I have always wondered what those loud noises are the semi-trucks make as they appear to be slowing down, but I don’t understand how that helps?
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What you are hearing is the engine brake. Diesel engines have much higher compression in the cylinders. For example, you could manually turn over a gas engine with a wrench, but it’s almost impossible to do so on a diesel engine because the cylinder moving the the chamber compresses the air. So, while driving downhill or slowing down, you can turn on the engine brake in a semi to slow the truck down without hitting the brakes. Usually there are different levels of braking you can select as well. Like others have said, when driving down a mountain, or long downgrade, you don’t want to use your brakes very much. They will get hot, and as they get hot they stop working, or even catch on fire if they get hot enough. So your options are to either downshift to a lower gear BEFORE you start downhill (if you pull your gear into neutral to try to downshift while going downhill, you’re basically screwed. You going to accelerate too fast to get the transmission back into gear, at least not easily) or use an engine brake to maintain a safe speed while going downhill.
Was driving down I-17 from Flagstaff to Phoenix Az one time in a semi that didn’t have an engine brake, and it was also an automatic transmission. There is a very long, 5-6 miles I think, downgrade on I-17. The engine in the truck I had was just too small for the weight I was pulling. RPM’s pegged out within a few seconds of starting downhill. Even after forcing the transmission into a low gear, it just kept accelerating, fast. Took me several hours to make it down that hill. Because it was roll down the hill a little ways, brake to a stop on the shoulder, wait 30-40 minutes for the brakes to cool, then repeat. One of the scariest drives in my life honestly.
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