Fuel injection for model diesels, how can it be done?

[Kaleb]

I've been thinking of trying to build a small "true" diesel engine down the track, but the injector is something that leaves me scratching my head on how to make it work properly in these small scales. I have seen others do it successfully, and a couple of very promising builds on this forum, but I'm still not sure how I could pull it off.

 

[MuellerNick]

Reverse engineering Diesel engines, I found that you need 1mm^3 per 100 cm^3 of engine displacement for a good idle.
A drop of water*) has 0.25 ml. That are 250 mm^3. So you take a drop, divide it into 250 parts and inject that amount. That's all.
The smallest commercial Diesel I know of, has 350 cm^3. So that is a hint what is doable.

Why do real model Diesel engines exist, despite of what I said above? To my knowledge, the can't regulate the amount injected.
This is only a wild claim by me, and I stand corrected if I someone shows me an injection that actually does vary the injected volume.

I was talking about pure mechanical injection. Controlling it with electronics is a completely different story.


*)
Not a raindrop, they vary a lot. But a drop from the tap.


Nick

 

[old-and-broken]

http://mbi-online.org/mems-fuel-injector

 

[MuellerNick]

Kaleb asked for injectors, not the jets.
For small jets, "open jets" are the way to go. They have no needles or other mechanical elements. Just a tube, simply cut off.
There is a bit of physics behind them, so they are not as simple as they look.


Nick

 

[cfellows]

Are you asking about the fuel injector or the injector pump. Model diesel engines I've seen have the injector (with nozzle) in the head, with access to the cylinder. The injector pump is a separate device driven by a cam. They have a governor which controls the injector pump by way of tapered pins between the cam and the plunger on the injector pump.

Here is one such engine.

[ame="http://www.youtube.com/watch?v=-PvZ6S8cadk"]http://www.youtube.com/watch?v=-PvZ6S8cadk[/ame]

This fellow, Find Hansen, also builds hot bulb engines which use a similar injector and injector pump setup. If you look at all of his videos you can kind of see how they work.

His web site also has some useful information and pictures:

http://www.findsminimodelhotbulbengines.dk/

Chuck

 

[mu38&Bg#]

Mechanical fuel injection varies the pump displacement to obtain variable injection volume. It's been this way since Rudolph diesel invented it, until the electronic injector.

In small scale the challenge is a pop off type injector with a suitable nozzle if it's to be efficient. The pump is also tricky to avoid leaks. It's unfortunate that Mr. Hansen does offer more detail of the pump.

Greg

 

[gbritnell]

I contacted Mr. Hansen through Youtube and he said he didn't want to share his development of his model diesel engine.

 

[Kaleb]

As far as the pump goes, that seems straightforward in principle, but my problem is how do you make something that will atomise the fuel sufficiently for it to ignite? Especially under the very high compression ratios required?

 

[mu38&Bg#]

The atomization is a problem in a model scale injected diesel for two reasons. First, the distance in which fuel must be atomized is much shorter than what is available in full size cylinder. We can consider injecting from the side of the combustion chamber to have the entire length for the fuel spray to break up. Second, the volume injected compared to the orifice area results in low velocity. At the home shop level miniscule holes are not easy to make, an even reduced to a single orifice, the size can be quite large in comparison to the injection volume. The typical injector nozzle has several Poor atomization means poor fuel consumption, but may result in a running engine. The Dux design by Martin Alewijn was published in Stricly IC and featured a simple injection pump and an injector with a scratch for an orifice.

Pump issues in a model engine also include wear and tolerances. Diesel injector pumps are usually made of high hardness steels finished to extremely tight tolerances.

The smallest injection system I know of is about 3HP. These are seen in the smallest Chinese diesel utility engines and there is also the Cosworth UAV two stroke 80cc diesel engine.

I have a diesel injection project on my list, but it's in the design stages and not a priority.

 

[Jasonb]

I see Johan Van Zanten recently joined this forum hopefully he will reply but he has made a diesel model with proper working injection, the pump runs at 150bar. (2175 psi)

There are some photos of his model here

http://www.model-engineer.co.uk/albums/member_photo.asp?a=6778&p=123122

J

 

[MuellerNick]

I have refreshed my memories about open jets (what a exciting bed literature).


Well, it is not just plain open (but it can be), but has a smaller diameter at its end.

The problem is getting both cross sections "q" and "µf" right.
With both the same, more fuel than was pumped in will leave the jet. There is a shockwave running towards the jet that is reflected. If it lacks the smaller diameter, there is no reflection.

If the small diameter is too small, more than that what leaves the jet is reflected and the jet drips for a short time after the pump's stroke.



From: "Gemischbildung und Verbrennung im Dieselmotor"; Dr.-Ing. A. Pischinger; Springer 1939


Nick

 

[cfellows]

I did have a few correspondences with Find. Here is what he told me about making an injector pump (Quoted):

The brass pump cylinder is heated to the melting temperature of solder, use some flush (Flux?), put some solder inside the cylinder, dip the plunger (could be a 2 mm needle from a needle bearing) in engine oil and, put it in to the hot cylinder and let it cool down. The engine oil is to make a "little space" between the cylinder and piston, and to prevent the solder to stick to the piston. Most of the time this works.

I assume you saw the exploded view of one of his injectors on his web site? He uses spring loaded, closed injectors. The tip of the injector is shaped like a very narrow poppet valve and and I believe it is tapered 7 degrees.

Chuck

 

[Till]

The smallest commercial Diesel I know of, has 350 cm^3. So that is a hint what is doable.

Why do real model Diesel engines exist, despite of what I said above? To my knowledge, the can't regulate the amount injected.
This is only a wild claim by me, and I stand corrected if I someone shows me an injection that actually does vary the injected volume.

In 1942 there were direct ignition diesel engines with 3,2ccm and 7,3ccm displacement, fully adjustable injection pumps operated by a camlobe on the crankshaft (2-stroke). The pump was adjusted prior to the tethered flight, Injection pressure was 300atm.
Grüße aus Hamburg,
Till

 

[ChooChooMike]

I contacted Mr. Hansen through Youtube and he said he didn't want to share his development of his model diesel engine.

That's too bad , cause those are wonderful models he's made

On Sherline's Craftsmanship Museum:

http://www.craftsmanshipmuseum.com/Hansen.htm

 

[Chicken]

Hello all!

My name is Alex and I'm from Germany.
I have just registered to this great forum. Came here by google as I'm looking for some hits how to build an engine with solid fuel injection.

I have already successfully built several IC engines with electronic ignition running with regular gasoline, but now I want to have a running engine with Diesel injection.

Basically this project is already close to be finished, but the fuel injection is a rather big issue.
Of course, I know the site of Find Hansen and his very nice hot bulb engines.
Unfortunately it is very difficult to get some responst from Find, although he sometimes answered some of my questions.

My engine is a simple 2-stroke design and the injection pump is directly driven by a excentric on the crank shaft. The same excentric also operates the cooling water pump.
It took me quite some time to build a suitable injection pump as my piston always tended to leak and not build up much pressure. Anyway I think my current pump is good, is has a 3 mm piston, 1.5 mm ball valves and a piston stroke ranging from 0.00 to 1.00 mm regulated by a fly ball governor. It easily builds up to 200 bars of pressure.

For a wonder, the injector seems to be the most difficult part of this whole engine!
I have tried many different things with open and closed injectors but nothing is working in a satisfactory manner.
I've tried to build a injector similar to Find Hansens pictures, but this is also not working very well. Most of the time only a drop of fuel is forming at the injector outlet no matter what kind of spring load I adjust.
Sometimes I get excellent spray, but this only occurs when the injector valve is slightly sticking to the valve seat. But then it depends how the valve is beeing pulled back by the spring. Sometimes I get a good spray for maybe 20 or 30 engine revolutions, then the valve fails to fall back into the seat properly and I only get a drop again. This seems to be totally independend of the spring load?
My valve stem currently has 0.8 mm and the valve head has 1.5 mm tapered with 5° angle.

Open injectors never seem to work at all at these sizes.

Can anyone of you experts give me some additional hints on how to build a suitable injector for my engine?
This is quite some frustrating work and I sometimes was close to convert the engine to electric ignition with gasoline fuel

Thank you and best regards!
Alex

 

[BMyers]

Hello Alex,
Care to share you electronic fuel injection? Closed loop ? Open loop ? Microprocessor controlled ?

 

[petertha]

I'm not sure if this is helpful or applicable, but 'injection' seems to be gaining popularity in competition RC engines. All this came out after my era flying pattern, so I can't offer much more information than web links.

http://www.keimod.com/shop/product_info.php?manufacturers_id=13&products_id=740
http://www.centralhobbies.com/Engines/parts/yse%20DZ170cdiexp.html
http://www.yspower.co.jp/en/data/pdf/dz175.pdf
http://www.os-engines.co.jp/english/2007spring-summer/fs200sfi/index.htm
http://shop.singahobby.com/?q=node/14164

 

[mu38&Bg#]

I didn't see a mention of electronic fuel injection by Alex? All of the OS fuel injected engines are discontinued. YS mechanical injection is simple and functional. While both are interesting this thread is about diesel injection at many times compression pressure.

 

[Chicken]

That's right, I'm not using any electrically controlled stuff.
Just a very simple plunger pump with 3 mm plunger and 1.5 mm ball valves.
I attach an image of my pump setup. The left one is the injection pump, the right one the water pump.
I had to solder everything on the injection pump.
When first experimenting with injection pumps I had most of it screwed together, but I found that tiny bits of air get cought in the threads and the pump won't build up any pressure. After soldering everything together and making all passage holes as small as possible everything seems to be working now.

I'm just totally stuck with the injector. I somehow can't get it working

 

[BMyers]

My bad... Reading comprehension was lacking last night:wall: