124 Vehicle retarders

12.4.1 Engine overrun vehicle retardation

When an engine is being overrun by the transmission system, the accelerator pedal is released and no fuel is available for combustion. Therefore the normal expansion in the power stroke by the burning products of combustion is replaced by the expansion of the compressed air alone. Consequently, the energy used in compressing the air on its upstroke is only partially given back on the downstroke, due to the friction and heat losses, so that additional work must be done to rotate the crankshaft (Fig. 12.30).

Thus an external energy source is necessary to keep this crankshaft rotating when no power is produced in the cylinder. This is conveniently utilized when the vehicle is being propelled by its own kinetic energy: the work put into overcoming the pumping, friction and heat losses while driving the transmission and engine reduces the vehicle kinetic energy, causing the vehicle to slow down. The retarding torque required on the road driving wheels to rotate the transmission and engine consists of the torque necessary to overcome the engine resistances when there is no power produced multiplied by the overall gear ratio of the transmission system.

12.4.2 Exhaust compression vehicle retardation

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