Had not intended to post, for a while, but the issue of safe braking of a large vehicle is literally a matter of life or death.
[WARNING: the following is a rant
based on personal opinion and professional experience.]
Accelerating a vehicle requires an expenditure of energy, either heat
in the form of burning fuel, or gravity
when rolling down a hill. Decelerating a vehicle requires energy, usually as heat
and occasionally, as gravity
when slowing , by heading uphill.
The energy required ot accelerate a vehicle to a certain speed is the same as the energy required to decelerate from the same speed. Except for rolling resistance, wind resistance, and (rarely) uphill resistance, that energy is dissipated in the form of heat
generated by braking systems.
On a down grade, applying the brakes, and then releasing them, until the speed builds up, and then applying again, is a reasonable, effective method, if and only if
the braking system and engine braking (relative to the weight of the vehicle, and the slope of the hill) have sufficient excess capacity. This is typically so with cars, but less likely as vehicle weight increases.
Gasoline engines can provide substantial engine braking by virtue of the throttle being closed, which greatly restricts the air (and fuel) entering the cylinders. Diesels, which do not have a throttle plate, always
pull in a full charge of air, and at least the idle amount of fuel is injected. Therefore, diesels do not provide nearly as much engine braking as a gas engine.
To compensate for this deficiency, Various devices are used:
1. Exhaust brakes: http://www.brakesystemsinc.com/wm771.html
2. Hydraulic retarders: http://www.allisontransmission.com/s...t=OM1334EN.pdf
3. Electromagnetic retarders: http://www.dancoequipment.com/retarders/
[The above can also be used on Gasoline powered vehicles.]
4. Engine brakes: http://www.jakebrake.com/products/hd.php
Many Allison transmissions can be had with hydraulic retarders. I think (but am not certain) that they can be retrofitted to existing buses.
Electromagnetic brakes, and exhaust brakes, can be fitted to existing buses. Retarders generate a large amount of heat
, which must be dissipated. So do service brakes.
Which brings back to braking on a hill. Slowing from (shall we say) from 40 to 30 generates a lot of heat. Allowing the vehicle to accelerate back to 40 builds up equivalent kinetic
energy, which then must be dissipated as heat
by the braking system, the next time you slow from 40 to 30.
If your braking system can dissipate that heat, before you need to brake again, you will be ok. If it cannot, your brakes will fade, then fail, then burst into flame. Nobody wants that.
How to avoid this fate? You need to KNOW how much engine braking your engine and transmission can provide. You need to KNOW how your service brakes are, both in braking force and in heat
dissipation. You need to KNOW how steep, and long, and twisty the particular hill is.
Lets say that you become aware that the above factors make "brake and coast" on a certain steep, long, hill much too dangerous. What should you do?
What I, personally do, and have done for decades, is this:
1. determine (by considering the above factors) the safe speed to descend that particular hill.
2. determine which gear would provide maximum engine (and other auxiliary braking systems) braking.
3. allow the vehicle to accelerate ONLY to that speed/gear.
4. apply the service brakes (using light, even pressure) to maintain exactly the chosen speed.
If you allow the speed to increase, you will increase the heat your brakes must dissipate. If you pick a speed which is too fast, your brake system will have to dissipate a large amount of additional heat
. This is basic physics:
Energy equals mass times velocity squared
E = M x V x V
The mass (M) of your bus stays the same. At two different speeds (say 30, and 60mph) the energy your brakes must dissipate increases:
FOUR times as much! (It is actually very slightly LESS than four times, since rolling and wind resistance also increase with velocity.)
End of RANT.
Hope this helps.