[sbasak]

I have a confusion over brakes in automatic cars.

I manual cars, when running at say 40 km/h, I can gently apply brakes (without pressing clutch) to reduce speed. When I do it, the brake itself tries to slow the car down, also as I move my foot off from accelerator, the engine rev falls and engine also helps to decelerate the car. Just before dead stop, I can press the clutch and apply brakes fully (so that the engine doens't stall).

Now, there is no clutch (conventional sense) in automatics. So, when I press the brake there, does it disengage engine from wheels immediately? In that case, only brake will decelerate the car - no help from engine for braking (that's quite scary).

Or, it internally still applies the same mechanism as in manuals?

Any thought?

[nitrous]

Well, that is one safety 'feature' in manual cars

[Shan2nu]

I doubt the engine gets disengaged. It's just that the engine speed will drop to a certain designated rpm as long as the engine feels comfortable in that gear. Go below that and the tansmission shifts to a lower gear.

I've just driven an A/T once so far (CR-V 2.0) and i didn't see the tacho fall to the bottom when i pressed the brakes, the revvs reduced gradually, indicating that the transmission was still in action.

Shan2nu

[nitrous]

Shantanu, u're totally right.

But, when brakes fail, there is nothing called engine braking in automatics.

[Samurai]

About 90% of my driving has been on auto cars, and I don't think engine disengages while braking. It down shifts depending on speed.

[Samurai]

Originally Posted by nitrous

Shantanu, u're totally right.

But, when brakes fail, there is nothing called engine braking in automatics.

Why not? I can try moving to 2 or 1 or even park.

[Dippy]

Samurai is right. D is for normal driving and L 1 and 2 are used for engine breaking just as they are used for more torque for going up inclines

[Steeroid]

Originally Posted by Samurai

Why not? I can try moving to 2 or 1 or even park.

Not park, maybe but definitely 1 & 2. I always use those on automatics to get some engine braking...

[cnu]

Originally Posted by Samurai

I don't think engine disengages while braking. It down shifts depending on speed.

True. Engine does not totally disengage but the engine braking is very less when we are in Overdrive/Drive mode. To take optimum advantage of engine braking one needs to downshift manually.

Checkout http://www.canadiandriver.com/articles/jk/030806.htm

[sbasak]

But even if it downshifts, still it falls short of manual performance. In manual, one can apply brake even in keeping the car in 4th gear - thus reducing speed very drastically.

What will happen in case of emergency braking? In manual, if clutch is not pressed, the engine will simply stop and the car as well (though it may skid a bit and transmission may get damaged).

But in automatic, the engine will never stall. So, at the last moment, its still the brake only - no help from engine.

[Samurai]

Originally Posted by sbasak

But even if it downshifts, still it falls short of manual performance. In manual, one can apply brake even in keeping the car in 4th gear - thus reducing speed very drastically.

What will happen in case of emergency braking? In manual, if clutch is not pressed, the engine will simply stop and the car as well (though it may skid a bit and transmission may get damaged).

But in automatic, the engine will never stall. So, at the last moment, its still the brake only - no help from engine.

You are speculating here. Even I don't know how it works. But I have done enough emergency braking on auto cars in dry,wet,snowy and icy conditions, and most auto cars I have driven have provided excellent braking. Ok, there were few exceptions while I was driving some rental cars, for that you can blame those cars.

Not park, maybe but definitely 1 & 2.

Park is the desperate step.

Shouldn't this topic be in Technical Stuff section, what is it doing in What car?

[rohit_dce]

hi
in fact the automatic transmissin cars use a device called as the hydraulic torque converter which replaces the clutch for automatic transmission, as the brake is applied and u keep ur foot off the accelerator, the engine will rmain engaged to transmission and it gradually diengages as u apply the brakes to slow down. if u don't apply brakes and jus release the accelerator the the car will slow down very gradually.

one impotand differece between a clutch and a torque converter is that the engagement and disengagement of transmission is very very smooth in case of a torque convertor comapared to conventional clutch.

but u do get braking action of engine but not to the extent as u get in manual cars because u can't control the engagement of torque convertoer which happens hydraulics mechanisms within it.

[vincentt]

I think in case of automatic as mentioned, there will be a downshift of the gear on which the vehicle is driven. Because the vehicle normally drives the gear depending on RPM and other senses like the torque convertor which senses the power required etc.

When going down the slope there is the option mentioned in the above posts, to keep the vehicle with a kind of gear feel. Since I have never ridden an automatic on ghats, i never know that reverse engine feel either.

[turbo_c]

A "Torque Converter" is not really a clutch (as in two plates meshing together by friction), but basically 2 turbines coupled by the Auto Transmission fluid... The turbine connected to the engine (the "driving" turbine) spins in the torque converter enclosure pressurizing the trasmission fluid; as the speed of the driving turbine increases, the pressure of the fluid reaches a point where it forces the 2nd turbine (the "driven" turbine) to start rotating... the driven turbine is what is connected to the gearbox... how the gear box "knows" which gear to select is a different topic!

Coming back to the original topic of the thread: What happens when brake is applied in automatic cars?

The short answer is "nothing special"!

The brakes work exactly the same as manual transmission vehicles... they retard the spinning motion of the wheels. There is some amount of engine braking because the driver's foot has been lifted off the accelerator, but this is much less than a manual transmission vehicle because of the slippage allowed by the torque converter and also the fact that most (economy) auto boxes will select the gear ratio based on the speed of the vehicle and nothing else... it's like trying to use engine braking on 5th gear! I suspect that the "Auto" version of a given car will be equiped with more powerful brakes than the manual version for this reason.

When the engine is at idle speed, the brakes are strong enough to hold the wheels in a stopped state. If one just release the brakes (without touching the accelerator) the car will start to "creep" forward. Some cars (eg. Ford Crown Victoria) have a foot operated "parking brake" that can be engaged when the car is stationary at a signal and this is released automatically (mechanically) if the driver depresses the accelerator.

To go downhill, most auto boxes have a "2" and/or "1" mode where the gear box will be prevented from switching to higher ratios than these, providing sufficient engine braking. Our '82 Chevvy Caprice Classic in Kuwait also allowed selecting between "D" (Drive) and "OD" (Over Drive). It was a 4 speed with the 4th ratio being "Over Drive" ... that's what we used for most "normal" driving and the "Drive" mode converted it to a 3 speed (minus the overdrive) for spirited driving 8) ... not that it needed it too much with a 350bhp V8 under the hood!

- T u r b o C -

[rageshgr]

The torque converter has a liquid coupling, so when you lift off, the engine will still try to brake the car, but since some of the energy is dissipated in the torque converter, it wont be as drastic as in manual. If you use "L" and "2" modes, you get pretty good amount of engine braking, although it is due to the low gears than anything else, and sometimes such engine braking might be even more ferocious than you expect.