GR-1 Turbojet Project 2/21/04

Posted on February 21, 2004

During the last week or so I had researched how I was going to go about designing my combustion chamber or combustor as it is called. Looking at others designs, I got an idea of what would be an effective combustor. It seems that an incorrectly designed combustor will result in overheating and a loss of power. This is caused by the combustors inability to burn the fuel inside if its combustion liner where it belongs.

Blowing fire out of the tailpipe is not the goal of the turbine builder unless you have an afterburner of course. A well designed gas turbine will produce very little flame past the turbine rotor. The trick is to design a combustor that can efficiently burn fuel and produce a low exhaust gas temperature so as that the engine will not overheat. The typical exhaust gas of a turbocharger turbojet is from 850 to 1200 degrees Fahr. in temperature. Running the engine past 1300 degrees will risk a meltdown. Having no experience with gas turbines left me guessing on the dimensions of my combustor so I started looking at examples of other turbines.
Most home-built turbines employ large combustors allowing for proper fuel combustion thus lowering the exhaust gas temperature. Commercial gas turbines have fairly small combustors relative to the turbine size which made me think that a small but efficient combustor is the way to go but more difficult to design. By making an educated guess on how large the combustor should be, I started finding the metal to make my combustor. I collected some scrap EMT (Electrical Metallic Tubing/conduit) from work and brainstormed on how I could use the pipe to create an economical combustor.

I took a 4” piece of EMT conduit and measured it as a candidate for the Combustion Chamber. Using the 4” seemed to be the ticket as I could use 2-1/2” EMT for the Combustion Liner. With an idea of what I was going to do, I started cutting the pipe. I wanted to make the combustor as internally aerodynamic as possible so I tapered the outlet to match the turbocharger flange plate that I purchased earlier. I used a paper template to estimate my cut lines on the pipe which worked awesome. After bending the pipe into shape I used a MIG welder to weld the joints together.


		This is a piece of 4” EMT (110 mm ID) and the paper template used to taper the pipe to fit the exhaust flange.


	These are the preliminary cuts that will make the taper possible


	Using an angle grinder and an 80 grit flap disk I was able to make a really cool looking taper.


	The hole for the compressor inlet is drilled and a inlet pipe is welded in place

I test fitted the turbocharger to the combustor and cut my hose to fit. So far so good but I still have a ways to go. I now have to create a bottom plate and a combustion liner to complete the unit. The combustion liner is responsible for holding the flame inside of the combustor and not allowing the inrush of compressed air from blowing out the flame. The cooler incoming air will not only oxygenate the fuel but will also keep the turbine cool as the air makes an insulating barrier around the hot gasses flowing through the turbine.
I intend to test the unit before I run it on the turbo to check that the combustion liner is doing it’s job. The liner will fit inside of the combustion chamber to create the combustor unit. The proper amount of holes must be drilled into the liner as this will determine how effectively the fuel will burn. If the liner cannot burn the fuel within the length of the liner, burning gasses will hit the turbine wheel and overheat the turbo. For now I have to focus on the bottom plate that holds the combustion liner.




		I cut the combustor’s bottom plate out of 1/4” steel plate and drilled six bolt holes to hold the plate in place. I welded 1/4”-20 stainless nuts inside the flange for the bolts to anchor to.


	I drilled sixty 1/4” holes into the combustion liner hoping that this would be a good start. I could always make the holes larger if the combustor fails to work.


	I welded the spark plug and fuel nozzle fittings into the bottom plate. A fuel nozzle was made out of a brass hose barb and drilled with a countersink bit. The hole in the middle of the fitting is about 1/8” in diameter.




		The prototype combustor is complete and ready for testing.

The completed combustor was ready for testing so I bolted it to my trusty engine stand and used a neon transformer to run the spark plug. I hooked up the propane tank and successfully created some fire balls in the driveway. I used the leaf blower to simulate the incoming air by taping the nozzle to a piece of hose on the inlet. I found a “cold” spot inside of the liner where the fuel was not burning evenly. I experimented with putting foil tape on the holes to see what it improved and after 3 hours of testing/modification I was content with it’s operation.

With a loud roar the combustor burned with a consistent, even flame.

You can see the area around the combustion liner which allows cool air to envelope the hot gas rushing out. This “bypass” is what makes the turbine run cool. The hole on the left is a spare bung for future fuels and the hole in the middle is the fuel nozzle. The spark plug is on the right.

The combustion liner seemed to be a success but I am not sure if it will work on the turbocharger. My next step will be to build a frame to bolt all of the engine’s parts to. I hope to build a frame small enough to make the engine portable. I am “fired” up to get this done soon!.

Well, till next time

Don Giandomenico

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