(and what makes a good exhaust brake?)
If you drive a diesel engine-powered pickup, SUV, or motorhome, you already know that there is precious little braking effect when you back off the throttle pedal. Few things can be more terrifying to someone driving a heavily loaded motorhome or a pickup pulling a trailer than a long downhill grade that progressively overpowers the vehicle’s braking system as the driver struggles to keep downhill speed in check. Moreover, when the vehicle’s brakes overheat and can no longer do their job, continued application only makes the brakes hotter, further reducing their effectiveness. To his horror, the driver is knowingly losing control of the vehicle. Speed continues to climb and there’s little or nothing the driver can do about it – at least not now that’s he’s already caught up in this dilemma.
Even when a vehicle’s brakes are capable of keeping the vehicle from gaining unwanted speed downhill, such loads impose significant brake wear and leave little safety margin for additional braking. Even then, sustained brake use can cause overheating that will damage some of the braking system components, such as the brake rotors.
In all too many cases, the vehicle’s brakes simply aren’t up to the challenge of sustained heavy braking. Conventional brakes are, after all, simply mechanical devices designed to convert kinetic energy (the vehicle’s movement) into heat energy, thus retarding or slowing the vehicle. This is typically accomplished by forcing stationary friction surfaces (either brake shoes or brake pads) against a rotating machined metallic surface (either a brake drum or a brake rotor) coupled to wheel rotation. The more pressure with which the friction surfaces are applied against the rotating surfaces, the more heat is generated and the greater the braking effect. So far so good, but both the friction surfaces and the rotating surfaces have temperature limits at which they begin to fail – both as effective braking components and structurally. The rotating brake drums or rotors can become literally incandescent, approaching the point at which the metal begins to lose structural integrity. By the same token, the stationary friction materials can get so hot that the composite material binding the friction materials together actually melts and begins to boil, releasing a thin layer of liquid binding material and hot gases that form a lubricating boundary barrier between the friction materials and the brake drums or rotors. When this happens, braking ceases to occur and a phenomenon known as “brake fade” occurs. Under these conditions, continued brake application does little or nothing to slow the vehicle. Fortunately, there are things that can be done to upgrade the braking systems that are standard equipment on many trucks and RVs.
Traditional brake upgrades have consisted of installing bigger brakes (an expensive procedure, at best) or special friction materials (if they are available for your vehicle) capable of higher sustained operating temperature before brake fade occurs. While both of these approaches will increase braking capability, one of the most practical techniques is to utilize auxiliary or supplemental braking capability to take some of the load off the conventional brake system.
To provide some supplemental braking, typically drivers downshift to a lower gear, allowing the engine to exert some braking force as the engine attempts to intake air against a closed throttle. Of course, this only works for gasoline engine-powered vehicles that have air throttles. Drivers with diesel engines, which have unrestricted air intake, face a different problem. Diesels provide little engine braking unless they are fitted with either an internal or an external device that uses the pumping action of the engine to do work. In either case, such devices on diesel engines are intended to create a pumping resistance that results in engine braking.
The internal mechanism built into some large diesel engines is called a Jake Brake®, so named after Jacobs Vehicle Systems® that originated it. This mechanism opens the exhaust valve of each cylinder at the top of each compression stroke. Thus, the engine has to work to compress the air, which is then released into the exhaust system. Unfortunately, the operation of Jake Brakes is fairly noisy, making a loud burbling sound as the vehicle decelerates. This is a fairly complex and expensive system that is usually found only on large trucks or some large motorhomes. It is also something that cannot be retrofitted to a “non-Jake Brake diesel” since it is incorporated into the engine’s original design.
An external supplemental braking system for diesel engines can economically be retrofitted to the vehicle’s exhaust system, thus the name “exhaust brake”. Such a device partially closes the diesel engine’s exhaust when the throttle is released. It does this by placing a shut-off valve in the exhaust system downstream from the engine. The valve uses an actuator to close it against exhaust flow, forcing the engine to pump against a restricted exhaust to create braking resistance. Such devices are easily retrofitted to diesel trucks and motorhomes of all sizes, and an exhaust brake upgrade can actually pay for itself in reduced brake wear and maintenance over the life of the vehicle. Best of all, there’s nothing that requires periodic service or replacement in a well-designed exhaust brake.
The primary advantages to exhaust brakes on diesel-powered vehicles are that they can provide engine braking to assist deceleration, help control downhill vehicle speed, reduce brake heat, reduce brake wear, and provide a greater braking reserve for the conventional brake system. They are also relatively quiet in normal operation and use. Of course, these benefits only occur when the exhaust brake system is operational, which is at the discretion of the driver.
The operation of supplemental exhaust brakes varies with the design. Most such brakes are very elementary and use a simple on-off switch to trigger a solenoid or a vacuum actuator to close the exhaust brake valve — essentially an all-or-nothing braking device. When installed, some designs create an exhaust restriction even when not activated, negatively affecting both performance and fuel economy. Other designs are more sophisticated. Gale Banks Engineering manufactures an exhaust brake — the Banks Brake — based on a unique design that optimizes exhaust flow, resulting in a power benefit and better fuel economy than other exhaust brakes can offer. The Banks Brake also features the exclusive computerized brake controller (CBC) module, an industry first. The CBC functions automatically when the system is turned on, calibrating throttle pedal location, “exercising” the valve and providing fast engine warm up on cold starts, and improving throttle response after brake activation. The Banks Brake also produces more braking force than other designs provide when activated. And for Dodge and Ford diesel vehicles with automatic transmissions, Banks also offers an optional electronic device called the SmartLock™ that automatically locks the torque converter clutch at a specified point and raises transmission line pressure while braking for improved holding capacity. By preventing excess slippage, the SmartLock also reduces transmission fluid temperature while braking, which greatly prolongs transmission life. GM and Chevrolet ’01-’04 Duramax diesel trucks already have a similar acting Tow/Haul feature built into their automatic transmissions and do not require the SmartLock module.
A well-engineered exhaust brake also includes safeguards to prevent any excessive exhaust system overpressure that might harm the structural integrity of either the exhaust system or the engine valvetrain. Unfortunately, not all exhaust brakes are created equal. There are designs that bypass so much exhaust flow that they provide little or no braking effect in some circumstances. Still other designs restrict or disrupt exhaust flow downstream from the turbocharger, impeding performance during normal driving. Premium designs, such as the Banks Brake, combine both system safety and reliability with effective supplemental braking capability while simultaneously increasing normal driving performance.
An exhaust brake should be considered as a worthwhile safety enhancement for any diesel-powered vehicle, and a virtual necessity for such vehicles when they are heavily loaded or towing trailers. Because adequate braking is so important, the quality and features of a supplemental brake should be a prime consideration when purchasing such a device. Thinking that you don’t need an exhaust brake because you’ve never had a braking problem in the past could be a risk. After all, you don’t want to discover the problem when you’re only part way down a long grade, gaining speed, and your vehicle’s brakes aren’t up to the task at hand (er, at foot). And even if you never encounter such a critical situation, the extended brake life an exhaust brake can provide will reduce your vehicle maintenance and give you greater peace of mind.