Why airbrakes?

Oooh, good point!

A hydraulic system works in a loop (closed system) so any heat generated by the brakes would transfer back to the master cylinder. And since brake fluid doesn't circulate when not in use a radiator wouldn't work. You'd need a dedicated cooling system.

Compressed air is constantly drawing in volume from ambient air. So there's more of a cooling effect. (Yes I'm aware of some thermodynamic laws about the air getting hotter under compression, but how much are we really talking about anyway? 20 degrees?)

 

The hydraulic brake fluid in your car doesn't "circulate" even when in use because brake lines don't form a "loop" of any sort. Climb under your car sometime. You have 2 lines coming out of the master cylinder...one traces to a junction block on the front where it splits left/right to each of the front wheels, and the other goes to the rear of the car to another junction block where it splits left/right. There is no return line. Heat generated from braking never makes it to the master cylinder, because the fluid never moves. It is compressed when you apply the brakes, and the pressure is released when you let up on the pedal.

 

Nothing wrong with hydraulic brakes. I would say they are probably safer for the average driver because hydraulic brakes activate instantaneously, unlike air brakes which have lag time and many more moving components which hydraulics don't have.

If heat is a concern, watch any driver in a 4 wheeler drive down a hill. Over half probably ride the brakes down the entire hill. Imagine how much heat that generates.

 

You can thank automatic transmissions for that. I have never seen one in a car that is capable of holding the car back on a hill. For that to happen, the torque converter would have to be locked in at zero throttle. The alternative would be to "hold" a gear at a high enough RPM to reach the converter's stall speed, and most cars will shift by then with the tps signal signifying no throttle input. Basically, the car is designed to coast down the hill, and you have to use the brakes to hold it back. That's also why I prefer my manual transmissions in cars...set the gear I want, and use the engine to control my decent.

 

Kill a automatic transmission car and you will coast. That allows you the restart option.

I had a high stall converter on my ford back in the day. Lock all 4 wheels down, spin the converter to 3 grand rpm or so, loose the brakes and off it goes. Some of my competition had line locks that held the front axle and allowed heating of the rear wheels to take off better. I was not a line car in my time it was a top ender.

Manuals rule. Most of you drivers should know that already. Autos are sweet but not that good.

 

I can manually downshift the autos on my last 3 pickups and they hold pretty good. Even my 6.7L Powerjoke @ 18k lbs and no exhaust brake holds decent on a 5% when dropped into 5th. Its running at 3k RPM though lol. My new F150 shifts actually pretty good. Its more speed dependant vs throttle dependent.

 

On a diesel (especially with an exhaust brake), they may have it set up to lock up the torque converter under the same parameters that would allow the exhaust brake to engage...otherwise, that exhaust brake would be pretty useless. In any case, light trucks are more inclined to have such a feature programmed into the computer than a car (primary focus being fuel economy) would just for the simple fact a car isn't generally going to be pulling a heavy trailer down a hill.

 

Many cars with automatic transmissions will hold hills just fine in automatic mode ("drive") when using cruise control. At that point they do lock up the torque converter and will even automatically downshift as necessary to prevent overspeeding. How many models have that feature? I have no idea, but it has been in cars for at least 17 years that I know of (my 2000 Mercedes did it and my wife's 2014 Chevy does it). Of course they all hold hills just fine (as has been the case with every car I have driven in the last ~40 years) in manual mode.

 

Gotta get yourself a car with a modern automatic transmission. The days of the runaway hydra glide are gone.
Most modern automatics have a manual mode. You can drop a gear and for the most part the transmission will engine brake.

It's funny sometimes after a long week in my truck I get in my wife's van and keep turning off the radio because it's the same spot as the jake brake switch on the Freightliner.

I guess "loop" wasn't the proper word. But the point remains, compared to a pneumatic system the fluid stays constant in the lines whereas air is continuously renewed.

 

Pretty much everything modern I've driven the TC stays locked up once its in the upper 2 or 3 gears. The HP and torque of these new engines combined with 6+ gears there is no reason to run a TC unlocked anymore to gain "half" a gear.