buliding a vaccum engine and need some help

[Gedeon Spilett]

hi Chizz
do I understand properly?
do you want to say that when you to launch the engine forward with the lit burner, you feel the vacuum in the cylinder at the end of the stroke, and the engine turns over violently backward without passing BDC ?
but you have got it, almost.
more heat, more muscle when launching from cold and he will start, normally.
I put one flywheel because I have found only one!
I also have fitted bronze bushes, ball bearings would be obviously better.
i have seen your pictures, and workmanship looks great
cordially

 

[NickG]

Chizz,

I thought you must have meant that! I'll go back to what I was originally going to post before I read yours again and changed my reply then!

I think if that's happening, the vacuum is forming before the engine gets past BDC (happening too quickly) and trying to suck it back. Try either retarding the timing slightly, so that the valve shuts a fraction later (check against Zephyrin's figure of 16 deg) BBDC. Or, let the engine heat up for 5-10 minutes before trying, or give it a warm up with a blow torch to speed things up. This is known as quenching, funnily enough, these things like to run hot, if the cylinder is too cold you get what you said, the hot gas cools too quickly before the engine has actaully passed BDC.

In a really fast running engine you might have the valve closing early which gives this feel of compression but as yours is really slow running I think you need the valve to close slightly later. Not much later though otherwise you'll get the gas contracting and just pulling air in through the port.

Hope this helps, it sounds like it's nearly there. If it still doesn't run, the only thing I can think is that it's excessive friction.

Nick

 

[chizz]

Thanks nick

maybe i will try that and take cooling fins of as they are a add on, the only other thing i thought it could be is the shape of the cam.

Zephyrin

you say the valve is full open at TDC mine only starts to open before TDC it is DEfornotly fully open at that point, any chance of a close up picture of your cam.

many thanks

 

[NickG]

Chizz,

leave the cooling fins, they will help sustain a temperature, if it gets too hot, it won't work but it does need some heat to avoid this quenching effect, you might simply have not waited long enough. Give it a good flame as well - a big one, you don't want to suck any cold air in.

Your comment to Zephyrin has confused me a bit now, as I said, I think you need to retard the cam a bit i.e get the valve to close a bit later but then it will open even later still... but if you advance it to get it to open sooner, then it will close sooner and that feeling of compression will increase further still as the piston will be trying to travel towards bottom dead center with the valve closed.

Hmm, to me it sounds like the profile on the cam can't be right, it's too severe, there is too much period where it's closed by the sounds of it.

Nick

 

[Gedeon Spilett]

Hi Chizz
you asked for a picture of the cam, here come pictures.
However the cam profile is not sufficient in its own to control the opening of the flap.
We don't have here a simple and needle like cam follower moving radially. the point of contact with the combined "spring and follower" is moving along the follower as the cam rotate and affect valve events, leading to more sharp slope I think (maths are a little too harsh for my gray cells now).
I draw and turned the best cam you can imagine, immediately modified by a few stroke of a file, adjusting flap movements experimentally. At the end, I put two cam close together on the crankshaft, and by turning them relatively, I had an adjustable cam allowing a lot of trials to be performed. It was a most enjoyable moment.
you may notice on the picture that the spring doesnot touch the cam when the flap is closed. I had to change the spring in a hurry last sunday to take the videos, and the new one is too short. But this doesnot affect greatly running. It is why I said that there are tolerances in the setting of the valve, provided the powerstroke occurs just in time.
I'm sure it is almost there

Zephyrin

View attachment cam_g3.pdf

 

[chizz]

thanks Zephyrin

those photos show me the shape of my cam is no too different to yours I had not thought about mounting to cams on that's a good idea.

also going from what a few have said about making sure it pulls no cold air in, and the port is quite wide on this engine i might remake the burner with a flat wick to cover more of the area.

thanks again for all your help all.

 

[Gedeon Spilett]

Hi, Chizz
I did a close look at your engine, a very nice work, the main bearing doesnot easy to do. I wonder what are the two little ear-like pieces that one can see on the back of the flap?
I think that the burner 's wick is too low, my engine run with the cylinder port completely surrounded by the flame.
Its a pity that no other of these engine, besides ours as far as I know, are built, they are so amazing to watch when running, as you will see soon!
Zéphyrin

I also add the cover picture of the french magazine "Le bricoleur", something like "the handyman" where I have found the plan of the engine.

 

[chizz]

hi Zephyrin

i think the pieces you are referring to on the back of the valve maybe the spit pins that are pinning the valve rod to the valve these need trimming down which i have not done yet, also about the burner i am thinking about making it again with a flat wick so the flame completely covers the port.

i do hope my engine works it will make are engines rare as it seem no one wants to build this design alot of people have said it looks like it could not work, but you have got it running so you have proved that theory wrong hope i can.

the main bearing were a bit awkward to make but i am a cnc machinist by trade so i have the resources. not something i think could be made on a manual mill.

 

[NickG]

Nice videos zephyrin, the large engine sounds and speed is much like my internal valve one due to the metal to metal contact of the valve .

 

[Gedeon Spilett]

Hi NickG

yes the noise is amazing, however I did not understand where he come from, for me it is not a simple mechanical rattling, a large part seems also due to air movements trough the valve port, I don't have measuring devices in my own workshop to experiment further, it is a pity because I sure that some energy is lost in such a noise. Being retired 6 months ago, I will never dare go back at work with my engines to do some experimental tests, just thinking at the face of my ex colleagues...
Zéphyrin

 

[chizz]

happy day got my engine running it was running slow but that is good enough for me today, at least I no I'm doing something right it ran better when i added small torch to it so i think i need a bigger flame or a different position.

well happy will try and get a video on soon.

 

[TroyO]

Congrats!

I was checking in to see how it went. I had never heard of a vacuum engine before so the whole theory of operation was new to me. Thanks for posting about it, and congrats on making a runner out of her!

 

[chizz]

Thanks TroyO

Making these engines is new to me to my first vacuum engine so happy i got it going, has inspired me to maybe design my own now i know the fundamental requirements.

Chizz

 

[Gedeon Spilett]

Hi
good news Chizz, may i suggest that you keep major tweaking after your lecture, which would be a great show with your engine running... so keep it running!
this engine cannot be a fast runner anyway, too much drag, too long stroke, too heavy valve flap and so long.
congratulation
cheers
Zéphyrin

 

[NickG]

Well done Chizz - can't wait to see the video.

Nick

 

[chizz]

hi nick

and thanks

video is on the questions and videos forum.