Quote:
Originally Posted by JDM4E
The question was: why - and even if - AT drops power at hi-rpm, and where the AT loses power when the transmission is locked. It was a genuine technical question.
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I'll try to tackle this. Worth noting that I'm not specialized in transmissions per say, but there is a lot of overlap with my area of engineering. From a technical standpoint, there two main reason why a traditional planetary automatic transmission will always have some efficiency losses even when it's locked:
1. In order to lubricate and to create sufficient hydraulic line pressure to energize the clutches and brakes, an oil pump is required. This is usually driven off the torque converter which is driven off the engine, and thus will have inherit losses, just as any accessory driven off the engine does.
2. Transmissions that use wet clutches (like the automatic in the twins) will have drag losses caused by the shearing of oil between rotating (but open) clutch plates. In this sense, DCT's have the advantage in that they only have one open clutch at a time, where as a traditional automatic could have 2-4 open at any given time depending on the gear selection.
Additionally, in practice AT's aren't truly "locked" a whole lot. This is to improve shift quality, durability, and NVH. Of course, any slippage will result in a power loss and it generates a considerable amount of heat. Tell me if you guys have any questions or something is unclear.