quarta-feira, 3 de setembro de 2014

Reports and power tests with Stirling engine at 1800 rpm cans

Initially after building the engine has peak 1860 rpm in free rotation. The next day, a new test has not passed 1820 rpm. A week later, after a repair in the cold cylinder, began a nightmare, not surpassed 1700 rpm and so each test worsened until it peaks at 1350 rpm.

My first suspicion was the displacer piston and bearings.

So I decided to make the displacer piston replacement, which now went from 25 mm to 35 mm in length, thus eliminating the dead area (10 mm) above the existing project, see in Fig. 1. Again dropped to 1400 rpm, but the torque was wonderful, even keeping the same courses for each piston. So in fact was proven, there were no problems with the displacer piston.

Stirling engine manual, displacer piston made ​​in steel wool
fig. 1

I decided to change the bearings (fig coins. 2) by 5 cents to 10 cents and got worse, maximum 1350 rpm and constant 1220 rpm! With this change, again I confirmed that I was on the wrong track.

Stirling Engine Manual, plain bearings with coins
fig. 2

I was getting frustrated, how much more alter, was the worst performance. I honestly wanted to put the engine in the closet the disappointment!

The torque was the best of all my engines cans, but did not reach the incredible initial velocity.

Some physical effect, aerodynamic, thermal problem or the material was occurring, the friction of the new bearings at low speeds is apparently little air leakage barely existed for a homemade motor! And there was still no concrete explanation for the feared problem! If engine had not hit 1800 rpm in the first tests, I would not be so stubborn.

My friend Elton commented, if this problem could be due to the hot cylinder (Figure 3)! I started to analyze the comment of Elton, and parts made ​​sense because it was the first engine which did not use a can of Coke as hot cylinder! Then disassembled again and made ​​a new hot cylinder.

Stirling Engine Manual, hot cylinder
fig. 3

I did the tests and finally the solution to the dreaded problem !!!

In this first test, the engine reached 1700 rpm, dropping to 1550 rpm and after two minutes running, now operates in constant 1600 rpm.

As was reported, I swapped the hot cylinder with a new one, but to take it apart, warped the piston rod displacer, and saw something nasty. With rapid movement of the displacer piston, the steel wool is compacted and loose from the stem, or the piston is 35 mm long, but the wool is found only in 27 mm long and the remainder is transformed in a gap. Again made ​​a new displacer piston and I fixed a flat iron in the middle of the piston, to solve this new problem, due to the high speed.

The results with are described below generator, and the water temperature varied between 65 to 75°C.

So I made the first tests with the generator, with the new displacer piston of 35 mm in length.

  4 leds          1179 rpm         3,89 V . 0,083 A = 0,32 Watts
10 leds          1130 rpm         3,43 V . 0,043 A = 0,30 Watts
30 leds          1180 rpm         3,31 V . 0,092 A = 0,30 Watts

Free rotation, peak 1830 rpm and 1600 rpm constant!

This engine performs better than the previous version (1600 RPM), both in speed, torque and power.

I want to measure the mechanical power of the engine, but first I need a mini precision scale. This engine is simple with impressive performance, fun to watch it in operation!

Now I reached the limit of this simplified design, there is scope for improvement in performance, but you need to change many things, hindering the construction. Have a sneaking suspicion that this project with a suitable generator could produce up to 0.50 Watts. I also do a test with another generator and follow the suggestion of Miro, test engine with two generators!

There are numerous other possibilities for change:

- Strengthen the crankshaft;
- Two wheels to stabilize the engine;
- rolling in the crankshaft;
- Change the bottom of the hot cylinder;
- Change the piston stroke of work;
- Replace the diaphragm by a glass syringe 50 ml;
- Do the balancing piston;
- Improving the airflow toaster oven;
- Make new test with a new type of torch, to test the limit of the hot cylinder casting;
- Install bushings in the headstock, to increase resistance;
- Studying a way to cool the first head;
- Install a water pump;

Thanks to all who follow this trajectory,
Regards Leandro Wagner.

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