filtering data by removing a specified frequency (noise)

Hi, I have Pressure-crank angle degree data for an Internal Combustion Engine, before combustion starts, the curve is realistic and smooth, after the combustion starts, a pressure wave, of known frequency, travelling through the combustion chamber resulting in disturbing the original pressure increase curve, how can I remove this known frequency from my data ? I tried with fft, butter, and filter but I couldn't done, any kind of help will be greatly appreciated, thanks in advance,

[Image Analyst]

Some screenshots would help. Like, what does your data look like normally, and with the pressure wave rippling though it? And what do the spectra look like for those two cases?

[Mamdouh]

thanks a lot for your concern and reply, I attached my case, the horizontal axis is the crank angle degrees (theta), while the vertical is the pressure in (bar), the engine rotates with 5000 revolutions per min., the disturbance frequency is 252 kHz, starts with the ignition, causing the saw teeth shape, and I wanna remove it, the regular shape is almost similar but without this oscillation,

Answer 1

[Image Analyst]

Take the Fourier Transform. Look for the spike at that frequency (252 kHz) and set it to zero. Then inverse transform.

Or, alternatively, you could try a Savitkzy-Golay filter (sgolayfilt in the Signal Processing Toolbox) for a one-liner solution. The SG filter is a sliding polynomial filter - it replaces the element at the center of the sliding window by the polynomial fit, at that location, of the data inside the window. (Demo available upon request.)

[Mamdouh]

thanks again, Image Analyst, really, I appreciate your fast and informative replies, regards

[Marlene Berke]

Could you explain how to set the spike at a particular frequency to zero?

[Image Analyst]

You need to find out what indexes of your signal encompass the spike, then do this:

yourSignal(index1:index2) = 0;

[Martian]

Hello Image analyst , Its really useful topic for me . Can you give demo of how to use Savitkzy-Golay filter for this particular case and also how to determine disturbance frequency. Thank you in advance.

[Image Analyst]

No, but I can attach some pre-existing demos I've made using the function.

[Martian]

Thank you so much for your reply.