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AUXILIARY SYSTEMS — HOW THEY WORK The auxiliary kit in
Page 59
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our test group falls neatly into two categories: compression engine brakes and hydraulic retarders.
The former work on the principle first pioneered by Jacobs in its 'Jake Brake' as the pistons approach top dead centre (TDC) on the compression stroke, the Jake pops the exhaust valve, expelling high pressure air. Without the normal 'bang' no energy is returned to the piston in the expansion stroke and the net loss of energy is called engine retarding', Activation is usually mechanical via a hydraulic oil circuit, while braking can typically be set up to work on three or six cylinders via the footbrake, dash or steering column switch, or by lifting your foot off the throttle.
With the exception of the Voith fitted on to the back of the Actress Telligent transmission, the retarders on test were fully integrated into the gearbox. In simple terms, they all work on the hydrodynamic principle based on oil circulating between a rotor and a fixed stator, creating drag that slows down the truck.
The amount of retardation normally varies by up to five stages and, in combination with a truck's cruise control (and also using what ZF calls its Bremsomat' mode), you can set them up to maintain a steady downhill speed.
Many electronically controlled retarder systems also incorporate 'brake blending', in which the normal exhaust brake or a compression engine brake can also be dialled in to provide extra retardation as part of the auxiliary braking function. Adding an auto box like AS-Tronic, Opticruise or I-Shift means that downhill speed control can be a simple touch of a button with the engine, transmission and retarder all combining to work out the best gear and engine revs to get the best braking performance. Who said drivers don't have it easy?