A look at mechanisms that can assist the service brakes in slowing a motorhome.
By Mark Quasius, F333630
The service brakes are a motorhome’s primary braking system. Most coach owners know that riding those brakes will overheat them and cause brake fade, resulting in a lack of stopping capability that can lead to serious, even fatal, consequences. But there are times, such as when descending a long grade, when it’s necessary to control the vehicle’s speed for an extended period. An auxiliary braking system is the best way to accomplish that.
It’s important to remember that auxiliary brakes are just that — an auxiliary system. They are designed to slow a vehicle, not stop it. They do not replace the service brakes. They merely assist in certain circumstances.
The most common auxiliary system is an engine brake. The engine creates resistance to the driveline, thereby slowing the motorhome. Engine brakes generally fall into the categories of exhaust brakes, engine compression release brakes, and turbocharger brakes.
- An exhaust brake, located downstream of the turbocharger in the exhaust pipe, consists of a valve that chokes off the exhaust flow, which binds up the engine to increase driveline resistance.
- Turbocharger brakes operate similar to an exhaust brake but are within the turbocharger.
In addition to engine braking systems, some transmissions are equipped with a transmission retarder, which applies a braking load within the transmission.
- Engine compression brakes are installed in the engine and operate by modifying the exhaust valve timing.
Let’s look further at the differences between these systems.
Gasoline engines normally are not equipped with auxiliary engine brakes. A gasoline engine has a butterfly throttle valve that varies the airflow into the engine. When a vehicle descends a grade, simply backing off the throttle closes the butterfly valve and creates a vacuum that is applied to the pistons, thereby exerting some braking action on the driveline. This works best at higher engine speeds where the increased airflow has a greater effect.
Diesel engines have no butterfly valve to vary the air intake and create a vacuum. The air intake is wide open, and engine speeds are dictated by the fuel flow through the fuel injectors. As a result, when the throttle on a diesel-powered motorhome is released, the vehicle coasts much farther than a gasoline-powered coach of the same weight and size. Therefore, most diesel engines are equipped with an auxiliary braking system.
Exhaust brakes are quite simple. When deployed, they act as though someone stuck a potato in the exhaust pipe. Exhaust brakes are mounted downstream of the turbocharger. When engine braking is activated, a valve closes to choke off the exhaust flow, creating a drag on the engine. The valve is activated by an external linkage connected to either an electric or a pneumatic actuator.
Some brands, such as Pacbrake, recommend regular maintenance and lubrication to prevent the linkage from seizing up or wearing over time. Others, such as the Jacobs Exhaust Brake, do not specify lubricating the unit. If you have an exhaust brake, be sure to check the manual for service requirements.
Exhaust brakes do a good job of applying braking horsepower and are common on midsized engines, but for even greater engine braking capacity, an engine compression release brake is needed.
Vehicles equipped with an exhaust brake have only an on-off setting.
Most turbochargers produced today are variable-geometry turbochargers (VGT). The optimal aspect ratio for a turbocharger depends on the engine speed. If the aspect ratio is too large, the turbocharger won’t create enough boost at low engine speeds. Conversely, if the aspect ratio is too small, the turbocharger will tend to choke off the engine at high engine speeds.
A VGT varies the aspect ratio of the turbo via inputs from the engine control module (ECM) to match the aspect ratio to the engine speed. The VGT accomplishes this by moving an axial sliding plate inside the turbo. The ECM also can be utilized to change the aspect ratio to restrict the exhaust flow from the turbo and cause resistance on the engine. The new 2017 Cummins X15 engine utilizes turbocharger braking to substantially improve the engine’s compression release braking performance at lower speeds (1,900 rpm and below) in comparison to the previous ISX15 model.
Vehicles equipped with a turbocharger braking system generally have only an on-off setting.
A compression release brake is commonly referred to as a compression brake or a “Jake Brake” engine brake. It utilizes the engine’s internal components to create braking horsepower. Jacobs Manufacturing introduced the engine compression release brake in 1961, a few years after its invention by Clessie L. Cummins. The “Jake Brake” nickname was made popular by truckers. However, “Jake Brake” is not a general term but a registered trademark of Jacobs Vehicle Systems.
A typical diesel engine has four strokes, which account for two complete revolutions of the crankshaft. On the first stroke — the intake stroke — the piston moves downward in the cylinder, creating a vacuum. At the same time, the intake valves are open so that the cylinder fills with fresh air containing oxygen. Next comes the compression stroke, when the intake valves are closed and the piston moves upward. At the very top of the stroke, the fuel is injected into the engine and the power stroke begins. The air and fuel mixture explodes, pushing the piston downward to turn the crankshaft. The exhaust stroke is last. The exhaust valves are opened and as the piston moves upward, burnt gasses left over from the power stroke are expelled through the exhaust system. Then the exhaust valves close and the cycle repeats itself over and over.
The compression stroke creates resistance and consumes power as the piston moves up to tightly compress the intake air. But the power stroke more than overcomes this drag. The net result is that the engine makes more power than it consumes.
A compression release brake modifies this procedure to turn the engine from a power-producing unit into a power-absorbing device or compressor. It does this by opening the exhaust valves when the piston is almost near the top of its compression stroke. This allows the cylinder pressure created during compression to exist, but then this compressed air is expelled through the exhaust system so that there is no power stroke. All that’s left is the power-absorbing compression stroke, so the vehicle slows rather than accelerates.
This sudden release of compressed air can be loud, but if ejected into an EPA-rated exhaust system, which has a maximum sound level of 80 decibels, it won’t be noticeable. Many communities prohibit engine braking, which is a response to large trucks that have no mufflers or are running straight stacks. The noise from such trucks can exceed 100 decibels. U.S. EPA emissions standards enacted in 2004 and 2007 prohibit high-decibel noise emissions from trucks made after those model years. To combat the noise from older or modified trucks, some communities ban unmuffled engine braking.
Jacobs rates the 490A compression release brake used on the Cummins 400-to-450-horsepower ISL series engines as producing up to 370 braking horsepower at 2,600 rpm. Engines such as the 600-horsepower Cummins ISX15 can create 600 braking horsepower, which is impressive.
Vehicles equipped with a compression brake generally have multiple braking levels. Newer electronic engines use the variable geometry turbo brake, in addition to controlling how many cylinders are allowed to release compression. The braking level adjustment allows the driver to tailor how additional braking is provided to maintain speed at the desired level.
Transmission retarders are an alternative to engine or exhaust brakes. In this case, auxiliary braking happens in the transmission instead of the engine. Allison transmissions equipped with a retarder are identified by an R on the end of the model number. Typically, they also can be identified by a joystick control lever on the driver’s left-side console, although some older coaches may just have a switch.
Allison’s retarder utilizes a vaned flywheel inside the transmission housing. The retarder housing is fitted with internal vanes that align with the vaned flywheel. Transmission fluid is directed to the retarder housing and creates drag as it absorbs energy. The level of braking varies in relation to the pressure inside the retarder cavity. The kinetic energy absorbed from the rotating transmission output shaft is transformed into heat energy, which then is dissipated by the transmission cooler.
The braking action occurs on the output shaft, making it dependent on vehicle speeds rather than engine speeds. Therefore, low engine rpm do not alter the effectiveness of the transmission retarder. At high engine speeds, such as a downshift to a lower gear when descending a grade, the braking horsepower produced by the transmission retarder is similar to that produced by an engine compression brake.
Transmission retarders operate silently, so there’s no worry about local ordinances pertaining to engine brakes. But transmission retarders cost more than engine compression brakes, and larger transmission coolers are required.
The Allison 4000MHR retarder operates in five levels via the joystick and is available with three selectable modes.
- The manual mode allows the driver to activate the retarder by selecting one of the five joystick positions. Ideally, the driver starts in the lowest position and moves through progressively higher positions as needed.
- The full automatic mode operates much like a compression brake and comes on automatically when the driver’s foot comes off the throttle. The driver has complete control over the braking level via the joystick.
- The latched mode ties the transmission retarder to the brake pedal. The braking force applied by the retarder is directly proportional to the pressure applied to the service brakes. This mode utilizes an air pressure sensor on the brake pedal to detect how much pressure is applied, instead of the five-position joystick.
For anyone driving heavy vehicles in mountainous areas, engine brakes and transmission retarders can be lifesavers. They allow drivers to control their speed while descending grades and conserve the service brakes so they remain cool and available when really needed.
Compression brakes generally have two or three levels of braking, which the driver selects by a rocker switch in the cockpit. Depending on the grade, the low setting may still allow the vehicle to gain speed, while the high setting may slow the vehicle a bit too much. Toggling the switch between the various positions allows the driver to maintain a reasonable range of safe speeds during a descent.
Compression brakes also depend on engine displacement and engine speed to determine how much braking horsepower is available. For example, a typical Cummins 9-liter L9 450-horsepower engine is capable of approximately 370 braking horsepower, while a Cummins 15-liter X15 600-horspower engine is capable of 600 braking horsepower. So, engine displacement does matter.
Maximum engine braking occurs when the airflow is greatest, which is at the highest possible engine rpm. That’s why the transmission attempts to downshift into a lower gear to raise the engine rpm. The ECM and the transmission control module communicate with each other to raise the engine rpm as high as safely possible without over-revving the engine. As the vehicle slows, the engine brake eventually disengages once it reaches the minimum vehicle speed, which can vary between 0 and 30 mph, depending on how the chassis builder has programmed the ECM.
When operating with the cruise control in use and the engine brake turned on, the brake will not engage until the vehicle speed increases by a set amount (typically between 5 and 10 mph, depending on vintage) over the cruise control set speed. Thus, the vehicle can coast down slight grades and pick up a few miles per hour without applying the engine brake, yet the engine brake will engage if the increase in speed becomes excessive. Once the vehicle returns to the set speed, the engine brake disengages and the cruise control resumes command.
Service brakes distribute braking energy across all of the axles. That differs from engine brakes and transmission retarders, which apply braking force to the driveline; therefore, all that energy is present at the drive axle. That can be a problem when driving in snow, ice, or other slippery conditions. The lack of friction on a slick surface can cause the rear axle to lock up when the engine brake suddenly engages, and that can cause the motorhome to skid or the driver to lose control. In such conditions, the best practice is to disengage the auxiliary braking system and maintain safe speeds and following distances so that the coach can be stopped just by using the service brakes.
Excessive use of the engine brake also can be detrimental to the life of the service brakes. Spartan Chassis states that its service brakes are good for 350,000 miles under normal operating conditions. There is no need to attempt to increase this by relying on the engine brake all the time. In fact, the service brakes should be used regularly and firmly to keep the brake linings in optimum condition. If you always use the engine brake to assist in slowing the vehicle, you’ll use light pressure on the service brakes, and that will have a negative effect on their longevity. Reserve the auxiliary braking system for its intended use. You’ll maximize the life of your chassis and be rewarded with safe and enjoyable driving.
Jacobs Vehicle Systems Inc.