The main difference between the internal combustion engine and the diesel engine is that the diesel engine

Internal combustion engine use a spark to ignite their fuel mixture.  diesel motors use extreme heat and compression to cause the fuel to ignite.  since the compression ratio is significantly higher in a diesel engine, the air will have very high heat when compressed due to the fact that as you decrease the volume of a cylinder without decreasing the mass of the material inside, both pressure and temperature rise. as the piston approached top dead center, the temperature and pressure are so high that the diesel fuel will 'self-ignite" as it is injected into the cylinder. the fuel does not need to be mixed with the air during compression and ignition does not need to be forced by a spark plug because the fuel ignites as it is injected. thus, this is the main difference between the two. 

Rain is pulled by gravity which is a non contact force