Engine braking is a natural occurrence in diesel engines, something that truck manufacturers have used to assist drivers for years with the added benefit of less wear on brake linings.
Engine braking is something we also learn to rely on for control in the 4×4 world. It takes some adjusting to NOT dip that clutch and just to trust the engine brake to decelerate especially for hill descents where it’s a vital tool in assisting us, but how does it work?
Ok we know if we take our foot off the accelerator the truck is no longer being propelled by fuel the same as we slow down for roundabouts etc, but on hills! The engine braking system defies gravity holding the vehicle at a nice steady pace whilst we are trundling down steep banks navigating our route. The engine miraculously doesn’t stall just slows and it all works by the engine brake controlling the airflow.
In a petrol engine by removing your foot from the accelerator the engine braking system closes the throttle chamber causing a partial vacuum in the manifold restricting the engine from taking in air. By suffocating the engine it slows the pistons and creates a drag controlling deceleration.
Diesel engine braking system works principle the same but instead of affecting the intake of air like the petrol version a diesel motor is limited from exhaling via the exhaust. By restricting airflow from leaving the exhaust it creates a backpressure choking the engine that can no longer expel it’s spent gases forcing the engine to slow down. By limiting the engines exhalation it can no longer intake air efficiently so diesel engines have a double effect on limiting the motors power. Because of this double control on the diesels airflow it is capable of holding heavy loads on steep hills without using the brake pedal.
As well as the natural engine braking found on diesels, there are add on components to maximise the phenomenon of engine braking, these are exhaust brakes that reduce even more the escape flow of the spent gases.
Exhaust Pressure Modulator (EPM)
Similar to the above but controlled with a range of reactive butterfly exhaust valves instead of hydraulic pistons. These valves regulate exhaust pressure over the engine speed range in proportion to a controlled input signal. When used in place of conventional exhaust brakes, near constant exhaust pressure was obtained over a wide range of engine speeds. The resulting constant brake torque increased the absorbed engine brake power by up to 100%. The same device can also assist with engine heating, reducing warm-up times, and maintaining efficient engine operating temperatures at low duty cycles.
The Sliding Gate Exhaust Brake
This add on is the most commonly used exhaust brake or the variant that is an opening butterfly valve The Williams Exhaust Brake is the most popular exhaust brake in use. Both restriction devices are positioned after the turbo charger exhaust outlet, and both devices restrict exhaust flow when activated. These restrictors normally use air pressure to activate with the air control valve being controlled by an electrical circuit.
To activate the exhaust retarder, five requirements must be met. The ignition switch must be on, the clutch must be engaged, the throttle must be at idle, system air pressure must be adequate, and the driver must have activated the retarder. Engine RPM must be kept high for the most retarder effect.
The Jake Brake (Jacobs Manufacturing)
This again is an additional retarder or engine braking system fitted to diesels, a compression release braking system. It’s an extra loop fitted outside of the normal exhaust scheme with a set of hydraulic pistons, a solenoid and a check valve. These ‘Jake Brake’s’ are used especially for steep descents and heavy loads so when the vehicle decelerates the check valves open and prevent the ignition of any remaining air-fuel mixture. This makes the engine an air compressor creating a greater amount of drag. The advantage with the ‘Jake Brake’ is the operator can control the level of drag on the engine with a lever or button in the cab making it a more managed and advanced system.
Known as the engine compression retarder, it uses the engine’s compression stroke to absorb energy from the vehicle’s motion. Normally the piston compresses the cylinder air for the next power stroke, and this compression of the cylinder air takes energy. That energy is normally provided by another cylinder that is in its power stroke.
The engine compression retarder uses up vehicle motion energy during the compression stroke of each piston because the other cylinders are not producing power during their power stroke time interval. No (or insignificant) engine power is generated, because the throttle must be at the idle position for the retarder to operate.
During the retarder mode of operation, when the piston approaches the top of the compression stroke, the retarder system opens the exhaust valves for that piston, which vents the energy which was absorbed from the vehicle’s motion. The retarder works by absorbing energy during the compression stroke, and throwing away this energy through the exhaust valves as the piston completes its compression stroke.
The most popular engine compression retarder is the Jake Brake. This retarder system is manufactured by the Jacobs Manufacturing Company, and their product is available for Caterpillar, Cummins, Detroit Diesel, and Mack engines, just to name a few.
Heavily laden vehicles are usually equipped with both the compression release ‘Jake Brake’ and exhaust brakes for added braking performance and control.
The Jake brake can usually be heard operating with a “throaty growl” from a truck as is decelerates towards a junction or on a steep decent, not very popular in some American towns as the marketing sign jokes.
For more information on engine braking, check out this video;