PyroJoes Recipe for Beginners
Posted: 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
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