18. Engine Retarders

One of the important reasons for higher efficiency of a diesel engine at part loads than a conventional spark ignition engine is because of the missing throttle in the inlet duct of the engine. However, for the same reason, the braking of a diesel-powered truck by the pumping power of the engine alone is insufficient and hence, the load on service brakes on the wheels is not graceful. Insufficient braking on slopes or during down-shifting of gears, causes over-speeding the engines and endangers parts under dynamic inertia forces. For example, limiting design speeds are in the increasing order: valve train, connecting rod, flywheel. For handling heavy trucks and buses, retarder is necessary. The exhaust brake invented by Clessie Cummins, named Jake brake is the first effective solution and it is still common in low tonnage vehicles, because it is cost-effective. With increased power density in terms of hp/litre capacity of modern diesel engines, an exhaust brake is not enough to retard high tonnage vehicles. In this chapter, the theory of the required braking power (hp/ton) for a given vehicle and the pumping power of a given engine (hp/litre) is derived; later, their estimation is described, Then, various systems and devices of different capacities are described for a designer to make a suitable choice.