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.
Slowing down truck takes energy. Usually all energy go to heating up brakes. Too much heat bad for brakes.
Downshift use engine to absorb energy too. Loud engine noise instead of hot brake.
I believe you are referring to Jake brakes. That really loud noise they make is engine brakes (jakes) not downshifting (mechanical braking).
A driver can engage his Jakes which will open a valve in the cylinder during the compression stroke so that way gasses will exhaust out of the engine rather than pushing the piston down and creating more power.
Downshifting is using the transmission and engine to slow down. You can only gain so much speed at each gear ratio (unless you have want to turn your engine into a bomb) which is why if you floor it in 1st gear your engine will shoot up to 7,000 rpms but you will only go like 15 mph.
As you increase your gear ratio (the amount of times the wheel will spin for every rotation of the input shaft/engine) you will need less engine rpms to go faster. Thats why when you downshift your engine rpms shoot up. You’re doing 65 mph in 5th gear with 4k rpms, but the lower gear would need you to be going 7k rpms to get the same speed.
Basically when you make the engine work harder to spin the transmission with no gas, it’s going to slow down.
Normal manual vehicles do not have Jake brakes but they can downshift because it is better than hard braking. Semi trucks have Jake brakes but they can also downshift if they have a manual transmission. When you use your engine and transmission to slow down, you’re avoiding the use of your brakes, which keeps them cool. Heat will cause damage and can even cause malfunctions with your brakes.
Occasionally I’ve seen signs that say “no engine braking “. Why?
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