Several retardation methods can assist in slowing diesel-powered motorhomes in traffic or on downgrades.
By Peter D. duPre
If you have owned a gasoline-powered and a diesel-powered motorhome, you don’t need to be convinced of the advantages of the diesel engine. Most RVers know that diesel engines produce more torque at much lower rpm than the equivalent gasoline engine, offer enhanced fuel economy, and have a longer service life. In fact, for hauling heavy loads, the diesel power plant can’t be beat, which is why it is the mainstay of the heavy trucking industry and is also widely used in the type A motorhome industry.
In all internal combustion engines, when the throttle is released, engine compression helps to slow down the vehicle. When going downhill with the throttle released, the engine acts like a large air compressor as it absorbs energy and slows down the vehicle.
Deceleration occurs from the compression effect and the internal friction of moving parts in the engine and drivetrain. A large diesel pusher can weigh as much as 45,000 pounds or more, and proper use of the service brakes will prevent fading brakes caused by overheated brake surfaces.
Engine Compression Braking
Almost all heavy trucks and most large motorhomes use an engine compression brake, which modifies exhaust valve timing so that when the throttle is released, the valves open as the piston hits the apex of the compression stroke. The energy gathered in the compressed air is released through the exhaust valves, and the compression stroke now provides hold-back power through the drivetrain. One common brand is the Jake Brake, so called because it is manufactured by Jacobs Vehicle Systems.
The engine compression brake is a very effective and efficient way of establishing a controlled speed on downhill grades and of saving wear and tear on the service brakes. When it is activated, the engine compression brake makes a rumbling sound, which is why some communities across the country have road signs indicating that the use of unmuffled engine compression brakes is prohibited. That is of little or no consequence to motorhome owners, however, since all motorhomes use muffling devices. While engine brakes can be relatively expensive, the enhancement to safe motorhome operation is priceless. A less costly but still effective auxiliary engine brake is available. While the end result of enhanced safety through engine braking is the same, the two systems work differently.
Instead of releasing maximum engine compression through the exhaust valves as with a compression brake system, an exhaust brake keeps compression in the engine by redirecting backpressure from the exhaust system back to the engine to retard engine speed. This is accomplished by the installation of a butterfly valve somewhere between the turbocharger and the catalytic converter (newer models) and the muffler. The valve closes to restrict exhaust flow, which results in keeping some of the compressed air in the exhaust system. The increased backpressure creates resistance to the movement of the pistons, slowing the crankshaft’s rotation and ultimately helping to slow the vehicle.
Since all diesel-powered RVs sold in the past two decades are also turbocharged, the exhaust brake typically is installed on the exhaust or outlet side of the turbocharger. A dash-mounted or a foot-operated switch typically is used to activate the valve. On units with an air brake system, the cylinder that activates the butterfly is air operated; coaches without onboard air can add a compressor or use a vacuum-style cylinder. When the throttle is depressed again for acceleration, the unit automatically turns off and normal engine operations resume.
A number of different exhaust brake systems are on the market today. The exhaust brake will assist in slowing a motorhome on a downgrade, sometimes without the need to apply the vehicle’s service brakes unless a complete stop or sudden slowdown is required.
Regardless of which system is installed, it is the engine rpm that determines the amount of braking exerted by the exhaust brake. The higher the rpm, the higher the braking effect in the engine and the greater the retarding power. Downshifting often is necessary to attain the maximum amount of exhaust braking power, which occurs at or near the engine’s top operating rpm, or redline. Some exhaust brakes are calibrated at the factory to operate with a specific engine so that excessive engine backpressure does not develop and damage the engine. Another method in the aftermarket is to supply an exhaust brake with a specific-sized hole in the butterfly valve. If the exhaust flow were completely blocked by a closed valve, the pressure in the exhaust system would rise until either the exhaust system failed or engine damage occurred. According to factory engineers, if exhaust pressure exceeds 55 psi on Chevy/GM DuraMax diesels, 60 psi on Cummins diesels, and 40 psi on Ford diesels, engine damage could occur. Care should be taken to not exceed the rating of the engine valve springs in selecting an exhaust brake.
As with any mechanical system, most exhaust brakes will need some regular preventive maintenance if they are to work properly, particularly on RVs that see only limited service. Luckily, for most units, maintenance procedures consist mainly of inspection and lubrication of moving parts, which is performed when the engine and exhaust system are cold. Make sure you use a synthetic lube and not a petroleum-based product, as the latter type has an extremely low flash point and could cause a fire.
While factory installation of an exhaust brake is pretty common on motorhomes, not all come with an exhaust brake from the factory. Many older units may be missing this additional or auxiliary braking device. The good news here is that exhaust brakes are readily available in the aftermarket. Some require exhaust pipe cutting and welding, usually necessitating the removal of part of the exhaust system, which can be accomplished by any experienced RV service center or muffler shop. When properly installed, however, an exhaust brake will greatly add to the positive driving experience and considerably reduce wear and tear on the service brake system.
Other Retarding Systems
While engine compression brakes and exhaust brakes are the primary ways of adding braking assistance to diesel-powered vehicles, they aren’t the only options. Manufacturers also employ driveline retarding devices that effectively help with braking. These are transmission retarders and driveline retarders.
Transmission retarder. This system utilizes the transmission fluid to create backpressure to assist slowing. The transmission retarder is accessed by a switch on the driver’s console and is activated in one of two ways: by a dash-mounted lever or joystick or by application of the service brakes.
On manually controlled units, the driver moves a lever into various positions. The more you move the lever from its static position, the more braking action is applied. You must manually downshift your transmission to help this device operate more efficiently. You must also carefully monitor the transmission fluid temperature. Experts suggest that when activating a hand-operated transmission retarder, the driver should minimize the initial application of the control lever and slowly move the lever, as increased deceleration is necessary to minimize heat buildup. The other method of activation utilizes three pedal-mounted sensors to detect the amount of brake pedal pressure being applied and automatically engage the retarder. Using a transmission retarder eliminates the concern of excessive buildup of engine exhaust pressure, but again the transmission fluid temperature must be monitored to prevent overheating.
Driveline retarder. A very efficient but rarely used device (which appears in less than one percent of new coaches) is a driveline retarder, which uses an electromagnet mounted between two driveshafts to create an opposing magnetic field. This causes the driveshaft to resist turning, thereby slowing the coach. The driveline retarder is engaged by pulling on a dash-mounted lever. Driveline retarders offer a very effective method of slowing a vehicle, but the systems can be expensive to install, because they require driveline modifications.
If you own a large diesel pusher, should you opt for an additional braking system? Many RVers feel more secure with an engine compression brake, an exhaust brake, or a transmission or driveline retarder installed on their motorhome. If your coach is not so equipped and you’d like to enhance your driving experience, especially when traveling down a steep downgrade, perhaps one of these systems is for you. Do your homework: consider the benefits, weigh the costs involved for purchase and installation, and decide what’s best for you.
Brake Systems Inc.
2221 N.E. Hoyt St.
Portland, OR 97232
Gale Banks Engineering
546 Duggan Ave.
Azusa, CA 91702
Jacobs Vehicle Systems Inc.
22 E. Dudley Town Road
Bloomfield, CT 06002
Engine brakes, exhaust brakes
19594 96th Ave.
Canada V4N 4C3
P.O. Box 1822
Blaine, WA 98231-1822
Exhaust brakes, engine brakes
AP Products Inc.
200 Jay St.
Coldwater, MI 49036