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Fuel injection systems

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Roger B:
Most of my future projects require fuel injection systems, both paraffin (kerosene) and diesel. These will be around 20 - 25 cc per cylinder and as the smallest commercial engines with mechanical injection are around 200 cc per cylinder I decided that I need to start experimenting and developing some manufacturing techniques.
The first step will be a fuel pump and injector hopefully suitable for a 25 cc engine. The fuel pump will have a plunger diameter of 2mm and a target stroke of 2mm. The injector will have 1.5mm diameter needle with a 0.35mm diameter nozzle.
I hope to trial this on my current horizontal engine injecting petrol into the inlet with a later step of injecting paraffin into a vaporiser.
This will require lower pressures than for direct injection in a diesel engine, but petrol is of low viscosity and has no lubricating properties.
I have made a start on the injector with a body from 10mm square black bar. To make the nozzle concentric I need to drill the hole from the needle side so I had a go at drilling a deep (12mm) hole in the end of some brass bar turned down to 1.5 mm diameter. This seemed to work (the centre drill has a 0.5mm diameter tip.
Next I need to look at making a needle and seat cutting tool from hardened silver steel (drill rod).

Roger B:
I had decided to try and grind the D bit and the needle as with these small sizes even at full speed (2000rpm) the cutting speed is minimal. For a previous project I had made a tool post support for my Proxxon hand tool, this is not so rigid but was sufficient for some trials.

The lathe bed was protected with a sheet of plastic covered with a damp paper towel. I arranged the setup so that the sparks would be downwards. This required running the lathe in reverse.

I was able to grind 30 and 45 points as well as parallel without the Proxxon colliding with anything.

The D bit was hand filled to half thickness and then hardened by heating to dull orange and quenching in water.

I am happy with the basic techniques but they will need some refinement.

Roger B:
Next I tried some of the steps for making a nozzle to see if I could drill the 0.35mm hole from inside.

Starting with some 6mm round brass I drilled a 1. mm hole 20mm deep and then reamed it out to 1.5mm. Using my D bit I then went down to 24mm which would leave around 1mm for the nozzle hole.

Finally I put my nozzle drill in, marked the expected starting depth with a marker pen, put a drop of oil on the brass shaft, started the lathe at 2000rpm and carefully pecked until the resistance went away.

The drill came out complete and removing the bar showed the hole had gone through  :whoohoo: 

Hi Roger

Very interesting project - many has tried and few has succeeded in making a working injection system in miniature size.

I for one hope that you will be among them   :ThumbsUp:

That way the rest of us can learn  ;)   .....  :popcorn:

Roger B:
Thank you  :)

I am well aware of the level of the challenge I have set myself  ::) From these first trials I need to improve the finish and fit by an order of magnitude to have any chance of success  :headscratch:

Maybe my machines and I can reach this, but anyway it will be good experience. The 1.5mm silver steel I am using is actually 1.49 mm. This is within specification,  but if my 1.5mm reamer cuts on size the clearance is already too much before I start lapping/polishing.

I have source of reamers in 0.01mm steps, but they are not cheap  :( Acrolaps also supply needle laps in this size range so I have some ideas for the next steps.

Valves for the injection pump are another challenge. The current Bosch type jerk pump with the spiral spill edge is out of the question in this size. I need to look to earlier designs with inlet and exhaust valves with variable stroke or fixed stroke with a separate spill valve. The earliest Benz design I have a drawing of used cone valves and the Lanz Bulldog used cone valves backed up with ball valves. Good cone valves 3 or 4 mm in diameter will be interesting to make :thinking: .

I am always happy for people to learn from me, even if it is how not to do things  ;)

Suggestions of how to do things better are also welcome  :)


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