Big brakes on rigs
Interesting info.
The retarder on the engine makes a load of noise.....used to live in a house on a steephill.....
Brakes are there to be used, not abused …
Big rigs possess sophisticated systems to prevent big crashes
On a downhill stretch a mechanical horse and trailer cannot be “reined in” by wheel brakes alone.
The friction linings will soon overheat and brake fade set in.
“Brakes are there to be used, not abused”, is what truck drivers often hear during training.
Wheel brakes should be used as little and as briefly as possible.
The danger of brake fade is of course also present in a passenger car, but here the driver has two advantages.
Firstly, the mass to be held back is smaller, and secondly, if it runs on petrol some braking is provided during the closed-throttle operation.
But the big diesel rigs have no throttle valve in the inlet manifold, which means the engine cannot be slowed down by throttling losses.
So what means of braking do these trucks have?
Several auxiliary braking systems are used on the big rigs. We will focus on three of them – engine brakes, hydraulic retarders and electromagnetic
retarders.
ENGINE BRAKES
With no throttle valve to help them, engineers devised a different form of engine braking for the big diesels.
For each cylinder they put an extra lobe on the camshaft, which opens an extra exhaust valve at the top of the compression stroke, exactly at the
moment when the compressed air (we assume there’s no combustion, because no diesel fuel will be injected when the engine is on overrun) will start to
push the piston down on what is supposed to be the power stroke.
During normal operation the stem of this valve telescopes, so the valve remains closed. But the stem is locked by a solenoid when the driver engages
the engine brake.
Then the valve is pushed open at the right moment on each engine cycle. Now the engine gets nothing back for all the hard work it does to compress the
air in the cylinders. The engine becomes in effect a huge compressor.
This type of brake, known as a “Jake brake” among truckers (from Jacobs Vehicle Systems) provides powerful braking that can be regulated by
controlling the number of cylinders on which the brake is active.
It has the disadvantage of being noisy unless special silencers are used – the machine-gun noise heard when a truck reduces speed comes from a Jake
brake. So it is frowned on in built-up areas.
HYDRAULIC RETARDERS
These are fitted inside or close to the gearbox and work like a torque converter on an automatic gearbox.
Hydraulic oil is flung by a set of spinning vanes against a second set of vanes in such a way that it retards the rotation of the second set of vanes
– almost like a torque converter trying to counteract the rotation of the drive shaft.
It’s quiet, virtually free of wear and very powerful. Its action can be modulated by varying the amount of fluid introduced between the vanes. The
violent vortices created in the fluid cause considerable heat which makes an oil cooler necessary.
ELECTROMAGNETIC RETARDERS
These are often called Telma retarders after the name of their French manufacturer.
They work by having a metal disc, attached to the drive shaft just behind the gearbox, surrounded by strong electromagnets fixed to the frame of the
truck.
When the drive shaft is rotating and current fed to the electromagnets, the disc will be spinning through the magnetic fields.
In these circumstances electric currents will be generated inside the spinning disc, and furthermore the magnetic fields which these “eddy currents”
create, will always oppose the fields of the surrounding electromagnets.
Thus the disc, and with it the drive shaft, is slowed down by the repulsion between opposing magnetic fields.
This is a silent, wear-free way of providing retardation that can be regulated by varying the current fed to the electromagnets.
A lot of heat is generated inside the spinning disc at full power, and in spite of natural air cooling the disc can glow white hot at times.
For this reason retarders were banned from trucks carrying flammable liquids.
|
