The air-brakes of a truck, particularly when loaded and with a trailer, are hardly sufficient for long downhill stretches of road. Although nowadays, these vehicles are fitted with disc-brakes, the brakes get too hot, except
perhaps when crawling along at very low speed.
A sustained-action brake is compulsory in trucks. In this case, the engine-brake and the retarder often work hand-in-hand, mostly it can be sensitively applied from the driver's seat. Now, one could think, that the engine-
brake consists mainly of a pneumatically operated flap in the exhaust-system, placed even before but mostly after the turbo-charger.
It may be, that this is sufficient for smaller vehicles, e.g., a light distributer truck, but definitely not for heavy utility vehicles. The next stage is a type of decompression, which prevents the power-stroke from accelerating
the piston unnecessarily.
The second stage of the engine-brake thus uses the back pressure caused by the sluggish outflow of the exhaust fumes and in addition, also the braking caused by the compression. This occurs either through a small
additional valve in the combustion chamber or the hydraulics in the drive of the exhaust valve. The important thing is, to keep the combustion chamber open in contrast to the closed outlet port.
Exactly the same as with the engine, one speaks of performance which is being bled off or more precisely, which is transformed into heat. This may be, in lighter vehicles, as little as 50 kW. In heavy utility vehicles this
process can produce as much as up to 300 kW of braking power. Together with the retarder, this then amounts to 735 kW (1000 HP). Of course, all this is in addition to the normal braking system.
The times when there was a button in the leg room, are mostly now a thing of the past. Very often it is controlled by a lever on the steering column. When used more often and more variably than in the past, the engine
brake can distinctly reduce the amount of wear and tear on the brake pads and discs. This fits in well with the longer service intervals, even more so in the truck field than in that of the motor car. 04/16