this post was submitted on 08 Sep 2023
379 points (96.8% liked)
Asklemmy
43812 readers
920 users here now
A loosely moderated place to ask open-ended questions
If your post meets the following criteria, it's welcome here!
- Open-ended question
- Not offensive: at this point, we do not have the bandwidth to moderate overtly political discussions. Assume best intent and be excellent to each other.
- Not regarding using or support for Lemmy: context, see the list of support communities and tools for finding communities below
- Not ad nauseam inducing: please make sure it is a question that would be new to most members
- An actual topic of discussion
Looking for support?
Looking for a community?
- Lemmyverse: community search
- sub.rehab: maps old subreddits to fediverse options, marks official as such
- [email protected]: a community for finding communities
~Icon~ ~by~ ~@Double_[email protected]~
founded 5 years ago
MODERATORS
you are viewing a single comment's thread
view the rest of the comments
view the rest of the comments
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
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.