A Jake brake, also called an engine brake, turns your diesel into an air pump that works against the truck. It slows the rig on a long downgrade so your service brakes (the regular wheel brakes) don't have to do all the work and overheat.
Brand names aside, the idea is the same on every rig: let the engine soak up some of the energy that gravity keeps feeding the truck.
Why does a loaded truck keep speeding up on a downgrade?
Gravity. On a long downhill, a heavy truck keeps gaining speed unless something absorbs that energy. Interstate rigs can run up to 80,000 pounds gross, which is the federal maximum on the Interstate System, so there is a lot of energy to deal with.
There are only two places for that energy to go: the brakes or the engine. Brakes turn it into heat. The engine can turn it into work against the pistons and exhaust.
What goes wrong if I just ride the service brakes?
Using the service brakes continuously on a long grade makes them overheat. Hot brakes fade, which means they stop the truck less well the harder and longer you lean on them.
That's the trap on a mountain: the brakes feel fine at the top and weak when you need them most. CDL manuals warn about this in their mountain driving sections, and it's the main reason engine braking exists.
How does the engine turn into a brake?
With your foot off the throttle, no fuel is going in, but the pistons still move. On the compression stroke each piston squeezes the air in the cylinder. That squeezed air then acts like a spring and pushes the piston back down, handing most of the energy back. So a plain closed-throttle diesel slows you only a little.
A compression-release engine brake changes that. Near the top of the compression stroke, it pops the exhaust valve open and the compressed air rushes out. The energy spent squeezing that air is not returned, so the engine drags against the drivetrain and the truck slows.
In short, the engine does the work of compressing and then throws the energy away through the exhaust instead of getting it back.
How do I use it on a long grade?
The CDL manual approach is to plan before you tip over the crest. Here is the basic order:
- Pick your gear before you start down, not halfway down the hill.
- Switch the engine brake on.
- Let the engine brake hold your speed.
- Use the service brakes in short, firm applications (snub braking) when you need to knock speed off.
Engine braking pulls harder at higher engine RPM, so a lower gear means more retarding effect. The limit is the engine's rated or governed RPM, the top speed it's built to turn. Stay inside it. Most engine brakes also have settings, often low, medium and high, which change how many cylinders work. Check your own engine's operating guidance for what yours offers.
Steep grades in some places have posted runaway-truck-ramp signs and grade warnings. Rules and signs differ by state and province, so read what's posted for each hill.
When should I turn it down or off?
On ice, snow or other slippery roads. Engine brake drag acts only on the drive wheels, and on a slick surface that can break them loose and cost you traction. Manufacturers and CDL manuals advise turning it down or off in those conditions.
If the road is slick and you're on a grade, that makes planning your gear and speed even more important, because you have less margin everywhere.
Why is it so loud, and where can't I use it?
The bark is the sudden release of compressed air through the exhaust. It's much louder with a missing, modified or worn-out muffler, which is why unmuffled engine brakes draw complaints.
Many towns post signs restricting or banning unmuffled engine brakes, in both the US and Canada. Local rules vary on what counts as legal and how it's enforced, so watch for the signs and obey them. A well-kept muffler helps both your neighbors and your standing with local law enforcement.
The bottom line on engine brakes
The engine holds your speed so the service brakes stay cool for when you truly need them. Choose your gear early, use the engine brake, snub the service brakes as needed, and back off the engine brake on slick roads.