With the combustor housing completely built after the redesign, I decided to tackle getting the fuel and air to the housing. The diffuser was first to be fitted up to the engine. I had some problems when I leak tested the diffuser and found that my welds were cracking and leaking! It turns out that the filler rod I was using for my welds was not the best quality and was made of an alloy that had a different thermal expansion rate than the metal. After switching to a better filler material, I had the holes fixed and sealed up tight.

Inlet diffuser welded to the combustor housing

After carefully aligning the inlet diffuser to mate with the combustor housing, I used my new plasma cutter to make the opening in the outer wall of the combustor. The diffuser was then tack welded into position before final welding after assurances that everything still lined up correctly. I took my time to keep everything straight, since I didn't want a repeat of my previous disaster with the stainless combustor housing.

Patch on inlet

With the inlet diffuser welded securely, the engine started to look a bit more like a jet engine should! The patch on the inlet diffuser is the one point where I had to remove a section that was badly contaminated with the old filler rod. I guess the batch of rod was just not up to par, but the new rod seemed to do the trick and allow me to fix the previous problems. As a general suggestion, do not skimp on filler material. The little bit of money you save will not be worth the cost of potentially ruined parts.

Flame tube mounting

A support system was made for the flame tube to keep it centered in the combustion chamber. I welded nuts to the outside of the chamber and threaded long bolts through until the ends of the bolts would snug up against the flame tube. The idea here is to barely loosen the bolts before running the engine, and then re-tighten the bolts once the engine had been run for a bit so that it can settle in with the proper amount of expansion in the flame tube. This should keep the flame tube centered while allowing it to float at the ends. If I did not allow for the expansion, then the flame tube could buckle when subjected to the extreme temperatures while running the engine.

Brass fittings

To start the plumbing of the fuel injectors, I made a trip to the hardware store. They were having a sale good for 20% off anything you could fit in the shopping bag they give you when you enter the store. I though it a perfect time to stock up on the brass fittings I would need for the fuel system.

Fuel injectors

I also received my order from Mc Master Carr which contained all of the fuel injectors I had ordered. They are actually water misting nozzles from Hago nozzle company, but are quite suited for use in DIY jet engines. I purchased a few different sets of nozzles with different flow characteristics so that I could determine through testing which would be the best for use in the HR-1A.

The fuel manifold

I was looking at a few different ideas for plumbing the fuel injectors and also had to find a way to securely mount them to the top of the combustor. I created a pair of fuel manifolds that would be mounted to the top of the combustor's end cap and allow the fuel injectors to be threaded into them. I used the mill to fabricate the manifolds from 1 inch square aluminum bar stock.

Threading the manifolds

After determining the positioning of the fuel injectors in relation to the end cover of the combustor, I transferred the measurements to the manifold and drilled it out on the mill. I then threaded the holes to allow me to screw in the injectors and their housings. I also transferred the measurements to the end cap of the combustor and drilled it out so that the end cap and manifolds would be a perfect match. The mill makes this work so easy since you can accurately move the piece into position instead of having to use what boils down to "close-enough guesswork" on the drill press.

Manifolds attached to the end cap

Thankfully my measurements paid off and the manifolds fit the first time. You can get a really good idea here of how the fuel system will function. The brass fitting on the left side of the manifolds is where the fuel enters the system. The nozzle holders are on the opposite side of the combustor cap.

Injectors fitted on manifolds

The manifold is held on with a threaded brass fitting that goes through the end cap and screws into the manifold. The fuel flows through the cap and fills up in the small barrel of the brass fitting allowing it to pass through the screen on the back of the injector for filtering. If the brass fitting was any smaller in diameter, the fuel could not flow around the basket and could not enter into the injector. There are just a lot of small parts involved in plumbing the fuel system on this engine!

Fuel injectors

The four injectors thread into the brass fittings and make for a nice clean set up inside the engine. I chose to use four injectors on this engine so that I could get the fuel to atomize more efficiently at lower pressures. Normally, lowering the pressure will cause the droplets of fuel to get much larger, reducing the ability to complete a full burn before the fuel has been carried away through the turbine of the engine. That would mean burning fuel is entering the turbine! Not the best idea if you are trying to keep your turbine blades within reasonable temperature limits. The smaller injectors will make a finer mist of fuel for easier burning, but having more of them means that I can keep the total fuel flow in the range the engine needs for proper operation.

Flame tube end

Another difference between my engine and many others, is that I like to keep the end of the flame tube away from the end cap of the combustor. I think that it allows for better flame tube expansion, as well as for better cooling since the incoming air to the combustor will flow around the entire outside of the flame tube. I had to transfer the hole pattern for the fuel injector locations to the end of the flame tube so that I could drill openings to allow the fuel to get into the center of the flame tube.

Looking down the combustor

With the end of the flame tube drilled out, I could easily fit the injectors down into the flame tube. This view from the turbine end of the combustor shows the arrangement of the injectors in the center.

Inside the flame tube

The fuel injectors protrude into the flame tube by a half inch or so and should pick up the air from the primary holes easily, keeping the burn in the flame tube right up near the end cap. I am not worried about the heat since the other side of the flame tube will have the nice cool air flowing around it to keep the temperatures down. The outside of the combustor cap should be almost cool enough to touch while the engine is running, but I will have to check the temps once I fire her up for the first time.

Assembled combustor

I guess all of my measuring paid off! Everything fit the first time around on this go, and I couldn't be happier. The fuel system is compact, and rock solid, plus it just looks good if I must say so myself.

Ready to fire

With the combustor finished, I will have to move on to plumbing the oil system soon. A test firing is close, and I can smell the Jet-A burning! The last step is to do an atmospheric test and mount the spark plug. With any luck, I should have the engine ready to go on the go kart soon.

Thanks for stopping by, and be sure to check back for the atmospheric tests of the engine. I'll get the updates up as soon as I can. See you again soon.

Gary Richards

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