this post was submitted on 08 Sep 2023
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If you take an engine out of a car and try to spin it by turning the crank shaft, it will be hard to turn because the cylinders need to compress air (it's required before adding fuel and spark to explode that compressed air so it expands).
When that engine is in the car, and you don't add fuel and spark, then the cars wheels have to turn the engine and compress that air, thousands of times per minute. That force that the wheels have to send to the engine to spin that engine slows you down.
I'm thinking you think the engine itself has a brake on it.. No.
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 in a car without the ECU (car from the 50s?) you can’t engine break?
I'm not exactly sure how it worked in practice, but if it's anything like simple aircraft engines with carbs, there's a mixture control that you'd use in addition to the throttle to control air intake.