this post was submitted on 08 Sep 2023
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Using engine brakes can cause your car to not use fuel in some cases.
I've read and heard this from different sources (even driving instructors) and I don't get how it's possible. Your engine is still running, doesn't it use at least as much as it does while it's idling?
Edit: thank you all for your answers. I knew how the engine brake effect worked, my confusion was about exactly why the engine didn't consume fuel in the process. I now understand so thanks all.
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.
Look it depends on the age of the car, but let's take an old manual car for example.
On those cars, there's a fuel map to rpm. There's actually a few maps including throttle and ignition timing. But think of a spreadsheet of rpm and fuel at a certain throttle load.
At 0 throttle: The map says to stop the engine from stealing at under say 800 rpm it needs to have fuel added at rpms lower than that to speed up the engine to avoid stalling. At 800rpm it needs a consistent amount kind of a known amount that keeps it in equilibrium. At over 800rpm it needs less fuel the more rpm it has over the idle 800rpm until it's zero fuel.
And you'll feel that, you'll feel that moment the car starts adding fuel because if you're only engine braking to a stop your car will get near that idle rpm and your engine will start adding power to avoid a stall, and your braking will diminish.