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In a manual car, when you up shift, you need to wait for the engine RPM to drop to a certain value so that the engine speed matches with the speed of the gears you want to shift into. On my manual car, it can take even half a second for it to drop to a suitable RPM. If you don't wait for it to drop then you either strain your clutch or upset the engine and the car judders before the engine speed matches that of the transmission, especially in lower gears (1st to 2nd, 2nd to 3rd).

How do automatic cars, especially those equipped with double clutch double shaft trannies, manage to drop the engine RPM so fast to allow for smooth acceleration when up shifting?

I searched online but couldn't find anything. Is it because of a lighter flywheel, more gears and smaller difference in gear ratios between them, do they employ some sort of braking mechanism to actively force the engine rpm to an appropriate value?

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This is more of a theory type question, rather than trying to solve a problem. Since it is not related to a specific vehicle/transmission, this will be a very general answer.

First, you need to know transmissions do not shift in order to change to a certain RPM. The RPM changes are due to gear ratio changes to deliver torque that moves the car; the higher the gear, the slower the engine turns and vice versa. Driving at a specific speed in a certain gear will produce a specific RPM each time the load conditions are the same. If you drive faster in the same gear, the RPMs will increase.

Basic automatic transmission function is it uses hydraulic pressure, valves, switches and clutches to shift up and down. There is a governor that monitors output shaft speeds and directs oil pressure to the valves switch to certain gears at appropriate speeds. Faster the shaft speed, gear shifts are up. The opposite happens when slowing down. It does all of this without disengaging the engine.

New technology has allowed for integration of electronics into the transmissions. Due to the electronic communications between the engine, drive train and transmission components monitoring vehicle speed, engine speed, load and such, communicate with electronic servos and switches inside the transmission which allow for faster, smoother, and more accurate shift points.

Sources:
http://auto.howstuffworks.com/automatic-transmission1.htm
http://transmissionrepairguy.com/how-does-a-transmission-work/
http://web.mit.edu/2.972/www/reports/automatic_transmission/automatic_transmission.html

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The auto trans is using hydraulic clutches to keep the engine in the highest torque / power band and changes rapidly between the gears - why there are now 5,6 or 7 speeds available also cvt (constant variable transmission) .

Some agricultural machines are designed with the engine running at constant rpm and a variable hydraulic pump controls the power delivery according to demand.

  • Ok, so the computer that manages the transmission decides how to operate each of the hydraulic clutches in such a way to allow for enough slip so that the engine gracefully slows down to the appropriate RPM? – user1969903 Oct 5 '17 at 7:00

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