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It is generally accepted wisdom that it is a bad idea to coast or glide a car equipped with a conventional hydraulic automatic transmission. Coasting (running with an idling engine that is disconnected from the drive train) is said to be at best unnecessary, and at most causing increased wear. Gliding (running with the engine turned off) is said to damage the transmission due to lack of lubrication because of the oil pump not working. For the same reason towing of automatic transmission vehicles over longer distances is said to be damaging.

Now it happens that some modern hydraulic automatic transmissions offer automatic coasting and gliding. For example, this description of PSA's EAT8 states that it supports coasting (between 20 and 130 km/h) and gliding (up to 20 km/h). How is this realized mechanically? The EAT8's oil pump being conventionally powered by the engine (as can be seen on the exploded view under the above link), does this mean that there exists an additional clutch to fully disconnect the drive train from the transmission or that some other measures where taken to limit the negative effects?


Additional sources:

  • A PSA press note clarifies that "free wheeling" in the EAT8 gear box is in fact coasting: "returns the engine to idle and disengages the gearbox with each lift of the foot off the accelerator for speeds between 20 and 130 km/h"

  • A response on quora contains a slide from a PSA presentation on the same topic.

  • The cited URL (a description of PSA's EAT8 transmission) says nothing at all about the oil pump, nor about whether this gearbox has automatic coasting or gliding. – David supports Monica Oct 8 at 3:53
  • I clarified the question accordingly. The oil pump is visible in the exploded view and looks conventional. The cited URL does state that the start & stop system (=gliding) works up to 20 km/h, and that "free wheeling" (PSA's term for coasting, see additional source above) is possible between 20 and 130 km/h. – spheniscus Oct 8 at 6:42
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Let's start by saying that absolutely nothing about coasting wears the transmission at all or what so ever. The most basic transmission has at minimum two clutches, forward and reverse. If a clutch wore while it was not engaged then every transmission would blow out reverse. While driving forward the reverse clutch is not engaged and since we drive forward 99.9% of the time, clutches like this can't be allowed to wear. These types of clutches either hold or completely release, nothing in betwen.

Gliding is all about lubricating the final drive, the differential. There are three possibilities; full-pressure lube, drip lube (I'm sure there is technical term but I'm not sure what it is off the top of my head), and splash lube.

  1. Full pressure lube injects pressurized oil into bearings. I have never seen this type of lube system in a transmission personally but I'm sure they exist. For obvious reasons, this type of system can't glide.
  2. Drip lube systems pour oil onto what needs lubrication. It is common to use the oil returning from the transmission cooler. When the pump no longer pumps the components starve for oil. The critical distinction between splash lube and drip lube is that drip lube uses a dry sump. The components of the final drive are not submerged in oil. This type of system can glide with the addition of a small circulation pump in the oil cooler lines. I have seen exactly one car like this flat towed behind an RV.
  3. Splash lube uses a wet sump. This means that the final drive is touching oil. They don't have to be fully submerged but the action of them turning splasher lube enough to lubricate. This is what manual transmissions and rear differentials use. This type of system can glide all day long with no problems.

It is likely that they are using splash lube for the final drive and that lets them glide.

  • Thanks! I found a detailed video that explains how this transmission works. At 15:15 it seems to confirm what you surmise on the mode of lubrication. – spheniscus Oct 14 at 8:35
  • @spheniscus The idle off feature works by trapping pressure in the clutch responsible for 1st gear. Normally when the pump shuts off the pressure bleeds down. This will release all gears and effectively leave the transmission in neutral. Once the engine starts it would take some time to build that pressure up and shift into 1st gear. By trapping pressure in 1st gear the car can move as soon as the engine starts without having to wait for the pressure to build. – vini_i Oct 14 at 11:46
  • Thanks! It turns out that the EAT8/AWF8G contains a "hydraulic accumulator", so this is where the pressure is kept: retraite-psa-sochaux.com/wp-content/uploads/2018/10/… – spheniscus Oct 16 at 14:12

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