Vapor lock questions for the hotter climate guys

[cygnusx1]

Technically, it's not superheated the whole time, because the rail pressure keeps it in a liquid phase even though it's above it's STP boiling point. However it passes through a superheated state just before flashing, as the pressure in the injector column drops upon firing.

In fact, the snap action "shock" of the pintle opening might be enough of a catalyst to start the boil.

Edited July 28, 2011 by cygnusx1

[doradox]

. Fuel has to atomize to burn. Not vaporize (it's about droplet size). Vapor would require a different condition to ignite than present in the IC engine.

Atomization helps bring about vaporization which is what fuel needs to burn. Read the conclusions on page 4 and 5 of this document.

http://nepis.epa.gov/Exe/ZyNET.exe/91002JP9.txt?ZyActionD=ZyDocument&Client=EPA&Index=Prior%20to%201976&Docs=&Query=APTD0964%20or%20fuel%20or%20atomization%20and%20vapor&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=pubnumber^%22APTD0964%22&QFieldYear=&QFieldMonth=&QFieldDay=&UseQField=pubnumber&IntQFieldOp=1&ExtQFieldOp=1&XmlQuery=&File=D%3A\ZYFILES\INDEX%20DATA\70THRU75\TXT\00000007\91002JP9.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h|-&MaximumDocuments=10&FuzzyDegree=0&ImageQuality=r105g16/r105g16/x150y150g16/i600&Display=p|f&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results%20page&MaximumPages=-1&ZyEntry=5

[cygnusx1]

Absolutely. However if the fuel injector injects a teaspoon of droplets, perse, vs a teaspoon of vapor, you have a totally different atom count. X-TAU is all about fuel depositing and vaporizing AFTER the injector. I has to come out of the injector as a mist of fuel droplets.

You link does not work, but I think I am familiar with that document.

I have used silicone tubing sliced lengthwise, and slipped over the injector bodies, and the fuel lines as insulation. However it didn't work too well. I don't think silicone was the right choice. It needs to be less thermally conductive and maybe even reflective. There are products designed to do just this. Search summit and jegs for heat control products for an easy idea.

Edited July 28, 2011 by cygnusx1

[sblake01]

Try this link. The original one is so long that it would be almost impossible to properly parse it for use as a website link.

[cygnusx1]

That is pretty much some of the basis of the CVCC from Honda, I think. The problem see with vapor in the injectors is simply from the fact that if the injector fires for 2.5ms to atomize liquid fuel, you get X amount of fuel in the chamber. If it opens up for 2.5ms and the fuel vaporizes through the nozzle path you are getting MUCH less than X amount of fuel to the chamber. This all clears up once fresh cool fuel makes it down to the pintles. Usually it takes about 30 seconds or so, once the engine fires, in my experience with the stock EFI. Now my car is a different story. It's not stock, it generates a ton of heat, no hood vents, and the billet rail does a great job of transferring heat into the fuel. With the car shut down hot, even with 30-40 psi still on the gauge, the fuel in the injectors exceeds the boiling point at atmospheric P. Even if I let it idle on a hot day the mixture will begin to lean out as the pulsewidth of the injector remains steady. The fuel rail is recirculated and I have two pulse dampers in the loop. My injectors are Bosch 440cc so they are at a pretty low duty cycle at idle which might explain why they can't cool themselves off with fuel. Perhaps?

The energy of one gallon of gasoline is roughly 112,000 BTU

A fully evaporated (vaporized) gallon of fuel occupies about 160 gallons.

Therefore, one gallon of gasoline vapor contains only 700 BTU.

You would need to inject 160 times the volume of vapor as you did liquid to get the cylinder fired off properly. So, if there is nothing but vapor in the injector OR the fuel is vaporizing coming through the nozzle, and the liquid phase fuel is not flowing through fully, you will be very lean.

Edited July 28, 2011 by cygnusx1

[FastWoman]

I did a little experiment today. I had run an errand and parked my car in the shade. I didn't get the engine very hot at all, but I suppose it was hot enough. When I restarted after only a couple of minutes of sitting (literally), I had such a rough start that the engine died. Before turning the key again, I thought I'd try ZTrain's suggestion of holding my fuel pump priming switch for quite a long time. I let the pump run for maybe 1-2 min, and.... STILL A ROUGH START.

I must have done a pretty thorough job of flushing fuel vapor from the rail and replacing it with cool fuel, but still no-go. I now suspect the injectors get hot enough to vaporize, fuel, which bubbles, rises and condenses in the fuel rail, while new bubbles form. Then when I start the engine, I have to inject small vapor pockets for a short while until the injectors are cooled by the incoming fuel, and until the intake manifold is cooled by the fresh air rushing through.

If that's the case, I suspect there's no amount of insulating that could be done to completely resolve the problem.

And again, I suspect this could relate to the formulation of gasoline -- luck of the pump, as it were. I'm having more problems with this last batch of gas than I usually do. I got it at a station that's not my usual place.

I wonder whether I would be able to hear the fuel boil if I put a stethoscope to the injector after engine shutoff. Unfortunately I don't have a mechanic's stethoscope. Next time I'm at HF...

Edited July 28, 2011 by FastWoman

[doradox]

That is pretty much some of the basis of the CVCC from Honda, I think. The problem see with vapor in the injectors is simply from the fact that if the injector fires for 2.5ms to atomize liquid fuel, you get X amount of fuel in the chamber. If it opens up for 2.5ms and the fuel vaporizes through the nozzle path you are getting MUCH less than X amount of fuel to the chamber. This all clears up once fresh cool fuel makes it down to the pintles. Usually it takes about 30 seconds or so, once the engine fires, in my experience with the stock EFI. Now my car is a different story. It's not stock, it generates a ton of heat, no hood vents, and the billet rail does a great job of transferring heat into the fuel. With the car shut down hot, even with 30-40 psi still on the gauge, the fuel in the injectors exceeds the boiling point at atmospheric P. Even if I let it idle on a hot day the mixture will begin to lean out as the pulsewidth of the injector remains steady. The fuel rail is recirculated and I have two pulse dampers in the loop. My injectors are Bosch 440cc so they are at a pretty low duty cycle at idle which might explain why they can't cool themselves off with fuel. Perhaps?

The energy of one gallon of gasoline is roughly 112,000 BTU

A fully evaporated (vaporized) gallon of fuel occupies about 160 gallons.

Therefore, one gallon of gasoline vapor contains only 700 BTU.

You would need to inject 160 times the volume of vapor as you did liquid to get the cylinder fired off properly. So, if there is nothing but vapor in the injector OR the fuel is vaporizing coming through the nozzle, and the liquid phase fuel is not flowing through fully, you will be very lean.

So what I said about vaporized fuel being required for combustion as opposed to not combusting as you had proposed was correct. And my thoughts questioning the injector's capacity to flow the volume of vapor required to support combustion were on track. What you are saying makes more sense now that we have generated a hypothesis that makes physical sense. Interesting.

Steve

[twsutt]

Has anyone read the owners manual for the 280z (I know it's in the '75) about starting the car after the engine has been running and the car has only sat for a short time? I was surprised to see that it instructed one to press the gas pedal all the way on to restart. I expect this is to flush out the injector/fuel rail.

[doradox]

Try this link. The original one is so long that it would be almost impossible to properly parse it for use as a website link.

Thanks. Much better.

Steve

[Z train]

I was doing a little light reading and found this little tidbit in the 76 FSM.

[ATTACH=CONFIG]46950[/ATTACH]

Damn-ya beat me to that one.

[Captain Obvious]

Technically, it's not superheated the whole time, because the rail pressure keeps it in a liquid phase even though it's above it's STP boiling point. However it passes through a superheated state just before flashing, as the pressure in the injector column drops upon firing.

In fact, the snap action "shock" of the pintle opening might be enough of a catalyst to start the boil.

STP has nothing to do with this. It's all about vapor pressure at elevated temps. You're nowhere near STP.

Also, your use of the term "superheated" has become is a little confusing to me, so it might be prudent to make sure we're talking the same thing...

What's your understanding of "superheated"?

[cygnusx1]

Just make sure your system is holding pressure after you turn off the car. That is usually the first thing to eliminate when faced with this issue. How long it should hold pressure? I can't find that number anywhere so in my logical opinion, "long enough for the engine to cool down a bit" ;-) If you do hold pressure, then you are seeing exactly what I am talking about...assuming of course all other components of the EFI check out...and it's a pretty long list. I like to compare specs to the FSM right from the ECU connector where you have access to all FI components. Of course theres more stuff that needs to be checked out. It's so easy to say something else is causing the issue, and it CAN'T happen, but I know that this CAN happen due to a vaporizing fuel issue.

Edited July 28, 2011 by cygnusx1