The turbine has a flange to which the combustion chamber will be bolted when the plumbing is complete. A flange plate had to be fabricated that would match up to the turbine. The central part of the combustor is the flame tube. Once the flame tube is complete a test of the combustion chamber under pressure can be done.

Layout for the turbine flange

A pattern was laid out on a blank piece of 1/8 inch plate steel. The four holes for the mounting bolts to the turbine were drilled to a smaller size so the holes could be used to secure the plate to the milling machine. The holes will be brought to full size as the plate is finished. 4 3/4 inch holes were drilled in the flange in the inside radius corners of the center opening and then the material in the middle was milled out.

Flange on the milling table after machining

Here you can see the plate after milling. It is on my new micro mill (from, you guessed it , Harbor Freight). The mill was a birthday present from the wife. God I love that woman! A wonderful spouse, soulmate, and business partner!

Flange next to the turbine it will bolt to

Here you can see the plate and the turbine flange where it will mount. The mounting holes have been opened up to accept 3/8 bolts.

Flange bolted to the turbine

A test fit of the plate shows that it is a perfect fit! You can see it held on by 2 bolts here.

Lip has been welded on to the flange

The plate was then turned into a flange by the addition of a protruding lip. 2 pieces a 1/8 inch plate were formed with radius curves for the corners and welded onto the plate. The pipework from the combustor will attach to this flange. The spatter from welding with flux cored wire will be cleaned later, and the whole combustor will probably get a good bead blasting to make it look nice. I am thinking about using gun blueing to give it a nice heat resistant color too.

Pattern for the flame tube holes

This is the pattern for the flame tube. It was produced using information from numerous sources. The basic principal is this, the area of all of the holes would equal the area of the inducer of the turbo. I will try to show you with a few calculations , and will make this easy for the beginner to understand.

Flame Tube Hole Pattern Calculations

First find the total area of the inducer of your turbo. The inducer is the section of the compressor intake wheel that you can see with the cover on. Our turbo has an inducer of 2.9 inches.

Area of a circle = pi * rē or radius * radius * 3.14
Since our inducer diameter is 2.9 inches, divide by 2 to find the radius.
2.9 / 2 = 1.45
1.45 * 1.45 * 3.14 = 6.60 sq. inches Easy!

So the area of all of the holes in the flame tube should be as close to 6.60 sq. inches as possible.

Primary, secondary and tertiary (or dilution) holes from left to right

Now, the area must be divided up into 3 zones, the primary, secondary, and tertiary (or dilution) zones. The primary zone provides the air needed to start combustion without blowing out the flames. This would be the area depicted as the small holes in the above photo. The secondary zone provides the air needed to sustain the burn, shown as the slightly larger holes in the middle. The tertiary zone provides the remainder of the air which is used for cooling the hot gasses before they reach the turbine section of the turbo.

The primary zone gets 30 percent of the air flow, the secondary gets 20 percent, and the tertiary gets 50 percent. So use the following calculations to get the total square inches of each area.

Easy again, now to find out how many holes to put in each area. For a large turbo like this we will use holes that are 1/4 (.250) inch for the primary zone, 3/8 (.375) for the secondary zone, and 5/8 (.625) for the tertiary zone. Find the square inches for each hole and then divide that into the area for each zone to get the number of holes needed. Use the two following charts to see the calculations.

So, using the above information, I rounded off the numbers to produce a flame tube with 40 1/4 inch holes in the primary zone, 12 3/8 holes in the secondary zone, and 10 5/8 holes in the tertiary zone. The length of the flame tube was calculated at 4 times the inducer so 4 * 2.9 was close enough to 12 inches so I used that. The diameter of the flame tube should be twice that of the inducer or roughly 6 inches, but because of our size constraints I am using a much smaller design of 2.5 inches. I would highly suggest to the aspiring builder that you double the diameter of the inducer for large turbos, or triple for small turbos to get a good diameter of the flame tube. Then keeping a 1/2 to 1 inch gap around your flame tube, size the combustor housing. An example is that a combustor with a 6 inch diameter flame tube should have a 7 to 8 inch diameter combustor housing.

The rest of the layout was simple math. I found the circumference (distance around the outer edge of the circle) of the pipe and used that to create an image in photoshop that was the appropriate dimensions.

Circumference = diameter of flame tube pipe * pi
so circumference = 2.5 * 3.14
Our circumference is 7.85 around the flame tube pipe

Thus I made the dimensions for the layout 7.85 inches wide and 12 inches long then I used guides in photoshop to aid in the placement of the holes. I also added crosshairs to the holes to help in finding the center for center punching. I laid out the primary as 4 rows of 10 holes, the secondary as 2 rows of 6 and the tertiary as 2 rows of 5. You can see this in the following image, which is a scaled down copy of what I created. I then used the registration marks in photoshop when I printed the image so I could line it up properly on the tube. Photoshop can put registration marks on printed copies, and basically they are little crosshairs at the edges of a picture. When I wrapped the image around the tube, I just had to line up the overlapping crosshairs and the layout would be exactly positioned on the tube where it needed to be.

Template for the flame tube

Flame tube layout.

Template lined up on the stainless tube that will become the flame tube

Here the layout is taped to the flame tube and the lineup is perfect! Look closely to see the crosshairs on the dots to see where I need to punch.

The flame tube gets center punched

I carefully center punched every hole to make sure the drill would follow the punch divot. This keeps the drill from skittering off, especially on rounded items like the pipe.

Drilling the flame tube in the drill press

All of the holes were drilled on the drill press. I started them as small as I could and sized them up 1/16 of an inch larger with each successive pass. This will keep the holes in better alignment, and makes the stainless easier to drill. The flame tube is made of 304 stainless pipe with a wall thickness of 1/16 or .0625 inch. Stainless is hard to drill, so the speed must be kept slow and the pressure on the bit must be firm. If the drill bit builds up too much heat from friction by turning too fast or there not being enough pressure to keep the cut going, then the steel will work harden.

Countersinking the flame tube in the drill press

After drilling the holes, I used a considerably larger size drill for each hole to countersink the holes for better air flow. I would bring the bit down just enough to cause a flared hole, then set the drill stop to keep that height before continuing to the rest of the holes. The drill stop keeps the bit from ever going far enough in to just make the hole bigger. Yes it is a cheap countersink method, but I didn't feel like spending a small fortune on countersink bits large enough for this one project. As you can see, it worked quite nicely.

The flame tube slides in to the combustor

Here you can see again how the flame tube slides right into it's home in the combustor.

The ring on the end guides the flame tube

Going, going....

The flame tube is held in place by rings on both ends

GONE! The flame tube slides into another ring in the back of the combustor that holds it into place in the center. The end cap keeps it from moving at all. So, I added the end cap and decided to test the combustor.

The first test of the combustor with the flame tube installed

I used the prototype electric starter setup (wont show you cause it is so rigged its not even funny) to spin the turbo to produce air movement and fed it some fuel. Had to light it by hand since I don't have an ignition circuit built yet. A couple of times I fed too much fuel before spin up and there were some harrowing experiences there. Lloyd lost the hair from his hand lighting it. Once the starter got the turbo up to about 5000 RPM the flames sucked right up inside!

The swirl created inside the flame tube by the mixture of
incoming air and fuel being burned

Here you can see the swirl induced by the flame tube and the air coming into the combustor at an angle. I am feeding it with a can of Butane used for filling lighters. This combustor must be efficient, because we really spun up the turbo and got an EGT (exhaust gas temperature) of 1300 degrees Fahrenheit from just the butane. The flames were well confined and the heat output was tremendous.

Video of the First Combustor Test

Click here to see a video of the run.
516K windows media player file.

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