Jim's Technical Combustion Spud Gun:
Closed Chamber Studies

Introduction

This is the first set of studies done on my technical (research) gun. These results are measurements made on the chamber without a barrel. The barrel port is sealed with a 2" threaded plug.

A drawing  and a photo of the chamber are shown below.

The chamber is constructed of 3" Sch. 40 pressure rated pipe with a 3" cleanout adapter on one end and a 3"-2" reducer, 2" threaded adapter and 2" threaded plug on the other. There are three spark gaps situated as shown in the drawing. The spark gaps are connected in series with a "spark detector" using wires with alligator clips. Any combination of the three spark gaps can be used. The spark source is a "100KV" stun gun. Propane is measured with a syringe and injected into the schrader valve after the stem is removed from the valve. Inside the chamber is a 40mm, 12V brushless CPU fan mounted near the breach end. The chamber is also equipped with a 200 PSIG dial pressure gauge and a 100 PSIG tire pressure (TP) gauge. The tire pressure gauge is used as a peak pressure recording gauge.

The chamber contains a piezo transducer as a pressure sensor. The output of the piezo is recorded using the sound card of a PC. The signal created by a "spark detector" is recorded on a separate channel by the sound card. The spark detector consists of a neon bulb (Ne-2) and a phototransistor (PT). When wired in series with the spark gaps the neon bulb acts as another spark gap and flashes when the spark gaps spark. The light output of the neon bulb is detected by a PT placed next to it. The output of the PT is recorded by the sound card. The spark detector marks the point in the recording when the chamber was fired.

The chamber has some leakage problems. When pressurized to ~90 PSIG with a compressor the leak rate is about 10 PSIG/minute. Most of the leakage occurs around the piston inside the tire pressure gauge. I have tried two different TP gauges from different manufacturers and they both leak around the piston at high pressures. If the TP gauge is removed and the hole in the chamber plugged, then the leak rate drops to about 1 PSIG/minute. The studies reported below were carried out with the TP gauge installed (and therefore the higher leak rate). Hopefully, at the high speed of the combustion process the leakage will be insignificant. The pressure drop in 1 second would only be about  0.17 PSIG.

The TP gauge also seems to have a problem with the very short duration of the pressure spike. It appears to read high due to the high velocity that the gauge scale is accelerated to during the firing process. To offset this I rest my finger on the gauge's scale to add a bit more friction to the scale as it moves.

For more information about the chamber, sensors and electronics see my build page.

The firing sequence is;

1. The stem is removed from the scrader valvee.
2. Propane is measured with a syringe and injected through the schrader valve.
3. The schrader valve's stem is replaced.
4. The desired spark gaps are connected in series with the spark detector and stun gun using wires with alligator clips
   
5. The fan is turned on (if it is being used).
6. A recording is started in Audacity on the PC.
   
7. The chamber is fired by triggering the stun gun.
8. The recording is stopped and the file saved as a 16 bit stereo wav file at 48 KHz (CD quality audio).
9. The chamber is purged, with the fan running, by pressurizing via the schrader with a compressor to ~90 PSIG and then venting back to atmospheric pressure. The pressurize / vent cycle is repeated three times. Three repeats should replace 99.7% of the gases in the chamber with fresh air.
   

Parameters

• 107in³ chamber of 3" diameter Sch. 40 PVC
  
• 70cc Propane in air
  
• Temperature 51F
• PC sound card line Input gain down 1 tick from maximum
• 100 PSIG tire pressure gauge (gauge reads high by 5.5 +/- 1.1 PSIG relative to the 200 PSIG dial gauge)

Results

When fired the closed chamber makes surprising little noise, just a faint "plunk" sound. The sound of the gauges rattling is almost louder than the sound of the combustion. The dial pressure gauge spikes to somewhere near midscale (100 PSIG) and then immediately drops back down to zero.

A screen shot of the Audacity recording window is shown below. This firing used the chamber fan and all three spark gaps. The wav file is here.

This chamber firing was done with the fan running and all three chamber spark gaps. The upper trace is the piezo signal and is related to the pressure in the chamber. The polarity of the piezo signal is arbitrary, the falling trace is a result of the polarity being reversed between the piezo and the sound card. With this wiring, a falling signal represents a rising pressure in the chamber.

The lower trace is the "spark recorder" signal. The highlighted region starts at the spark that ignited the chamber and ends at the peak maximum for the piezo signal. The status bar at the bottom of the Audacity window indicates that this time range was 25.6 mS.

As a first study I examined the piezo "pressure" signal obtained when the chamber is fired without the fan running and a single spark gap compared to the signal obtained with the fan running and all three spark gaps. A graph of the two piezo traces is shown below.

Comparison of No fan + 1 spark (1g) vs. Fan + 3 sparks (1j):

The above graph was generated by converting the wav files to tab delimited text files, using Wav2Txt, and then graphed in Excel. The two traces were aligned by the spark detector signals so the times shown are measured from the ignition event for each firing.

As you can see, the peak piezo signal occurs much earlier with the fan running and three sparks than it does without the fan and only a single spark. The times to the peak signals were ~25mS for the fan + 3 sparks firing and ~49mS for a single spark and no fan.

I have made a series of measurements with the closed chamber in which I varied the use of the fan and the number of spark gaps used. The data is summarized in the table below.

Table Notes:

1. When fan is used for mixing but not during firing there was a 5 minute wait with the fan off before ignition.
   
2. Spark gaps used; B = breech end, C = central, M = muzzle end.
   
3. Time from the spark signal to the maximum signal deflection (peak) and the next zero crossing.
   
4. Relative to the longest time to the zero crossing point.

The "TP Gauge w/Friction" column gives the peak pressure reading obtained with the tire pressure gauge.  The two shots that used both the fan and the full set of three spark gaps pegged the TP gauge. The theoretical peak pressure for the combustion of propane in air at 51F is 129PSIG according to GasEq.

There are two data sets that were obtained using the same conditions. Shots #6 and 10 were both done with all three sparks and with the fan running. The timings are very similar for the two shots.

The piezo signal is not directly related to the actual pressure in the chamber. Piezo transducers produce a voltage that is proportional to the rate of change of the pressure with respect to time. In addition, the sound card also modifies the signal since the frequency is at or below the low end of the audio spectrum. The piezo signal recordings look like they are approximately the first derivative of the pressure versus time signal. In the absence of a method to convert the piezo signal to a true pressure signal we will instead just use the characteristic shape of the signal to identify a reference time for the combustion process. We could use the time to the top (actually the bottom) of the peak as the reference point. In the table above these values are given in the "Time To: Peak" column. Since the piezo signal actually looks more like a first derivative signal I believe that the zero crossing time is probably a better estimate of when the peak pressure occurred in the chamber. The zero crossing times are shown in the "Time To: Zero Crossing" column.

In the graph below the time to zero crossing is scaled by the slowest burn rate (no fan, one spark).

Graph Notes:

1. The "Relative Burn Time" is relative to the longest time to the zero crossing point.
2. "Fan: - -" is no fan for mixing or firing, "+ -" fan for mixing but not firing, "+ +" fan for both mixing and firing.
3. "Sprk - + -" is the central spark gap, "+ - -" is the breach end spark, "+ + +" is all three spark gaps.
   

Conclusions

There are several observations that can be made from this data. It appears that;

1. The pressure in the chamber caused by the combustion process lasts for only a very short time. Based on the response of the dial gauge, it looks like the pressure drops back to about atmospheric pressure in less than one second after firing.

2. Thoroughly mixing the fuel results in the fuel burning about 20% faster.

3. Having the fan running during firing increases the burn rate by an additional 15~20%.

4. Three sparks give a small increase (a few percent) in burn rate if the fuel is well mixed.

5. Running the fan during firing and having all three sparks gives another small increase of a few percent.

6. Three sparks and the fan gives an overall decrease in burn time of 40~45% compared to no fan and a single spark.