Saturday, November 30, 2013

Jet Engine - Part 4

Over the last few weeks I've done a pile of work on my jet engine project. It's been on the back burner (afterburner? haha!) for the last few months while I focused on riding, the Wilbur Mine project, and work--which pays for the fun stuff.

Unfortunately, most of the work I've been doing is math: calculating all the thermodynamic transfers from the compressor to combustor to turbine, so I could determine the design requirements for the combustor and develop a fuel injection system. This exercise proved to be quite challenging since it's been about 20 years since my last thermo class, and many important details for the parts I have and system I want to design had to be deduced from a pile of measurements, web info, and educated guesses. I'm not going to go into all the details here just now. For one thing, this blog lacks an equation editor; for another, it would be more useful if I proved out my assumptions before going to all the effort of showing my work.

For now I will say that I found an extremely useful paper from an MIT engineering student who went through the same design exercise as a 4th year project. However, that design used a different turbo than mine and adopted a propane fuel source. Since I want to use kerosene, I needed to recalculate the entire design starting from a different chemical (combustion) reaction, and using a different performance map for my turbo (a Garrett VNT-15 from a VW TDI). The operative concept is calculating the enthalpy of reactants and products through the three stages of compressor, combustor and turbine.

I've completed the calculations and come up with a preliminary design spec. The next step is to design the combustor itself, now that I know the required fuel flow, input and output temperatures, and mass air flow through for my desired operating point.

Meanwhile, I took a break from the math and got my hands dirty. Machining a new main shaft journal bearing has been a real nuisance. I successfully made most of the part to 0.0001" tolerance compared to the original, then took it to a machine shop to bore out the critical internal diameter which includes two steps (the bearing surface) and a recessed area to allow pressurized oil to reach the bearing surfaces. I was unable to make a boring tool fine enough to cut the inside to the required profile, and was reluctant to buy the required tool for about $250. Hence the machine shop. But they screwed up the diameter so now the whole part is junk. Even if I got the interior right (which I'm confident I could), there was the other problem of how to machine an offset circle in one end to accommodate the anti-rotation plug. That would've required a 9mm end mill plus a way to mount it and the part for machining. All told, I was looking at several hundred dollars of tooling.

The easy solution was to order a bearing rebuild kit, which I did for $78 from Amazon. This proved essential anyway, because when I dry-fitted the turbo with my junked new bearing, I discovered that one of the thrust bearings needs to be replaced as well. Plus there's a redesigned oil plate cover in the rebuild kit which is supposed to improve cooling. Bottom line is the kit should solve a bunch of problems and let me move on to more fun things. It arrives next week.

Today's job was sand-blasting all the cast components to remove rust and prep them for painting with Very High Temperature paint. This paint is good to about 2500F and will be applied to all the hot components to prevent further oxidation. The paint is really a ceramic material that sprays on and must be cured at high temperature. Fortunately I now have my glass kiln up and running, so I can cure the paint to 1700F if necessary, which is well above the anticipated ~1100F operating temperature I expect from the jet engine. As you can see, the clean cast iron looks quite different from the crusty lump I started with. This turbo is is in really poor condition--lots of surface corrosion which has eaten away substantial metal--but it should hold together OK. The important bits are there and it doesn't need to take further abuse from a car engine.

The next step was to fabricate a flange for turbine inlet to which I can attach the combustor. This is going to be a tricky operation and it may well fail: I need to weld the mild steel flange to the cast iron turbine housing. It is metallurgically possible to weld cast iron to steel using a nickel filler, but the challenge is keeping everything at the same temperature so the iron or weld don't crack. Cast iron contains a lot of carbon compared to mild steel, which makes it brittle. This problem is exacerbated by the grungy iron of the turbo, which has been soaking up even more carbon from the hot exhaust. The trick will be to heat everything to the same temperature in my kiln before MIG welding it, peening the weld periodically to remove stresses, then cooling the whole welded assembly slowly. This will be a task for John Branje, my local welding expert, who offered to perform the delicate operation for me in my shop.

The flange was cut from 1/4" plate using a combination of angle grinder, jigsaw, and drilling according to a template I made based on the inlet shape. Lots of filing later, it ended up looking pretty good. It'll be important to clean everything before welding, because any contaminants could reduce the ability to weld. I also bevelled the outer edge of the casting to 45 degrees to allow a bit more penetration for the weld bead. Not sure how easy it'll be to get a MIG in there.

Tomorrow I'm going to try painting the all the turbo housing components except for the turbine scroll, which I'll do after it's welded and cleaned up. Fingers crossed that I can pull this off! Otherwise I'm back to square one on this project, and will have to look for a different turbo design.

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