one.

Hmm, it opens here in Read-Only mode, so if you want to try different strokes or conrods, it looks as if you have to first save it under a slightly different name and then open the newly saved version.


Too much caution in computer programs nowadays, once upon a time they used to do what they were told to do..

[john.p.clegg]

Thanks Bill

That sorted it..

John

[billwill]

There is one minor layout error on the spread-sheet, the distance shown as h in the diagram (above) is column labelled [g] on the spreadsheet. And Gudgeon is spelled wrong.


~~~~~~~~~~

I would be grateful if any member that has done these calculations before, would check my method; I might have got it wrong.