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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When you downshift, the RPM on your engine increases without more input of fuel. That is referred to as Mechanical Breaking. Because you use the momentum of the wheels to speed up the engine instead of fuel. It’s safer to, for example, use a combination of mechanical breaking and regular breaking when going down hill.
You conserve brake pads by shifting to a lower gear and getting rid of your extra kinetic energy this way.
To understand what happens you need to understand how an engine works:
* The piston moves down, sucking in air and fuel.
* The piston moves up, compressing the air and fuel to a very high pressure. This obviously requires quite a bit of force to compress the air that much.
* The compressed air and fuel are ignited, resulting in heat and gases from burning that have an even higher pressure than what was previously compressed. This pushes the piston down, with a very high force.
* Valves open to let the smoke out and the piston moves back up to push all the smoke out.
Then the cycle repeats.
So basically what you have in an engine is multiple cylinders, with one piston in the process of the explosion that creates so much force, and the other cylinders in the process of using some of that force to compress the air and fuel that they have in the cylinder.
If you cut the fuel to the engine, there will be no explosions and no force being generated, but all of the compression of the air still happens, so bottom line you have to manually put a lot of force into trying to turn an engine by hand, because of all the compression that happens inside.
So semi trucks use that for braking. They keep the transmission “locked” into a lower gear so that the wheels are forced to keep turning the engine, and the huge resistance from the compression phase in all the cylinders can act as a huge brake and slow down the truck.
What you are referring to is Engine breaking. The main purpose of this is to slow the vehicle down without using the breaks (extended use of the breaks can cause them to heat up and eventually fail).
There are two different methods of using the engine to break. Engine breaks, which open cylinder valves at a different timing, allowing the engine to waste work rather than generate it. The second option is the use of a Jake Break, which is basically a valve that restricts the exhaust flow, resulting in the wasting of work.
In order for either system to work the truck needs to be in the correct gear which results in downshifting while slowing down.
An engine can be thought of as similar to a rubber band. The faster the engine goes, the farther the band is stretched, the harder it is to keep it that way.
Shifting is like using a lever with your rubber band. It keeps the engine turning at an easy pace but let’s the wheels turn faster.
When you are already going fast, down shifting forces the engine to work much harder. Since you also lift your foot off the gas, it’s like letting the rubber band relax slowly. The engine will slow itself down, and the lower gear will slow the truck.
The engine, transmission, etc of the vehicle are far larger than the brakes alone so using them to slow the truck causes less overall strain. The brakes also heat up very quickly. So when the truck must slow down for a long time engine braking can be essential.
Using the engine as part of breaking is essential for small engines like ATVs and Motorcycles where they have sequential shifters. However, it is best to keep the engine RPM lower to reduce engine and transmission usage.
I would say it is not bad, rather it is not necessary for most vehicles.
First we have to understand some fundamental key points.
* A gasoline engine uses a throttle body which throttles the amount of air which can enter the engine. In modern fuel injected engines, the amount of air coming in is read by a Mass Air Flow sensor and the amount of fuel injected into either the throttle body (Throttle Body Injection), intake manifold (multi-port fuel injection), or cylinders (direct injection) is adjusted. In older (carbureted) engines, this was done via the venturi effect (essentially more flow draws more fuel, watch Smarter Everyday’s episode on carbs if you need a visual). Overall process is simple, execution is not (there is a lot of voodoo that goes into how carburetors work).
* On a gas engine when you let off the throttle pedal (which closes the throttle, really returns it to the idle state) you create a vacuum between which the cylinders have to fight on the intake stroke thus slowing the engine down. Many newer vehicles will actually stop delivering fuel when the throttle is closed in a downhill driving situation to conserve fuel.
* Diesel engines don’t have throttles. Engine speed is metered according to the amount of fuel injected into the cylinders (all diesel engines use some form of direct injection). Without the throttle, there is no vacuum to pull so the engine has to be slowed down in a different manner.
* Compression release method, commonly referred to as the Jake brake for the company that initially designed it (Jacobs Vehicle Systems), works by cracking the exhaust valve during the compression stroke which in turn slows down the engine.
* Exhaust brake method, uses what is essentially a throttle body on the exhaust to restrict the flow of exhaust gases (thus slowing the engine down on the exhaust stroke). Most exhaust brake methods are done via a variable geometry turbocharger these days but is still a common upgrade for older diesel engines with fixed geometry turbos which don’t have compression release systems available (such as the Cummins B series, International/Powerstroke light duty engines, and Duramax engines).
* Worth noting that at one point one of the US pickup manufacturers was playing with variable valve timing to simulate the function of compression release braking in conjunction with exhaust braking via the VGT but I don’t know if it ever made into production.
Ok like you are 5:
An engine wants to not turn. It wants to just sit there. Normally adding gas and spark causes little explosions inside the engine, which force it to turn and move the truck.
When you stop adding gas to the engine it slows down as fast and as much as it can. If it is connected to the wheels of the truck (in gear, clutch disengaged), it will slow down the truck as well. Which gear the driver has the truck in will determine how effective the engine can slow the truck down, lower gears working better (and being louder).
I read the assignment.
The purpose is to slow down the truck without using the brakes. The reason you don’t want to use the brakes is because after a while they get hot and risk catching on fire.
When you let off the accelerator, the wheels turn the motor more than the motor turns the wheels… Which causes the wheels to slow down (like a brake) because turning the motor is difficult.
Down shifting makes the wheels try to spin the engine even faster, which is harder to do, so it slows the wheels down even more.
This works with fixed-gear bike as well…. Except instead of a motor, the wheels cause your peddles to move, which causes your legs to move. The more you try to resist your legs from going up and down on a down-hill, the slower the bike would go.
The loud noise is called “engine braking” and, sometimes, “Jake brake” (as in, “using the Jake brake”).
Simply put, with diesel engines, you can, essentially, open the engine to reduce the compression, slowing the vehicle down. This lets out a lot of the angry engine noises.
This is the what. The why is to save on the normal wheel brakes. Trucks are heavy and often heavier still because of the loads they carry. Relying on wheel brakes alone can cause them to overheat and fail. This is bad for crashy reasons. A mix of braking techniques increases safety.
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