Why exhaust brakes are so effective on diesel-powered vehicles.
Diesel engines control engine speed and power output by throttling the amount of fuel injected into the engine. A diesel has no air throttle. Because it has no air throttle, a diesel engine offers virtually no engine braking when the driver lifts off the accelerator pedal. There just isn’t a pumping loss to retard engine speed as the piston descends on the intake stroke. Air is free to enter the cylinder, restricted only by the flow capacity of the air cleaner, turbocharger compressor, intercooler, intake manifold, cylinder head port and intake valve opening. This can be disconcerting to a driver that is used to the engine braking produced by a gasoline engine, and it can be downright unnerving to the driver of a heavily-loaded diesel pickup or motorhome on a downhill grade, especially if the vehicle’s service brakes begin to overheat and fade. That’s why exhaust brakes, such as the Banks Brake, have become so popular for such diesel vehicles.
Before we go any further, we should mention that an exhaust brake is not a noisy compression brake, or “Jake” brake such as used on diesel big rigs and dump trucks. A Jake brake works by using hydraulic pressure to momentarily open the exhaust valve at the end of the compression stroke, venting off the compressed air into the exhaust system. That’s where all the noise comes from. The braking of a Jake brake occurs because of the pumping loss compressing the air, and then eliminating the compressed air “rebound” on the power stroke. Additionally, there’s a pumping loss as the piston descends on the power stroke with both valves closed and no combustion.
An exhaust brake is a device that essentially creates a major restriction in the exhaust system, and creates substantial exhaust backpressure to retard engine speed and offer some supplemental braking. In most cases, an exhaust brake is so effective that it can slow a heavily-loaded vehicle on a downgrade without ever applying the vehicle’s service brakes. But what really goes on inside the engine when an exhaust brake is activated, and how does exhaust backpressure slow the engine with enough force to slow the entire vehicle?
To understand how an exhaust brake works, we first need to understand how a diesel makes power – or more specifically, torque. After a diesel has ingested air on the intake stroke and compressed it on the compression stroke, fuel is injected directly into the cylinder. The heat of compression ignites the fuel, and combustion occurs releasing more heat to increase the pressure of the compressed air in the cylinder. This pressure then pushes the piston down on the power stroke to generate torque on the crankshaft. The more fuel that is burned, the more heat that is generated and the greater the pressure acting on the piston. And of course, more pressure means more torque.
Now let’s look at the exhaust stroke for our diesel. Normally the piston just pushes the exhaust out past the open exhaust valve, and through the remaining exhaust conduits until the exhaust gases reach the atmosphere. When the driver releases the fuel throttle, almost no fuel is being injected into the cylinders, so there is very little more than the air that was initially ingested on the intake stroke to be expelled on the exhaust stroke. In fact, if we put a pressure sensor in the exhaust system, we’d find that the exhaust pressure is very close to zero during deceleration because the exhaust system can easily handle the exhaust flow. This means the pressure in the cylinder is also very close to zero on the exhaust stroke under these deceleration conditions, and zero pressure on the piston top during the exhaust stroke offers no resistance to the piston as it rises.
Now let’s add an exhaust brake downstream from the turbocharger. An exhaust brake is basically a valve that can be closed in the exhaust system to restrict (but not totally close off) exhaust flow. This valve closes when the driver releases the fuel throttle. Under these conditions, the exhaust flow from the cylinders is bottlenecked and rapidly builds pressure in the exhaust system upstream from the exhaust brake. Depending on engine speed, this pressure can easily reach up to 60 PSI maximum working pressure. Maximum working pressure is limited as part of the design of an exhaust brake. In this example, that same 60 PSI also remains in the cylinder for the entire exhaust stroke (exhaust valve open) and exerts 60 PSI on the piston top to resist its upward movement. We can think of this as negative torque, slowing the engine for a braking effect. We might even think of this as just the opposite of the power stroke, and in effect, it is. Thus, you can see that simply restricting the exhaust flow can generate substantial braking. That’s what makes an exhaust brake so effective.
The above example is simplified, and it gives a good illustration of how an exhaust brake works, but there’s a little bit more you should know. Theoretically, if we completely closed the exhaust path, we could build very high exhaust pressure to act on the piston during the exhaust stroke for even greater braking. We could, at least in theory, slam on the exhaust brake to the point of sliding the rear wheels – not a good plan when towing a trailer downhill. Consequently, we want to temper the exhaust brake action for a reasonable, controlled degree of braking. Besides, an exhaust brake cannot close the exhaust system completely for a number of other good reasons. If the exhaust is completely closed, the pressure in the exhaust system will continue to rise until either the exhaust system ruptures or engine damage occurs. In fact, according to the factories, letting the pressure in the exhaust system exceed 40 PSI for Ford diesels, 55 PSI for the Chevy/GM DuraMax, or 60 PSI for Cummins diesels, can actually damage a diesel engine. Here’s how. If the pressure in the exhaust system, which also bears against the back side of closed exhaust valves in a multi-cylinder engine, exceeds the valve springs’ ability to hold the valves on their seats, the exhaust valves would be forced open and the pistons could strike the valves, causing severe engine damage. Consequently, an exhaust brake must vent some exhaust flow through the exhaust system to keep the peak system pressure below the danger point. The pressure at which the exhaust valves can be blown open depends on the valve spring seat pressure and size (area of the valve head) of the valves used. This is a carefully engineered setting on all Banks Brake applications: a careful determination is made to produce maximum practical braking without causing engine damage. If it weren’t for the potential valvetrain or exhaust system rupture problems, the exhaust brake system could be designed to create incredible engine braking force, theoretically going so far as to lock up the engine!
Okay, now that we know how an exhaust brake works, let’s consider a couple of other things that are related to exhaust brakes and this line of thinking. First, if creating backpressure in the exhaust system generates negative torque and engine braking, then any exhaust system restriction that prevents free exhaust flow during cruise conditions or full throttle operation actually detracts from power output and fuel economy in the same way. It’s like a mildly effective exhaust brake that’s always on. This is why the free-flowing exhaust system pieces in all Banks Power systems, from Git-Kit to PowerPack®, and including the Monster exhaust systems, for both gas and diesels, have such a positive impact on both power production and fuel economy.
Secondly, when closed, an exhaust brake stalls the turbine section of the turbocharger. The exhaust brake must open before the turbo can spool up again to provide boost on demand. The Banks Brake is engineered to open at speeds below 15 MPH, when very little exhaust braking is available anyway, to quicken turbo response when the driver again steps on the fuel pedal. Only the Banks Brake has this computerized feature. This is just one of the functions of the CBC (computerized brake controller). And just as noted above, any restriction in the exhaust system downstream from the turbocharger will negatively affect the turbo’s ability to generate boost under normal driving conditions, and especially at full throttle. Here too, the Banks Brake, which is designed to actually enhance exhaust flow out of the turbine in the open position, improves both power and fuel economy, as well as supplemental braking when needed.
Are you seeing a pattern here and why Banks products are designed the way they are? It’s all related to airflow and engine pumping efficiency. At Banks, we know diesels. We’re engineers. We do it right!