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Formulas for Plotting

Engine Building Fundamentals

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Discussion and questions related to the course Engine Building Fundamentals

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I was hoping you would be able to provide the formulas that you used to plot the following examples from the Rod to Stroke Ration lesson.

Piston Position vs Crank Angle vs Rod Length

Piston Acceleration vs Crankshaft Rotation vs Rod Length

I want to be able to plot these out on my own to use when designing an engine rebuild.

Sounds like a high school trigonometry problem. Give it a try with a spreadsheet.

Have you considered using engine simulator software? -- this is the type of calculation that they do. Here is an example:

Desktop Dyno

If all else fails, Wikipedia has an entry for exactly this:

Piston Motion Equations

As David said, check out the on-line options for free software - I've done and plotted those calculations, plus jerk, way back in the day with a calculator at 5 degree points and I strongly recommend you avoid it unless you have a lot of time and patience.

For the most part, it's going to come down to what's available unless you REALLY want to design everything, hoping you can improve things.

As a rough rule of thumb, the longest connecting rod will give the best results - this is going to be limited by the block height (with some engines you can get aftermarket taller blocks) and the height of the gudgeon/piston/wrist pin in the piston which is going to be limited by the ring pack and lands.

Even then, you're going to have to be aware of how the change in piston velocities around the overlap period will affect breathing in that critical area.

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