the reason Diesel engines are injected is not only the lack of vacum, but the fact that the heat of the air in the cylinder is what actually ignites the fuel. No spark plug. the injector (in one without glow plugs) is where the spark plug would be. glow plug diesel engines have a 'pre-combustion chamber' inside the head where bothe the injector and the glow plug are at. the glow plug is only used before the engine is cranked.
the fuel is injected right before TDC and explodes, forcing the pison down. this is also why diesel engines are built so much 'sturdier'. they have more compression pressure, and stronger 'shock waves' to deal with. pisons and cylinder walls are built more like modern gas racing engines. high nickel steel blocks instead of cast iron. forged aluminum pistons instead of cast. forged steel rods instead of cast iron. forged steel crank instead of cast iron. things lke that.