Safe engine braking in automatic cars

Question

When I was learning to drive, I took my lessons in a manual car. On one of the lessons I was performing a reversing maneuver and I controlled the car on both the clutch and the brake pedal... which I now understand was stupid...

The instructor explained to me that controlling the car with the clutch and brake pedals at the same time is bad because it can burn out your clutch assembly.

Now I drive an automatic car (with triptronic gear shifters) and one thing I am wondering is: Will the clutch (or the equivalent of the clutch) burn out in an automatic car if I am shifting down a gear while braking? (Lets say I am coming up to the red traffic light). Or is the computer in the car, clever enough to know that it needs to do the equivalent of pushing the clutch pedal down if I am braking and shifting down a gear at the same time??

I hope my question makes sense.

Thanks for your time, Dan.

Answer 1

Your instructor properly objected because the clutch/engine and the brakes do opposite things: the former adds energy to the car (speeding it up) and the latter removes energy (slowing it down). If you use both at the same time, then you're just pumping energy from the engine into the brake pads, to no good end. If at the same time you're feathering the clutch then the clutch, too, can find itself dissipating an arbitrary amount of energy. Not good.

In your automatic, the shifts take a specific amount of time; you can't "ride" an automatic torque converter unless you're on the gas. True, stepping on the gas while stepping on the brake is analogous to what your instructor didn't like, but that's not what you're describing. Just using engine braking to slow down the car while you also press on the brake won't be a problem.

Answer 2

The torque converter in a "standard" automatic transmission does the job of a clutch, and it doesn't really wear out in the same way as a clutch does - a clutch is two dry pieces of material holding each other through friction (kind of like 2 sheets of sandpaper pressed against each other), and each time the clutch is slipped those surfaces rub against each other instead of moving in sync, causing that grippy material to be slowly worn off. In a torque converter, you have something somewhat similar to a turbine with a fluid solution inside of it - the "friction" is the turbine blades against the fluid it is filled with, instead of two surfaces contacting each other. This doesn't cause the same type of wear because the surfaces aren't abrasive against each other.

This is part of the reason automatic transmissions are kind of common in drag racing - in automatic cars, you can abuse the torque converter by stepping on the brake AND gas, up until the point where the torque converter stops locking up. This is generally around 2-3k RPMS, which means when you let off the brakes, the car launches in a more favorable position of the RPM range. You could also do this in a manual car (slip the clutch as much as possible) but you would be absolutely destroying your clutch, similar to holding a car still on an incline via the clutch. Anytime that you are applying more power than you have wheel movement means that the clutch must be slipping, and that means those friction plates are rubbing against each other instead of moving in sync, and therefore are wearing away. So you don't really have to worry about wearing out a torque converter from slipping/engine braking.

EDIT: to answer the question more clearly - the torque converter in an automatic transmission doesn't operate the same as a clutch. But you can still brake and operate the clutch on a manual car as well (heel-toe shifting, check my comment below) and it isn't "good" for the clutch, but is a valid driving technique, and as long as it is executed properly you shouldn't be putting much wear on your clutch anyways while downshifting.

Answer 3

It very much depends on the type of car you have but most paddle shift cars employ an automatic gearbox so are effectively clutchless (relying instead on a torque converter). Some systems employ a robotically operated manual gearbox with clutch (or two clutches on systems such as Volkswagens' DSG).

However, the situation you are talking about; slowing down whilst braking and downshifting, is very different to riding the clutch as the downshifts will be completed so the higher gear is disengaged and the lower gear engaged. This is a legitimate driving technique in either an auto or manual.

The system in your car will have built in safety features so that if you ask for an inappropriate gear, the computer should prevent that gear from being selected at that time.