this post was submitted on 08 Sep 2023
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Of course not. I know it's not an actual brake but it comes from the engine's resistance to spin on higher rpms, so when you shift to a lower grear the rpm goes up, which "activates" this resistance.
What I'm confused about is the relation between idling and engine brakes.
Even without giving it additional gas the engine is still idling, so on a level road you could travel with a certain speed without pressing the gas pedal.
So what happens when you're going downhill, you don't press the gas pedal and the engine brake effect kicks in? Does idling not consume fuel anymore?
I think I'm missing some information that would put everything in its place for me.
When you're engine braking—like when you downshift and let off the gas—the ECU often cuts off fuel to the cylinders. The throttle valve is also closed. In this scenario, your RPMs are maintained by the car's forward motion, which is connected through the drivetrain back to the engine.
So yeah, you're not using any fuel in that case, but you're still turning the engine over. The wheels are essentially driving the engine instead of the other way around. That's how you can have RPMs but no fuel flow during engine braking. The energy to keep the engine turning is coming from the car's inertia.
A common example would be going downhill. You downshift to a lower gear, take your foot off the gas, and let the engine do the work to help slow you down. You'll see the tachometer showing RPMs, but fuel flow is minimal or even cut off, thanks to our friend the ECU.
So basically because you have more than enough rpms to maintain idling the engine knows to turn off fuel injection until it needs to exert force again?
I'm thinking of a scenario when you start on a level road, reach a slope going downhill, then reach a level road again. Then the engine first consumes fuel, then it shuts it off, then eventually on again, without me pressing the gas pedal at any point?
yeah, exactly. I don't know enough about the implementation details to know if it is actually consuming 0 fuel though but there's not much work the engine is actually doing.
Great, this explains everything, thank you