Vapor lock questions for the hotter climate guys

[doradox]

The confusion here is in the semantics between true 'vapor lock' and 'vapor lock type symptoms'. In the combined 18 years of the 'joy of ownership' of mid to late 70s Datsun EFI vehicles the only time I ever experienced that was when the check valve went bad on my 810. Replaced it - no further problems. It gets well over 100 degrees everywhere I've lived here in those 18 years. If it happens on a carbed car it's vapor lock if it happens on an EFI car is a mechanical problem that manifests itself with vapor lock type symptoms. When everything is right, it will never happen. That can't be said of carbed cars. Even with wrapped fuel lines the fuel pressure is only, what, 3-5 psi.?

I would tend to agree with you.

Steve

[doradox]

I will repeat. Fuel that sits under pressure, behind the injector pintle, becomes superheated. Superheated is a technical term. What happens to superheated liquids when a catalyst is introduced or the pressure is suddenly reduced? It flash vaporizes. This DOES happen inside of the injectors when the pintles are fired from a superheated state. Nissan, not me, installed a cooling arrangement in an effort to keep the injector bodies from becoming holding chambers for superheated fuel. Again, not my idea, it was the idea of Nissan engineers. The flash vaporization on hot start, can also happen in the fuel rail as the pressure sine waves bottom out, as the BATCH injectors all fire at once. The fuel damper is too far away to absorb this wave fully. Sequential injectors minimizes the amplitude of the sine pressure waves in the fuel rail, which can prevent this effect from creeping into the rail. When it happens, you have a situation where you are injecting vapor into the combustion chamber. Vapor will not burn. Thus it's a sort of vapor lock. Call it what you will. Yes, it is probably fuel dependent.

Now, can anyone answer why the cooling fan for the injectors was added?

Do have a source for your assertion that vapor will not burn? I was under the impression that fuel needed to vaporize into air in order for combustion to take place. Something about the fuel molecules needing to be distributed among the O2 molecules so the oxidation reaction (burn) can take place. Maybe vapor isn't the technical term you were looking for.

Steve

[doradox]

Been working on EFI Datsun's myself for over 25 years. I see vapor lock issues, even when the system holds pressure. It very well could be the fuel formula we get here in the Northeast.

So you are or were an auto tech?

Steve

[cygnusx1]

Mech. Engineer but it's irrelevant. 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. I am thinking that the key to the differing experience has to do with A)Regional fuel quality and B)Condition and rate of pressure drop of the sealed system of Northeast vs Cars in warmer climates.

I am not sure if cars in the warmer climates experience this, but in cold weather, the hoses in the Z's EFI tend to leak. There is actually a recall or service bulletin for the issue. It may be that the cars exposed to colder climates, do not hold pressure long enough or high enough to ward off vapor-lock, unless the owner has taken remedial actions. That's why I asked about what the spec in the manual is for holding system pressure, in case you wondered. I can't seem to find it anywhere. I know it's not indefinite though. The "cold weather fuel line leaking", is not isolated to Datsuns either. Later model Subarus also had a similar recall, last year.

Edited July 27, 2011 by cygnusx1

[cygnusx1]

I am going to research the chemistry/physics of fuel atomization vs vaporization and the effects on combustion, but I know the short of it. Fuel injectors are not good vapor injectors.

[doradox]

I am going to research the chemistry/physics of fuel atomization vs vaporization and the effects on combustion, but I know the short of it. Fuel injectors are not good vapor injectors.

Being designed for liquid fuel I don't imagine they are.

FWIW I'm a BSME Purdue. Sold the shop, moved to the midwest, went back to school.

Any chance the difference in volume of gas vapor vs. liquid is causing trouble?

Steve

Edited July 27, 2011 by doradox

[Captain Obvious]

I did a little digging into the vapor pressure of gasoline and learned that the petroleum industry and US government have standardized on the "Reid Vapor Pressure (RVP)" test. It specifies a test temperature of 100C and measures the resultant vapor pressure in psi.

The vapor pressure is constrained by government regulations and is tightly controlled by industry. The limits change by location and time of year, but from what I found, the vapor pressure of your typical pump gas (at the RVP test temperature of 100 degrees C) is between 5 psi and 11 psi depending on where you live and what season it is.

Vapor pressure will decrease as temp goes down and go higher as the temp increases.

So how hot do the fuel get and what pressure is it under?

Being as how I'm not an ME, it's probably not a good idea to wade into the middle between two of them, but I've never been very bright...:tapemouth

[doradox]

I am going to research the chemistry/physics of fuel atomization vs vaporization and the effects on combustion, but I know the short of it. Fuel injectors are not good vapor injectors.

I was thinking about what you said. "Fuel injectors are not good vapor injectors"

Is it possible they simply are unable to flow enough vapor to get the mixture rich enough to ignite? Gas volume vs. liquid volume is very roughly 800 to 1. That would be a lot of vapor to pass through the injector nozzle.

Steve

[ShakotanLife]

I have been looking for something to wrap my fuel lines in. I cant really find anything that looks alright and will get the job done. can anyone link me to a product they know of that will work?

Thanks guys

[cygnusx1]

I did a little digging into the vapor pressure of gasoline and learned that the petroleum industry and US government have standardized on the "Reid Vapor Pressure (RVP)" test. It specifies a test temperature of 100C and measures the resultant vapor pressure in psi.

The vapor pressure is constrained by government regulations and is tightly controlled by industry. The limits change by location and time of year, but from what I found, the vapor pressure of your typical pump gas (at the RVP test temperature of 100 degrees C) is between 5 psi and 11 psi depending on where you live and what season it is.

Vapor pressure will decrease as temp goes down and go higher as the temp increases.

So how hot do the fuel get and what pressure is it under?

Being as how I'm not an ME, it's probably not a good idea to wade into the middle between two of them, but I've never been very bright...:tapemouth

Haha don't worry, I my engineering degree is mostly evidenced by the grease under my fingernails. I am no threat for sure. I am only a threat to myself. Good data that you found. Funny, I just got off the phone with a good friend of mine who is also an engineer, and worked on fuel systems for snowmobiles, now he is with NASA, and he gave me almost the same info you just did. He said that typical cold region Winter fuels can definitely vaporize in Summer conditions found in a fuel injection system. And he confirmed that fuel vapor is a completely different phase than atomized fuel. Fuel vapor is much closer to air than it is to fuel. He also said that it is possible to have a "geyser" effect. When the injector opens, the superheated fuel can instantly vaporize and apply back pressure to keep liquid fuel from entering the stream.

[FastWoman]

The vapor pressure is constrained by government regulations and is tightly controlled by industry.

Controlled? Yes.

Tightly (or well)? I very seriously doubt it.

Remember, you can buy up to 30% ethanol gas at the pump, even when 10% is the maximum allowed by law. There's definitely a control problem, and the mechanics in my area have their hands full, trying to fix the consequences.

[Captain Obvious]

Controlled? Yes.

Tightly (or well)? I very seriously doubt it.

Yeah, You're right. I probably should have said "the upper limit on vapor pressure is supposed to be tightly controlled"

Research indicates that the intention of the gov't is to reduce the amount of fuel evaporation into the atmosphere as much as possible while still allowing the fuel to work well in application. The way they do this is to dictate an upper limit on the vapor pressure for different locations at different times of the year. How the manufacturers achieve that vapor pressure limit seems to be mostly up to them, including the composition and ethanol content of the fuel.

My research turned up two things that always seem to be true:

1) Winter fuel (RFG) is allowed to have a higher vapor pressure than summer fuel, and...

2) California requires a lower vapor pressure than most other states.

And he confirmed that fuel vapor is a completely different phase than atomized fuel. Fuel vapor is much closer to air than it is to fuel.

Of course they are. You remember the difference between "suspensions" and "solutions", don't you?

And you mentioned the concept of superheat earlier and I forgot to ask... Why do you think the fuel in the injectors is superheated? Seems difficult to achieve.

Have we strayed far enough from the OP's question yet?