Does shifting conventional automatic to N at standstill decrease fuel consumption and improve torque converter lifetime? [duplicate]

Question

Let's consider a car with a conventional automatic transmission including torque converter. When at standstill, it is possible to have the transmission either at "D"rive or "N"eutral. My understanding is that when at "D", the input shaft of the torque converter is spinning. This in my opinion wastes some fuel (because the input shaft of torque converter is spinning but the output shaft is not, and therefore, there is some power loss in the torque converter). Also, in my opinion it should lead to a reduced torque converter lifetime because the torque converter is spinning and slipping unnecessarily. Of course, because this is not a friction device like a clutch, continuous slip should not be catastrophical.

According to this reasoning, one should shift to "N" when at standstill, especially if the car does not have a start-stop system that shuts off the engine at stoplights. Is my reasoning correct? Any idea how great the magnitude of these two effects (fuel usage, torque converter lifetime) is? Or is it actually the case that the longevity of the shift lever or the transmission is reduced so much that it doesn't make sense to shift between "D" and "N"? My expectation is that the torque converter lifetime is probably a negligible effect, but the fuel economy effect may actually save you some money on fuel costs.

Of course, the answer to this question may dramatically change if the car has some new type of automatic-like transmission. For example, on Toyota hybrids you don't want to change to "N" because doing so depowers both motor-generators and thus eliminates the only possibility of charging the high-voltage battery pack. Also, on dual clutch gearboxes, both clutches are disengaged when at standstill, and therefore, there is no extra load on the engine. However, on conventional CVTs, there is usually a torque converter, and therefore, the answer should apply.

Somewhat similar question: Automatic transmission: Is shifting to neutral while approaching stops bad? ...although this question does not discuss the effects of fuel consumption, only briefly mentioning it.

Answer 1

Well, I am guilty of shifting to Neutral when stopped or stopping. I've been doing it for years and I can say that it had absolutely zero effect on my gas mileage. When in D while stopped, the engine RPMs are lower (because the transmission and brakes are applying a load on the engine), while in N, the RPMs are always a little bit higher because the engine has no load against it. The fuel consumption of lower RPMs with a load vs higher RPMs and no load comes out in the wash. Besides, how much time do you spend at traffic lights? It feels like a lot, but comparatively, it really isn't. Most of your fuel consumption comes from accelerating the car once that light turns green.

So why have I been shifting to N while stopped or stopping for years? I find it easier to stop the car (I have better brake performance) with the tranny NOT trying to propel the car while I'm trying to stop. I also don't like the feel of the engine load under my brake pedal when stopped. I'm sure all these little things add-up as wear on the driveline, but do keep in mind, all that extra shifting will put extra wear on your shifting cable. I busted one last fall right after hitting 300,000 km.

Answer 2

The torque converter is always "spinning" when the motor is running, it is connected to the motor's crankshaft using a flex plate, and, the converter drives the transmission pump.

The process of putting into neutral is internal to the transmission by disengaging internal clutches and friction plates but the converter still "spins".