Thermal Efficiency Discussion

[1project2many]

We do not have tailpipe emissions testing here. All emissions testing relies on OBDII monitoring and reporting. I have not found any long term negative effects from using cetane improver over the last 10 or so years. The recommendation to use Boric acid came from a chemist who once worked in the oil industry. Most easily obtainable forms of Boric acid are not soluble in gasoline. So I use a particular brand of cetane improver which contains such chemical.

from what i understand with diesel, there is a very short delay after the injectors open and start flowing fuel before the fuel ignites. being compression ignition, they have a comparatively small window that they can start injecting in without risking there being enough heat in the cylinder to ignite the fuel. this window would be pretty centered around TDC with maybe some slight skewing towards the ATDC side. i would assume compression ratio dictates window size to a large degree. in a simplified and slightly inaccurate sense, the injectors inject fire. i have no idea how long combustion would still be taking place after the injector quits flowing fuel, probably dependant upon AFR? in any case, fuel that combusts(and gives pressure rise) after 90* ATDC would give diminishing returns, though would still give more output as long as there is still oxygen left in the cylinder to combust with.

Diesels have come a long way and I'm not up to date with the latest tech but the old engines with a prechamber used timing curves that aren't that different from what we expect in a gasoline engine today. The diesel in my '82 mercedes has injection at idle set to begin around 24 deg BTDC and advances from there. The reason the mixtures have such a range is because of the way fuel and air are mixed. In the gas engine, a lean mix means fuel molecules are farther apart in the chamber. While you may initiate a reaction, you will not carry one across the chamber. In the diesel there's a high concentration of fuel in the area of the injector even if it's a very lean mixture. Honda was able to run very lean mixtures in a carbureted engine using a similar idea by using stratified charge. Look around for CVCC head design and theory.

if you were using a spark ignition, direct injection engine(note no mention of gasoline), you don't have to rely upon compression for the heat necessary to start fuel combustion. with a fast enough flame speed and an injector that was capable of flowing huge amounts of fuel in a very short amount of time, if the injection event started(and having a spark event VERY soon after injection begins) say at 75* ATDC and was completely done by 85*(and combustion stopped shortly after 90*), i don't think it would be of consequence if the fuel auto-ignited? this isn't really a good example due to completely ignoring engine speed, which is probably its downfall, along with unrealistic flame speed with conventional ignition systems, common fuels and the injector flowrate required.

Sure it would be of consequence. Look at some possibilities from unintended autoignition immediately upon injection. You'll have a highly concentrated charge with the outer portions in contact with O2 and the inner portions O2 starved. You'll alter the burn rate and peak pressure changing specific output and efficiency of the engine. You'll create excess carbon and other byproducts of incomplete combustion. And you might sustain damage to the injector if it's expected that combustion will happen away from the injector tip. If the system is designed to inject then combust, you'll have unexpected results. Maybe we could cause it to operate in diesel-like fashion instead?

I seem to remember reading about liquid propane injectors being developed that were intended to inject air and fuel at the same time. Maybe the idea was to use the air to add heat to the fuel and help with phase conversion. But mixing air and fuel during the combustion event imo is the best way to ensure a homogenized combustion environment. It also puts us closer to getting rapid combustion ATDC and can offset reduced cylinder pressure at TDC achieved when using other than Otto cycle. Another effect is there's less time for heat to conduct to the block and piston which means more energy can be used to drive the crank.

Oil burners today are rated from 85% to 100% efficiency. Steam is still an excellent medium for transferring heat energy. Maybe the days of external combustion aren't gone yet?