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Background

I am building an engine from scratch for the sole purpose of becoming a better mechanical engineer--I'm currently a second year undergrad. The engine I'm designing is a single piston 4-stroke gasoline engine. I have no intention of using it on anything yet and only want to make it run reliably.

Problem

I'm using fuel injection and I've heard I need to control the pressure on the line fairly accurately. I'd like to avoid using a fuel pump in favor of something I can actuate with a microcontroller. One of the harder things I've found in mechanical engineering is that it is difficult to find what parts exist that can solve your problem. So my question is, what are some ways that I can do this if I don't use a fuel pump? I really want to become a good engineer so any insight is greatly appreciated.

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  • $\begingroup$ how do you have pressure at all with no pump? $\endgroup$ – agentp Mar 24 '17 at 4:02
  • $\begingroup$ I should rephrase the question. I'm trying to build and maintain pressure without a fuel pump. I was considering something like a solenoid or linear actuator that could apply a force. Or a threaded device that is electronically controlled to incrementally increase pressure. I will try to reword my question. $\endgroup$ – Klik Mar 24 '17 at 5:37
  • $\begingroup$ "One of the harder things I've found in mechanical engineering is that it is difficult to find what parts exist that can solve your problem." Yes. $\endgroup$ – mart Mar 24 '17 at 7:24
  • $\begingroup$ Hey, in the comments to my answer you see us guessing about the 'constant pressure' requirement - please take a look and elaborate what exactly you need. Maybe my answer missed the point enitrely :) $\endgroup$ – mart Mar 24 '17 at 11:23
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    $\begingroup$ you seem to be envisioning something novel and basically unrelated to the way "fuel injection" systems work. Anything "you heard" about fuel pressure is out the window. You need to edit your question with far more detail about what you are asking. $\endgroup$ – agentp Mar 24 '17 at 11:48
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Edit to clarify
As of now, The question is a bit unclear. I imagine you want a largely pressureless fuel system, each cylinder has it's own solenoid pump that replaces the fuel valve on this cylinder and is controlled by your microcontroller. So the fuel supply is not done via supplying a constant pressure to a carburetor that does the mixing, but by directly dosing fuel into each cylinder. Since you are building a sort of direct injection engine, you would likely not control the air flow, but just the fuel flow between lean and stochiometric. In common rail you'd have one high pressure fule lin and solenoid valves, instead the OP wants to have a low pressure line and solenoid pumps.

This is the assumption I base my answer on, if this is not the engine the OP wants to build everything below is moot.
End edit

You are looking for (search terms) solenoid driven diaphragm pumps or solenoid driven piston pumps. These exist as metering pumps. The working principle of diaphragm pumps is explained on wikipedia, in metering pumps with solenoids there is only one chamber and the solenoid works in tandem with a spring. AFAICT these are mostly metering pumps for pharmaceutical or chemical industry.

Most of these pumps each stroke will deliver the same volume. Is this acceptable? If not, variable stroke solenoid pumps exist, but I'm not sure you'll find ones where you can adjust the stroke online, on the fly.

I've found one supplier where the volume per stroke is in the µl range. Is this acceptable?

The supplier linked above states a frequency of 0-20Hz for it's pumps, so in a 4 cylinder engine you get max 80 Hz = 4800 rpm a most.

You will likely need some pressure on your fuel line upstream of the pumps or they won'T fill reliably. Maybe it's sufficient to mount the fuel tank a few m up, and have a rather fat pipe and manifold upstream the pumps.

Edit to clarify: For fuel injection, you need a precise control over the volume. In common rail injection, this is achieved by maintaining a constant pressure upstram of tightly timed solenoid valves. When replacing the valves with pumps, you wouldnt need a constant pressure to achieve control over volume flow, 'just' enough pressure to overcome static pressure in cylinder and transport losses. Control over fuel amount would be as god or bad as your pump.

In summary, I think what you need exists but it will be likely expensive and may not work very well in the context of your engine.

Or, if your goal is to become a better mechanical engineer, you invent/design a variable stroke diaphragm metering pump where the stroke volume can be adjusted online, qickly, via analogous signal and the individual strokes are controlled by a digital signal.

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  • $\begingroup$ A big problem seems like it will be that he needs constant pressure, where these pumps are rated on displacement. I'm not sure how well they operate to reliably pressurize. $\endgroup$ – JMac Mar 24 '17 at 9:55
  • $\begingroup$ I'm not so sure: The reliable pressure requirement AFAIK comes about so: You control the fuel injection (Volume per stroke of engine) by opening the valves, if the pressure upstream the valves fluctuates so will the volume injected. However, these pumps would control volume directly. However, I'm not a big expert on IC engines so maybe this is all wrong. $\endgroup$ – mart Mar 24 '17 at 10:17
  • $\begingroup$ I believe the pressure requirement comes about because of the required pressure for mixing. It's one thing to pump a volume of fuel into a cylinder. You need to pump it the right pressure through a nozzle to get the right air/fuel ratio. I'm not super experienced with IC engines either, so I may be wrong as well. $\endgroup$ – JMac Mar 24 '17 at 10:24
  • $\begingroup$ I believe you two are correct in what you're saying about the fuel injectors. From what I've gathered on the subject, the pressure has 2 effects. The first is that it helps atomize the fuel into the cylinder, as JMac described. And the second is that the volume of fuel injected is a function of the pressure. It's been described me to that the fuel injectors will remain open for an interval of constant length. Therefore, if I vary the pressure slightly, I can control how much fuel will be injected. I can do this to a certain degree, because there is a threshold for atomizing the fuel. $\endgroup$ – Klik Mar 24 '17 at 20:41
  • $\begingroup$ @mart The idea to mount the gas tank up high is actually genius. I'll look into these ideas! Can the solenoid pump maintain a constant pressure and be adjusted? $\endgroup$ – Klik Mar 24 '17 at 20:46
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Without a pump you will have trouble maintaining flow. The pressure gauge on a fuel regulator I looked for maxed out at 140 psi.

You can build pressure with an actuator but it will only run till the cylinder you make is empty. If you use a large actuator and a large cylinder (the size of a gas can) you might get it to work but that is not an efficient way to do it.

If you are insistent on not using a pump, consider using a fuel bladder or making a cylinder and just putting a weight on top until you get sufficient pressure.

If your concern for the system is that you worry about the pump running but not moving much or enough fuel to keep it cool (which can be a problem) you should install a return line to the tank with a pressure regulator. That way you can keep constant pressure and keep the pump cool. This will also limit the amount of pressure change as the fuel flow to the engine changes.

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  • $\begingroup$ Or, if you are looking to accurately inject a precise amount of fuel in each cylinder that you can vary, you could build a system using solenoids and a piston like chamber. The easiest method I can think of would be to have the solenoid ram a rod into a larger chamber. You can vary how far the rod rams in to vary how much fuel is displaced. You could fire it with the same signal that opens the fuel injector but you might want to delay the injector a bit to allow the rod to start moving first. You will still need something to supply the chamber with fuel. $\endgroup$ – Gwydionforge Mar 24 '17 at 14:48
  • $\begingroup$ I think that approach would be difficult because the engine will operate at about 1,000 to 2,000 rpm. Building pressure with an actuator is something I've thought of, but I'm not sure which actuator could be used to do this. $\endgroup$ – Klik Mar 24 '17 at 20:43
  • $\begingroup$ The actuator only moves something. Pressure must be built with what is moved. If you fashion a piston and cylinder you will make pressure depending on the force and area of the system. The engine will operate at 1000 ~ 2000 rpm, but what is the frequency and duration the actuator will need to fire? On a 4 stroke engine, you get combustion every other revolution, the duration and stroke of the actuator will depend upon the geometry of your cylinder. A solenoid could fire fast enough to keep up with your engine, but you might need more than one as i described above to achieve the needed pressure $\endgroup$ – Gwydionforge Mar 24 '17 at 21:54

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