DIY GDI --> DIY EDM

[jharvey]

I've been dreaming of a DIY GDI, and I'm thinking of modifing a small engine by changing the spark plug for a plug with a hole down the middle. The fuel would be injected through the specail spark plug. So where do you find a spark plug with a hole in it, you ask. You don't you make it. This brings me to DIY EDM, and I'm wondering if anyone out there knows much about EDM.

A bit more background, I've been thinking of different ways to get a hole in a spark plug. Recently I found a spark plug with a funny prong for the other side of the electrode. This would allow fuel squirted down the middle of the plug to pass by the prong with out direct contact. Causing the fuel charge to be placed in the center of the cylinder, rather than swirled around and evenly distributed like normal. However, with out a hole down the center, it doesn't like to squirt fuel. The plug is about 1.5 inches (about 35mm) long so drilling a hole with a conventional drill really isn't an option. Drills like to wonder to much and you wouldn't come other the other end in the middle. I've been thinking of having someone drill it via laser, but lasers often want a focal point as well as then loose their strength the father away you get. It would likely be difficult and expensive to drill the hole this way. So far the best I've come up with is Electronic Discharge Machining EDM.

I'm picturing an electrode made from 3 or 4 mechanical pencil refills, with a mild varnish or something to bond them together. This would create a channel down the center making them into a reasonable long straw. Then with a little rubber hose clamped to one side, and a fuel pump, I can pump kerosene down the center. This should create the washing action desired for discarding removed material.

To me it sounds low enough cost to give it a try. I've got a couple power supplies, one does 30 amps at 12VDC, another can do an amp or so at up to 60VDC. I think both should work well for EDM. What I don't know is anything real about EDM. I don't know what voltages are required or preferred, or how fast they work. Is pecking a good approach. Should I stop penetration and let it burn, then once I see the current has dropped plunge some more. These are items I simply don't know.

If the power supplies at arms length will take a day or two to drill the hole, I'd want to build a robot to do the pecking / paused burning. However if it will only take half an hour, I'll simply put it on a drill press and have at it.

Does anyone know specific details for EDM, like what voltages are preferred / required, what current draw to expect, how to predict how long it will take to burn a hole with the power supplies I have, ect.

[Fred]

All I know is that fuel pressure is hugely high and for good reason. I also know that OEM implentations are not reliable yet, not outside of warranty anyway, and that it will be quite a challenge to get a safe system setup and working at all, let alone well.

Good luck though :-)

Fred.

[jharvey]

I'm by no means competent in this subject, but here's some of my thinking anyhow. When I'm done, I'll definitely be more knowledgeable then I am now.

My understanding about the high pressure injection is that it allows you to inject during combustion, just like diesel. This allows for better control of the flame front and pressures under ultra lean operation. The temperature of the burn is largely relative to the pressure of the burn/compression. Normal methods require the pressure and ultimately temperature to stay below the flash point until ignited. After ignition the temperature is typically design to be kept below 1400 degrees (c or f? not sure must be f). Above that temp produces a high level of NO's, below it reduces the HP per CC rating. Most normal engines are designed to have these specific ranges maintained. Simply advancing the spark doesn't allow the burn temp to increase much. Your mechanical limits for detonation and erosion have been mechanically tuned into the engine. I understand the injection pressure is much higher, and I'd bet the compression ratio is also much higher, as well as the dome is significantly different. Going with GDI in this sense is very unlikely and very hard to do I'm sure.

If you inject the fuel then compress (like I intend to play with), the charge will be kind of smushed with a high concentration of fuel in the center. So the air and fuel won't be mixed very well, and getting a complete burn will be hard, just like with manifold injection. I suspect that having more droplets that convert to gas during combustion, may cause some variations in HP vs manifold injection, but I don't expect much of a variation from the normal.

Many small engines run full tilt all the time, so call them fixed RPM. To allow the power level variations between loaded and unloaded, you have to starve the engine of air. This mean less air when you burn, so naturally you will have more CO vs CO2, ect. If you try to not adjust the air pressure with a manifold charge, you end up with too little fuel, and no ignition when you spark. This is because the fuel to air ratio isn't flammable when you try to ignite it.

However, if you inject through the electrode, it should be easier to keep the fuel ratio flammable in the presence of ample air. Well flammable at the point of spark that is, the front will have to take it from there.

I don't picture this type of GDI allowing for significant increases in HP per CC. It may allow for some improvements in emissions. I also see the possibility of removing the throttle body. I also see the removal of XTau.

I won't know until I drill a hole down the center of this plug and give it a try. So for now EDM is what I'm trying to learn more about.

[longracing]

Have you found any heads where the plug is slightly offset?
Either that or build the head up with aluminium and machine the spark plug so it is slightly offset.
That way you could place an injector directly above the bore without having to go through the spark plug and maybe reshape the combustion chamber for better squish.

Would you have a lot of trouble insulating the injector from the high voltage ignition if you went through the center electrode?

You can always file the ground electrode back if you need to move it from blocking the injector spray ( with the hole through the center electrode). If you make it to a blunt point it will concentrate the charge across a smaller area and make it more likely to jump the gap (try it with an old set of plugs sometime). This is why some manufacturers make plugs with fine electrodes.

[jharvey]

About the resistivity and reactants of gas, I don't know. Is it a resistor to DC? Does it have a high reactants to AC? Not sure about both. I suspect it has a high capacatance, so it's reactants increase as frequency increase.

I am sure it can be isolated, what I don't know is how hard it will be to isolate it.

Even if I find a head with an offset plug, I really need the spark to be very close to the injected fuel. The plug I found with a hole in the electrode is this one http://www.e3sparkplugs.com/diamondfire.htm They make some crazy sounding claims, but who cares, I just want it because it's got a hole in the middle of it.

[longracing]

I think building the head up with aluminium weld so you could reposition the plug would be successful. If you use a small diameter plug from a small engine with extended reach electrodes you could be able to get it within 4-5mm of the fuel/air stream.

Perhaps you could use a check valve where the fuel/air is injected to the head to allow the use of a conventional injector further up stream? (That is if you inject the mixture early in the compression stoke before the cylinder pressure is too high)
Or the fuel could be injected into an airstream that is then forced into the head at a higher pressure.