Sunday, 13 January 2008

A new beginning

I decided to follow the construction plans for Thomas Kamps engine given in the Modellers' World book with a few modifications to allow me to use my own compressor impeller and turbine.

By now I'm beginning to regard making a complete new engine as just a couple of evenings' entertainment.
With modern tools, there's no geat deal of graft (hard work) involved. It's more a matter of working out the easiest way to do each job.
For example, cutting out the blades and the blade slots for the diffuser assembly shown here was done on a woodworking bandsaw and took about 10 minutes.

Only thing is, it's getting expensive in terms of materials. A big lump of aluminium like this costs real money.
So as an aside, I've started experimenting with a homemade forge to recycle aluminium scrap - It looks promising, I was able to melt down a pile of swarf and tiny offcuts to make a respectable ingot,
just using some scrap timber and barbecue coal as fuel. The turbine starting blower made an excellent bellows.
The hearth itself was just a loose circle of driveway bricks (known as 'pavers' in the UK). Some of these cracked with the heat but without any fuss.

For a crucible, I cut an old disposable gas cylinder in half (the type that is used for argon in portable welders)

I ran some tests on this compressor/diffuser assembly. I mounted it in the engine casing with a manometer attached so that I could measure the pressure. Then I drove it using an electric motor.
At 18,000rpm, I got a pressure rise of 5 inches of water. (with all the holes in the case completely closed off.) So that gives me a measure for the best the compressor can manage at that speed.

With the engine running, if I can get better than 18000 rpm at a casing pressure less than 5 inches, I reckon it should self-sustain. Anything less than that speed will result in the gasses being blown back through the compressor.

(18000rpm sounds like a reasonable idling speed to aim for)

Here's the rear of the compressor/diffuser assembly, with the drive shaft mounted. Note the oil feed pipe that injects oil into the (3) cooling air slots machined into the turbine shaft mounting.
Air and oil mist are driven down these slots by the engine compression and into the shaft tunnel.

This arrangement seems to work. After the first test run, the bearings looked slightly discoloured but not burned or coated. There was still liquid oil visible in the ballraces afterwards.

The combustion chamber - It looks a bit nasty in the photo because I didn't bother to clean off any brazing flux before running.

A couple of problems came up when building this - I didn't have the correct material (thin stainless) and had to make do with some 1mm mild steel
that was a bit thick to shape - especially the dished front. I tried spinning it in the lathe but gave up and resorted to bashing it into a hollow former with a ball pein hammer.

Then when it came to the holes, the drawings didn't agree with the text as to hole number and placement.

Some of the holes are supposed to be recessed in a hollow to form a sort of venturi effect
but the metal was too thick to allow this.

Still, it should be better than the old one anyway.

The fuel injector ring is mounted in place at the rear and injects fuel into 'sticks' (pipes) that project from rear to the front of the chamber.

This makes all the difference to maintaining a stable, torroidal flame in the front of the chamber.

Even at full blast, there was no sign of flame at the engine exhaust.

The burner ring being tested on propane. The feed pipe is 3mm steel and each nozzle is a 1mm hypodermic needle brazed in place.

The assembled engine with the outer casing removed. The thing sticking out the top of the casing is a brass nipple to measure internal pressure.

This engine was tested using the new impeller and a turbine wheel recycled from the old engine.

The starting speed was 5000rpm. When the fuel was turned on and ignited at the exhaust (I dispensed with the spark ignition), the flame fired back into the combustion chamber immediately.

I found that controlling the gas supply allowed me to vary the engine speed from 5000 to 16000 rpm. At the latter figure, the turbine exhaust began to glow red-hot.

These results were encouraging, 16000rpm is a new record and the speed control was excellent, but the engine still failed to self-sustain.
Taking away the starter air-blast caused the turbine to simply wind down.

I put this down to the turbine wheel itself, which doesn't fit the housing terribly well. The turbine housing itself is not of the best quality too. I made a couple of mistakes that left the housing out-of-round.

I need a new turbine wheel and back end (Nozzle guide vane) assembly


In theory, any gas turbine should work, no matter how badly made, if you can get enough heat energy into it. Thermodynamics says so.

The trouble is that all the thermodynamic calculations in the world won't tell you if a particular home-made turbine or compressor wheel is going to be 90% efficient or 9% efficient
and the efficiency of the compressor/turbine system determines the temperature that the engine runs at.

Below a certain efficiency level the necessary gas temperature is above the melting point of the components, or you can't supply enough fuel. Either way, it won't work.

I spent a lot of time trying to think of a way to measure efficiency of the compressor and turbine. But without building some sort of calorimeter device that can accurately measure the power supplied or absorbed at the drive shaft , I can't see a way to do it.

The obvious step of attaching an electric motor to the shaft allowed me to get some speed/prssure values that were interesting (suggesting that the compressor efficiency was just a little down on Schreckling's design ), but the motor power (current*voltage) was obviously being mostly wasted in the motor itself.

I'm not saying it isn't possible, it just isn't worth the investment of time and trouble. I may as well just keep experimenting to improve the efficiency of each component part.

The various construction books on the subject are little help. If you pick through the maths carefully it all makes sense until you come to the critical question of efficiency, then it all breaks down into assumed values and 'according to experiments' (not detailed)
The two main books, Schreckling and Kamps disagree wildly.
Schreckling gives an assumed value for his compressor as 70% efficient.
Whereas Kamps gives a figure of 0.86 for what he calls the 'pressure ratio' of the same compressor :-

2 . Cp . T[(P/Po).286-1] / u2

This is basically efficiency as defined by Schreckling(P37), times 2 so Kamps is saying the Sch. compressor is 43% efficient. Other values given are 65% for the T. Marbore and 1.16/2 = 53% for another, homemade.

No comments: