I’ve heard people say that fuel injection automatically turns off when you let the car coast in gear, but if I completely let off the accelerator in first gear, the car will only slow down to 3-5mph (depending on the car) and will then stay at that speed indefinitely like a lil’ cruise control, suggesting that the car was always injecting fuel at engine idle ratio?
I understand this kind of question has been asked before, but as this element doesn’t check out, I don’t believe it has actually been completely answered.
In: Engineering
In a gas powered car (both fuel injected and carbureted) the throttle controls the amount of _air_ that goes into the engine. So lifting off the accelerator closes the throttle and cuts off the engine’s supply of air/oxygen. There’s an idle mechanism that reopens the throttle a little if the RPM falls too low to prevent a stall.
In a modern (~1980s+) fuel injected car, the “mass airflow sensor” measures how much air is going into the engine and then electronics determines the correct amount of fuel to inject. When the throttle is closed, that amount is zero. In modern cars, it’s all computer controlled, and can adjust based on load, temperature, RPM, etc. Older cars used simpler electronics or even mechanical fuel injection systems that just used the throttle position, and not the airflow directly, as input.
Carburetors work differently and are purely mechanical, but also provide fuel at roughly the correct ratio to the air being fed through them.
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