Confused about using no gas when braking

[DarkNinja75]

I've heard this mentioned a few times and I'm trying to make sense of it. Apparantly, when you downshift and brake, your ECU tells your fuel injectors to stop feeding gas to your engine (I think), so the friction between all the components help slow you down. If that's about right, when you come to a complete stop and are idling (in a MT), does your ECU tell your fuel injectors to give just enough gas to keep the engine going?
It's confusing me because I thought the only way to keep an engine going was constantly having the gas ignite inside the pistons. While slowing down it's understandable that inertia would keep the engine going. But at an idle it seems like it would have to be receiving gas to stay running.

[dakmor]

Seems an awful lot of engineering just to stop a car when you have brakes for that.

I doubt it's true.

[twin88cam]

ecu feeds your cylinders enough gas so it can idle down when down shifting, if the ecu fed no gas, your rpm's would drop almost instantly, would be like turning your ignition off while in gear at 3 grand

[Piloter]

If the ECU didn't cut fuel, it'd still be injecting at the same rate, right? That would be the equivalent of keeping your foot down and still trying to brake/downshift. Wouldn't work too well to slow down when you're trying to speed up, now would it? Hence fuel cut...

[Dragonmaster]

The injectors are throttled back to provide enough fuel to produce the minimum idle speed when you let off the gas. They do not totally stop providing fuel.

[toyotechwv]

Throttle Position Sensor

[94_Rolla_Guy]

The fuel delivery is proportional to air delivery, simply put the ECU adds fuel in the ratio of approximately 14.6:1 to the air regardless of anything else.

Your throttle body which is connected to the gas pedal is the air valve so from your input to the throttle governs the air, the amount of air governs the amount of fuel delivery and the air and fuel delivery are directly related to power, now take into consideration

Constant speed = balanced power output to overcome all forces of drag and friction

Acceleration = More power output than all forces of drag and friction

Decelleration = Less power output than all forces of drag and friction

when you decelerate the engine is making less power output than all forces of drag and friction and the process reverses the wheels turn the engine and an engine basically being an air pump, the engine makes hi vaccuum trying to get more air but can't, it burns only enough fuel air mix to maintain idle and thats what slows the car down

there is a little more too it than that but thats the basic idea

Try this, go up to 30km/h or so in second gear (1st is to jerky) then take your foot off the gas the car will slow (Decelleration = Less power output than all forces of drag and friction) to idle then maintain about 5km/h it maintains 5km/h because, Constant speed = balanced power output to overcome all forces of drag and friction

[Donald]

Remember that when you are braking you should have taken your foot off the accelerator pedal, therefore the throttle valve is closed.
The throttle position sensor tells the ECU that the throttle is closed, so the ECU in turn tells the injectors that the engine needs less fuel.
The O2 sensor then tells the ECU what the burn ratio is, and the ECU regulates the fuel even more to minimise unburned fuel in the exhaust. At this point the combustion pressure is just about insiginificant.

While all this is going on, your engine is experiencing what is called compression braking. The pumping action of the pistons in the cylinders helps slow the car down.

[Dragonmaster]

Originally Posted by Donald

Remember that when you are braking you should have taken your foot off the accelerator pedal,....

Unless of course you are a great standard driver and heel toe shift to match the rpm's to your transmission when downshifting for a corner and braking at the same time thus causing less wear on your clutch and being ready for maximum acceleration when hitting the apex of the corner.

Welcome to driving school!!!

[Donald]

Originally Posted by Dragonmaster

Unless of course you are a great standard driver and heel toe shift to match the rpm's to your transmission when downshifting for a corner and braking at the same time thus causing less wear on your clutch and being ready for maximum acceleration when hitting the apex of the corner.

Welcome to driving school!!!

Touche!
I was thinking automatic.

[toyomoho]

The engine would have to go back to idle mode (or close to it). Some Toyota’s and other manufacturers engine setups have a vacuum dashpot that moderates the idle down when the gas pedal is released. Part of the emissions system.

[91gtssilvertop]

its called decel fuel cut. when your decelerating with your foot off the clutch pedel in gear, the inertia from the moving vehicle drives the engine through you gear box keeping it spinning. there is no need for fuel at this point because the engine will keep turning whether fuel is present or not. Once a predetermined RPM is met, fuel injection will kick back in to keeep the engine running at a lower speed and to prevent stalling. Its just a way to keep emissions down and increase fuel economy that little bit.

[Donald]

Originally Posted by toyomoho

The engine would have to go back to idle mode (or close to it). Some Toyota’s and other manufacturers engine setups have a vacuum dashpot that moderates the idle down when the gas pedal is released. Part of the emissions system.

The idea behind this was to avoid a sudden rich situation when the throttle plate closed. The dashpot held the throttle open based on manifold vacuum.

[Flashmn]

Unless of course you are a great standard driver and heel toe shift to match the rpm's to your transmission when downshifting for a corner and braking at the same time thus causing less wear on your clutch and being ready for maximum acceleration when hitting the apex of the corner.

Initial D!

[twin88cam]

i tried hell toe first day had my car, cost me a clust cable and my 1st gear plate