Solenoid diesel injection pumps

[The Ignoble Troll]

I'm all on board with polymer use / o-rings. I agree that they could be used more often instead of relying on metal to metal fits. Its one of the ideas I've been kicking around.

My designs (all virtual so far) are for a unit injector with cam actuation and wedge control in place of a helix and rack. The injector and pump are actually 2 separate pieces, sealed with an o-ring between them and an o-ring where it seals to the cylinder chamber (that last one maybe optimistic..).
I plan to use a plunger type pump and use a gland o-ring seal at the top end, but I want most of the sealing for the high pressure to be done by the plunger fit.
Another item I'm struggling with is the size of the check valve, even at 1/8 scale that I'm trying to design to with a piston of ~30mm diameter (1-3/16"), the check valve ball and components end up being tiny - 1mm ball and itsy bitsy springs. I need to work on watchmaking to works with parts that small.

I probably need to concede to reality and use an external check valve to the pump, and use slightly large parts to simplify things to start.

The plan is to use a mushroom type injector, seems the easiest to make.

Here is a section view of what I've drawn up so far, with the injector and pump in a cylinder head beside a valve. For scale, the OD of the pump and injector are 3/8" (9.5mm)
View attachment 164002

I like it! Do you have a cad of the plunger driver? Curious how you plan on transmitting the force to the plunger.

 

[The Ignoble Troll]

Excellent discussion, everyone!
I am still in the unit pump/injector wannabe group and have at least a half dozen versions of the "so close you can taste it," category in the boxes of failures. Designing to a minimal number of critical sealing surfaces seems to be the most workable goal for the home shop, but I have take apart a yanmar pump-line-injector setup, and a detroit diesel unit injector, both of which use the lapped plunger with rotating helix spill design for volume control. Both of these in stock form produce a perfect cloud of vapor-fine mist when operated. The fit of the plunger into the pump body is an order of magnitude better than anything that I can produce. The fit is almost magically good, LOL. But they are made to work for 1,000's of hours. I would be happy with minutes.
Our nemesis of scaling factor is one problem. If you try to make a pump of 1/3 scale, the pump volume will be 1/27 BUT, the area of the sealing surfaces will be 1/9. So you essentially have 3 times as much leak potential as a full sized unit. I was told by a retired diesel expert, that any type of elastomeric material in the high pressure side will lead to inaccuracies in fuel delivery. I am still holding hope that a good governor will be able to overcome some of that issue. But the real injectors all seem to have an abundance of both flat and cylindrical lapped surface, all of which are critical.
Seal material choice is critical. Look up compatibility info with diesel fuel and as already noted, Viton and teflon might be the best possibilities, except for a few very expensive specialty compounds. Some material might work perfectly today, but after sitting for 2 days have swelled and locked up.
I am at the point that most of my failures have actually somewhat worked, but none have worked "well", at least for any length of time. Attention to detail, tenacity, project burn-out, are all critical factors. I am again, at the almost-there on another variation and I remain optimistic. Others have succeeded, so there is probably no reason that any of us should not be successful, too.
Lloyd

I follow your build with great interest, especially the blower build. That was some prime trouble shooting and adapting.


"If you try to make a pump of 1/3 scale, the pump volume will be 1/27 BUT, the area of the sealing surfaces will be 1/9. So you essentially have 3 times as much leak potential as a full sized unit."

Thank you for that tidbit. I hadn't thought about it that way...

 

[HMEL]

I just finished moving and am months away from getting my shop to be functional. That being said I am picking away at the issues I will need to solve to build a small two stroke diesel.

Currently my thoughts are a 1" bore with a 4" stroke.

My initial plan is to build a single cylinder proof of concept unit and supply it with shop air at 15psi.

From there I want to build a 5 cylinder version that will use a bought roots blower as the air supply.


I want to start with the fuel system and perfect that before building the engine.

To that end, I plan on designing a small injector that will use minimal lapped parts and a separate fuel pump to generate the flow to run the injector. They will be connected with some sort of schedual 80 small bore stainless pipe.

For the fuel pump I am thinking that the easiest solution is to do away with mechanically driven pumps and to use a series of solenoids with a common plunger. One coil would extend a 1mm diameter plunger into a true plunger pump, the other coil would take the place of a return spring and withdraw the plunger.

With off the shelf solenoids, a 1mm plunger pump should easily generate 10,000psi.

The stroke length and timing would be arduino controlled. The dual coil solenoid pump would only need a single double pole relay to control it.

I have a few quadrat sensors somewhere that could be used to adjust timing, injection duration and injection volume.


The plunger pump will have a positive pressure feed pump, which will also allow for a fuel filter to be used.

Can anyone see any foreseeable problems with this set up?

Very interesting post. I have been researching this subject myself. I believe its possible to mechanically drive the pump but have had a very difficult time with the issue of fuel metering. The smaller the air flow the more difficult it seems to be. I like the idea of solenoid but have never given the thought you could control the pump stroke that way. I have also considered the injector design and I am of the opinion you just need to get the right amount of fuel into the cylinder at the right time. At a models size I have doubts that the spray pattern is going to be all that important. That is just an opinion and does not mean its true. The injector will have to be balanced so the pressure area works against the closing spring. I would not be surprised if you put a solenoid on that. I could not set up a electrical system like you propose as to me it would be an additional learning curve. I am also thinking that you might be able to use a spring to return the plunger but removing it does lower the force the solenoid needs to pressurize the pump. I cant see any reason why your design concept would not work given the electronics are fast enough for all the variables. I hope you are able to continue your work on this as many are very interested in this subject. Good Luck

 

[ninefinger]

I don't have a good drawing or model at the moment as its been a few months since I was looking at this with some dedication, and Fusion and / or I have managed to break my assemblies with too many redesigns and edits.
In words, as its a 2 stroke, the cam actuates rockers for the exhaust valves and the plunger with different lobes for each, exactly as in full size practice for the engine type I wish to model. The difference is that between the lobe for the rocker(*) there will be a moveable wedge. I think Roger B has shown this style quite well, just having a hard time finding the post / forum he posted it in, but in his case the cam lobe acted directly on the wedge and plunger. I'll be drawing it again sometime, as I plan to build a test bed for just the fuel system before I go and try and build an engine. Again, following how Lloyd-ss made a test bed for his injector (if I'm remembering correctly).

(*) - I may try designing the moveable wedge to be at the other end of the rocker - between it and the plunger pump. Depends what will work (1st priority) and what may look more scale (secondary priority for now).

 

[Lloyd-ss]

................................. edit

(*) - I may try designing the moveable wedge to be at the other end of the rocker - between it and the plunger pump. Depends what will work (1st priority) and what may look more scale (secondary priority for now).

FindHansen has at least one good YouTube video (among his many excellent videos) demonstrating the wedge adjustment for fuel volume and for timing adjustment. I think it was in a video of his dark green vertical diesel where he has all of the parts laid out and explains them. I am not sure, but I think the only time in the operating cycle that the governor will be strong enough to move the wedge is when there is the momentary lash clearance in the valve train during each revolution.
Lloyd

I follow your build with great interest, especially the blower build. That was some prime trouble shooting and adapting.


"If you try to make a pump of 1/3 scale, the pump volume will be 1/27 BUT, the area of the sealing surfaces will be 1/9. So you essentially have 3 times as much leak potential as a full sized unit."

Thank you for that tidbit. I hadn't thought about it that way...

Thank you for the kind words, but more like half trouble shooting, and half fumbling. The build of the roots blower was where the scaling factor became painfully obvious to me, and I had to make the blower bigger than expected to compensate for leakage.

(Edit- I had to rethink the leakage factors. Still not sure how correct it is..)
For example, with a 1/4 scale blower, the blower volume is 1/64, but the lineal length of all the friction seals along the lobes of the blower is still 1/4 of the original. So even if you can cut the seal clearances in half in the model, the losses along the seals might be 1/8, but the volume pumped is still only 1/64. So the actual leakage potential of the model might be 8 times as great as the full sized unit. Luckily, you can overdrive the blower quite a bit to make up for some of the losses and won't be an obvious uh-oh in the model. The blower will just make more noise and the flow increase is (I think) the square of the blower speed increase. Having a belt drive to the blower makes changing the overdrive ratio a bit simpler.
Lloyd