This will take me a while to get to my question, so please bear with me. Where's ernesteugene when I need him ?

We all know that diesel fuel doesn't evaporate anywhere near as fast as gasoline (has to do with vapor pressure and is beyond my question). Further, it appears (from observation) that evaporation of liquids is highly associated with surface area exposed (more surface area exposed, the higher the evaporation rate) and to temperature (the higher the temperature of the liquid the faster the evaporation rate). Evaporation is the molecules of the liquid disassociating from the liquid into a gaseous state. SO, you say?

Well, when the diesel injector shoots a small measured amount of fuel (anyone know the amount?) into the cylinder of a diesel engine the fuel is in a spray of very fine droplets (which is probably in the micron size) the surface area of that fuel increase exponentially (if I knew average droplet size I could figure the change in surface area of the fuel...anyone know the average droplet size?).
Though the droplets are very small, they are still liquid droplets. Visualize the droplets floating in the atmospheric air that has been stuffed into the cylinder. Now as the piston begins coming up on the compression stroke, temperatures begin to rise in the cylinder dramatically (gas law).

Now my question (about time you say):

Do the fuel droplets vaporize (that is, go from the liquid state to the gaseous state...like evaporation takes place at a greater rate at a higher temperature) BEFORE they are ignited by the heat of compression? I think that the fuel droplets vaporize before combustion actually takes place...but that is just my opinion.

So, I guess I am ultimately asking, are diesels burning a liquid or a gas?

 

There are several differences between diesel fuel which is more like the vegetable oils that sometimes replace it and gasoline. Two significant differences are the flash point and the vapor pressure. The compressibility of the fuel before it will ignite is an entirely different question.

"Every liquid has a vapour pressure, which is a function of that liquid's temperature. As the temperature increases, the vapour pressure increases. As the vapour pressure increases, the concentration of evaporated flammable liquid in the air increases. Hence, temperature determines the concentration of evaporated flammable liquid in the air." Flash point - Wikipedia, the free encyclopedia

Flammable liquids require different concentrations of its vapor in air to sustain combustion. One definition of "flashpoint" is "The lowest temperature at which the vapor of a combustible liquid can be made to ignite momentarily in air." flash point: Definition, Synonyms from Answers.com

The flashpoint for gasoline is -45 deg F. The flashpoint for diesel fuel is from 100 to 130 deg F depending upon the grade. http://www.engineeringtoolbox.com/flash-…
"It is the liquid’s vapor rather than the liquid itself that ignites when mixed in certain proportions with air in the presence of an ignition source. ... Volatility is the tendency or ability of a liquid to vaporize. Vapor pressure is a measure of a liquid’s volatility. A high vapor pressure usually is an indication of a volatile liquid ... and, possibly, an increased fire hazard." http://web.princeton.edu/sites/ehs/hazar…

Under NFPA ratings gasoline is recognized as a "quickly vaporized and highly flammable liquid" while diesel is "Flammable when moderately preheated or exposed to high ambient temperature." and can become more dangerous at normal high temperatures: http://www.industrialgasplants.com/nfpa-…

Don't throw a lighted match into a pot of diesel fuel that has been sitting under the summer sun in Arizona but in the middle of a Maine winter you might get away with it.

BTW I didn't stay @ a Holiday Inn last nite but I do know how to search the web.

 

Thanks loads Neal. I was sitting blissfully content with my place in the universe, swirling c******nay with a cool breeze tempering the golden afternoon sun, and you have to go pointing out something strange that requires an answer before I can possibly go to sleep.

So I thought about it. Where my thought train goes over the cliff is the effect of compression. High pressures cause vapors to condense back into liquid. Is the vaporizing effect of these high temps sufficient to overcome the coalescent effect of compression pressure?

 

Also lets please don't forget that there is already extreme pressures in the cylinder when the injection event begins to occur.

 

Correct...but the relative difference in pressure when the poppet is released in the injector dwarfs (it must, right?) the 400-500 psi at maximum in the cylinder. From the perspective of the fuel, it is under extreme pressure going into a depressurized environment.

 

In a pretty neat safety training video, they said liquid doesn't burn! But the vapors are what actually burn. Like said above what makes diesel burn is more heat cause of it's higher flash point. Now I'm sure this topic could go real deep, but this is as far as my knowledge goes. Oh yea, also the reason why you don't want to try and burn cold diesel. I'm guessing this is why the fuel is circulated like it is on our trucks.

 

Correct. I believe the ratio is 7:1, meaning the intensifier piston pressurizes the fuel at 7 times the oil pressure. So, at idle the fuel is forced through the nozzles at 3500 psi and up to 21,000 psi at WOT. I am sure a fair amount of atomization occurs at that sort of pressure, especially since the nozzle ports are quite small. I have no clue if atomization is the same as vapor though...

Last summer I had glow plug issues and had a hard time starting my truck in the mountains (7500 ft). Looked like the unburnt fuel was coming out the exhaust in vapor form...although I imagine it was a little of both and only the vapor made it all the way out the exhaust tip while any liquid was trapped along the way.

 

Atomization is NOT the same as vaporization (using the accepted meaning of the word atomization). From a huge amount of reading I have doing on this topic, I have discovered that the word atomization in diesel injection means in the range of 20-60 micrometers (1 micrometer = 1.0 × 10-6 meters) mean diameter of droplets as they emerge from the injectors. The smallest droplets vaporize but the larger droplets begin breaking up due to the drag in the high pressure environment within the cylinder. As the pressure builds (as the piston comes closer to TDC), the heat generated ignites the vaporized fuel, which in turn ignites the larger droplets that have not been vaporized yet. Vaporized fuel burns very efficiently and maximum energy is released, with less efficient release of energy from the larger droplets. It appears that the breakup of the droplets is highly associated with the pressure the fuel is under relative to the cylinder pressure and that may be why injector pressures have gotten so high (along with emissions requirements). The higher the pressure difference the faster the droplets travel which causes them to break up sooner due to drag, which means we are probably looking at pumps and injectors built to increasing tolerances and capable of higher pressure. The VW 1.9 TDI has injector pressures of almost 30,000 psi!

So I think I have my answer to the question: Do our engines burn a liquid or a vapor? The answer is BOTH! If we could somehow get all of the droplets to vaporize prior to ignition we would get better fuel mileage (due to more efficient energy release) with lower emissions, - a win-win! My thanks to everyone who shared their thoughts and I am off to think about this some more .

 

you nailed it. "flame front propagation" was a subject in bmw school.

the science behind 100hp per liter engines!

kurt