By Russ and Tiña De Maris
If you’re thinking about a big Class A rig, fired by a diesel engine, listen up. That great big behemoth is mighty comfortable, but once it starts rolling it can be a challenge to slow down.
Diesel engines, unlike their “gasser” counterparts, respond a bit differently when you take your foot off the accelerator. Get off the accelerator in your gasoline rig, you feel the immediate change, particularly on the down hill with the gear selector dropped down into the lower range. But just throwing a diesel pusher into a lower gear range doesn’t give anywhere near the kind of “compression braking” we’re used to having. Hence, most diesel pusher manufacturers include an engine braking system to help out.
There are two types of engine brakes on a diesel motorhome. If you don’t have either – one is available as an aftermarket option. Let’s take them for a drive.
The first is technically called a “compression release brake,” that many refer to as a “Jake Brake.” Originally a takeaway from it’s originator, Jacobs Vehicle Systems, Jakes work internally inside the engine. When braking is called for, the system opens your engine exhaust valves at the top of the compression stroke, releasing some of the compressed air into the atmosphere, removing that stored up energy from the engine. Without this available energy available pushing the piston back down, the engine has to expend energy to pull the piston down, thus slowing you down.
Exhaust brakes, often called “Pac Brakes,” from the name of a popular manufacturer, don’t work inside the engine but rather downstream (as you probably already guessed) in the exhaust system. Simply put, a butterfly valve is inserted in the exhaust, downstream from the turbocharger. When braking is called for, the butterfly valve restricts the amount of exhaust gas released from the engine, which builds up more compression in the engine, causing a slowdown.
There are more technical details we could go into, but that’s the basic rundown on how these braking systems work. At this point, the usual question will be: Which is better? Experienced RVers will mention that Jakes will provide greater braking over a wider range of engine RPMs – that’s because an exhaust, or Pac, brake gives more braking effect when the engine is turning over faster, and less as RPMs head downhill. That may be a moot point if you have a motorhome without any engine braking system, because once the engine is built you can’t go back and “add” a Jake system, but you can buy an aftermarket exhaust brake.
Now, how about in practice? How should engine braking affect your downhill slide? Ideally, applying engine brakes should preclude the need for “stepping on” the service brakes. Your rig’s brake linings should last much longer, and your fears of overheating brake rotors and loss of braking power should vanish. That may not always be the case — you may need to add some service brake action depending on road conditions, speed, loaded weight, etc.
ONE THING TO KEEP IN MIND, however, is this. If you’ve bought a motorhome with an engine braking system, it’s probably tied in automatically with your transmission system. You call for engine brakes, the unit automatically tells the transmission which gear to switch to. The idea behind this is to ensure “not too much” gear action is called for – you don’t want to over-rev your engine or throw too much at your torque converter. Should you choose to manually down-shift, keep an eye on your tachometer, and know your rig’s “happy RPM” ranges.
A final note for “Jake” users. You may find your system has a switch to allow for a two-stage application of the engine brakes. One setting affects half the cylinders on the engine, giving you braking power. Kick to the next setting and all cylinders are bumped into braking action, giving you a definite “no questions asked” aggressive brake. Take your time to experiment with it to learn your needs.
And a couple more notes for all: When your engine brake is engaged, are your brake lights kicked on? Most manufacturers make this provision. If yours doesn’t, it might be wise to kick on your hazard flashers to warn traffic behind you of your slowing situation. And maintenance? Read the fine print in your owner manual. Jake-style brakes will probably require no maintenance, while exhaust brakes may require a little lubrication.