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
The car will only start injecting fuel again once the RPMs come down towards idle. Fuel injection is computer controlled, so it can take many inputs into account when deciding whether to inject fuel, and if so, how much.
When you take your foot off the accelerator. You are just telling the cars computer something.
Your pedal is connected to a computer, not the engine directly. (Old cars this is not true).
It is technically up to the computer what it actually does with the fuel and throttle bodies based on how much pedal and other factors like your speed.
Whoever programmed your car decided that at really slow speeds, no pedal travel still should add a tiny bit of fuel and air to maintain a low speed.
In Carburetted engines, there are two fuel circuits, one for idle which is always spraying minimum fuel adjusted by the idle control screw irrespective of throttle input and one connected to your throttle directly that increases revs proportional to input. I’m assuming the same is either present or at least emulated by the Engine ECU.
The engine still needs fuel to continue idling (to stay ‘on’) so the computer injects enough to keep it running. If you’re not in neutral i.e engine and drivetrain ‘connected’, this amount of engine output at that rpm will still be enough to generate some turning force through the drivetrain to the wheels. Low gear and low rpm -> low speed.
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.
In addition to the other answers — the car completely stops the fuel injection for an indefinite amount of time if it’s coasting downhill (and maintaining speed).
It’s called over-run fuel cut. When you take your foot off the accelerator, you’ll notice the rpms don’t drop at all (in a manual) or only drop a bit (automatic). This is because the vehicle is now turning the engine as it is still moving. In this situation, fuel is not required, so once throttle position reaches zero, the engine load goes below a certain threshold, and the rpms are above a certain number, the computer cuts fuel. As soon as any of those criteria are no longer true, fuel comes back. You can’t feel this in most cars because the thresholds are minimal, and engine load is next to nothing.
Well the other responses are mostly correct, but it shouldn’t really cut fuel completely or the engine runs too lean which causes problems like backfiring.
followup: Coasting down a long freeway hill, speed 60, tach at 2500 rpm or so, is the engine taking any fuel?
GM calls it “Deceleration Fuel Cut-Off” (DFCO) and uses these parameters with values unique to the vehicle model:
RPM: above 1400 rpm
Speed: above 40 mph
Coolant Temperature: above 30f
Engine Load for activation: below 30 kpa
Engine Load for deactivation: above 32 kpa
Amount of spark retardation: 20 degrees
Delay: 1000ms
KPA is from the Manifold Absoute Pressure (MAP) sensor that measure the vacuum/pressure in the intake manifold. MAP generally increases as the throttle is opened. Each engine model with have a specific MAP value at idle speeds, such as 35 kpa, which increases to around 100 kpa at full throttle (equal to atmospheric pressure outside of the engine).
Deceleration with the throttle closed creates high vacuum (low pressure) because the throttle is closed so the engine is having suck in what little air it can. This produces a MAP value in the 20 KPA range.
For example, If the vehicle is moving faster than 40 mph, the RPm is above 1400 rpm, and the MAP value is under 32 KPA, then the computer shuts off the fuel injectors (pulse width goes to 0.0 milliseconds).
When any of the conditions is no longer met, the computer resumes regular fuel delivery.
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