system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do different engines need different fuels? What is the difference between these fuels?
gpt
So, you put fuel into an engine and the engine then also takes in air, and then either the spark plug sparks it, causing the fuel to combust OR with diesel fuel, compressing the fuel causes it to ignite on its own. The difference in octane rating (89, 91, 93 etc) is that the higher the rating, the less it "knocks," or, basically combusts too soon while it's being compressed inside the piston. In higher performance vehicles, the higher octane fuel is needed for "timing" - the turning of the crankshaft in time to the combustion of the pistons as the system turns around. Basically, octane ratings relate to how soon the fuel combusts inside the piston. In real time, we're talking about milliseconds difference, but this timing makes a HUGE difference over the life of the engine. If you ran 89 octane in a vehicle that needed premium, you probably wouldn't see a huge difference immediately, but over time the valves, cams, pistons - everything associated with combustion and timing - would be thrown off and would wear funny. Not only  that, but you would start to get that characteristic pinging or knocking sound that comes from premature firing.