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ignition problems for propane

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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ignition problems for propane

Unread postAuthor: bennettjackson » Wed May 11, 2011 8:20 pm

ok so ive finally got around to testing my propane and used the burnt lake fuel tool and found that if i have a chamber volume of 216 ci, factored in the 14.7 psi of atmospheric pressure and with a 1/2" x 6" meter pipe i would need about 70 psi in my regulator, so why wont it ignite sometimes, and when it does its a little dud, any ideas? should i just play around with different pressures? thanks
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Unread postAuthor: jackssmirkingrevenge » Thu May 12, 2011 12:34 am

I got around 109 psi.

metre volume is 1,17 cubic inches

metre volume plus chamber volume is 217,17 cubic inches

4% of total volume (ie volume of propane required) is 8,68 cubic inches at atmospheric pressure

Metre pressure is therefore 8,68/1,17 x 14,7.

Problem with fuel tool/your data input/my calculations?

Could it also be that your ignition system isn't reliable enough, or you are not venting the chamber properly between shots?
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Unread postAuthor: bennettjackson » Thu May 12, 2011 6:38 am

ill give it 108 pso a shot thanks, ill let ya know
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Unread postAuthor: Technician1002 » Thu May 12, 2011 8:49 am

Depending on the physical layout sometimes the length of pipe between the meter valve and the chamber will contain a significant volume of the fuel. The calculated pressure is a starting point.

From the calculated meter volume, bracket the pressure by 5% up and down to tune in your results.

The calculator assumes no space between the meter valve and the chamber so all the fuel is injected into the chamber with none remaining in the plumbing between the meter and chamber. This often leads to a lean mix if this pipe has significant volume. When building this pipe is best to be short as possible and small diameter. The second meter valve should be as close to the chamber as possible for reliable fuel injection.

Here is a good example of a meter with minimal pipe between the chamber and meter. I picked it from the showcase.
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Unread postAuthor: jackssmirkingrevenge » Thu May 12, 2011 9:02 am

Technician1002 wrote:The calculator assumes no space between the meter valve and the chamber so all the fuel is injected into the chamber with none remaining in the plumbing between the meter and chamber. This often leads to a lean mix if this pipe has significant volume. When building this pipe is best to be short as possible and small diameter. The second meter valve should be as close to the chamber as possible for reliable fuel injection.


Since technically the meter already contains 14,7 psi of air (I assume most designs aren't purged, and the meter gauge reads 0 at atmospheric pressure) opening the meter valve means the propane will flow towards the chamber and a homogenous mix of air and propane will result, the calculator should assume that the meter should contain enough propane to give 4% concentration in the total chamber+meter volume. Any connecting piping should be included in this volume.

Anyone care to check my calculations, I made them this morning over a hurried coffee ;) they don't coincide with the burnt latke results (tried to input the data myself and got the same result as bennetjackson) so either I've got it wrong or the calculator is flawed.
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Unread postAuthor: Technician1002 » Thu May 12, 2011 9:06 am

Fuel meters should be purged of air before use so for the first shot they should contain 0 PSI of propane. A couple of meter cycles without a shot and venting the chamber should take care of that. They can then be stored with both valves closed so they are primed and ready for another day of shooting.

Any connecting piping between the meter and chamber should be counted as part of the chamber volume. Unfortunately this volume does not tend to mix well with the rest of the chamber volume and thus can mess up your chamber mix ratio.
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Unread postAuthor: jackssmirkingrevenge » Thu May 12, 2011 9:24 am

Let's say the meter contains only propane, which means that when the meter gauge is at zero there are in fact 14,7 psi of propane in the meter (in our case, 1,17 cubic inches of propane at atmospheric pressure).

We need to calculate the amount of propane needed for meter plus chamber, because unless you have a plunger on the meter that can totally empty it, you need to have a correct 4,2% mix in both meter and chamber.

So, I'll do it to the correct percentage this time.

Meter + Chamber volume is 217,17 cubic inches.

4,2% is 9,12 cubic inches of propane at atmospheric pressure required.

This, by the way, assumes that the propane is going to displace some of the chamber air. If the chamber were perfectly sealed, you would have to calculate 4.38%, but anyway.

9,12 cubic inches of propane in a 1,17 cubic inch meter gives us 114,6 psi of propane in the meter assuing the gauge reads -14,7 psi at atmospheric pressure. Assuming the gauge reads zero at atmospheric pressure, the psi required in the meter is 99,9 psi.

For a gauge reading zero at atmospheric pressure and a non-purged meter, you need 114,6 psi in the meter.

This is well short of fuel tool's predictions...
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Unread postAuthor: Technician1002 » Thu May 12, 2011 12:34 pm

In cool weather Propane might not give the required pressure. An alternative to building the meter larger is to use a lower pressure and a multiple charge. For example the Sureshot cannons meter 2 half shots of propane.
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Unread postAuthor: saefroch » Thu May 12, 2011 3:08 pm

It's always possible to heat up the can with body heat, or just use a larger meter.
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Unread postAuthor: bennettjackson » Thu May 12, 2011 10:46 pm

thanks everyone, however im not able to get my pressure up past 100, if anyone could find out what size pipe i would need for a 80 or 90 psi charge that would be much appreciated, thanks
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Unread postAuthor: Technician1002 » Thu May 12, 2011 11:19 pm

The max pressure is related to the temperature of liquid propane. As mentioned before you can use 2 half charges. Simply calculate for 1/2 and give it 2 shots of fuel.

Start with 2 shots at 50 PSI and adjust from there.

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Unread postAuthor: jackssmirkingrevenge » Thu May 12, 2011 11:22 pm

bennettjackson wrote:thanks everyone, however im not able to get my pressure up past 100, if anyone could find out what size pipe i would need for a 80 or 90 psi charge that would be much appreciated, thanks


Simply double the length of metre pipe and you'll need half the pressure ;)
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venting?

Unread postAuthor: bennettjackson » Fri May 13, 2011 3:49 pm

do you know if i need ventilation holes to let the 4.03% if air out instead of just adding in propane?
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Unread postAuthor: Technician1002 » Fri May 13, 2011 4:21 pm

No need for holes. A little leakage past the projectile will do fine.

As mentioned earlier, play with adjusting the mix up or down a little to tune in the mix for your conditions. The mix is not super critical. It will have good performance over a fairly narrow range. You may need a chronograph to find the absolute peak performance.

As you have noticed, when the mix is more than a tiny bit off, the power drops off and it becomes difficult to get ignition.

As you adjust the mix in the right direction, you will notice the increase in power right away.
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Unread postAuthor: bennettjackson » Fri May 13, 2011 5:04 pm

alright thank you and just so i know, how much psi does a new tank of propabe have?
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