PyroJoes Recipe for Beginners

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PyroJoe
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PyroJoes Recipe for Beginners

Post by PyroJoe » Fri Sep 24, 2010 8:50 pm

Well design is a tough subject, but I will throw in my 2 cents.

What makes the topic of performance and ratios a touchy subject is the variations of function that the engine is required to perform.

It is nearly like asking a group of people what is the best boat? Some will say a tug boat, some will go into detail about the fastest hull and keel design of sailboats throughout history. Others will talk about "go-fast" planing hulls and hydrofoils and go on about prop sizes. Some will chat about their favorite row boat. How about those Ice Breakers? Ask the question in a pub near a sea port and your liable to start a fight.

The following is my own recipe for how to design a easy starting propane engine with reasonable performance and throttle.

Lets start with the exhaust, It is most difficult to roll a long skinny tail pipe, so a builder usually wants to find a section of existing pipe for this. The INNER DIAMETER is extremely important to the rest of the engine, so measure it with great care. It will be the most important measurement you make on the engine, and will control the rest of the design.

The value is so important that from here forward I will refer to the exhaust diameter as CALIBER.
Ok, so you have measured the caliber, how long should the tailpipe be? 20 CALIBER tends to work well.
So that pretty much has the tail pipe measured out.

Lets look at the Combustion Chamber. Typically a easy starting engine has a chamber of about 2 CALIBERs inner diameter by 4 CALIBERs in length (as seen in the "Muffler Shop Special" in the above plans section). A cone transition from the cylinder to the exhaust pipe helps performance, but adds complexity. If you do use a truncated cone, try to keep the volume of the cone and the remaining Chamber near the SAME as a chamber that has no transition.
What this requires is a stretching the length to greater than 4 CALIBERs to facilitate more volume. But....be forewarned as to much transition over 5 CALIBER will become problematic. I typically find 20 degrees to be a good transition cone angle. As a side note it is best for a beginner to start with a Chamber near 3" inner diameter for a first engine, which puts the CALIBER near 1.5".

So that pretty much pegs down the size of the Combustion Chamber. There are probably a million questions why to use such a basic volume for the chamber, my own answer for this is Cavity Resonance nothing more. Blow over a empty cola bottle and the sound it produces is cavity resonance. Most of my best engines have this property, sometimes to a frightening degree.

Once when grinding near the opening of a big engine the grinder blew air across an opening and the resonance gained amplitude. For a second or two it was akin to grinding a sizeable blaring fog horn. My nerves aren't easily rattled but this was unexpected. Imagine two cola bottles 6 inch diameter, and over 12 inches tall coming into resonance.

On to intake design.
Although I have a great respect for the Lockwood design, I think it should be avoided like the plague by someone new to pulse jets. In my work I have found that intakes larger than the exhaust are nearly opposite of what is needed for aerodynamic valving. It is much better to have a intake of smaller diameter than the exhaust for ease of starting.

In testing many engines that have the same diameter intakes as exhaust, it is just not worth the difficulty in starting. Therefore I repeatedly will reduce the diameter of the intakes to 0.88 CALIBER (typically 70% to 88% caliber works well). This tends to add just enough containment that the blast will bias more to the exhaust than the intake and the engine will jump to life much easier. I will also try to bias the blast by applying the cone to the exhaust side only. REMEMBER... this is a design method for pressurized propane fueling using injectors and will have little use if applied to non-pressurized fueling.

Intake length is little tricky, but has worked well at 2.67 CALIBERs. Flare the open end if possible.



PART 2
As you become better at staring and running your engines, you may attempt pushing the engines more and see how they do. Eventually there will be a point that no matter how much pressure/fuel run through the injector, it just doesn't do much more than dramatically increase/push blue flame out the intake. Yeah it spits it back out!

If you have reached this point, have a little celebration for its time to goto the next step.
Tail expansions help performance on most but not all engines fueled by propane injectors.

It is best to start with a conservative expansion, of the last 7 CALIBERs of the tailpipe leaving 13 CALIBERs of the original pipe. The truncated cone for the expansion is typically near 9 degrees (this example is 9.53) and leads into a 2 CALIBER diameter cylinder 1 CALIBER in length. The engine will show little difference other than the noise will become more pronounced, sometimes it is slightly easier to start.

After a review of the small performance changes, it is time to make a bigger shift, moving the expansion a little over 4 CALIBERs forward, leaving approx. 9 CALIBERs of the original tail pipe. The cylinder at the tail increase from 1 CALIBER in Length to a whopping 5 CALIBERS! The noise level will increase considerably. Now when you push the engine it sounds like a trumpet match between two archangels.

It is of paramount important to review injector designs, I especially like the three point injector by GRIM, and of course the Rosscojector in the above "Looking for plans? Look Here" topic. I just can't put enough emphasis on injectors. Each minute working and learning about them will save hours, improve performance and waste less propane. Always keep an eye to what works.

Time needs to be allocated for working with the injectors, usually this work is overlooked or taken for granted. If it takes 10 hours to build an engine, I often allocate about three hours for the injector construction. Even if the engine is built perfectly and the injector is whipped together in 15 minutes, you may observe only 1/2 the total potential of the engine.
Happy weldings :wink:
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Re: PyroJoes Recipe for Beginners

Post by Mike Everman » Tue Sep 28, 2010 4:26 am

Very nice, Joe, thanks!
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Zaitsev
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Re: PyroJoes Recipe for Beginners

Post by Zaitsev » Wed Dec 22, 2010 12:40 am

Great info, would it still work if you use two smaller intakes, that the inside adds up to .88 caliber and have them facing to the rear?(like a thermo)

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Re: PyroJoes Recipe for Beginners

Post by PyroJoe » Wed Dec 22, 2010 2:59 pm

Yes it still works with two smaller intakes. What I do is multiply the .88 caliber by 0.71 This gives two areas roughly equal to the .88 caliber area like this:

.88 calibers= 0.6082 area in caliber units

.88 calibers x 0.71= 0.6248 caliber diameter each =0.3066 area ea. x 2 intakes = 0.6132 area in caliber units

Not perfect, but is close.
I haven't had a chance to try it in thermo style yet, "hard folding" has produced a shorter engine.

thanks, Joe

Thanks Mike for the encouragement, I hope this is useful to some.

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Re: PyroJoes Recipe for Beginners

Post by Zaitsev » Wed Dec 22, 2010 3:53 pm

Thanks for the help, One more thing i just remembered, how thick does the steel of the chamber compared to the rest of it?

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Re: PyroJoes Recipe for Beginners

Post by PyroJoe » Wed Dec 22, 2010 4:41 pm

The part of the design I typically use thick material (125%-150%) is the cone section and the narrow section of the tail pipe.
They experience the highest heat, and are furthest from the ends. That specific area will typically experience low internal "vacuum" pressure and the outside air will try to "crush" those areas inward.

Usually not a problem in small engines but when engines are built large, near or above 2" in tail diameter the problem tends to show.


Many more engines have been crushed than exploded outward.

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Re: PyroJoes Recipe for Beginners

Post by PyroJoe » Fri Jan 21, 2011 4:41 pm

Here is information on folding an engine, there are differing opinions and options from what I will post, but this is what works for me.

My goal is to clarify/reveal some of the mystery of how to fold an engine. The caliber method helps visualize this somewhat, and have included the centerline distance represented in red as a reference.

REPOST-"If you have a piece of pipe that looks about the right size for something you want to build, the pipe diameter becomes the measure. Units are diameter.

It is often neccessary to have this section thick enough to avoid crushing under the low pressure part of the cycle. It represents the tightest roll with the thickest material of a build.....i.e. the highest pain in the posterior to roll/fabricate."

if Caliber is confusing, then don't measure, use a short cut section of tailpipe as the measure.

It is best to keep tight folds in the tail closest to the combustion chamber.


The key here is the 20 calibers/centerline distance of the tail is maintained, either in linear form or folded as seen in the second folded engine.

The radius of the bend can vary to accomodate the design or use of the engine. It is wise to fold in a manner that allows room for a intake augmentor/ejector when thrust is a important design parameter.

If you can imagine a fold so tight it becomes a simple opening of one caliber between the combustion chamber and the tailpipe(shown as the yellow caliber), this is what I term "hard folded", as there is not a tighter fold available. Of course this is nothing new as I basically patterned after a jet from our predecessors. the front is extended about half a caliber to preserve the combustion chamber volume/length. It eliminates the need to find/purchase/create the bend in the pipe. It does require more work in the cone transition of the CC.

Hope this helps.
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PyroJoe
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Re: PyroJoes Recipe for Beginners

Post by PyroJoe » Fri Jan 21, 2011 5:00 pm

First one I hard folded:
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Re: PyroJoes Recipe for Beginners

Post by radrider33 » Mon Mar 28, 2011 5:42 pm

Hi, I have a few questions about the designs on this page. The plans have been really helpful, as I'm making a pulsejet for my senior design project. In order to keep things simple, I'm planning on sticking with the simplest design on the page, with just two straight sections of pipe, the chamber being directly attached to the intake and exhaust with no cones. my questions are: which injector designs would be best for this engine, and what should the injector/spark plug placement be on the combustion chamber? I'm planning on making a jet with a 3" combustion chamber for the sake of being able to acquire pre-rolled steel.
Jay

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Re: PyroJoes Recipe for Beginners

Post by PyroJoe » Mon Mar 28, 2011 8:18 pm

Given that propane is the fuel:
A perpendicular cut Rosscojector works well for engines between 2" to 3.5" CC diameter.

A sparkplug could be applied near the mid-length of the combustion chamber although it isn't required. Typically the injector is placed just inside the intake and can be ignited by torch or lighter.

Pulsejets aren't the best choice for projects that are time critical. They require allotments of time to observe, adjust and modify.

If a cone is completely out of the scope then it becomes important to smooth/round the exhaust port transition from the CC to the tail pipe. If possible flare the exhaust pipe slightly.

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Re: PyroJoes Recipe for Beginners

Post by radrider33 » Fri Apr 01, 2011 2:22 am

Thanks so much for the information. I have about 2 months time frame and so i'm hoping that I can get a prototype built and then work out the kinks. I'll try a plug-less model; I can always add it in later on. Thanks again.

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Re: PyroJoes Recipe for Beginners

Post by ganuganu » Tue Sep 27, 2011 3:54 pm

joe, please clarify my doubt here i have designed an engine which does not have any cone only straight pipes. here i dont know which type of intake diameter to use can you say me..here is the design..

here iam using four intake pipes 90 degrees to eachother.
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Re: PyroJoes Recipe for Beginners

Post by PyroJoe » Tue Sep 27, 2011 6:31 pm

The caliber is 8cm, so 8x0.88= 7.04cm
which gives a total intake area of about 38.93 sq.cm.
38.93/4 intakes gives about 9.73 sq.cm per intake
a 9.73sq.cm intake is near 3.52cm in diameter

So it appears 4 intakes that are 3.52cm in diameter should work.

This is somewhat a partial blended engine so don't be surprised if you have to cut the intake length from 19cm to near 16.4cm.

Real interesting project, keep up the good work.
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Re: PyroJoes Recipe for Beginners

Post by ganuganu » Wed Sep 28, 2011 2:29 am

do you recommend me to use 1.7cm dia(for each) intake pipe or 3.52cm dia pipe(for each).

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Re: PyroJoes Recipe for Beginners

Post by PyroJoe » Wed Sep 28, 2011 1:05 pm

3.52cm dia (for each)

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