Porsche 12 Cylinder Fuel Injection

[Roger B]

A Compression Ignition (CI) engine uses the Diesel Cycle, its speed and torque are controlled only by varying the amount of fuel delivered. A Petrol (methanol) Injection (PI) uses the Otto Cycle where the its speed and torque are controlled by the amount of AIR and FUEL mixture delivered through a carburettor or by injection. The air throttle and the fuel injection halves need to work in perfect harmony to deliver the correct amount of fuel in all throttle/ speed/ load conditions. A carburettor does that automatically by air flow though the venturii past the fuel jet.

How are you proposing to get the air side to control the fuel side?
Mike

My experience suggests that an open loop system will work, although it is obviously not optimal. My 25cc horizontal engine has a wedge adjustable injection pump plunger stroke which is not coupled to the throttle, it was set in the first trial and remains in that position. The engine responds well to the throttle, although it will stop if the throttle is opened rapidly, and will pick up a load without stalling. The speed dependent leakage past the pump plunger seems to match the engine characteristics quite well.

Here I am starting and running the engine at various speeds and then adding load via a switchable resistor bank. Max load is around 120W electrical. I don’t know the efficiency of the small DC motor I am using as the starter/generator, but I expect it is less than 50%.

<a href="https://www.youtube.com/watch?v=R7eOP3MIaMw" target="_blank">http://www.youtube.com/watch?v=R7eOP3MIaMw</a>


The vertical twin uses a cam to adjust the fuel rack in accordance with the throttle position. The profile was developed by measuring the rack position for optimum running at various throttle positions and then filing the cam to suit. This works well, there is a video clip of it running in reply 57 of this thread.

I would plan to use a similar system to control my version of the Lucas PI shuttle pump setting the shuttle stroke according to throttle position. This system was used on the competition versions of the Lucas PI system. With a slide throttle there was never going to be a nice balanced idle so the driver would have to keep blipping the throttle anyway. The road car versions were controlled by manifold depression.

[Vixen]

Hello Roger

Thanks for the detailed reply. I can now see how, on the horizontal engine, you use the wedge adjustment to manually match the fuel to a given throttle setting. Then, when you can gradually open the throttle, the engine speed driven fuel pump output (and air/fuel ratio) stays close enough and the engine gathers speed.

The vertical twin, which uses a cam to adjust the fuel rack in accordance with the throttle position, is clearly a refinement.  I was not aware that was what you had achieved to enable the the air side to control the fuel side.  It clearly works well, so belated praise is due.   

Cheers

Mike

[dieselpilot]

A concern I have with a complicated homebrew methanol injection, is corrosion. The Schillings system has simplicity in it's favor. Material selection for a Lucas type would have to be planned well, unless you can flush it for storage.

[Roger B]

If the metering is good enough the engine could run on petrol, possibly with a little two stroke oil to lubricate the pump.

Making the distributors would be another challenge due to the electrical breakdown clearances which do not scale. I would probably cheat and make two 6 cylinder distributors and add a dummy set of leads and plugs.

I have made a couple of the smaller injectors as a trial. The basic bore is 1mm and the needle is not lapped into the nozzle, just given a polish with a fine diamond hone.
The spray pattern is ok for petrol but not well enough atomised for diesel. In the test here the spray of lamp oil (kerosene) would not ignite.

<a href="https://www.youtube.com/watch?v=GHcJAl6gp80" target="_blank">http://www.youtube.com/watch?v=GHcJAl6gp80</a>

I then fitted it to the 12cc twin using a small piece of flexible tube as an adaptor. The engine ran quite well but is very rich due to the lower injection pressure reducing the leakage past the pump plunger. Manually leaning it off improved the running.

<a href="https://www.youtube.com/watch?v=yVcLcY32dkY" target="_blank">http://www.youtube.com/watch?v=yVcLcY32dkY</a>

[steamer]

That's awesome Roger!
I had the same thought about the distributors.   I think (2) 6's is a good idea, considering the size of the distributor caps which is 1.08" ( 27.5mm)

I am forever amazed with your work!  and as always I want to thank you for creating and publishing this information, and all the work to do so!

I am full tilt boogie at work, and taking care of my daughter.  I try to get my weekends back for shop work, but the weather hasn't cooperated with me lately!

I promise I am reading and trying to get some testing on my own regarding this.    Be patient....Im working on it.

Dave

[Roger B]

Thank you Dave     These experiments are my ‘fun’ with this hobby.

This small multi-cylinder low pressure injection system is also needed for one of my future projects. I was thinking of using helix controlled pumps, but the very short leakage paths mean they do not work so well with the lower viscosity of petrol. Maybe the shuttle pump will be better. I am starting to build a two-cylinder version based on the one in picture 06 at the start of this thread.

I have purchased the drawing of Schilling’s fuel injection system and have a few more details:

The fuel pump uses 11 tooth MOD 1 gears 4mm thick. This seems to have a much higher capacity than the one that Mike proposed. There is no mention of the operating pressure and no details of the relief valve spring to allow me to make an estimate.

The injectors are very similar to model engine carburetor needle valves. There is a 0.6mm orifice and a 10° tapered needle with an M3 thread. He does describe the system as a carburetor without a float chamber.

[Vixen]

Thank you Dave     These experiments are my ‘fun’ with this hobby.
                                              ................
I have purchased the drawing of Schilling’s fuel injection system and have a few more details:

The fuel pump uses 11 tooth MOD 1 gears 4mm thick. This seems to have a much higher capacity than the one that Mike proposed. There is no mention of the operating pressure and no details of the relief valve spring to allow me to make an estimate.

The injectors are very similar to model engine carburetor needle valves. There is a 0.6mm orifice and a 10° tapered needle with an M3 thread. He does describe the system as a carburetor without a float chamber.

Hello Roger,

Do the Schillings drawings shed any more light on the size of valve/tap mounted across the gear pump? As I read the drawings, this valve diverts some (most??) of the pumps output back to the inlet side, thereby limiting (controlling?) the fuel flow available to the 12 injectors.

Mike

[dieselpilot]

Extra pump capacity is just bled off. Cost is some efficiency (fuel heating), if that matters.

[Roger B]

Mike, there are no details of the bypass valve or the pressure regulator. I think, however, that it is a low pressure system, 1 or 2 bar.

The Schillings injector has a 0.6mm nozzle and a 10° needle with an M3 thread. In his notes he starts with the needle ½ turn open and then opens it further to obtain good running. An M3 thread has a pitch of 0.5mm so if we open the needle 1 turn it will move 0.5mm giving, with a bit of trigonometry, a needle diameter of 0.513mm. (0.5 x 2 x sin 5)

This gives an anulus for the fuel flow with an area of 0.076mm2 (π x ((0.6 /2)2 – (0.513/2)2)

A this is equivalent to a hole of ~0.3mm diameter (2 x √ (0.076 x π)).

For various reasons the anulus fill have a lower flow rate for a given pressure than a simple hole and will give a better atomisation/distribution of the fuel. This suggests similar nozzle sizes to those in the earlier discussions.

Please check my calculations before using them for anything real