Poor Mans Low Compression Direct Injection Engine

[TurboDiverArt]

Hi All,

I'm sure someone has thought of this as I'm certainly not the smartest person on this board. Since lowing the compression is generally considered a good thing all around. What would happen and has anyone tried simply removing the pre-cup from the head and smoothing the rough edges? I would think removing the pre-cup insert would knock the compression down some. Not sure how much but it's got to bring it down somewhat. In addition you would not have direct injection and not lose some of your heat by the use of a pre-cup. A few obvious problems I see is that the flame front is not going to travel evenly and easily across the piston top but then again, neither does it when shooting out of the pre-cup slot either. Additionally, cold starts would be harder, not only from the drop in compression but also you now have to heat a larger area vs. the smaller concealed "cup" area.

I'm sure there are reasons why this won't work or is a bad idea, I'd love to hear them. No, I'm not planning on trying this on my engine. I

[DmaxMaverick]

I've wondered about this myself. One problem I see right off is, you can't just remove the cups. You'd have to modify the cups, or cast new heads. The cup cuts across the fire ring of the head gasket. If you were to create new heads, just place the injector in the center, and use DI pistons. Hey, they already did that, and called it Duramax.

Give it a shot and let us know how it turns out.

I think, in the end, you'd have to be a pretty rich poor man to pull it off.

[dieseldummy]

The other problem with no precup besides head gasket sealing would be getting the proper swirl effect on the fuel. In IDI motors the precup creates the swirl to properly combust the fuel, in DI motors it is a combination of the injector and dish in the piston. To get this right you'd have to do extensive computer modeling and trial & error with parts. Assuming that you could come up with a combo that would work I don't think that the bottome end would hold up too long...

Just my .02

Justin

[DmaxMaverick]

IMO, swirl is irrelevant in IDI 4 cycle Diesel engines. The fuel should be combusting upon injection. The spray pattern is more of an issue to get the fuel in contact with O2. There will be no effective swirl inside the precup.

In a 2 cycle Diesel, swirl would be detrimental.

[dieseldummy]

IMO, swirl is irrelevant in IDI 4 cycle Diesel engines. The fuel should be combusting upon injection. The spray pattern is more of an issue to get the fuel in contact with O2. There will be no effective swirl inside the precup.

In a 2 cycle Diesel, swirl would be detrimental.

If one were to remove the precup then it wouldn't be an IDI diesel anymore... IMO, swirl does matter in an IDI. Without swirl there is no way for the fuel to combine with the air efficiently. If it didn't matter then why does most literature discussing the Ricardo comet precups mention swirl?

[DmaxMaverick]

If one were to remove the precup then it wouldn't be an IDI diesel anymore... IMO, swirl does matter in an IDI. Without swirl there is no way for the fuel to combine with the air efficiently. If it didn't matter then why does most literature discussing the Ricardo comet precups mention swirl?
Refers to swirl exiting the precup, to prevent burning a hole in the piston and distribute the heat.

The fuel doesn't have to "mix" with the air. It is sprayed into it, and combusts immediately (or is supposed to, anyway). The O2 is consumed as the combusting fuel expands into the cylinder. keep in mind, the entire volume of air drawn/forced into the cylinder is squeezed into an area smaller than a medicine cup. The force of the injected fuel will negate any swirl that took place before it got there, which won't be swirling anymore. What is the intake air swirling with before the fuel arrives? It may help with a cold start, but that's all. Swirl porting may work with gassers, but the fuel and air are pre-mixed before ignition.

Still, the 6.5 head has the injector positioned at the edge of the cylinder. You'd have to redesign the head for an injector to spray onto the piston, and have a piston that supports this.

[ronniejoe]

I agree with dieseldummy. Jim's fuel efficiency article discusses this in great detail.

The spherical shape of the precombustion chamber (with the precup in place) is designed specifically to create circular flow patterns to enhance atomization and burn.

I don't think anyone should even think about trying to run one of these without the precups... I'm sure you wouldn't like the results.

[dieseldummy]

The fuel economy article is exactly what I was thinking about RJ. Another thing I was thinking about were other IDI precup motors. I have on older IH tractor that is a precup engine. The injectors in it spray a very course stream of fuel, not a fine mist like most engines. Without swirl in the precup there is no way that the fuel would combust.

This carries over to our engines for the simple fact that there has to be some turbulence caused by air being forced through a small hole into the precup.(alot of turbulence, 10 PSI boost+21:1 CR= lots of pressure going through a hole the size of my pinky finger) Without that turbulence the fuel sprayed into the motor wouldn't burn evenly or efficiently. The largest diamond precups were probably the bigest hole possible without sacraficing power and fuel economy. Taking the cup clear out of the equation would be like making that diamond precup's hole 3-4 times larger...

[TurboDiverArt]

I've wondered about this myself. One problem I see right off is, you can't just remove the cups. You'd have to modify the cups, or cast new heads. The cup cuts across the fire ring of the head gasket. If you were to create new heads, just place the injector in the center, and use DI pistons. Hey, they already did that, and called it Duramax.

Give it a shot and let us know how it turns out.

I think, in the end, you'd have to be a pretty rich poor man to pull it off.
Yeah, and if GM put a D-max in a Suburban I wouldn't have to stay up late thinking of such things.... :rolleyes:

I believe increasing the size of the IDI outlet port does nothing for performance (I think John Kennedy tried bigger cups and found no extra power). But, increasing them (removing metal and increasing cylinder volume (lower compression) might be a cheap option. Not that I would ever try it unless the engine went south but I figured I would pose the thought and see if there are any other nut jobs out there like me thinking of such things late at night.... :)

Art.

[ronniejoe]

I'm running the same pre-cups that Kennedy did. I haven't measured anything yet, and wouldn't be able to separate out their contribution anyway. However, why did GM increase the size of the port on the turbo F engines then if it doesn't affect power?

Look at the pre-cups in Jim's article. The NA 6.2's have the smallest openings with the later turbo F 6.5's having the largest. The ports in the cups that I am using are bigger than any of those shown in the article and were probably developed for the 215 hp version that never made it to production. Seems to be a strange correlation if throat size has no impact on performance...

[TurboDiverArt]

The fuel economy article is exactly what I was thinking about RJ. Another thing I was thinking about were other IDI precup motors. I have on older IH tractor that is a precup engine. The injectors in it spray a very course stream of fuel, not a fine mist like most engines. Without swirl in the precup there is no way that the fuel would combust.

This carries over to our engines for the simple fact that there has to be some turbulence caused by air being forced through a small hole into the precup.(alot of turbulence, 10 PSI boost+21:1 CR= lots of pressure going through a hole the size of my pinky finger) Without that turbulence the fuel sprayed into the motor wouldn't burn evenly or efficiently. The largest diamond precups were probably the bigest hole possible without sacraficing power and fuel economy. Taking the cup clear out of the equation would be like making that diamond precup's hole 3-4 times larger...
All interesting discussions. Guess I should have read the entire thread before posting the above. I've never see one of our heads off the engine to see exactly how things line up and the angle of things. In fact, I've never seen any diesel head off an engine. Things I have subsequently learned that I didn't think about include the added stress the combustion event would have on the top of the piston. I'd think that a lot of the initial shock of combustion is taken up by the head and is contained within the pre-cup. Not having one, or having a bigger hole (like 3-4 times bigger like you mentioned) would probably transmit a lot of this shock to the top of the piston. If the crown didn't crack from the added stress the block webbing probably would.

I also didn't realize the position of the injector in the pre-cup. I would agree that if it were not pointing straight down on the piston face that you would not have very efficient flame travel.

All in all a very interesting discussion. I'm not about to try it on my engine as I hope to not have to take it apart for a long time. Hoping not to jinx myself but I think I've finally overcome the 180K blues where things on the truck broke, leaked or stopped working. Knock on wood, not the engine. Major items I think are fixed and now it's time to try to fix the smaller annoying ones....

Thanks,
Art.

[TurboDiverArt]

I'm running the same pre-cups that Kennedy did. I haven't measured anything yet, and wouldn't be able to separate out their contribution anyway. However, why did GM increase the size of the port on the turbo F engines then if it doesn't affect power?

Look at the pre-cups in Jim's article. The NA 6.2's have the smallest openings with the later turbo F 6.5's having the largest. The ports in the cups that I am using are bigger than any of those shown in the article and were probably developed for the 215 hp version that never made it to production. Seems to be a strange correlation if throat size has no impact on performance...
True, it is possible. I read Jim's article and that's actually what got me thinking of this whole thing. I do think going from the smallest pre-cup size to the larger ones did have a positive impact on performance and a negative on MPG. I was referencing John Kennedy saying in another thread long ago that he tried bigger than big stock pre-cup openings and didn't find any more power. Maybe it's a diminishing return issue and the limit has about been hit? Maybe this only applies with stock boost and compression. I would think that a larger opening would allow the additional O2 in a higher boost engine to enter the pre-cup without getting quite as hot as with a smaller opening. Remember, we are squeezing a large volume of air through a small hole, it