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.
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.