Gallery, Projects and General > Project Logs

The Return of No. 83, a Hot Air Engine

<< < (23/25) > >>

Country Bubba:
Ah the sweet smell of success! Happy for you.
 :beer:

vtsteam:
Thanks Dwayne, Country Bubba!  :beer: :beer:

vtsteam:
I've made up a Prony brake to measure power output, in conjunction with a tachometer ( https://www.madmodder.net/index.php/topic,13897.0.html ) and now that I'm ready for more tests, it's time to tear down and clean the engine. Inspection shows a few issues:

1.) Flywheel grub screw is loose.

2.) A wear pattern of the outer rim of the main bearings is visible on the flywheel boss. This means the flywheel was contacting there, rather than bearing against the inner journal of the ball bearing. I need a shim washer to eliminate that source of friction.

3.) The steel wrist pin is lightly marking the power cylinder. I need to substitute a brass wrist pin, or shorten the pin and plug the ends with some graphite plugs.

4.) I suspected that the displacer pushrod was loose in the displacer. It screws into the far end, and was originally sealed there with a high temp adhesive. I probably broke that bond when adjusting the pushrod length awhile back.

To check for leaks, I lowered the displacer into a glass of hot water. This expands the air inside the displacer, and any leaks immediately show up as bubbles. Sure enough, they popped right out of the screwed end joint, so I have to fix that. Otherwise the internal volume of the displacer adds markedly to the dead space of the entire engine, and with the internal volume expanding and contracting a little out of phase, absorbs power that should be going to the power cylinder.

5.) The tiny amount of oil from lubricating the external parts has worked into the power and displacer cylinders, congealing, and adding friction. It was washed out with soap and water.

6.) The finish inside the displacer cylinder is slightly rough to the touch from boring. I didn't think much about it before because it's a non-contact surface, but I now imagine that it must produce air resistance at speed as it shuttles back and forth. So I think I'll hone all of the test displacer cylinders to a smoother (and similar) finish for testing. I don't think lapping is necessary, but maybe some day I'll try that as well to see if it makes a difference.

nickle:
Running on wood pellets as originally imagined is a fantastic outcome. Well done. On the cylinder roughness... It may actually help in some circumstances. A turbulent boundary layer caused by roughness can actually reduce overall drag under certain conditions... Think dimples on a golf ball type thing... Smooth golf balls are higher drag and don't go as far. I've heard some engine tuning types actually prefer rougher ports when porting performance engines for the same reason. I can't recall how to set up the math's as it's been an awful long time since I learned about it. Before and after figures would be interesting and maybe a plan to roll back to rough surface finish if that works better. Either way, well done on getting it to run so well.

Nick

vtsteam:
Thanks Nickle for your thoughts  :beer:. I did wonder if the roughness might contribute to heat exchange, which I think is the biggest concern of all in hot air engines, especially if increasing their size (which I intend to do). We'll see. And yes, a possible experiment is to try otherwise identical cylinders with different internal finishes.

BTW, one other variable, besides smoothness, is the increased side clearance created by honing the ridges down. Does that help or hurt? Well could be both, also. More clearance reduces air resistance, but decreases heat exchange.

The number of variables in hot air engine design is huge compared to IC or steam. That's because of the gas heat exchange requirements within the engine itself. Also why they are still so subject to experimentation..... and so therefore, interesting.

Navigation

[0] Message Index

[#] Next page

[*] Previous page