Supporting > Engine Ancillaries
Porsche 12 Cylinder Fuel Injection
Roger B:
The possibilities for fuel injecting Dave’s Porche 917 engine have been discussed in various places at various times so I thought I would try to bring my thoughts on the various concepts together in a new discussion thread.
The full size engine had a diesel derived Bosch mechanical injection system. There were two rows of 6 helix controlled pump elements with a central control rack. This required left and right hand helixes. (picture 01) I have seen the injection pressure quoted as 17 bar.
This may be technically feasible to model but approaching a scale size would be difficult. A means of adjusting the output (helix angle) of the individual elements to ensure that the same amount of fuel was delivered to each cylinder would have to be devised.
As the injection timing is not critical as in a diesel a simpler wedge controlled pump may be a better option. This was used on an existing model Porsche 917 engine (pictures 02, 03). I have some designs for wedge controlled pumps but have not bult and tried them (picture 04). One open question is the fuel inlet system. Using a port in the side of the pump cylinder gives clean closing but requires a pressurised fuel feed to avoid vaporisation. If a ball valve is used the ball needs to be closely constrained or spring loaded to give quick and consistent closing. It may still be necessary to pressurise the fuel feed (~0.8-1.0 bar).
Another option is a separate metering pump and distributor. This in theory reduces the need for balancing the quantities to the individual injectors. A number of diesel systems have used this concept. A 6 cylinder unit for a 4 stroke engine requires 3 injections for one revolution of the crankshaft. This either requires a multilobe cam which must be very precisely made or a 1-3 step up gear. The full size units, as far as I am aware, all used a multi lobe cam system. The Lucas mechanical petrol injection system avoids this problem by generating the injection pressure (7 – 7.5 bar) with a separate electric pump and setting the fuel quantity with a shuttle valve in the middle of the distributor sleeve. (pictures 05, 06) I think that it would be possible to make a model version of this for up to 6 cylinders.
Electronically controlled injection is also possible. Daniel Hunziger has produced a running example, however finding small enough injectors is difficult.
http://www.youtube.com/watch?v=xqMk87vOMsw
Roger Moore made his own injectors for a ¼ scale Ferrari Type 049 Formula 1 engine. Quite a challenge and you also have to produce some form of ECU.
http://www.youtube.com/watch?v=7JXXfkMclB4
Another option is a continuous flow system like the Bosch K – Jetronic. The problem here is maintaining equal fuel distribution across all the injectors when they are all connected to one metering valve/system. Bosch appear to achieve this by very precise machining of the outlet slits in the metering valve and very precise machining and setting of the injectors.
Schillings produced a simplified version with the injectors fed from a relatively large diameter manifold and individually adjustable injectors which were set to give approximately equal exhaust temperatures. He appeared only to use this system with methanol fuel and glowplug ignition which will run with a wide range of fuel air ratios. (pictures 07, 08)
Roger B:
The injectors for manifold injection can be quite simple. They need to incorporate a non-return valve so that the pipes remain full of fuel. A small orifice may be sufficient, (picture 09) although the 0.2mm version I have made may still be too large, otherwise a poppet style injector as used with the Lucas PI system should work (pictures 10, 11, 12).
I don’t see a need for the diesel style needle injector, which would also require a return piping system for the fuel that leaks back past the needle.
Vixen:
Roger,
Mist spray nozzles, similar to your (picture 09) in the above post, are readily available on e-bay with nozzle diameters down to 0.1mm.
Search for '0.1mm mist spray nozzle', there are loads to chose from, including bare nozzles and others which incorporate a non-return valve. The nozzle inserts appear to be made of a ceramic. They are intended to operate from around 20Bar with flow rates from 10ml/min. No indication of how they would perform at lower pressures, hence flow rates.
It looks like you could easily modify the outside diameter to make them smaller.
Could they form the basis of a constant flow, manifold/ port injection system, similar to the Schillings design?
Cheers
Mike
steamer:
Wow!
I would like to thank you both for your thoughtful effort! I have been wondering about the electronic as well as the manifold/port injection scheme. Thus far the electronic version was one I was rather keen on as it could be controlled with an Arduino which I believe is how Hunzinger is operating his. The challenge has been size. The fuel injection system on the 917 is rather iconic! A snakes den of plastic lines leading to intake trumpet mounted injectors with a flat slide throttle.....It has a "Look" all it's own! and is something that kinda needs to be right to make it work visually, to say nothing of making it run!
I wish I had the skill and talent of Mr Moore, His workshop equipment would rival any Aerospace company! Winding 0.0007" diameter wire injection coils with his CNC Hardinge for instance is amazing! Alas, I seem to be missing that particular piece of equipment....
I have seen that model of the 917 before, and I've mentioned that it lacks this visual, and is also different in many fundamental points, like not having the gear driven output shaft like the real one, no cooling fan, ect. That is one of the reasons I've not given that model my attention, though just getting a v12 to run is an amazing feat in and off itself and I commend the builder of that model for his/her fine work.
The Hunzinger and the "Schilling" solutions seem to be the path that seems plausible for my shop, and the "Look".
Or I could just go 2 carbs .....lol well if this model comes down to which carburation I am going to use, I think I'll be doing just fine.
Thanks again! And I'll check out those sprayer nozzles Mike!
https://www.hagerty.com/media/news/road-legal-porsche-917-in-monaco/
steamer:
^^^^^^^
By the way , that beast of an engine was held in the car with 3 THREE 8mm bolts......
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