GRV-2 Jet Bike Project 11/29/09

Posted on November 29, 2009

       OK, where did we leave off? Oh yes, the burner duct support bracket is finished at this point and in need of a duct cradle bracket. The duct cradle’s job will be to evenly support the afterburner duct while still allowing it to “slide” back and fourth during it’s heat expansion cycles. This is super important or else the expansion will bend the frame and possibly push the turbo out of alignment.
       I decided to build the cradle out of 3/16” thick x 1” wide 304 stainless steel bar stock. The bar was carefully formed to match the contour of the burner duct which should afford the most support.

       I welded a couple of mounting tabs to the bottom of the cradle. I also added a small band clamp keeper tab to the outer side of the cradle. This will help keep the “t-bolt” clamp in place (the clamp will be used to hold the burner duct to the cradle).

       The cradle was now ready for installation. I used a set of 5/16” grade 8 bolts to secure the cradle to the support bracket as seen below. I then installed a stainless “constant tension” t-bolt hose clamp to the cradle and duct.

       This special t-bolt hose clamp has a spring assisted tension adjustment. This spring should help the clamp survive the expansion and contraction of the burner duct. However it will remain to be seen just how well it will work once the burner is in operation. I may have to install some ceramic spacers under the clamp if the heat is too great. 

       OK, the next step is to plumb the fuel injectors into the fuel delivery manifold. I decided to use a four port pressure manifold to evenly devide up the fuel pressure for all four fuel nozzles. Each fuel nozzle will be fed with a 3/16” steel fuel line which will come from this manifold.

       A set of 1/8” NPT x 3/16” 45° flare fittings were installed into the manifold in preparation for the steel fuel tubing. The white thread sealant is Permatex high temperature thread sealant (Cat # 59214).

       The manifold was bolted to the rear frame assembly as seen below.

       A set of 90° flare fittings were installed on the mixing chamber. Permatex anti-sieze compound (Cat #133A) was used in leu of white thread sealing compound on these fittings. The anti-sieze will hold up better than the white sealant when subjected to the 1200° F exhaust temperatures.

       It was now time to start fabricating the fuel injector lines. I used 3/16” OD x 0.028” wall corrosion resistant carbon steel tubing for my injector lines. This stuff is easy to bend and is flareable.

       A tubing bender was used to help form the injector lines. Luckily I have some experience in pipe bending (electrician) which saved some time.

       Now in a perfect world all of the injectors would be fed with even length fuel lines. This would of helped to promote even fuel pressure at each nozzle. However the system is already at a disadvantage due to the nozzles being at different elevations. Either way the bottom nozzles will get a slightly higher fuel flow than the top nozzles which is hardly worth worrying about (IMO).

       One factor I needed to consider when designing the mixing chamber nozzle array is the “post-fuel bleed down effect” (this is my own term so don’t bother Googling it). The so called bleed down effect is where the remaining fuel in the fuel lines trickles out of the injector nozzles after the fuel has stopped flowing from the fuel manifold. This is mostly caused by gravity and will effect the lowest positioned nozzles in the array.
       This trickle is somewhat harmless although it can cause carbon build up on the nozzles as well as narrowing of the nozzle orifice over time. An optimum fuel delivery scheme would be to purge the fuel from the injector lines with air each time the fuel flow stops. This system would also help keep the standing fuel in the lower injectors from boiling off due to the heat of the ducting.
       In my case I decided to go simple and use a set of robust 1-piece nozzles which should be able to handle the carbon build up without effecting performance too greatly.

       I tried to keep the fuel lines to the top injectors as short as possible. This should hopefully reduce the volume of fuel that will most likely siphon to the lower injectors after the fuel has stopped flowing.

       At this point I could not wait to see how the engine would sound with the new burner duct installed. I was very curious to see if there would be some kind of vibration or howl caused by the step transition. I fired up the engine to see how the duct would behave and was surprised to discover that the jet exhaust was more or less muted by the duct. The larger nozzle coupled with the long duct acts sort of like a muffler, go figure :oP

       The next step was to figure out how to ignite the fuel air mixture in the burner duct. I had originally envisioned a spark plug system that would be installed into the burner duct but I was not sure where the plug should be installed. I contacted one of my jet builder friends Grant Merriman and he suggested that the plug should be somewhere half way to three quarters down the burner duct. He said that he achieved the most reliable results from this configuration.
       Armed with this information I set out to build a spark plug fitting that could be welded into the burner duct. This fitting will serve as a threaded hub for the plug and also provide a small “pocket” for the spark to ignite the fast rushing air fuel mixture. This fitting will also serve as the “side electrode” or anode of the plug which will widen the spark gap tremendously.
       I started out with a NGK B8ES motorcycle plug and a 2” OD x 1” long piece of 304 stainless rod (for the spark plug fitting).

       I have discovered through my experiments that a factory spark gap used on a standard spark plug is generally too small for reliably igniting diesel fuel. A wider spark gap seems to work more efficiently as it has a better chance of making contact with the tiny droplets of diesel fuel that pass through the spark. However, widening the spark gap requires some modification to the stock plug.
         The B8ES plug I am using will need to be trimmed up a bit to widen the spark gap. I had performed this modification on the GR-7’s combustor plug a while back and it was fairly easy to do. The objective is to remove part of the threaded end that holds the side electrode. This exposes the center insulator about a 1/4” and enables the use of an external electrode (anode).

       After clipping off the side electrode I used my lathe to trim down the threads of the plug as seen below.

       Now that the plug has been trimmed up a bit I can move on to the plug fitting.

       I chucked up the 2” piece of 304 rod in the lathe and started shaping the plug fitting.

       A 12.8 mm hole was bored through the spark plug fitting in preparation for tapping. The drill was stopped just short of the end of the fitting so that a “ring” electrode could be machined into the fitting later on.

       I proceeded to tap the hole with a M14 x 1.25 hand tap.

       The narrow unthreaded end of the fitting was now machined into a ring electrode. This narrow hole will act as a side electrode for the modified spark plug. The face of the plug fitting was also beveled to improve it’s aerodynamic profile in the burner duct.

       I used the lathe again to machine a shoulder on the plug fitting. This will provide a good joint for the weld to adhere to when installing it into the burner duct.

       After a couple of hours I was done :oP The new plug fitting was ready for installation.

       The small pocket or cup at the end of the plug (seen below) should help to “scoop” out some of the fast moving fuel air mixture for ignition. I can only hope that this design will work especially because it will be welded into the burner duct permanently :0P

       Once again I am a little closer to firing off the new afterburner assembly. By next week I should be able to perform a live fire test on the burner and see if my new plug works or not :0}

       Please join me again next week for the continuation of the GRV-2 project.

Till then dream big my friends!!!

Don R. Giandomenico


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