Quote:
Originally Posted by GreenHornet
Supercritical fuel injection
Link = Supercritical Fuel Injection | MIT Technology Review
Wow seems pretty high tech 
In a nutshell they are preheating the diesel before injection. They heat the diesel to 450 degrees Celsius under high pressure. They say they achieve it by utilizing the engines exhaust.
To me I think this would be difficult to achieve sustained high temps consistently. Some kind of electrically controlled flash boiler or transducer may be needed for system reliability. One would also have to be careful of starving the diesel engine. To me this would be like the ultimate way of leaning out the engine. We all know when you lean it out to much on a diesel it just dies on you! However by utilizing water/meth injection in conjunction may provide the needed fuel boost for steady reliable operation under load. Just up the meth to water ratio to improve combustion and prevent water lock.
I have heard of quite a few people trying to do this with gas cars as its much easier to vaporize gas fuel. However most people have mixed results as its supposedly hard to control and obtain the right air/fuel mixtures under the various driving conditions. To me the system would need to be variable in its operation to be effective. It would have to be able to adjust to the varying engine loads to be effective.
The other side similar to this supercritical fuel injection is fuel evaporation. Again used in gas tech and would not be applicable to diesels as they do not evaporate near as easily as gas.
Here is a link to one such unit I came across some time ago that seems to work reliably = 1st AGS Gasoline (Manufacturer of the Fuel Vaporizer MPG System) Carburetor Vaporizer Fuel Adapter
I think the term supercritical is simply a fancy term for vaporizing the fuel and is something that has been experimented with for along time. I think this is on the lines of the holy grail to supreme fuel economy. Supreme fuel economy would be in my opinion getting over 240mpg. It is encouraging that somebody is trying it with diesel because to this point there has been little attention given in this area of potential gain for diesel technology.
This research sets the stage for catalytic fuel cracking and is the holy grail of supreme fuel economy  If a person was able to achieve this in an diesel or gas auto of average size they would be able to easily achieve 200+ mpg figures However we probably would not see them around to long LOL!
So some interesting concepts to think about as it pertains to increasing diesel engine efficiency.
GH.. |
The MIT and Transonic research is not just simple heating of the fuel for the sake of improved vaporization ( layman's terms ). Super Critical is a term used to describe the point in a standard phase diagram where the three states of matter come together. For diesel fuel, this is at several hundred degrees Centigrade and several thousand pounds per square inch pressure. But, one cannot just simply heat and pressurize the fuel to the critical point as time is a factor. At those elevated temperatures and pressures, carbon forms into solid deposits within Milli-seconds. Transonic has gotten around this by preheating the fuel to sub critical levels and then using an electrically heated injector pintle to add the last bit of heat at the very last moment.
This exercise is not just to get better vaporization. It changes the combustion profile of diesel.
In standard diesel injection, the fuel has a slight heat release delay as the shear caused by the fuel velocity forms small droplets that start to vaporize and form combustible mixtures which then can oxidize and start the flame. The heat released follows a curve and so does cylinder pressure as more fuel is fed into the flame front. Droplets do not quite completely evaporate and thus form carbon solids ( soot ) even with modern 30K psi injection systems. Yes, the high pressure common rail systems reduce soot, but they just produce smaller amounts of it in smaller sizes.
With super critical diesel injection, the high pressure plus the high entrained heat cause the fuel droplets to not just evaporate and start combustion. The fuel droplets do evaporate but combustion is delayed as the fuel has enough enthalpy ( energy ) to add to the hot air and the kinetic energy ( more enthalpy ) and endothermically produce dissociated combustion sub species. Some people call these sub species combustion precursors. But, in reality, they are part of combustion. They are the left overs after the long chain carbons (dodecane - a major component of diesel ) have been broken up. They include very reactive hydrazine and hydroxyl molecules as well as others. This reactive cloud penetrates deeply, mixing into the hot oxygen rich air absorbing energy until it spontaneously starts combustion in full. The flame front is large and broad in comparison to the narrow one found in standard combustion. Practically all the fuel is burned as droplets have additional time to evaporate and very little soot is formed. Also, due to the rapidity of final combustion, Nitrogen has not the time to form NO as it is much more "lazy" then the more reactive Carbon, thus reduced NO is a byproduct.
The combustion delay followed by a sharp heat release means less negative work (pressure rise before top dead center - TDC ) and more positive work ( pressure increase after TDC ). This results in more power produced for the same amount of fuel. Transonic is reporting greater than 50% thermal efficiency at partial power output. This compares favorably with the very large diesels found in ships and power production.
You mention catalytic cracking as a means to "super fuel efficiency". But, in essence, that IS what is taking place within a super critical injection system. Yes, we could save several tenths of unity of lost energy by performing the cracking outside the combustion chamber via recycled heat and catalyst chambers, but you would just push the 60% thermal efficiency level. Apply such an engine to my 4200 pound Dodge Hemi Magnum and I would go from 25 miles per gallon (mpg) to above 50 mpg. I would be VERY happy with this but it is NOT "super fuel efficiency". However, apply this engine to the Vetter style streamlined bikes or some of the slick constructs on this forum such as OLD MECHS streamlined tadpole and you may just get your super fuel efficient vehicle!
And on a side note, I was one of those college kids in the 70's who bought plans from the back of Popular Mechanics and built vapor carbs and so forth. I built GEET type reactors as well as HHO generators. I know how they work and how they don't. After several undergraduate degrees in science and engineering and beyond, I am still here. The government does not do away with people who have knowledge of new technology, they hire them! There is no great conspiracy.