Quote:
Originally Posted by Isaac Zachary
A few things come to mind, however.
- Gasoline is comprised of several liquids that boil from some 35 °C or 95 °F to some 200 °C or 395 °F. So heating it to water temps of around 200 °F or 97 °C isn't going to boil everything, but still could cause vapor lock with the lower temp boiling liquids.
- Yes, the fuel rail is at a high pressure, but the compression stroke creates even more pressure than the fuel rail. So what would happen to those liquids that were liquid at 45psi, vaporized at 14psi (or whatever atmospheric is) and then get compressed back to 150psi or so?
- I still feel it is imperative to take into account the total heat energy in the air and fuel. The question is how much heat energy does the charge need to keep all or at least a certain percent of the gasoline in a vaporized form. For an example, boiling fuel also reduces temepratures. It takes a lot more heat energy to heat a liquid to a boil and boil it all out than to pressurize it and heat it up 10 or 20 degrees past it's boiling point. When such a pressurized liquid hits a pressure drop to atmospheric some of it boils off, but the rest just cools down to just under it's boiling point and stays a liquid.
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As to point number 1:
The majority of the gasoline is comprised of C8 to C9 carbon chains. A small percentage of heavier oils are contained but comprise such a small amount as to be immaterial to the discussion. The small amount of lighter carbon chains are bound by weak intermolecular bonds to the longer heavier chains (surface tension). These dominate and reduce the boil out of the light components. Think about how long it takes for pure rubbing alcohol to evaporate on a warm day. Mix it with water and it takes much longer. At only a few percent alcohol, the evaporation rate is dominated by the water. This is why you do NOT get vapor lock even if your fuel rail is at 40 C and the dissolved butanes, pentanes, propanes, etc. have far lower boiling temperatures than 40 C which is the typical fuel temperature on a normal summer day.
As to point number 2:
You are forgetting the ideal gas law. As you rapidly compress the fuel air mixture in the cylinder, the temperature rises too. You know this to be true for diesel engines to the point they auto ignite the fuel injected at the top of the compression stroke. Also, temperature is simply a measure of the vibrational kinetic energy of the gas. Gas turbulence is energy into the system. Most modern engines have both tumble and spin designed into their intake tracts resulting in additional energy contained in the fuel mixture to promote gasification as well as to prevent recondensation and drop out.
Also, recondensation takes TIME and CONCENTRATION. It is not as simple as reaching the condensation point (pressure and temperature) of a fuel and expect it to instantly drop out as liquid droplets. It takes time for the individual molecules to find another partner molecule to Coalesce into a pair, find another molecule to join them . . . and so on, until you get a droplet. Throw in the fact that the majority of the molecules (a factor of 14.7 parts by mass) are air molecules and you start to see how the fuel will stay in a vapor state well beyond the milli-seconds level event even an idling engine will experience.
As to point number 3:
Once the fuel is injected out of the orifice, the fuel stream experiences internal as well as external shear and collisions. This is added to the energy sum of the fuel/air mix. The velocity differential between the moving air stream and the fuel stream also adds to this sum. A well-designed injection system will take all of this into account and will result in a combustion mixture that is almost all in a vapor phase. As pointed out above, once in a vapor phase, there is not enough time for the fuel to coalesce, agglomerate and condense.
Do you want an everyday operational proof of what I have just stated? Direct injected spark ignited gasoline engines are now known to produce carbon soot particulates. They inject at bottom dead center or after the intake valves close. They don't have the time to evaporate the liquid droplets entirely. Just as you mentioned, the increasing cylinder pressure makes vaporization more problematical. It is only a small mass percent of the total fuel injected but that soot is a sign of incomplete vaporization. Instead of tens of PSI of fuel pressure, Direct Injection engines use thousands of PSI in an attempt to form smaller droplets that can evaporate more rapidly but the problem still remains. Port injected engines on the other hand, do not produce soot particulates AT ALL! This shows you that the vaporization is complete otherwise any droplets would turn into carbon with the heat from the flame front. They do produce partially burned hydrocarbons and carbon monoxide but that also shows that vapor is what is being incorrectly burned, not liquids. The small amount of carbon particulates collected in laboratory settings is attributed to oil slip - lubricating oil getting into the combustion chamber.
Do you want an interesting aside?
A company called Transonic Combustion used high pressure injectors (up to 8000 psi) with fuel heated (300 C) to put the hydrocarbons within the critical point. Critical liquids have unusually high diffusion rates. They claimed a reduction of 50% fuel use (reduced brake-specific-fuel-consumption)as well as claims of ultra-clean emissions without after treatment. Unfortunately, their website seems dead and maybe so is their company.