Gas engines must stay within a certain range of fuel/air ratio in order to run. Spark ignition means the mix must be rich enough but not too rich to be ignited. You can find charts that tell the explosive mix for fuels and if the mix is out of that range it won't light. Gas engines are set for 14.7:1 for the best compromise between power, mileage and emissions. They will run as rich as 12:1 or as lean as 17:1 but the rich mix makes soot and high CO and hydrocarbon emissions. The lean mix will make more NOx emissions and overheat valves and pistons. No fuel burns as a liquid...it must be vaporized to burn. Gas is injected on the intake stroke then compressed this allows it to vaporize. The flash point for gas is around -35F the flash point for
diesel fuel is a minimum of +125F (by federal law). Most is more like +140F or even +160. The diesel fuel is injected near the top of the compression stroke when the chamber is at +1000F or more. The fuel lights immediately from the heat. A small amount of fuel means little power (as in at idle) a lot of fuel means much more power. The only time fuel/air ratio matters with diesel is the max amount of fuel that can be injected with a certain amount of air. That will depend on engine speed and turbo boost. Fuel proprties charts list the mix as being 14.7:1 for diesel (by weight not volume). I know a guy who retired from GM's EMD division and he told me by volume anything richer than 25:1 on diesel would make a lot of smoke. Some engine makers list that spec and usually say 21to23:1. Diesel is backwards from gas...diesel gets hotter when you add more fuel and cooler when fuel is cut back. The only time lean can be a problem for diesel is if the engine as at full temp, full load, full boost, and full throttle...if the fuel is cut off suddenly then it can melt pistons nearly instantly.
That is why all the pulling tractors and truck use an air damper on the intake as an emergency shut off. Whan people mod a diesel high exhaust gas temps are a big problem. Adding more fuel with bigger injectors or remapped ECM's can over fuel and cause engine damage.
Bigger turbos are expensive but usually the only real solution to high egts from over fueling. Most diesel makers say a max of 1350F for
any length of time and 1250 for contiunous duty. If it hits 1500 or more you had better back off, change to a lower gear or if boost is below normal stop and see why it dropped. Most small light or med
duty diesels do have aluminum pistons. Most large heavy duty engines do have steel domes. Nearly all have oil spray cooling on the pistons. The light/med duty engines don't run as much boost and are not tuned for as much power. Modern diesels can make more than two pound feet torque for every cubic inch of displacement. Try that with a gasser! So, unless they are under hard throttle diesels are always leaner then the max allowable fuel/air ratio. All now have fueling based on boost pressure to prevent smoke during start outs and gear changes. Remember when trucks made a cloud off black smoke with every gear change? Now when boost drops the ECM cuts the fuel rate back below the max for that rpm and boost level.
I hope this sheds some light on your question.
|