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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Under normal operation you let fuel into the cylinders which burns pushing the cylinders down and eventually using that energy to turn the wheels.
When you are off throttle no fuel should be going into the engine thus it is providing no energy. Instead the energy the truck has from moving is used to move the cylinder up and down against friction and also compressing any air in their providing a braking effect. The faster the cylinder is turning the more firction and the more braking their is.
Big trucks can take this even further with a J brake which lets the compressed air out at max compression making the noise you hear, this stops the air acting as a spring and returning some of the energy once compressed.
If you are hearing sudden loud popping sounds from a truck, it’s the engine brake, or “jake brake.” This is different than the RPMs of the engine slowing it down by changing gears – it restricts airflow into the diesel engine and therefore forces the engine to slow down. When a driver has it turned on, it begins as soon as they lift their foot from the accelerator. In other words, it is kind of like an enhanced coast.
You also might be hearing the fan come on, or going into high speed. It’s hot, and engines have a lot of difficulty staying cool with 100degree air. The fan runs a whole lot this time of year.
The foot brake is an air brake system. People have already mentioned slowing down with changing gears. And the jake brake is the 3rd form of braking. Should the brakes ever become disconnected from the air supply, that is the emergency brake, to ensure should the mechanical systems fail, the springs in the brake drums engage brakes to make sure the vehicle doesn’t roll uncontrollably.
Shifting down means now the wheels are pushing the engine instead of the engine pushing the wheels. You haven’t hit the accelerator, so fuel is being injected — so now the engine is acting like a big air compressor driven by the wheels. Compressing air requres energy and serves to put backpressure through the drivetrain to slow the wheels.
I think you’re thinking about the engine brake. It’s using decelerative forces in the engine to slow the speed the wheels are turning at. Shifting down, and slowing down the vehicle with heavy loads with minimal wear on the brakes.
Jake-breaking is what they call it some places. You don’t have to touch your brakes…. I used to jump from 3rd gear to 1st sometimes in traffic, in a passenger car, as 2nd gear is worthless in rush hour.
Try watching a Japanese show where they experiment ski jumping different tire size rolling downhill.
Imagine a small round rock pebble (gear), if you roll it down a hill, it wouldn’t be moving as fast.
Now imagine a huge rock boulder, if you roll it down a hill, it will pick up more momentum and speed.
Just think of the rocks as if they’re gears on a semi-truck. That’s why they shift to a lower gear when going down hill to minimize speed.
Also, they shift to lower gears going uphill too. It’s a lot easier to push a small pebble up a hill vs pushing a huge boulder up a hill.
As pointed out this is engine breaking and it will apply to cars. If you have been properly trained how to drive you should know about this, but really few people are.
Case in point,
I was coming down a canyon in Utah from my favorite resort. It is pretty common to smell brakes from people not knowing how to do this. On my last trip we are about 2/3 of the way down and we come upon a mid size SUV with smoke coming out from the front wheel wells and two very concerned 20 somethings.
I stopped and explained they should actually keep going but keep the car in Low 2 (an automatic), and this will keep the car below 30 or 40 mph. They seemed to understand.
We continued in ahead and stopped for our usual snack, maybe 15 20 minutes, and then continued back to our hotel. 5 min late we are stuck in a line of 20 cars, doing 35 mph.
The same two guys did not realize ( ok I did not explain it all) that once they can drive normally they can shift to D, normal mode… they had just kept driving in L2 for miles.
I felt bad for that rental car….
The purpose of downshifting is not only slow down but to have more control of the engine.
To downshift, you:
– lay off the gas
– once it’s at 500rpms, put the clutch in, shift out of gear (it’s in neutral now)
– hit the gas so the engine revs up to about 1500rpms
– push the clutch in
– downshift to the next lowest gear.
Trust me, downshifting is so, so important when it comes to coming up and down steep hills on highways. plus it makes stopping so much easier.
If you’ve ever ridden a 10- or 12-speed bicycle, those employ a similar concept as a transmission on a car. 1st speed is the biggest gear.
Bigger gears are better at producing torque (think more leverage, which equates to better ability to turn/twist). Smaller gears are better at producing speed. The difference with bicycles opposed to cars/trucks is that “ticka-ticka” sound when you’re not pedaling. That’s essentially the bike equivalent of coasting. It’s like being in neutral when you’re not pedaling.
Now take those concepts and apply them to a vehicle with a transmission that can be put in and taken out of gear manually, whether you’re trying to accelerate, slow down, or just maintain a consistent speed while going down a hill.
The tendency of a lower (larger) gear to want to create torque but not want to go as fast as the higher (smaller) gear works in both directions. If it’s engaged when you’re going slow and you hit the accelerator, it’s good at speeding you up. If you engage the low gear while you’re going faster and don’t hit the gas, the speed of the wheels will cause that gear to spin at a really high rev (hence the sound) until the weight of the engine being forced to turn without any fuel slows the vehicle to a speed that the gear “wants” to go.
Engine breaking is extremely important for driving in the mountains, and absolutely critical for large, heavy vehicles as it assists their brakes which tend to overheat and become molten mush when overused.
None of these answers are ELI5 enough for me.
#ELI5
Pretend you’re on a bicycle. It’s a simple bicycle, just one gear.
Now pretend that you can’t coast! So…
* If the pedals move, the wheels will move.
* If the wheels are moving, the pedals are moving.
Got it so far? The wheels are **fixed** to the pedals, there’s no coasting at all!
Okay. So you start pedaling, and you go maybe 3mph while you’re pedaling. Neat.
Now you go over the top of a hill, and you start going downhill. And you gain a lot of speed! Too much speed! What do you do?
“Well for one, I’ll stop pedaling”.
Nope, can’t coast. You can’t stop pedaling. This bike is fixed so that when the wheels move, the pedals move. So now the pedals are moving a LOT faster than you really want to go!
So now what?
Well, you put your feet on the pedals and you TRY YOUR HARDEST to **slow the pedals down.** You could call it “trying to pedal backward” but you get the idea. Your feet are on the pedals, and you do your very best to slow the pedals down!
You can’t stop the pedals, because you’re going downhill too fast. So you do your best to just slow the pedals down a little. This is hard work! So you’re grunting and groaning and making all kinds of noises!
**Some trucks** are set up kind of like that. Some trucks, if you put the transmission into a gear that is “slower” than the speed you’re actually going, then the truck will try to “stop the pedals” by using the ENGINE.
Instead of the engine making the wheels go, they’re trying to make the wheels stop. Just like your feet on the pedals going downhill.
What you hear is the engine working really hard to do that.
##THIS IS ELI5.
This is far from complete, and very far from totally accurate. So go ahead and add corrections, but realize I left out a lot ON PURPOSE.
I know. I didn’t explain HOW the engine/gears do this. This is ELI5.
I know. I didn’t explain the difference between engine braking and actual Jake brakes. This is ELI5.
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