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hybrid fueling

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.
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hybrid fueling

Unread postAuthor: jakethebeast » Sun Jan 23, 2011 12:24 pm

good evening spudfiles!

i know that here are tons of topics about hybrid fueling but still, i dont understand it.

Im building my first hybrid, tomorrow im going to buy fuel meter parts and few others and make the ingnition system. If i know right, for 1x mix i need 7.3 psi of propane in the chamber? and for 2x mix is it right that i put 17.6psi propane and 1 or 2bar of air in it? is it really this simple? :D
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Unread postAuthor: Lockednloaded » Sun Jan 23, 2011 1:26 pm

I don't know where you got those propane pressures, but in any combustion you need ~4.2% propane in your chamber, but in a hybrid you multiply that by the mix number, so at 10x you need~42% of your chamber volume worth of propane. As for air you need one atmosphere of air pressure which is about 14.7psi, but it verys from place to place. There is already one atmosphere worth of air in your chamber, so at 2x you would need to add 14.7psi so that you have two atmospheres worth of air. Got it?
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Unread postAuthor: DYI » Sun Jan 23, 2011 1:36 pm

I would advise you to search harder, but doing so would achieve nothing as the topic which very thoroughly answers ANY sub-200X hybrid fueling question is directly above the one you just posted.

Also, it appears to be time for a rant; 7.3psi? REALLY? How the hell did you arrive at that? Can you not see how blatantly wrong that is just by looking at the simplified equation for the propane/oxygen reaction? Calculators are fine things, but you need to have at least some intuition as to what kind of numbers you're looking for.
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Unread postAuthor: jakethebeast » Sun Jan 23, 2011 2:51 pm

well it wasnt that much of, read that thread again and it was 6.44psi. BUT i can be totally of
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Unread postAuthor: DYI » Sun Jan 23, 2011 3:52 pm

well [sic] it wasnt [sic] that much of [sic?], (I)? read that thread again and it was 6.44psi. BUT i [sic] can be totally of [sic]


That wasn't particularly coherent, but the gist of the first sentence seems to be that you're reading one or two lines of SB15's excellent guide, and completely ignoring the calculations. If that's the case, I'd strongly advise you to stay away from hybrids altogether, lest the community as a whole be unfortunate enough to receive negative publicity when you hurt yourself using a device you do not understand.

Otherwise: Yes, it was "that much off". By an entire order of magnitude, and then some. Do the calculations for yourself, on paper, and in a few minutes you will notice what you would already know, had you taken time to read the thread carefully - your initial number was off by a factor of twelve.

As to the second "sentence": "Oh look: you fail at English AND hybrids." translated (via BabelFish) through Dutch --> French --> German --> Traditional Chinese and finally back to English yields "Oh seeing: They are lack the lors English and to the hybrid", which, after creative shuffling of punctuation and capitalization, sums up your situation quite nicely.
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Unread postAuthor: MrCrowley » Sun Jan 23, 2011 4:07 pm

and 1 or 2bar of air in it?

HA!

The HYBRID FUELING 101 (introduction to hybrid fueling*) topic can be a little difficult to grasp at first. The mistake I always made was using the example of a 100ci chamber in my calculations where you divide (I believe it's the first calculation) by chamber volume. While I would use my chamber volume for the initial first calculation, I would always divide it by 100 which is wrong if your chamber isn't 100ci.

I suggest you do a lot more research before asking for more help. While it is okay to ask if your figures are right, it'd help if you had some idea of what you were doing.


*this is a core course and must be taken in your first year of study if you plan on majoring in Hybrid Science.
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Unread postAuthor: jimmy101 » Mon Jan 24, 2011 9:05 pm

Hybrid fueling isn't all that hard to understand, just take your time and start with something simple.

For a 1X mix you need 4% of the chamber's volume as propane. If you are fueling into a closed chamber then the pressure in the chamber goes up by 4% when the fuel is injected. Since atmospheric pressure is about 14.7 PSI adding 4% more stuff will raise the pressure (14.7)(1.04)=15.3 PSI. That is an absolute pressure. Most pressure gauges are calibrated to "Gauge Pressure" which basically means the reading is 14.7 PSI less than the actual pressure and the gauge reads zero when open to the atmosphere. When fueling for 1X you need to get the gauge pressure to go up by (14.7)(0.04)=0.6 PSIG. 0.6 PSIG = 15.3 PSI

For a mix greater than 1X you can just multiply the 0.6 by your hybrid number. So, for a 3X mix you pressurize the chamber (0.6)(3)=1.8 PSIG. After pressurizing with the fuel (this is all calculated for propane at 4%) you then pressurize the chamber with air. For a 3X mix you can just pressurize until the gauge reads (14.7)(2)=29.4 PSIG. (The other 14.7 PSIG of air is already in the gun, but it read zero. 29.4 PSIG = 44.1 PSI)

Actually the above air pressure will be a bit low. You actually want to multiply 14.7+0.6 by your hybrid number, not just the 14.7. But the difference is pretty small.

You don't need to know the volume of your chamber for this approach to wrok. However, you do need a reasonably precise pressure gauge. Many gauges are only calibrated in 2 PSI steps and are pretty inaccurate below 5 PSIG. So this method won't work with a cheapo pressure gauge since you can't come anywhere near an accurate measurement of 0.6 PSIG (1X), or 1.2 PSIG (2X), or 1.8 PSIG (3X) ...

The only time you really need any more math than the above is when you are using a pressurized meter. Then you have to take into account things like the amount of fuel left in the meter pipe, the pressure in the meter etc.

EDIT: BTW, if you do find an accurate pressure gauge that reads down in the vicinity of a couple PSIG, and you leave it connected to the chamber when you fire the gun, your expensive pressure gauge will probably be converted into a single use instrument.
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Unread postAuthor: Lockednloaded » Mon Jan 24, 2011 9:20 pm

jimmy 101, correct me if I'm wrong, but doesn't that methods, known as manometric fueling, only really work with higher mixes and lower chamber volume?
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Unread postAuthor: saefroch » Mon Jan 24, 2011 10:39 pm

I think the method jimmy101 has described is known as "chamber fueling" and is usually recommended for higher mixes. That's because the error in a normal analog gauge for a 1X or 2X mix could be all of the fuel. By using a meter, you measure a much higher value with the gauge, and measurement error becomes less significant.

Simply put, to get absolute pressure from gauge pressure it's a simple addition of your local pressure, which is normally 14.7psi, but if you want to be really accurate, you can check the weather and use the actual atmospheric pressure value. Instead of working around with the funky 14.7, I'd just crank everything through in atmospheres, then convert to whatever you're measuring in later. A 1X mix is just the air in the chamber and 0.042atm propane, and an online calculator will tell you that's .62psi, try reading that on an analog gauge.

jimmy101 wrote:BTW, if you do find an accurate pressure gauge that reads down in the vicinity of a couple PSIG, and you leave it connected to the chamber when you fire the gun, your expensive pressure gauge will probably be converted into a single use instrument.
I purchased a digital 0-300 psi gauge, reads to a tenth of a psi. Error is 2% :D Simply placing a ball valve between the gauge and the chamber will prevent most damage, unless you're SB15. But even so, just because it's only supposed to be used to a certain pressure, the way digital gauges work it can probably handle far past there, and will actually shut itself off if you go much past the intended operating range.

In addition, if you do the "air-through-meter" fueling setup now advocated by the site's hybrid enthusiasts, you don't have to worry about any fuel left in the meter. You add the fuel first, then leave the meter open to the chamber and add the air or oxygen as if you're chamber-fueling.
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Last edited by saefroch on Mon Jan 24, 2011 10:41 pm, edited 1 time in total.
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Unread postAuthor: MrCrowley » Mon Jan 24, 2011 10:41 pm

Lockednloaded wrote:jimmy 101, correct me if I'm wrong, but doesn't that methods, known as manometric fueling, only really work with higher mixes and lower chamber volume?

Why low chamber volume?

DYI thought it would be best used on higher mix hybrids but I haven't had a problem as low as a 4x mix. I imagine if you had a tiny chamber volume it would be more difficult to use than a larger one.

I'm quite grateful to DYI for this method, I find it much easier to use than volumetric metering. By the way, I think the accuracy for the gauge I used was about 1.5% F.S. (full scale). It was a gauge from a tyre inflator used on drag cars I believe, so it had to be quite accurate.

I purchased a digital 0-300 psi gauge, reads to a tenth of a psi. Error is 2%

Is that 2% full scale? If so, doesn't that mean if the gauge reads, say 140PSI, it really could be 140PSI +- 6psi?
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Unread postAuthor: Lockednloaded » Mon Jan 24, 2011 10:49 pm

I'm having a whole lot of trouble finding an accurate, low-pressure, and most importantly cheap, pressure gauge. I've seen digital ones for tires, but I doubt their accuracy. 3847K11 on McMaster-Carr is my best bet as of now
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Unread postAuthor: MrCrowley » Mon Jan 24, 2011 11:17 pm

You should be able to get a decent one for $30USD easily enough on eBay. Don't buy the crap branded ones, always look up the brand of the gauge and if they make good stuff then buy it (assuming the accuracy is good).

Mine came off this, bought secondhand for $20NZD (Moroso is the brand btw):
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Unread postAuthor: Lockednloaded » Mon Jan 24, 2011 11:34 pm

regulator with digital gauge would it work well at mixes up to 5x?
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Unread postAuthor: MrCrowley » Tue Jan 25, 2011 12:01 am

It says it's accurate to 1PSI which isn't great since that is higher than a single 1x mix for either propane or MAPP (10x mix is 6.44PSI with propane but with this gauge it means it could read either 5.44 or 7.44 I believe). At 1.5% F.S., my gauge would read any number +- 0.23PSI.

You can work out the error in the fuel:air ratio with a gauge like that though. I did it for my gauge (which was 1.5% F.S., I think) and realised that at its worst, the fuel:air ratio would be between 4.2% and 5.0% for MAPP gas (or something like that), which isn't that bad really. You'd probably have a higher margin of error using the volumetric method.
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Unread postAuthor: Lockednloaded » Tue Jan 25, 2011 12:08 am

with the mcmaster gauge I've got %2± so at a 5x mix using 2.4~psi of propane I could potentially have 1.92 or 2.88 psi, not good. far too inaccurate. I assume 1.5% is the minimum

EDIT, found a 1% accuracy gauge, but it goes up to 30psi and has 5 psi numeral marks, .5psi graduation marks. Its a "Glycerin Filled Gauge"
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