Tuesday, January 15, 2013

Jet engine project - Part 2

I hadn't really inspected the main journal bearing closely up to now, so when I finally did I was dismayed to discover that in fact it shouldn't be in two pieces. Funny what a little grimy oil can obscure!

That little ring on the right is supposed to be attached to the piece on the left. I thought maybe I'd snapped it off somehow when I took apart the seals, but closer inspection suggested that it probably torqued off last time the turbo operated. The brass in the portion of the journal inside the smaller piece was galled, probably from heat. I suspect the bearing lost oil pressure or maybe had a blockage at this end, and allowed the hot end of the bushing next to the exhaust turbine to overheat. Not a happy donor car at all. Hopefully there isn't a problem with the end seals around the thrust bearings. Visually they seem fine.

Close inspection of the shaft suggests it's still serviceable. I took a zillion measurements to get a sense of the dimensions and tolerances I'm dealing with. It's all metric apparently, but most of my tools are in inch so that's what I'm going to use, although some of the numbers seems weird. There's a good 0.001" of clearance to work with between the shaft and journal which would be filled with the pressurized oil that serves as the working bearing. The shaft still measures consistently within 0.0001-0.0002" along its bearing points, so any deviations above the surface won't penetrate the oil skin into the brass proper. Any microscopic grooves in the shaft bearings shouldn't have any impact on performance. They certainly pass the fingernail test. I will need to measure run-out as well, but I don't anticipate it being a problem. 

After searching the internet for a replacement journal bearing, all I could find were complete rebuild kits with parts I don't think I need (famous last words?). I can't afford to pay $120+ for this part. There's a proliferation of el-cheapo options from some guy named Rothenbacher who runs various internet parts stores which all seem to actually be the same business. However, online reviews suggest that parts quality is extremely poor and the businesses are awful to deal with. Two apparently reputable parts companies that I contacted never got back to me. 

That leaves me the the sole option of making my own journal. It's not a technically complicated part--it's just brass, after all--but it'll be vital to get the inside journal dimensions just right. 

So, I spent an entire evening making careful measurements and thinking about a strategy to make this piece. 

First off, my lathe wasn't up to snuff for these tolerances. It took an entire evening to tear it apart, polish the ways, and generally tune it up to eliminate unwanted play. It was due for some TLC anyway since I moved shops. Some test cuts in scrap brass showed I could consistently achieve cuts within a few ten thou, so I know I'm now in the ballpark. Yay Busy Bee. 

Second, I lacked some important tooling to work brass at this scale and achieve smooth finishes and the profiles I needed. To make tooling, I needed to use my grinder--and it's had a chronic wobble on the fine stone that I needed to use. So another evening was spent truing the grinding wheel and carefully carving out a cutter for fine surface work, and a roughing-out tool--both in HSS. Roughing out the journal with these tools showed they worked perfectly and well within the tolerances I sought. A few bucks of type 360 brass rod later, and I'm on my way to fabricating this critical part.

The hardest two cuts will be first the inside of the journal between the actual bearing surfaces (critical to allow just the right amount of bearing oil to pass through the journal), and then the milled-out dimple on the end which takes an insert to prevent the bearing from spinning inside the turbo. I don't have an inside cutter small enough for the former, and I'm nervous about rigging the finished bearing in a four-jaw chuck to cut the latter. Then I had the idea of making an inside cutter from an HSS drill bit, and modifying another HSS drill bit as an end mill. I'll experiment extensively with both tools as well as using the four-jaw chuck before committing to the procedure. 

I'm fairly confident I can make this part. Yes the tolerances are tight, but I the think film of pressurized oil will be somewhat forgiving within the tolerances I can achieve. As long as there's plenty of oil and enough freedom to prevent metal-on-metal contact, even my home-made journal should work fine. And I don't expect it will look any worse the the OEM one.  

Now it's off to make more tooling!  

Jet engine project - Part 1

If you're reading this blog, chances are it's because you're a gear head and so you may find this project interesting. In this case, the motor is the project and the wheels are optional.

I've always wanted a jet engine to play with. Nothing beats the scream of a turbine spooling up to 150,000 rpm and the oily smell of burning kerosene. Makes me think of travel and adventure.

Since I can't afford to buy a jet engine, my only option is to build one. This has been done before by many garage hobbyists and isn't really that complicated, but there are a few design challenges that will keep things interesting. 

If you look at an aircraft jet engine, it's basically a tube with a bunch of big internal fans inline with the central axis. Air enters the hole at the front of the tube and is compressed by the first few fans. Somewhere near the middle fuel is injected and ignited with the compressed air. The burning fuel expands rapidly and is forced to exit the rear end of the tube, first passing through more fans which are connected to and drive the front compressor fans. The hot air that is pushed rapidly out the back produces a lot of thrust. One of the most successful versions of a jet engine used in some older military aircraft had only seven moving parts in the main tube. Pretty simple concept for such impressive performance. 

One of the simplest jet engine concepts uses a car turbocharger for the compressor and gas turbine, with a combustor connecting the two components and a few support systems to provide oil pressure for the shaft bearing, ignition, and fuel control. A car turbo is designed for an almost identical purpose. However, instead of running on car engine exhaust, I'll be running on a stream of burning kerosene, pressurized into the exhaust turbine by the air compressor that normally would pump air into the car engine's cylinders. From a conceptual point of view the design is quite similar to an aircraft jet engine. However, in my version the air doesn't run in a straight line through all the fans; it has to follow a number of 90 degree bends to connect all the pieces. For that reason it's less efficient and doesn't generate much thrust, but if all goes well it'll sound and smell like its aviation kin. 

For my design I've started with a junked turbocharger from a Volkswagen diesel, probably a Jetta. My brother has a buddy who knew of one and since I could get it for free, the price was right on budget.

I forgot to grab a picture of the turbo when I got it, but it looks pretty much like the one above. It was in poor condition with lots of rust, seized fasteners, and extensive soot from what appears to have been damaged piston rings on one cylinder. It had been unceremoniously cut out of the engine bay with a torch and hammer. I wasn't sure it could be salvaged and the configuration and shape of parts was extremely inconvenient for a jet engine concept. It would need a lot of work. But it was free!

After two nights of vicious hacking and twisting with various tools, I'd managed to get most of the components apart. However, several key bolts and screws were seized into place with rust and no amount of loud swearing got them free. Eventually I had to drill them out, carefully avoiding damage to the iron casting itself which could not be repaired easily.

As you can see, the exhaust manifold is cast in one piece with the exhaust turbine. I wanted just one hole to which I could connect a combustor. That meant sawing off the manifold with a hacksaw. The leftover manifold will be good for practicing how to weld a steel flange to the turbo housing so I can attach the combustor. Welding to cast iron requires a few tricks but again, thanks to the Web I was able to find some excellent advice on what to do.

Meanwhile, on the fresh air side of the turbo, I had disassembled the compressor to inspect the oil bearing. You can see the little compressor wheel and the underside of the scroll casing it sits inside. I also had to drill out a screw because the stupid thing had seized even there, immersed in oil. The gold coloured parts are the oil bearing for the turbo shaft. Initially I was concerned about bearing play. Too much, and pressurized oil would leak out around the bearing and into the hot side being fed by the combustor. That would result in a lot of smoke and the rapid self-destruction of the whole turbo. However, after inspecting closely and comparing with examples on the web, it looks like I lucked out on the bearing. It should hold up for my purpose. 

Back on the hot side I faced a significant challenge: how to remove the scroll case cover from the scroll case so I could remove the exhaust turbine, inspect the bearing at this end and service some other components. As you can see from the assembly clamped in the vise (turbine shaft sticking straight up), the scroll case cover had seized into the case and no amount of whacking on it would break it free from its rust. I'd pretty much given up on gaining access and decided to try harsher methods: namely, a chisel. By putting the flat end of the chisel on the case and hammering on the point so I wouldn't accidentally damage the cast iron scroll case, I was finally able to crack the rust and tease off the cover. However, I forgot to wire the case to my vise and the damn thing flew sideways to the floor when it suddenly released. I quickly stuck out my foot to catch it before it hit the concrete floor and saved the case but whacked my leg with the heavy casting. Ouch.

 Now almost all the parts were free. The exhaust turbine was a sooty mess from the engine and the variable inlet vanes barely moved. More screws had to be drilled out to free these parts for cleaning.

Finally I had the whole thing apart and could see what I was dealing with. It looked pretty good under all the crud. Some epic filing on my hacksaw cut cleaned up the future flange point for the combustor. Other parts were rigged into the lathe to shave off rust. Much elbow-grease later I'd cleaned the worst of the corroded mating surfaces. Lots of debris remains to be thoroughly cleaned out before I can reassemble the delicate bearing, but I'm not going to do that until I can get the outside bead blasted to clean it of rust flakes and other crap. There's a high temp paint that will look good and should work up to 2000F. 

Today my son and I hit the fastener store in Ottawa for some replacement bolts and screws. Not everything matched exactly so I had to modify several fasteners on my lathe to get the right shapes and sizes. By the end of today I was able to dry-fit my modified components and re-orient the hot and cold scroll cases so they pointed in the same direction as needed to eventually position a combustor between them. Next step is the bead blasting and thorough cleaning in prep for reassembly. Then I can blank off the holes and work on a scheme to mount the turbo in a frame and get oil into and out of it. I've decided to aim for a "steampunk" look for the finished engine, sort of a cross between Victorian-era design and science fiction (check out this steampunk computer and motorbike). This gives some exciting possibilities for gauges and controls and I'll be hitting a nearby antique store to look for cheap old industrial junk I can modify to work. So far I haven't seen a steampunk jet engine.

Compared to what I started with a few days ago, this is looking rather promising!