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Maybe this will help... When the piston pressurizes air in a cylinder, the air temperature rises as others have correctly stated. How much it rises depends on the incoming temperature, the amount and the rate at which it is compressed. The quicker you compress it and the more you compress it, the hotter it gets. Well into the hundreds of degrees F. I don't have a chart handy but seems to me the shop manual for my 70D says as much as 500° F. Not real certain but I do feel safe saying in the hundreds of degrees. Of course if it's only 10°F and the cranking speed is low it's not going to be so high a temperature and that's why cold diesels can be hard to start. The rest of the story comes from the ability of the pump with the injector's help to "time" the injection. When the fuel comes out of the injector nozzle it is not a squirt of liquid or really much of a fine mist. It's more like a fog. We all know it doesn't matter what the fuel, the smaller the particles, the easier it is to burn it up because it more easily combines with oxygen and burns. That's why we split firewood into little pieces and call it kindling. Each fuel has what is called a flash point. That is the temperature at which it burns. Raise its vapors to this temperature in the presence of oxygen and it will burn. May not continue to burn until more vapor is produced but the vapors will light up. See an easy to understand table of flash points at www.engineeringtoolbox.com/flash-point-fuels-d_937.html. On that table the flash point of #2 Diesel is listed as 125°F. Now put all those parts together into the engine. The injector pump squirts liquid #2 into the injector. When the injector can hold it back no longer because the pressure has reached it's "breaking point" the fuel bursts out the nozzle into the combustion chamber as a vapor or microscopic droplets of fuel. Those microscopic droplets have a flash point of 125° F and there is oxygen in this chamber already heated to maybe 350 to 500°F well above their flash point and that temperatiure is probably still rising. Those droplets of fuel BURST into flame! Then the typical diesel "knock" is heard, smoke comes out the stack & we're ready to do some work! Doesn't make any difference if it's a 4 stroke, a 2 stroke or a pile driver. They all work the same. Squirt a little #2 through a small enough nozzle at high enough pressure into hot enough oxygenated air and fire will happen. Helpful? Hope so. Later.
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