The spark plug fitting is now ready for installation into the burner duct. Last week I had made the decision to mount the spark plug fitting at about 10” past the step of the burner duct. Hopefully this will insure that the fuel air mixture will light reliably. Ultimately it will take testing to see if this is a wise decision.
I removed the burner duct from the mixing chamber in preparation for the install. The support cradle system that I built earlier makes for easy service of the burner duct.
I used a 1-3/8” hole saw to cut a hole in the side of the burner duct. Cutting oil was used to keep the saw cool during the cut.
A Dremel tool was then used to grind the surface of the hole in preparation for welding. This will insure that there are no impurities on the surface that may contaminate the weld. The light gold color you see on the metal was caused by heat the duct received when I ran the engine earlier.
The spark plug fitting was dropped into the side of the duct and TIG welded into place. I used argon gas to back purge the burner duct as I did earlier on the duct nozzle.
It was now time to clean the weld with a Scotch-Brite wheel. I used blue painters tape to mask off the area to be cleaned.
The argon shielding gas did a great job at protecting the inside of the burner duct :0) You will notice that the plug fitting protrudes in about 3/8” from the inside wall of the duct. This is in case I decide to add an anti-howl liner which in some cases can quiet down the screeching noise sometimes caused by unstable combustion. An anti-howl liner is a perforated liner screen installed down the length of the burner duct which may reduce the destructive vibrations caused by this howl. I may also experiment with lining the burner duct with a ceramic coating to reduce heat transfer. The duct will certainly get hot considering there will be no internal cooling air as there is in the combustor.
Now that the plug is finished I can move on to the ignition coil.
I am going to use a standard ignition coil from an early model pickup truck to drive the new spark plug. I have had very good luck using these for gas turbine projects so why stop now ;0)
I welded together a support bracket for the coil to be installed on the rear frame of the bike.
Two 1/4”-20 bolts were used to secure the coil in place just in front of the rear wheel. This position should be close enough to keep the length of the coil wire short and reduce resistance in the line.
A 7 mm silicone coil wire was installed on the support bracket as seen below. Three high temperature silicone insulated straps were used to support the coil wire.
I reinstalled the support bracket and connected the coil wire on both ends.
I now needed to test the ignition system so I dug out my old mechanical coil driver that I made for the GR-1 project. The driver uses an electric motor and cam to open and close the coil points. A condenser is also connected to the points which helps reduce arcing at the contacts.
I connected the driver to the coil for a spark test. I was very happy to hear a loud audible buzz from inside the burner duct. I used a telescoping mirror to inspect the spark gap as seen below.
It was now time to get a temporary fuel system together so that a live fire test could be performed. The Summit Racing fuel pump (Cat #SUM-G3138) I had purchased earlier was unpacked and installed on a plywood test board. I ran the pump through a series of tests to see what kind of flow rates it could handle and was quite surprised. The pump is capable of producing as much as 80 PSI at a flow rate of 30 gallons per hour!!!
The test board I used to bench test the pump was retrofitted for use as temporary fuel delivery system for the afterburner. The plumbing included a bypass relief valve and a delivery metering valve which will be used to adjust the fuel flow to the mixing chamber nozzles.
I set up the fuel system and ran the engine to see how well the fuel would vaporize. I made a couple of pretty big “smoke” clouds in front of the house ;0) However I dared not try to ignite the burner for fear of alarming the neighbors. This stuff is already pretty loud, the last thing I want to do is aggravate my tolerant neighbors by kicking it up a notch :oP
I made plans to test the afterburner at the Highland test site with the help of my good friend Sgt. Adam Hicks (who has recently returned from serving our country in Afghanistan). We trailered the bike out to the site on a Saturday and set up the test equipment in preparation for a live fire test. At the time I really did not know what to expect when firing off the burner but I was sure to find out ;o) I started up the GR-7 and allowed the engine to acclimate to operating temperature. We then tested the fuel flow through the nozzles to see how well the pump can move diesel fuel. After creating some seriously huge smoke clouds it was time to try igniting the fuel air mixture for a burn. The coil driver was attached to the battery and the engine was throttled up to about 50,000 RPM. I gave the signal to Adam to open the fuel delivery valve and POP - POP - POP barked the burner duct. It was quite startling as the pressure waves hit us in the chest much like a sonic boom. Once we got over the surprise of how loud the burner was we resumed testing. We proceeded to experiment with fuel pressures as well as engine RPM’s to see where the “sweet spot” was for reliable ignition of the fuel.
After a short while we became more comfortable with the noise and started to test higher burn rates. It was clear that the burner was producing a lot of thrust considering that the 300+ pound bike was lurching foreword as the burner ignited. It was a real shame that we did not bring a scale to see what the static thrust was :0\
At this point we had fired the afterburner an number of times over the period of an hour. We had found the sweet spot for ignition at about 44,000 RPM with a fuel pressure of about 25 PSI (20 PSI actual due to pressure differential). The burner easily ignited at these settings with very little “pop”. However the burner is extremely loud which solidifies my decision to test the burner at a remote location.
The engine was eventually throttled up to full power and 35 PSI actual fuel pressure was applied to the nozzles. The roar from the burner was impressive at this point and actually started to screech as we approached full throttle. The burner duct was heating up quite nicely and I’m pretty sure it was glowing at one point. Little did I know that the rear tire was absorbing all of that infrared heat produced by the burner duct as it was starting to smoke :0P We shut down the engine and cooled off the rear tire and fender before packing up the show and heading back to the shop. By now the burner duct had been transformed from the shiny stainless duct to the dull brown pipe.
For the most part I learned what I needed to learn on this last test. I now need to build a proper fuel delivery manifold that will allow me to adjust the amount of fuel being delivered at certain parts of the throttle cycle. If all goes well I will have a two stage fuel delivery system that will bump up the fuel flow when needed. This proposed system should save fuel and allow the engine a little more throttle control opposed to a fixed fuel flow system. Oh, I will also need to build a heat shield for the rear tire ;oP It is pretty clear that the burner duct will get super hot during extended runs. Other than that I am pretty excited to see the burner get one step closer to completion. A big chunk of the work is already finished with only a few details left to sort out. Hopefully this new afterburner will propel the GRV-2 to new speeds. I guess we will have to wait and see :0)
Please join me again for the next installment of the GRV-2 jet bike project!!!
