Well Gang,
Some of these explanations are close and some are seriously off base. After driving fire trucks for over 30 years and, coping with all kinds of "Auxiliary" braking systems, you learn quite a bit. Of the BASIC four kinds of auxiliary braking systems that were available to us as options over the 30 years I was on the FD, we used three of them at various times and, on certain pieces of equipment.
The first, which we soon disliked, was the "Retarder" or, otherwise known as a "Transmission Retarder". That system simply utilized the transmission, transmission fluid and valving, to restrict the ability of the driveshaft to turn while descending grades, off ramps etc. We found out that we didn't like it because of the damage to the transmission it did in many cases. That system creates a SERIOUS amount of heat in the ATF and the corresponding effect trans parts.
There are some earlier model motor homes, mostly diesel, that utilized the Retarder. But, it soon fell by the wayside due to the same reasons we found out years ago.
The second Auxiliary braking system we used and, is still in use today, is the "Jake Brake". Incedentilly, the as I'm lead to believe, the term "Jake" was applied to that braking system simply because, the inventor of that braking system tried to sell it to the few diesel engine builders decades and decades ago and, no one would buy it. So, the only company that stepped up to the plate and purchased the rights and patents to it was, "Jacobs Mfg". at the time.
And, it's the same company that manufactured the drill chucks, in all of your stand-up, bench model and, hand held drill motors. It's since gone through many changes and, ownerships etc. But, it's still referred to as the "Jake Brake".
Now, to its operation. Briefly, a gasoline engine, in it's four cycles, (1) INTAKE, during which the piston starts at the top, and, at the moment, the intake valve opens and a fuel/air mixture is introduced to the top of the cylinder as the piston is moving downward towards the bottom of that stroke. (2) COMPRESSION, the piston, which is now gone down to the bottom and, is starting its way up, compresses the fuel/air mixture to a point that, it's fairly compressed at the top of it's stroke.
(3) POWER, at that time, the spark plug fires and, explodes the fuel/air compressed mixture and, that explosion sends the piston back down with fierce force.
(4) EXHAUST, at the bottom of the power stroke, the piston, with all the burnt gases/air etc., is beginning to travel back up. All the while, the exhaust valve opens and, all that burnt mixture is expelled out the exhaust valve and, consequently, out the exhaust pipe.
But, on a diesel.
You have much of the same as above only:
When the operation starts out, the piston starts at the top, and travels downward, with the intake valve open. As it hits bottom, the intake is closed and, the compression stroke is started. At the top of the compression stroke, that AIR ONLY, NO FUEL, is compressed so much, (normal compression ratios for the average diesel engine is around 20:1 - 22:1) that it is super heated air.
At that precise moment, fuel is introduced THROUGH AN INJECTOR that is right at the cylinder head and combustion chamber. At that precise moment, that mixture of the injected diesel fuel and hot air, EXPLODES!
Thus, you get the "POWER" stroke.
Now, as for the Jake brake, the engine operation is the exact same only, at the top of the compression stroke, the fuel is shut off via components in the rocker assembly and, also at that top of the compression stroke, the exhaust valve is OPENED. So, all of that compressed air, is not put to use as power, instead, all the power that was used to compress that air, is now instantly relieved, without any benefit of power. But, there IS a benefit. And that is, all that energy to compress that air, is now USED AS BRAKING for the vehicle.
And, not only that but, through a switch and a bit of technology, one can control just how many cylinders are used for the "Jake" brake operation. In our fire trucks, the Jake brake switch on the dash, in the early model fire trucks, had two positons, high and low. Back then, we mostly ran Detroit 8-V53 series, 8-V71 series and, the last of those, 8-V92 Silver series.
The Jakes on those were effective, no doubt about that but, not nearly as effective as they were in our later engines.
The newer Detroit engines we used for quite a long time are what's known as the Series 60 Detroit. It's a straight six engine but, the standard fire trucks were 450 HP versions and, our larger ladder trucks came with 500HP versions of those.
Those engines, were fitted with Jakes that had 3 positions on the dash, high, low and, intermediate. The driver could utilize any position depending on driving conditions, emergency operations etc. But, in the low operation, the Jake system only utilize 1 cylinder for slowing. It worked but, very limited in it's effectiveness.
In the intermediate position, the Jake system utilized 3 cylinders. That was the normal position for most of us drivers. It was effective enough for most braking conditions and, even for emergency driving.
But, in the HIGH position, the Jake system utilized the FIVE cylinders of the six. Talk about some slowing and stopping power, yahooooooo. IT was so powerful in that stage, it was too powerful and impractical for city driving. It would lunge everyone forward if used improperly.
The third auxiliary braking system we were introduced to in the mid '80s is called the "Telma Retarder". That retarder is not anything like the old transmission retarders. This retarder is basically a disc brake, located on the drive shaft, just in front of the differential. But, on both sides of the disc are a series of coils about 3" in diameter. The face of those coils, are positioned approximately 1/16th away from that rotating disc.
Near the drivers right hand, was lever, on the dash with, four detented positions. In "P-1", there was a certain amount of amperage applied to those coils. That in effect, turned them into magnets. They tried like he... to grab ahold of that rotating disc. Ain't happening.
In position "P-2" it was more juice added to the coils, effecting more pull on the rotating discs. And on up to P-4, where maximum of almost 400 amps were applied to the coils for maximum magnet effects. Talk about some serious stopping power, all without touching the brake pedal and, even a better benefit, NOTHING WAS TOUCHING ANYTHING, NO FRICTION WHAT SO EVER!!
The fourth type of auxiliary braking system is of course, the "Exhaust Brake". As I recall, we only utilized that system in very, very few rigs. And the main reason was, the diesel engines in those, were not capable of the installation of Jake brake assemblies. Those were mainly Brush fire trucks. And, you all know how those Exhaust brakes work so, I won't mess with explaining those.
Sorry for the long dissertation but, I thought I'd throw out how things worked as well as what else was out there. Incidentally, most of our later fire trucks were outfitted with both the three-stage Jake brake and, the Telma Retarder. Now THAT'S SOME SERIOUS STOPPING POWER, ALL WITHOUT TOUCHING THE BRAKE PEDAL.
The cost of the secondary braking systems far, far out weighed the cost of the brake jobs on all of our trucks over even a short period of time.
So, with the proper use of a Jake system and or, even an Exhaust Brake system, your service brake life is seriously extended.
Scott
