Oh believe me I know about samples and windowing. It's annoying as crap ;) Hence why I asked the question.

As for the 3 million/min... why do you make me do math? I hate math! so you'd be sampling at 48khz unless you wish to avoid windowing then you'd need to sample at 96khz. Is the timing sensor hall or optical? (I'm trying to estimate the sensor error to guesstimate the perturbed error)

so your calibration uncertainty is 0.04% which ain't bad. If you can nail down the timing uncertainty it will be easy enough to get an uncertainty plot for the pressure distribution.

Windowing occurs when you sample something too slowly and the result is a different reading altogether. For instance say you take a sample of a simple sine wave with a frequency (i'll keep it simple) of 4khz. If you sample that same sine wave at some other frequency that is less than 4khz you could get some surprizing results. Windowing would occur and you may end up looking at a cosine wave with a frequency of 2.3khz and a different amplitude that fluctuates over time. Not exactly what you're looking at? For DAQ's it is a good rule of thumb to sample at twice the frequency than what your expecting to be as your function. IE: if you are ever going to see 48khz in a function you'd want to sample double that to ensure you don't have window the function.

There is also another windowing occurance which usually helps in DAQ systems and is fairly easily explained in the following wiki article:
http://en.wikipedia.org/wiki/Window_function

I don't mean to be contrary, but I received the same 'sample at double the input signal frequency' advice from textbooks and some professors. Other professors with more real-world industry experience advised that data will be much more useful when sampled at no less than 5-10x the signal frequency. Try sampling a 100Hz sine wave at 200Hz, the plotting the results... it's not going to look much like a sine wave. If you logged a fuel injector its operating frequency, the indicated duty cycles would always be 0%, 50% or 100%... Personally I'd like to at least know +/-10%.


80kHz sounds like a very reasonable sampling rate. Assuming the sensor responds quickly enough, an 80k sample rate will give you one datapoint every ~0.5 eccentric shaft degrees at 6500 RPM.

Compare this to the RX-7 'crankshaft' position sensor which has one tooth every 30 degrees... I wonder if your ECU's ignition timing accuracy is better or worse than +/- 0.5 degrees? If your logger has a spare input it would be very interesting to monitor the ignition trigger signals in addition to chamber pressure, either by tapping into the 0/5V signal going from the ECU to the ignitor or by tapping the wire between the ignition coil and ignitor (careful, this will be over +20V due to inductive flyback... make sure the logger can handle it).

You're right. I forgot the word 'least' after at.

This is the way we can get more area under the curve. Notice that the end that changes most is the leading peak pressure end. If you could view each firing of the rotor it would look similar to the five curves shown with not much movement of the trailing tail.

http://i287.photobucket.com/albums/l...ustionlate.jpg

Good news and bad news.

As the mean effective pressure has increased through changes in timing a new problem has arisen. Preignition!

The in-chamber testing can show us the beginning of combustion pressure from the small ignited kernel of flame front, but it is so small at first that the pressure change usually only shows an increase after about 5º of eccentric movement (15º BTDC advance starts showing pressure at 10º BTDC). Imagine our surprise when we started seeing a pressure rise starting at 28º BTDC!

The crazy thing is that it made more power and no real detonation!

Barry

http://i287.photobucket.com/albums/l...reignition.jpg

The curve is ugly, but I like the timing! You should be about peak power at this point (12-18 degrees ATDC for peak pressure).

Keep up the good work! (Try not to hurt your engine, I don't know how safe it is to run a turbo at optimal ignition timing. Most turbo cars I have seen run REALLY retarded.

Notice that this is not the pressure chart but the burn rate. It is ugly but fairly typical. There seems to be a wave cycle of burning.

The proper term for what is happening is probably auto-ignition.

I think Honda has the answer with their two stroke experiments into Active Radical Combustion.

“When the spark plug fires and ignites the fuel mixture, some of the fuel is isolated from the resulting flame by the exhaust still in the cylinder, and does not burn. What Honda has done is to develop a way to ignite all the fuel in the cylinder by using the properties of auto-ignition, and has termed this process Activated Radical Combustion. This title is derived from the way fuel actually ignites. When the fuel is brought to the right pressure and temperature, the molecules break down into what are known as active radical molecules. These are highly unstable chemical compounds which are an intermediate step in the actual combustion reaction. When hot exhaust gas remains in the cylinder, it contains a small percentage of active radical molecules; when these are combined with the incoming fuel charge, the resulting mixture begins to auto-ignite at lower temperature that a pure gasoline/air mixture. What we currently associate with auto-ignition is engine knock, a phenomenon that occurs when the fuel ignites before the spark plug fires, while the piston is still on the up-stroke”

I am working with a Gorilla (Jonathan) and a Trout2 (Jack) on this project.

The real question may be… do we try to eliminate the reversion causing auto ignition or try to control this new ignition source with some type of valve?

Thanks for your input, we learn together,
Barry

I thought that the hand-drawn pressure curves above corresponded to the burn curve below. I didn't intend to add confusion

Could you show the pressure curve for burn initiation @ 28 BTDC?

http://i287.photobucket.com/albums/l...ssurecurve.jpg

Notice the light-off hump at 30º BTDC, also later when the exhaust port opens it has still has 132 psi and 1750ºC temp.

Barry

Peak pressure still at 38 degrees, if that engine were NA, I recommend that you advance the timing more. Turbo, you probably have it pretty much maxed out.

Are you sure that the exhaust port opening time on the chart is accurate? I would have expected the pressure to drop significantly more than 12 psi in 15 degrees of eccentric shaft rotation if the exhaust port were open. This is very close to the curve that would be followed with the port closed. I know that the exhaust port opens 75 degrees before BBDC = 195 ATDC, but I suspect that the trace may be inaccurate in scaling (which is weird, because the no-fire trace line looks Ok).

Can you continue the trace out to 250 ATDC? Your sensor should be able to read about that long.

You could be right. I am not used to looking at pressure curves for gasoline-fueled turbo engines. The valve opening on a piston engine is visible in the trace, even with their slow valve loft. Valve open actually looks a lot like the shift in the curve at 175 ATDC.