I know how a DCT works and what parts it has, but I've always wondered how it manages to shift very fast and still be smooth, while having little torque drop during shifts.In a manual transmission, shifting at high rpm(6000), while lifting the clutch at a reasonable speed, results in a slight forward jolt (jerk) because engine speed doesn't have enough time to drop to the corresponding rpm of the newly engaged gear.DCT-s can also shift slower during normal operation, and you can even see this by looking at the RPM clock.But when they do shift fast, does the flywheel take the shock or does a gradual (but still fast) clutch engagement solve this problem ?
The fast/smooth part you're asking about is because there is a nearly continuous flow of power. In a normal standard shift, where the driver presses a clutch pedal to release the clutch, it also releases (or disconnects) the power between the engine and transmission. In a DCT setup, the computer is ready from one shift to the next. It has the "next gear" setup and ready to go for the shift, which is almost instantaneous (the source in Moab's article states it to be 8ms ... fast enough you'll not feel it).
Why you might feel the shift at some times and not at others could be the computer itself. The computer has to "guess" at which gear you'll want next. It can only set up for one "next" gear, so looks at its programming and decides, "Oh, they are in 3rd gear, they'll want to go to 4th next" and gets the transmission ready to shift to the next gear. If you are power shifting (getting on the gas pedal and shifting up through the gears), the computer has a pretty good idea you want to go from 2nd, to 3rd, to 4th, etc., and complies without a hiccup. Let's say you're just cruising around and have travelled up from 2nd to 3rd, and normally you'd want to go to 4th, but change your mind and drop it back into 2nd. The transmission may not have accounted for it, had setup for the shift to 4th, but now must go into 2nd. It has to change the gear from 4th to 2nd on that clutch, then can engage the clutch. In doing so, there may be a perceptible feel during that gear change. This may be what you're feeling at odd times.
Please note, not all DCT are made the same nor programmed the same. Some will behave better than others. Some will shift more imperceptibly at all times. It just depends on the make/manufacturer as to how well they work. You could expect something in a $250k Lamborghini to work just a little bit better than what you'd find in a $25k Ford Focus, so please take that into account when looking at DCTs.