So What Does Gasoline do to a Diesel Engine?

The issue is multi-faceted. It is a function from the completely different burn characteristics in the fuels (volatile and mind blowing gasoline versus high adobe flash point diesel fuel), and also the peculiarities of engine style in regards to how fuel is actually ignited (spark ignition compared to compression ignition). Gasoline will be formulated to resist auto-ignition in a spark engine (see octane), so this fuel released into a diesel engine possibly won't ignite or will certainly (much more likely) spark at the wrong time leading to severe detonation. Though diesel powered engine reciprocating components (pistons, wrist pins and linking rods) are built to withstand massive explosive force, the surprise wave effects of uncontrolled detonation can easily destroy them.

Besides the specific burn characteristics which diesel fuel and biodiesel afford to compression combustion diesel engines, the energy itself acts as a lubrication for the fuel pump as well as delivery system as well as the device train (recall that diesel-powered fuel is actually a very lighting oil). Running thin, lower viscosity gasoline through a diesel engine fuel system would deprive it for lubrication and also cause those sensitive elements to rub together (metal-to-metal) eventually destroying them.
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