I think that the rod through the sides rotates the interior which has the effect of controlling when the internal passages vent extra flow via a helix.
But don't quote me on that.
Request - Injector designs
- Thread starter The Ignoble Troll
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I think that the rod through the sides rotates the interior which has the effect of controlling when the internal passages vent extra flow via a helix.
But don't quote me on that.
I will speak from my experience :
Designing and machining an injector is not too difficult for many members here. It's really important to make sure it's 100% sealed - otherwise it will upset the fuel pump system.
Spray pattern : It's very nice - But does it matter?? It depends on your wishes. But if you want a really small injector - smallest in the world
- you have to trade off spring force and then don't expect it to look good - unless experimenting with a larger amount of fuel than the engine needs. That's the problem I often see !!!! - If you don't limit the plunger stroke to the amount of fuel the engine needs, don't expect me to trust that result.
Sealed : Machine an injector and test it with compressed air (1 or 2 bar) and dip the injector tip in water or oil and it will tell you.
Designing and machining an injector is not too difficult for many members here. It's really important to make sure it's 100% sealed - otherwise it will upset the fuel pump system.
Spray pattern : It's very nice - But does it matter?? It depends on your wishes. But if you want a really small injector - smallest in the world
- you have to trade off spring force and then don't expect it to look good - unless experimenting with a larger amount of fuel than the engine needs. That's the problem I often see !!!! - If you don't limit the plunger stroke to the amount of fuel the engine needs, don't expect me to trust that result.Sealed : Machine an injector and test it with compressed air (1 or 2 bar) and dip the injector tip in water or oil and it will tell you.
Roger B
Well-Known Member
To get a small diesel to run the fuel pump, the injector and the combustion chamber are all important. Sufficiently atomized fuel must be injected into high temperature and pressure air in such a way that combustion takes place rapidly but in a controlled way.
As Minh says control of volume and timing are important. The volume can be changed by altering the stroke however this also tends to alter the timing of the start of injection. I think Find Hansen has controls to adjust the volume and timing separately. The alternative is to copy full size practice and fix the timing of the start injection whilst controlling the volume with the end of injection using a helix in the pump plunger. The components will be quite small, a 20 cc cylinder will require around 2mm3 of fuel at full output, say a 2mm plunger and ~1.5mm stroke.
The injector needs to sufficiently atomise the fuel so the droplets are small enough to reach ignition temperature without much delay. This typically achieved using a very small anulus between the nozzle and the needle, either external with a poppet design such as Find Hansen’s or internal with a conventional needle design. Assuming a compression pressure of around 35 bar to reach ignition temperature the injection pressure will need to be 100 bar plus. With a 1.5mm diameter needle this would require a spring load of around 18N. For a poppet design the compression pressure will tend to keep the injector closed so the spring force can be less, for a needle design the compression pressure will tend to force the injector open so more force will be required.
The combustion chamber needs to be compact to minimize heat loss and needs to ensure the air is moving rapidly to allow good mixing with the fuel droplets.
As practical example this is my 20cc four stroke diesel with two different injectors. It has a helix controlled injection pump. The first is just a 0.2 mm diameter nozzle backed with a non-return valve. The engine won’t fire and unburnt fuel flows out of the exhaust.
The second is with a needle type injector, again with a 0.2mm nozzle. The engine starts as soon as I move the fuel rack to start the injection (this was a cold start).
There are various construction pictures of this and other engines here:
https://www.flickr.com/photos/153503473@N05/with/50539849686
Here are the two injector designs:
Roger B
Well-Known Member
To give an idea of the size of my injectors: ( I didn't have any matches )
)
You have done some excellent work. It is impressive because its difficult to go from a design to a working model. What is impressive you have the numbers to go with it although I tend to use imperial because I grew up with it. One question I have and its purely theoretical has to do with the timing of the fuel injector. It would seem that given a pressure and a nozzle area and the required mass of fuel there would be a time difference based on the nozzle velocity. This would mean that the fuel mass could vary based on the delivery pressure as well as nozzle diameter. I am going to assume that the injector opening time is based on the decay of the injector pressure. So just out of curiosity do you consider that in the build of the injector. I also understand that sometimes the best way to get to a answer is trial and error. Just curious.
Curious too, how do you idealize timing on a model engine, EGT or just by looking and listening?
A dynamometer would be the best way.
Roger B
Well-Known Member
Some interesting questions.
Firstly, I am an engineer on the Autistic spectrum so I need to understand how things work. I have some experience with small full-size engines via motorsport and preserved narrow gauge locomotives.
Before I start an engine design I will see what I can research and calculate. This the comes against the limitations of what I can actually make based on my limited skills and limited machine tools. Finally I have to see what I can test, again within my limitations:
Compression pressure; make a one way valve and attach a pressure gauge.
Injection pressure; make a test pump and use a spring balance to measure the force required to open the injector. Calculate the pressure.
Fuel flow; Point the injector into a measuring cylinder and record the flow for a given number of revolutions at different speed and rack settings.
If we assume that the injection is solid, no trapped gas, the injection will start as soon as the inlet port on the pump is closed and will stop when the inlet port is opened again by the helix. The spring in the injector will control the pressure in the system and hence the penetration and spread of the spray. The mechanics of the injector will also affect the spray pattern.
If you just want to optimise the engine a DC motor attached to the engine with a resistor load bank will give you a guide. If you want actual numbers you will need to build a proper dynamometer to compensate for friction and electrical losses.
This is my 12 cc four stroke twin petrol engine undergoing some load trials:
Firstly, I am an engineer on the Autistic spectrum so I need to understand how things work. I have some experience with small full-size engines via motorsport and preserved narrow gauge locomotives.
Before I start an engine design I will see what I can research and calculate. This the comes against the limitations of what I can actually make based on my limited skills and limited machine tools. Finally I have to see what I can test, again within my limitations:
Compression pressure; make a one way valve and attach a pressure gauge.
Injection pressure; make a test pump and use a spring balance to measure the force required to open the injector. Calculate the pressure.
Fuel flow; Point the injector into a measuring cylinder and record the flow for a given number of revolutions at different speed and rack settings.
If we assume that the injection is solid, no trapped gas, the injection will start as soon as the inlet port on the pump is closed and will stop when the inlet port is opened again by the helix. The spring in the injector will control the pressure in the system and hence the penetration and spread of the spray. The mechanics of the injector will also affect the spray pattern.
If you just want to optimise the engine a DC motor attached to the engine with a resistor load bank will give you a guide. If you want actual numbers you will need to build a proper dynamometer to compensate for friction and electrical losses.
This is my 12 cc four stroke twin petrol engine undergoing some load trials:
ninefinger
Well-Known Member
What is ideal? Low emissions (NOx), low particulates, max power, max efficiency, or an engine that starts and runs easily (i.e. fully controllable)?
As a model engine maker, I'm going to aim for the latter, and maybe play in the other sandboxes later...not sure how many of them can be met at the same time - look at full size practice - they are still chasing "ideal".
That's an excellent question, I would say runs smoothly, starts easily in this context.
Then it can power a scrubber to remove NOx and particulate
xander janssen
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Thank you for sharing your designs.
For my next engine I want to build a Diesel engine like those of Mr Find Hansen. I'm currently watching his YouTube movies to get some critical numbers e.g. bore, stroke, flywheel diameter and weight. Once that is all known, I will make a preliminary design based on these numbers. Then I want to build a first injector and pump just to make sure that I can build the critical components before building the entire engine.
I was thinking of using some ISO 6752 / DIN 9861 Head shape D punches as a poppet valve. These are hardened and already have a nice concentric head machined to them. On can get those with a stem diameter of 0.5 mm (~0.02 inch) and increasing in steps of 0.05 mm (~0.002 inch). Grinding of the head allows to change the angle of the poppet valve relatively easily
For my next engine I want to build a Diesel engine like those of Mr Find Hansen. I'm currently watching his YouTube movies to get some critical numbers e.g. bore, stroke, flywheel diameter and weight. Once that is all known, I will make a preliminary design based on these numbers. Then I want to build a first injector and pump just to make sure that I can build the critical components before building the entire engine.
I was thinking of using some ISO 6752 / DIN 9861 Head shape D punches as a poppet valve. These are hardened and already have a nice concentric head machined to them. On can get those with a stem diameter of 0.5 mm (~0.02 inch) and increasing in steps of 0.05 mm (~0.002 inch). Grinding of the head allows to change the angle of the poppet valve relatively easily
xander janssen
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@Roger B
Thank you for all your information. This will help a lot in building my own injector and pump. I made a metric calculation sheet for calculating several important parameters.
When I put in your numbers, all seems to match except for the pump stroke. You say "say a 2mm plunger and ~1.5mm stroke." but I get a stroke of only 0.63 mm.
Can you explain where my calculation is wrong? It seems that I'm overlooking something important causing me to be off by a factor of ~2.5 which to me seems a lot.
Regards,
Xander
Thank you for all your information. This will help a lot in building my own injector and pump. I made a metric calculation sheet for calculating several important parameters.
When I put in your numbers, all seems to match except for the pump stroke. You say "say a 2mm plunger and ~1.5mm stroke." but I get a stroke of only 0.63 mm.
Can you explain where my calculation is wrong? It seems that I'm overlooking something important causing me to be off by a factor of ~2.5 which to me seems a lot.
Regards,
Xander
Xander !
..
..
Roger B
Well-Known Member
The ~1.5mm is the maximum mechanical stroke. The cam lift is 3mm. The inlet port is 1mm diameter and I set the pump up so that the inlet port is completely open. This means that the start of injection is after at least 1mm of stroke, depending on how sharp the inlet bore and plunger are, leaving less than 2mm for the maximum injection stroke. I agree that the actual working stroke is around 0.5mm, depending somewhat on how much fuel leaks past the plunger. It is very difficult to measure the actual working stroke with a helix plunger.
I usually make trials with the injector feeding into a measuring cylinder and recording the fuel volume after a set number of strokes at different speeds and rack positions. I have attached the Excel table from one set of trials. The effect of leakage at lower speeds can be clearly seen. The speeds are the maximum speeds of a selection of electric drills.
I have been casually following along with this injector thread, and have read it all once, but still not sure I understand how a typical injector works as far as varying flow and /or timing.
If varying the flow affects timing, then how does an injector like a Deroit Diesel injector vary flow/timing ?
Similarly, how does a Lister diesel injector do the same ?
.
If varying the flow affects timing, then how does an injector like a Deroit Diesel injector vary flow/timing ?
Similarly, how does a Lister diesel injector do the same ?
.
Changing the flow affects the timing : Yes - But how much !!?
5 degrees of camshaft will be 10 degrees of crankshaft
10 degrees of camshaft will be 20 degrees of crankshaft
Like IC ignition engine, ignition timing and injection timing have a period of time for the engine to run well enough, like 10 --> 20 degrees BTDC / crankshaft ---> 5 to 10 degrees / camshaft
Homemade engine, With a spark ignition engine, determining the ignition timing is quite simple but with a diesel engine it is completely different - It depends a lot on the efficiency of the pump system when it injects into the high pressure place
And with a small plunger stroke of 0.3 - 0.8 mm, with 5 or 10 degrees of lobe is very small
Conclusion: There is a change but how much I don't know
5 degrees of camshaft will be 10 degrees of crankshaft
10 degrees of camshaft will be 20 degrees of crankshaft
Like IC ignition engine, ignition timing and injection timing have a period of time for the engine to run well enough, like 10 --> 20 degrees BTDC / crankshaft ---> 5 to 10 degrees / camshaft
Homemade engine, With a spark ignition engine, determining the ignition timing is quite simple but with a diesel engine it is completely different - It depends a lot on the efficiency of the pump system when it injects into the high pressure place
And with a small plunger stroke of 0.3 - 0.8 mm, with 5 or 10 degrees of lobe is very small
Conclusion: There is a change but how much I don't know
peterl95124
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there are a LOT of details in a Hansen A-frame engine to work out, I suggest starting with a spark ignition gasoline engine, then proceed to diesel, IMHO, YMMV, yada, yada, yada
!!!(for example, you are unlikely to want to cut your own helical gears, so what's available in the small size required here?, I acquired mine from HPCGears in UK, 45-deg RH helical, 48-DP (34-"effective DP"), 15-20 at crankshaft, 20-30 at camshaft)
Last edited:
For a Detroit: Flow is controlled by a rack and pinion. The injector has a helix cut into it and as the body rotates, that helix dumps excess fuel sooner or later - which changes the injected volume. See the rod that is transverse to the body? That's the helix revolving mechanism.
If you google a cutaway Detroit injector, you will see the helix.
ruben
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there are a LOT of details in a Hansen A-frame engine to work out, I suggest starting with a spark ignition gasoline engine, then proceed to diesel, IMHO, YMMV, yada, yada, yada
(for example, you are unlikely to want to cut your own helical gears, so what's available in the small size required here?, I acquired mine from HPCGears in UK, 45-deg RH helical, 48-DP (34-"effective DP"), 15-20 at crankshaft, 20-30 at camshaft)
Where could I get plans for that engine I love it
And some diesels have a fuel return line, going to the fuel tank.
If you have enough large diesels connected to a fuel tank, you can actually overheat the tank.
We had to install a chiller on the return fuel line, for the local 20MW power plant.
I assume a Detroit Diesel injector does not have a return fuel line.
.
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