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I *hate* to do what I'm about to do, and I've never done it on this forum, but it needs to be done: lay out my credentials. Credentials are an appeal to authority that leads participants in a discussion away from the merits of a case, but in this thread merits don't seem to be doing too well for the obvious reason that most of you participating in this discussion not only don't understand the science/engineering behind this, but you almost surely lack the education to understand that science/engineering. I'm slightly annoyed by the inability and/or unwillingness by some to be swayed by preponderance of evidence.

So, my credentials: I hold a degree in mechanical engineering. (Ohio State, class of 1999.) As part of that degree I was required to take a certain number of "technical electives" and mine were (1) internal combustion, (2) heat exchanger design, and (3) a graduate course in combustion science. That last item is the big cannon here; while I am not an expert in combustion science, I'm a lot closer than most engineers will ever be, let alone people without technical education. Further disclaimer: I do not now, nor have I ever, worked as a mechanical engineer in the field of internal combustion or combustion science. I'm just an educated guy with more than a passing interest in cars, who was lucky enough to use his education to further his interest.

Combustion science was hard. If you've got a college degree, especially a technical degree, you take dozens of courses in seemingly unrelated fields and you think "I'll never use these again, especially not together!" Then I sat down in the first day of combustion science and realized how wrong that was. To comprehend combustion science, you need detailed understanding of the following:

1. Chemistry: reactions, rates of reactions, heat of combustion, etc.
2. Heat transfer: convective and conductive.
3. Thermodynamics: energy and power balances.
4. Fluid mechanics, both turbulent and laminar flow.
5. Differential equations.
6. Linear (matrix) algebra.

Now, on to the questions. This is going to be long so if you're not into tomes, just move to the next post.

(1) How much better is fuel vaporization than fuel atomization?

Key to Understanding #1: the Perfectly Stirred Reactor
Combustion reactions thrive best in a vessel knows as a "perfectly stirred reactor" or "PSR". In a PSR, the fuel and oxidizer mixture is 100% uniform and also in perfect ratio. Literally, there would not be one extra molecule of fuel or oxidizer out of balance, and every fuel molecule would have the perfect number of oxidizers around it so that when the flame front swept through, the only thing that happened was pure chemical reaction at its maximum theoretical rate. This creates maximum process temperature for maximum useful work. Let's say that the PSR has the perfect ratio with the perfect distribution and that both are required.

Key to Understanding #2: Heat of Reaction
There is a maximum amount of heat energy that can be released by any combustion reaction. This amount is dictated only by the quantity of fuel involved; no other consideration is required. So for quantity X of gasoline, it can offer quantity Y of heat useful for work in an engine.

Key to Understanding #3: The Second law of Thermodynamics
The laws of thermodynamics are known to govern the world of Newtonian mechanics insofar as energy flows and balances are concerned. The first law says, "You can't win" meaning that you can't get more energy out of process than you put into it. The second law clamps down even more tightly, saying "You can't break even". This means that you can't get even 100% of the available energy out of a process; you are doomed to some efficiency below 100%, with the theoretical maximum efficiency being a factor of the temperature differences at the ends of the process. For internal combustion, this is in the 60's for gasoline. Practically, it won't even be half of that. This means that 10x gain is ridiculous; a 2x gain is probably the most for which you could ever hope, even that's ridiculous.

So, how much better is vaporization than atomization? First, let's talk about why it's better.

It's better because liquid fuel fails both the ratio and distribution requirements for the PSR. Liquid fuel, by definition, cannot locally be in the correct ratio with its oxidizers since liquid fuel contains kabillions of fuel molecules in a single drop that will have far too few oxygen molecules around it. And if it's out of local ratio, then it is also out of perfect global distribution. The net result is less total heat released by the reaction, released more slowly, and with incomplete combustion.

Vaporized fuel could be much better. But before you way, "A ha! I knew it!" we aren't done. Let's suppose that you have vaporized 100% of the fuel. Is this sufficient to make a PSR? Not remotely! That vapor still has to be mixed with the oxidizer to make a perfectly uniform distribution. This is extremely difficult to do in a piston engine because the flow around the valves and down into the cylinder is difficult to control.

So, how MUCH better is this legendary vaporization than atomization? Real-world answer: only marginally. It may be a few percent better than the old carburetor used to do; 10x gain is dismissed with raucous laughter by any knowledgeable engineer the instant that figure is quoted. Why only marginally? Consider that the difference between the two would be influenced by the following factors:

1. Size of atomized fuel droplets. Obviously, the smaller the fuel droplets, the smaller the advantage of vaporized fuel. Carburetors released fairly large droplets into the intake. But guess what? The intake had heat and turbulent motion that began the evaporation process before the mixture reached the cylinder. It turns out that carburetors weren't THAT bad insofar as fuel droplet size was concerned.
2. Degree of mixing in either case. Tumble and swirl provide an advantage for BOTH vaporized and atomized fuel. All intakes provide some degree of tumble and swirl while some are better than others. Since vaporization is not a guarantee of mixture, it turns out that its advantage is further diluted since both benefit from mixing.

Conclusion: You can get closer to the PSR with vaporization. You can get closer to the theoretical maximum heat of combustion with vaporization. And you can get closer to the maximum theoretical cycle efficiency, but not much.

The author then quotes a question posed by another member of the forum:
There are always two answers to a question like that. Either it's not true, or it costs too much. In this case, it's not true. Now you know why.

The author then quotes another comment posed by the same member:
Wonder no more. They have been, for more than 30 years. Fuel injection is an exercise in fuel vaporizing technology. Let's trace FI's development to see how this is true. Remember: fuel droplets vaporize in the intake even if you didn't intend for them to do that.

Throttle body injection, or TBI. The fuel injector replaced the venturi in the carburetor and injected liquid fuel into the intake air stream near the throttle plate. Instead of being drawn by a vacuum, the fuel was injected in a spray pattern under pressure. The net result of this was twofold. First, air/fuel ratio control was far superior when electronic control when lambda feedback was employed. Second, the fuel droplet size could be easily controlled (read: probably smaller) and those smaller droplets could more quickly vaporize in the intake, if only partially. Result: Improvement.

Multiport fuel injection, or MPFI. In this setup, there is one fuel injector aimed at each intake port. In most applications, MPFI was batch fire, firing once per crank revolution. This gave each intake valve TWO squirts of fuel for each intake event, and for most cylinders, the fuel spray was directed at a closed intake port. This represented a magnificent leap in fuel control. A major contribution was greatly improved fuel vaporization because the fuel was shot (a) under higher rail pressures for decrease droplet size, and (b) at a closed AND HOT intake port. Misted gasoline, when sprayed into a hot and closed intake port, vaporizes very rapidly. In fact, for most operating conditions, the degree of vaporization for MPFI is shockingly high.

Sequential port fuel injection, or SPFI. Architecturally, SPFI is just like MPFI in that it has one injector per intake port. Operationally, it is different because each injector is fired on its own, with the timing of the firing relative to its own cylinder's position. If you shoot the fuel at the closed and hot intake port at the right time, you can get good, uniform vaporization for all the cylinders.

Direct injection, or DI. DI injects gasoline straight into the cylinder. The pressures at which it does so are very high. Injecting the fuel into a very hot (because it's being compressed) and turbulent intake charge allows the gasoline to vaporize AND mix very quickly. It's a good way to get closer to the PSR ideal I described above.

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