Login    Register
User Information
Username:
Password:
We are a free and open
community, all are welcome.
Click here to Register
Sponsored
Who is online

In total there are 66 users online :: 5 registered, 0 hidden and 61 guests


Most users ever online was 155 on Mon Aug 15, 2016 1:40 am

Registered users: Bing [Bot], Google [Bot], Google Adsense [Bot], MSNbot Media, Yahoo [Bot] based on users active over the past 5 minutes

The Team
Administrators
Global Moderators
global_moderators.png CS

premixed fuel chamber.

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
Sponsored 
  • Author
    Message

Unread postAuthor: SpudFarm » Mon Aug 02, 2010 3:09 pm

I can't get involved into your discussions before I have first hand experience so I will rather ask you guys to give me some opinions on my valve design.

I want to keep the design as simple as possible but still effective. I want it to be quick to fuel and eventually vent.
I have been thinking about size for the valve for a while and I am stuck between 2" and 3" tee fitting housings. I want to use a golf ball sized barrel so it has to have some flow. It is going to be used at 5 to 10x mixes.

My problem is that I don't know how to calculate opening pressure and that sort of stuff yet.

What I want is a 2" housing but I don't know if that is possible with a 1.5" seat. If possible I will use a 1.75" seat but the problem is that I have no clue.

Keep in mind that this diagram is not done. Fueling ports ETC is not added yet.

Sorry about my crappy explanation but this is quite hard to explain for some reason.
  • 0

Attachments
Paint3.png
Paint3.png (8.64 KiB) Viewed 380 times
"Made in France"
- A spud gun insurance.
User avatar
SpudFarm
Donating Member
Donating Member
 
Posts: 2563
Joined: Sat Nov 04, 2006 9:39 am
Location: Norway Trondheim area
Reputation: 0

Unread postAuthor: SpudBlaster15 » Mon Aug 02, 2010 3:51 pm

Looks good.

You should be able to use a 1.5" seat in a 2" housing, as this is what I've done with my design.

Opening pressure is calculated by determining the area ratio of the front and rear piston surfaces, then using the relationship between pressure, surface area, and force to determine the pressure differential required to obtain a net force in the backward direction. Your equation would look something like this:

P<sub>opening</sub> = P<sub>pilot</sub>*pi*r<sub>piston</sub><sup>2</sup>/(pi*r<sub>piston</sub><sup>2</sup> - pi*r<sub>seat</sub><sup>2</sup>)
  • 0

People should not be afraid of their governments. Governments should be afraid of their people.
User avatar
SpudBlaster15
Major General
Major General
 
Posts: 2385
Joined: Wed Oct 18, 2006 11:12 pm
Location: Canada
Country: Poland (pl)
Reputation: 3

Unread postAuthor: SpudFarm » Mon Aug 02, 2010 6:58 pm

Thanks for your help!

As I am really bad when it comes to math I will have to try and use the calculations I think of as logical and get a equation that you can approve.
What I am about to do is to find the surface area (roughly as I don't have exact measurements of the parts I will use) and find the force acting on the piston.

1.5" seat:
1.5"= 3,81cm 1,905*1,905*3,14= 11,395cm2 surface area on seat.
2" piston:
2"= 5.08cm 2,54*2,54*3,14= 20,258cm2 surface area on entire piston.

So subtracting the area of the seat from the piston itself gives me 8,863cm2 surface area on the "ring" that the chamber pressure acts on.

I am pretty certain the above is correct but from here on out I am completely in the dark. I will play around and see where it leeds me.

Lets just start off with a rough 11x mix assuming a pre ignition pressure of 10bars.
That will lead to those loads:
Seat load: 113,95kg
Entire 2" piston load: 202,58kg
Ring load: 88,63kg
From here I am guessing that I will have to raise the pressure (in form of combustion pressure) in front of the piston (between the seat seal and the seal against the housing) to the point where the load on the "ring" is higher then the 113,95kg acting on the seat.

Once again I am in the dark and only doing math out from what I find logical.

I took the 113,95kg acting on the seat and divided it to the 88,63kg acting on the "ring" and got a ratio of 1,285. I then took the pressure of 10bar and multiplied it by the ratio between the seat and the ring and got a pressure of 12,85bar.
Now I want to check if there are any bond between those numbers so I find the force acting on the "ring" when subjected to 12.85bar and that turned out to be 113,889kg witch is pretty close to the force acting on the seat. The deviation is caused by me shorting down to two decimals.

What this seems to tell me is that this valve will open at slightly above 12,85bar.

I don't know if this makes any sense to you at all but if I am right I will be suprised. I would think that the load on the seat would simply rise as the combustion rises the pressure. I hope that you mention something about pilot pressure ETC that causes everything to make more sense to me.

I can try to write a shorter version of this if you want me to, this is just a "blueprint" to make sure I got it right.
  • 0

"Made in France"
- A spud gun insurance.
User avatar
SpudFarm
Donating Member
Donating Member
 
Posts: 2563
Joined: Sat Nov 04, 2006 9:39 am
Location: Norway Trondheim area
Reputation: 0

Unread postAuthor: SpudBlaster15 » Mon Aug 02, 2010 7:55 pm

You're on the right track, and very close to getting it right.

From here I am guessing that I will have to raise the pressure (in form of combustion pressure) in front of the piston (between the seat seal and the seal against the housing) to the point where the load on the "ring" is higher then the 113,95kg acting on the seat.


The forward force acting upon the piston is actually a function of the overall piston surface area, not the area covered by the barrel port during the valve's closed position. So, rather than using 113.95kg in this calculation, you would use your other figure of 202.58kg.

I hope that you mention something about pilot pressure ETC that causes everything to make more sense to me.


The first thing to remember is that the piston area which is covered by the valve seat does not matter here. The chamber pressure acts only on the 'ring' you refer to.

The pressure within the pilot chamber is what produces the net forward force on the piston prior to ignition. The pilot pressure acts upon the entire rear surface of the piston, which is why the entire cross sectional area is used to find the force required to open the valve from the chamber side.

Hope this helps, the explanation is a bit rushed as I need to leave in a few minutes.
  • 0

People should not be afraid of their governments. Governments should be afraid of their people.
User avatar
SpudBlaster15
Major General
Major General
 
Posts: 2385
Joined: Wed Oct 18, 2006 11:12 pm
Location: Canada
Country: Poland (pl)
Reputation: 3

Unread postAuthor: SpudFarm » Mon Aug 02, 2010 8:57 pm

I am getting closer here, but there are still bits I need to place right.

If I got it right now you have a perfect seal around the piston in the housing. Prior to fueling you pressurize the pilot chamber to a pressure higher then the pre ignition pressure to hold it sealed, then fuel directly into the chamber?
Then when the pressure infront of the piston gets higher then the pressure behind it, it will actually crack the valve?

I thought the piston would have to be forced from the barrel port and thus limiting you to using a port with less than half the surface area of your piston itself.

When you get time you could explain it in more depth. It is kind of hard to get it at first.

Thanks for your help again.
  • 0

"Made in France"
- A spud gun insurance.
User avatar
SpudFarm
Donating Member
Donating Member
 
Posts: 2563
Joined: Sat Nov 04, 2006 9:39 am
Location: Norway Trondheim area
Reputation: 0

Unread postAuthor: SpudBlaster15 » Tue Aug 03, 2010 12:50 am

If I got it right now you have a perfect seal around the piston in the housing. Prior to fueling you pressurize the pilot chamber to a pressure higher then the pre ignition pressure to hold it sealed, then fuel directly into the chamber?
Then when the pressure infront of the piston gets higher then the pressure behind it, it will actually crack the valve?


Fundamentally, yes. However, because of the area differential between the 'ring' on the front of the piston and the surface acted upon by the pilot pressure, the chamber will have to reach a much higher pressure than the pilot chamber to crack the valve open.

For example, if you have a piston cross sectional area of 5cm<sup>2</sup>, and the area of the 'ring' is 1cm<sup>2</sup>, this gives you a ratio of 5:1. Assuming you have 10bar in the pilot chamber, the chamber pressure will need to rise to 5*(10bar) = 50bar to equalize the directional force (Net force of zero). An increment higher than 50bar, and the net force will shift to the reverse direction, and the piston will move backwards, opening the valve.

The important thing to remember is that when calculating the amount of frontal piston area exposed to pressure (The area of the 'ring'), you must use the outside diameter of the piston seat, not the inside diameter of the pipe. So if you're using 1.5" nominal pipe as the seat, the figure you need to use is 1.9".

I thought the piston would have to be forced from the barrel port...


Once the valve has initially opened, the entire frontal surface area is exposed to chamber pressure (The pressure will of course begin to drop, as the barrel increases the volume of the system), and the net force in the direction of piston opening travel becomes MUCH greater. This is what allows the valve to initially resist motion at high chamber pressures, then fully open within a few milliseconds after the barrel port seal is broken.
  • 0

People should not be afraid of their governments. Governments should be afraid of their people.
User avatar
SpudBlaster15
Major General
Major General
 
Posts: 2385
Joined: Wed Oct 18, 2006 11:12 pm
Location: Canada
Country: Poland (pl)
Reputation: 3

Unread postAuthor: SpudFarm » Tue Aug 03, 2010 9:08 am

Got it! I am not entierly done reading about it but I think I know enough to start designing the valve to more detail.

I will write more once I have another diagram.

Edit: I made a diagram where I made the seat incredibly close to the housing diameter.
I just want to ask if this design could work. No calculations, just your judgement based on the looks of it.
If you tell me that this would work it will make the rest of the designing much easier.
  • 0

Attachments
concept.png
"Made in France"
- A spud gun insurance.
User avatar
SpudFarm
Donating Member
Donating Member
 
Posts: 2563
Joined: Sat Nov 04, 2006 9:39 am
Location: Norway Trondheim area
Reputation: 0

Sponsored

Sponsor
 


Unread postAuthor: SpudBlaster15 » Tue Aug 03, 2010 11:57 am

Yes, it should definitely work.

That looks about right for a 2" housing and 1.5" pipe seat. This will give you a pretty convenient surface area ratio.

Also, you have mad MS paint skills. :P
  • 0

People should not be afraid of their governments. Governments should be afraid of their people.
User avatar
SpudBlaster15
Major General
Major General
 
Posts: 2385
Joined: Wed Oct 18, 2006 11:12 pm
Location: Canada
Country: Poland (pl)
Reputation: 3

Unread postAuthor: SpudFarm » Tue Aug 03, 2010 2:49 pm

Then I hope to build this as soon as possible.

I will try to get the next diagram exactly to scale, I just have to find the dimensions of the tee and the seat.

I wasn't using paint for this one, I used something called "EMachineShop"
It allows me to make each part out of different materials and then turn it into a 3d model if I want to.
Then the program can calculate the cost for those parts made on CNC machines.
I recommend it!
  • 0

"Made in France"
- A spud gun insurance.
User avatar
SpudFarm
Donating Member
Donating Member
 
Posts: 2563
Joined: Sat Nov 04, 2006 9:39 am
Location: Norway Trondheim area
Reputation: 0

Previous

Return to Hybrid Cannon Discussion

Who is online

Registered users: Bing [Bot], Google [Bot], Google Adsense [Bot], MSNbot Media, Yahoo [Bot]

Reputation System ©'