This difference in compression leads us to all of the other differences between gas and diesel internal combustion engines. Take spark, for instance, or "ignition" as it's called in the field because it's what ignites the air-fuel mixture in the combustion chamber of an engine. A gasoline engine has a spark plug that is installed in the cylinder head. The tip of this plug makes an electric spark right in side the chamber, at exactly the right time so that the air-fuel mixture explodes and forces the piston back down to the bottom of the chamber. Here comes the big difference -- diesel engines don't have spark plugs. Rudolf Diesel knew from his studies in thermodynamics that if he could compress the air-fuel mixture enough, like 500 psi enough, he could get it to explode without an external sparking mechanism. Modern diesel engines do have what's called a "glow plug," which helps the engine run more efficiently even when cold, and helps the engine to start, but once it's going the engine has enough internal heat and compression to keep running. Rudolf Diesel also knew from his studies that a diesel engine would be many times more efficient than other engines, especially the popular steam engine which loses a huge percentage of its energy to lost heat via escaping steam.
There have been countless advances to diesel engines since they started being used in cars and trucks. Diesel reliability is amazing, with engines getting 500,000 miles without a rebuild on a regular basis. Turbocharging has given diesel engines more power so that cars and trucks will have better acceleration. Direct injection has made them run much cleaner than the smoky messes we saw in the 1970s. Diesel fuel prices have been on the rise for years now, so it's unlikely we will see many more diesel developments, but the diesel engine's place in history has been and continues to be very important.