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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.
Usually your engine uses fuel to turn your wheels.
When you engine brake, your wheels turn your engine.
the inertia from the cars current speed is used to spin the engine rather than the engine spinning the wheels, the resistances of the engine is what causes the slowing effect
modern cars with fuel injection can complete disable injecting fuel when not needed and can also adjust the timing of the injections for maximum efficiency
For the car to be "running" in those cases, the engine just needs to be turning to keep the alternator and potentially the power steering pump going. When engine braking, the rotation of the tires is locked to the rotation of the engine, so the inertia of the car keeps the engine turning without needing to use fuel.
Depends on the ignition system and everything of course, but it can be true.
And don't engine brake a two-stroke engine, as fuel is mixed with oil. No fuel, no oil... No more engine for you.
The fuel injectors are off when engine braking. It is the momentum of the car that keeps the engine running/rotating. That is why you are slowing down more rapidly because you're losing momentum into the engine.
It's not putting fuel into the cylinders. So it's spinning, but there are no explosions.
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 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.
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.
The most difficult part lf stanrting a car is gettinf the pistons to move, if your engine still has inertia (if you are going downhill for example) you can completely cut the fuel injection and it can starts again because the pistons that are still moving will compress the gas (and for diesel engine that's enough to ignite given enough temperature in the block, for gasoline the spark plugs will work as usual).
Of course if the engine has low inertia (it's spinning too slowly) the car will stall, but probably the electronic injection will compensate.
If you drive manual you can go down a hill without burning a single drop of fuel, not sure if automatic are smart enough to do it.
In my car with automatic CVT transmission, I have to downshift using the paddle shifter in order to force it to do engine braking. It doesn't do any engine braking if I simply let the gas pedal go.
It is probably still "engine braking" in that the engine is putting resistance on the wheels, but it's probably opening the throttle and halting fuel injection in order to maximize your coasting/minimize rolling resistance.
CVTs usually use a clutch rather than a torque converter, so they like to stay "locked up" to minimize wear.
I think. Not a mechanic, and I drive a manual.
In my car (it has a cvt transmission with a torque converter instead of a clutch), it doesn't seem to do any engine braking though. In a steep downward incline, if I let go of the gas pedal, the car would just keep getting faster and faster without any resistance at all. Force a downshift and acceleration will stop, and the fuel efficiency indicator shot up which indicate it doesn't use fuel at all (thus doing an engine braking).