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This is a question more to satiate my curiosity than something I want to apply in practice. It's also my first question here, so apologies in advance if I'm doing anything wrong.

I've recently started driving a car with a manual transmission, and while learning and doing some research on things like how to drive on efficiently, one topic gets brought up a lot, engine braking.

Everything here is what I know from my own research, so if I have something wrong, let me know. So from what I know, engine braking in a manual lets the wheels keep the engine spinning and all or almost all fuel is cut from the fuel injectors, which saves on gas.

I got curious as to how this works with an automatic and looked up how a torque converter works, and from what I've read, a torque converter only allows torque to be transferred in one direction so that stopping or rolling back in an automatic won't stall the engine, right? But a lot of automatics have a manual mode or a way to select lower gears, and when in those lowers gears, I do feel an effect similar to engine braking, in that my car will start slowing down and the revs jump up, does that mean the wheels are working to keep the engine spinning even while the throttle plate is closed? This doesn't really match up with how I think a torque converter works so I got curious but couldn't find much out of a google search.

Is the effect of engine braking in an automatic perhaps simulated by a computer? Or is there a way for the wheels to drive the engine in an automatic (and possibly save gas)?

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You can think of a torque converter as a essentially straight shaft under most circumstances meaning that it holds both sides at the same speed. It's only when the car is moving slowly or stopped that the converter "decouples" input from output to prevent the engine from stalling and to allow a smoother start and stop.

Most torque converters today are "lockup" converters which means that above a certain speed they literally lock input to output so that the slip goes to zero. That's because in practice a real torque converter does "slip" a little due to the way they work and the lockup eliminates that.

In some cars you can actually hear a "clunk" when the lockup happens but today most are pretty quiet.

So back to your question, for the most part an automatic transmission and a manual do the same thing when you let off the gas. The engine gets no fuel and the car tends to brake due to the engine loading the drivetrain. The energy that normally would heat up your brakes ends up in the engine and out the radiator.

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  • For the most part, I agree with what you're saying here. I do have contention with a few of the finer points, though. First, a lockup torque converter usually only locks up when certain conditions are met and the vehicle is in the highest gear. I'm not quite sure where you get the idea the engine heats up during engine braking? There isn't much heat produced during engine braking. There is some which occurs during the compression cycle, but that is pretty much negated on what would be the power cycle, any extra going right out of the tailpipe on the exhaust cycle. – Pᴀᴜʟsᴛᴇʀ2 Nov 20 '19 at 21:34
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    @Pᴀᴜʟsᴛᴇʀ2 I think he's just referring to conservation of energy. I think I'd agree with you that a significant amount of the energy dissipated is from pumping losses (intake and exhaust), but depending on the engine, a lot can be lost to friction, and thus is converted to heat. – Kitsunemimi Nov 20 '19 at 21:43
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    @Pᴀᴜʟsᴛᴇʀ2 A significant percentage of the energy dissipated in engine braking is transferred into heat during the compression cycle where it then moves into the coolant and out the radiator. Since the compressed air has now lost some of its energy it doesn't return as much during the expansion cycle. The rest is basically lost to friction. – jwh20 Nov 21 '19 at 1:12

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