Vapor lock questions for the hotter climate guys

[Zed Head]

Too bad we can't just open the horse's mouth...

http://www.lhup.edu/~dsimanek/horse.htm

[cygnusx1]

We could probably open the horses's mouth if we had a clear fuel rail and clear injector tubes. However what we do have: Difficulty hot starting some EFI Datsuns, rough idle when first started from hot, then we have datalogs of injectors feeding leaner mixtures as they heat up with a constant pulsewidth, and we have evidence that Nissan's engineers were still working on the issue from 1975-1983 by the design evolutions they made. Also, but not evidence, superheating, is relevant to this theory. In fact it's a critical point.

Counter evidence is that some EFI Datsuns have never exhibited this, and the manual says it can't happen.

We also know that fuel can boil pretty readily at engine bay temperatures, and atmospheric pressure....

And finally we have this:

The later generations of injectors are called SIDE-FEED because they are almost fully immersed in the fuel rail flow path. Why do you think they are bathed in the fuel path?

Edited July 29, 2011 by cygnusx1

[doradox]

Nice description of the gradient path, and I'm with you.

Assuming the temperature is high enough, somewhere between fuel pump and the intake valve end of the pintle the gasoline will cross the phase change line from liquid to gaseous. Still not sure exactly where, but it seems from yours and others experiences that it's in a place that affects performance for the first few minutes of operation. I'm buying it.

My only point of contention is that I'm still not seeing the "superheated" part and we are really running the risk of wandering off into the weeds with this part of the discussion so I will try once and then let it go.

I believe the use of the term on the geyser page is a misnomer as well. I know you had to pass thermo, right? Just because you have a hot liquid under pressure does not "superheated" make. That just means you have a hot liquid under pressure. With all imperfections in the rock surfaces and turbulence in the water, I can tell you that there's no superheating in a geyser. It's too unstable for that.

You can change phase and flip back and forth across the liquid/gas phase line all day by varying temp or pressure as your theory suggests without ever becoming "superheated".

In order for a liquid to be superheated, it has to incorrectly exist in liquid form when conditions place it in the gaseous area of the phase diagram. In other words, your substance is at a temperature and pressure that SHOULD result in a gas, but you are incorrectly a liquid instead.

Ask yourself the question "What is keeping it in liquid phase?"

If the answer is "Pressure.", then you're not superheated.

If the answer is "Uhhhhh... I don't know. It really SHOULD be a gas but it's not!!", then you're superheated.

Does that make sense?

Perfect sense.

Steve

Edited July 29, 2011 by doradox

[cygnusx1]

I well know what superheated means. Water at standard pressure that is at 213 deg F and not boiling, is superheated. The reason I bring superheated into my theory is that once the pressure is dropped, or the vibration of the pintle is started, THAT alone could bring the fuel to change phase, from a superheated state, or even just skipping the superheated state. Yes, it could all happen without ever being superheated. There is also hysteresis between the phases. So maybe there is some instantaneous point in time when the fuel is superheated, maybe it just vaporizes without ever becoming superheated. I don't know for sure. Fastwoman is right in the cooking pasta observation. Microwaving pure water, which boils the instant you drop something into it, is another example of superheated water gone wild. Superheated states, are highly unstable, which would make a liquid in this state much more likely to vaporize by a seemingly insignificant catalyst.

Edited July 29, 2011 by cygnusx1

[Captain Obvious]

Please forgive me for not knowing the intricate details of the Z's FI system, but...

You made mention earlier of the batch type injection concept where that all the injectors open at the same time for the same duration. Is that what the 280Z uses?

[doradox]

So maybe there is some instantaneous point in time when the fuel is superheated, maybe it just vaporizes without ever becoming superheated. I don't know for sure.

Also, but not evidence, superheating, is relevant to this theory. In fact it's a critical point.

So is superheating critical to your theory or is it something that maybe sort of might be happening? Can cogently explain why superheating is a critical point?

Steve

[cygnusx1]

I am pretty sure that the old fuel injection systems in the S30's use full batch fire. All the injectors fire at once so you do have quite a bit of pressure bounce. The fuel damper is way at the back of the car too. Superheating may or may not be critical to the theory. But I now think it is. I just figured that a single injection happens pretty quickly (2-5ms), so the if the fuel is phase changing, it's happening pretty quickly. A liquid in a superheated state would be the first, and fastest to flash. So if you have a column of liquid with a pressure gradient, "above boiling pressure" at one end, and "below boiling pressure" at the other end. Naturally, somewhere in that column there is a segment of critical pressure. There is also a place in that segment where a smaller segment of superheated liquid lives. That small segment may be the "seed" for the instant flash, so to speak. This is really getting microscopic, but it's all perfectly feasible and I think it happens. It happens when the conditions, and fuel quality, fit the mold.

[doradox]

I am pretty sure that the old fuel injection systems in the S30's use full batch fire. All the injectors fire at once so you do have quite a bit of pressure bounce. The fuel damper is way at the back of the car too. Superheating may or may not be critical to the theory. But I now think it is. I just figured that a single injection happens pretty quickly (2-5ms), so the if the fuel is phase changing, it's happening pretty quickly. A liquid in a superheated state would be the first, and fastest to flash. So if you have a column of liquid with a pressure gradient, "above boiling pressure" at one end, and "below boiling pressure" at the other end. Naturally, somewhere in that column there is a segment of critical pressure. There is also a place in that segment where a smaller segment of superheated liquid lives. That small segment may be the "seed" for the instant flash, so to speak. This is really getting microscopic, but it's all perfectly feasible and I think it happens. It happens when the conditions, and fuel quality, fit the mold.

Isn't it also possible that ALL the contents of the injector are already vapor before the injector opens? I think it would be more likely than your scenario where the exact conditions for the existence for some, however small, amount superheated liquid have to be present to cause the fuel to experience the phase change at exactly the moment required for it to cause a problem. So to my thinking, superheating isn't critical to explain vapor induced drivability problems in EFI cars.

I think your tortured hypothesis may be possible but not one that Occam's razor wouldn't slice the heck out of when speaking , in general, about drivability problems suspected to be caused by fuel, heat, and pressure not playing well together in an EFI system.

Steve

[cygnusx1]

Yup that is possible. But according to the manual, it's not. Then again, the earth was flat once.

[FastWoman]

Thanks for digging up that tidbit, Dave! I think that MIGHT be another mystery solved. Now I won't fret so much when my hot engine re-starts roughly. Maybe the solution lies in a different design of injector.

BTW, I forget who mentioned Datsun's tidbit about goosing the throttle for a hot start, but I don't think that will help much. I've tried goosing the throttle AFTER starting, and the engine still runs roughly. It doesn't even out until I pull out of the driveway and head down the road.

Edited July 30, 2011 by FastWoman

[cygnusx1]

There is not much you can do to clear the vapor, other than drive it a bit after it starts. Sometimes it barely idles, so you can rev it, but the best thing is to place a moderate load on the engine which will open the injectors fairly "wide". Leaving the hood open is usually enough to prevent it in the first place. I wonder if any additives can raise the vapor pressure of the fuel, for experimentation.

[doradox]

Yup that is possible. But according to the manual, it's not. Then again, the earth was flat once.

That, I think, stems from the definition of "vapor lock" which seems to have now evolved to mean any vapor related drivability problem. In the 70s maybe not so much.

Steve