What is exhaust braking? - is it something different than engine braking? - is it a feature that you activate or is it just an effect of driving a certain way - forgive the ignorance!

 

Wikipedia lists it as - An exhaust brake is a means of slowing a diesel engine by closing off the exhaust path from the engine, causing the exhaust gases to be compressed in the exhaust manifold, and in the cylinder. Since the exhaust is being compressed, and there is no fuel being applied, the engine works backwards, slowing down the vehicle. The amount of negative torque generated is usually directly proportional to the back pressure of the engine.
Since diesel engines lack a throttle valve on the intake manifold, there is no intake vacuum when the engine is not using fuel. The intake vacuum creates the drag effect felt in gasoline engines when going down a hill with the throttle closed.
Exhaust brakes are manufactured by many companies, including competitors Pacbrake and Jacobs. The brakes vary in design, but essentially operate as described above. More advanced exhaust brakes have exhaust pressure modulation (EPM) that controls the back pressure which in turn improves the braking performance across a range of engine speeds.

 

Does it work like that on Big Rigs? I know they're loud when they exhaust break and many cities including ours posts signs at the city limits (No Exhaust Brake).

 

Most of the big rigs have the engine brake built into the cyclinder heads. They create a loud noise when decelerating.

Our pickup have the restriction programed into the turbo. When exhaust braking is asked for, the Turbos close down and prevent exhaust gases from passing thru, hence creating the back pressure. 3rd party products such as PacBrake, Banks, B&D usually install some kind of butterfly valve in the exhaust stream. When the system ask for braking, the valve closes and the back pressure is created.

With exhaust braking, there is usually some kind of waste gate installed, when back pressure get too high, the waste gate opens and the excess pressure is vented out. This usually sounds more like a Whooosh than the Rat-a-tat-tat-tat-tat that the big trucks sound like

So to answer your question, our exhaust brakes don't usually offend the laws in most cities since they are much quieter than big rig engine brakes. You activate the Exhaust brake by engaging your Tow/Haul button. This not only tells the turbo to restrict the exhaust flow, but also to keep the Torque Convertor locked up. No coasting when you take your foot of the throttle.

The back pressure in the exhaust is usually defined by how strong the valve springs are in the engine. If you get more back pressure than the springs can tolerate, you will get valve float. So the waste gates are usually set at a pressure that is something less than the valve spring strength. The springs in previous generation Powerstrokes built by Navstar had 40 psi springs vs the 50 and 60 psi found in the competive diesel engines. Hence the Powerstroke engine never could develope as much exhaust braking regardless of the type of restriction placed in the exhaust. The 6.0L Powerstroke engine would develope 180 ft/lbs of braking. I have not seen any specs on what our current 6.7L engine produces.

 

Nope. Those are called compression brakes. They function by popping the exhaust valve when the piston reaches top dead center, preventing the compressed gas from forcing the piston down again. Basically turns the engine into a giant air compressor.

 

thanks.........

 

found it in the Diesel Supplement page 14 (5th printing) as follows:

ENGINE-EXHAUST BRAKING
This feature increases engine braking at higher engine speeds to provide
better grade descent control with less brake and transmission wear and
tear.
This feature is integrated with the tow/haul mode feature. When tow/haul
mode is switched on, the engine-exhaust braking feature will also be
active. For more information on tow/haul, see Automatic transmission
operation in the Owner Guide.