I was just searching for an answer to find how the idle speed of an engine is decided and who makes that decision. Is it the engine designer alone or is it based on the inputs from all the vehicle components (e.g. transmission, engine accessories including the engine)?
The more I searched, the more I got confused.
The engine has to be able to sustain operation smoothly (enough power to run itself and the gadgets you may have attached to it). It should be in a reasonable range for the accessories themselves at normal load: too low and the alternator won't charge, so you would run the battery down while idling, too low and you won't get enough oil pressure. Now, accessories can be made to run lower, but then you sacrifice the high RPM ability, which then causes a reduction in potential horsepower. It's all a big balancing act that usually ends up at a 750 RPM range idle for a typical car, sometimes a bit lower for a big engine, and sometimes up to about double that for small engines.
