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I will soon start my studying for a driver's license and I'm a bit curious about the behavior of the automatic gear.
I know that some modern automatic cars are equipped with a DSG gear while most are equipped with "standard" gear which is made of "complex hydraulic systems".

I felt that when a bus (probably not equipped with DSG) put the gear into D mode from N while still pressing the brakes, the bus started shaking and produce louder engine sounds. I assume that what is happening is that the clutch (or whatever) went into the friction point where the engine isn't completely connected to the wheels, but still transfer power (I took physics, I know the difference between static and dynamic friction) to the wheels, therefore the sound and vibrations.

In the DSG car I haven't felt it. I also know its a more efficient gear. Is it smart enough to completely "press the clutch" to separate the engine from wheels and only attach when the brakes is not pressed, or am I wrong? Is my guess about "standard" gear correct?

Thanks!

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1 Answer 1

up vote 3 down vote accepted

A standard automatic transmission doesn't use clutches in the sense of cutting off power from the engine to wheels. It uses what is called a torque converter which separates the power of the engine from the transmission through a viscous fluid. This allows the engine to continue to rotate even though you have the brakes applied and have stopped the transmissions input shaft. This just means the brake force is overcoming the force of the fluid trying to rotate the input shaft. Once you let off the brake the fluid will rotate the transmissions input shaft and you will start to move.

On the bus likely what happens is that in neutral the input shaft is spinning freely because it isn't connected to the stationary wheels once he put the vehicle in drive the input shaft was locked to the rotation of the wheels (none) meaning that the fluid is now pushing against an immovable object which in turn increases the load on the engine. This may cause the engine to drop in rpms causing vibration until the ECU can adjust to the load and increase the rpms back up to the idle speed.

I haven't studied DSG transmissions but what I understand about them is that they are two manual transmissions stuck together and electronically controlled. I assume that when you stop one drivetrain is in neutral (or clutch disengaged) and the other drivetrain is waiting in 1st with the clutch disengaged. Once you let off the brake the clutch engages putting you into 1st and the other drivetrain goes into second until you activate the shift. The difference being there is no torque converter which is putting a load on the engine while you are in drive but stopped.

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Thanks! Now I seem to understand. –  Mark Segal May 6 '13 at 4:34
    
+1 great answer. spot on. –  mac May 6 '13 at 20:56

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