ENGINE BRAKING | How Semi Trucks Slow Down Without Brakes | SCIENCE GARAGE

So if you're driving a manual transmission and you take your foot off the gas and you don't engage the clutch, you slow down. But what is engine braking actually doing? And is it bad for the engine? Engine braking, not breaking your engine! When you're driving a manual transmission and you take your foot off the gas without letting up on the clutch, you slow down. You can feel it. When you step on the pedal again, that power is right there when you want it. It's one of the main reasons people like driving manual transmissions so much. But what's going on in there when you engine brake? I mean, is that going to damage something like the transmission or maybe the engine? What the heck is a jake brake? We're getting to it. To understand either of those methods for slowing down, you've got to understand the engine a little bit. We've mentioned before that the engine's like an air pump. The more air it can pump, the more power it can make. Both gasoline engines and diesel engines work on similar four-stroke principles. But it's the differences between them that lets us engine brake in a gasoline engine while necessitating a special mechanism, the jake brake, in diesel engines. First, let's talk about engine braking in a gasoline-powered manual transmission automobile. We're also going to watch what happens with the throttle as the cycle runs its course and you'll find out why when we get to diesel in a second. The strokes are as follows. Intake, throttle open, valves open, air and gas mix, fills the vacuum created by the piston. Compression, valves close, piston moves up and the pressure builds. The power stroke, piston's driven down by combustion, generating power. And finally, exhaust, where the exhaust valve's open, the piston drives up and expels all that combustion waste. That's what happens when you give it the gas. The crankshaft turns the linear motion into rotational movement and it heads the output shaft, which then heads through the clutch into the driveshaft, powering the wheels. Easy. Alright, now you've taken your foot off the gas. Let's say you're just starting it up sitting in the driveway. Your engine's at idle. It's getting air and gas in a much smaller scale. Those pistons aren't moving fast, so they don't need that air quickly. The throttle isn't all the way open. The valves all still do their thing, but it's the limited amount of air from the throttle that requires less gas that keeps you at those low, low revs. Alright, now you're cruising in a manual at 55 miles an hour and you let off the gas. You're going to feel the car slow down, but the engine RPM will still closely approximate where they were when you were cruising with the gas. And that is because the wheels are now driving the engine. You put the clutch in, the driveshaft and the engine disconnect, and the revs will drop. You'll be at idling speed and you won't decelerate as quickly. So why are you decelerating when the driveshaft's engaged? Well that is thanks to the throttle body. This syringe will represent our piston. When the throttle body's open and the piston draws down, it gets to pull in as much air as it's needed to fill it quickly. When the throttle's at idle, this piston's going to be moving more slowly. So the throttle body, despite not being all the way open, still lets enough air in to fill the chamber as quickly as it needs the air, which is not that quick. Now if the engine is getting the air and the fuel for idling, and the wheels are trying to turn it faster than it idles, well this piston is being pulled down faster than the air can get past the throttle to fill the chamber. So it's like this. The limited airflow is creating a slight vacuum that slows down the descent of the piston. Now think of four or six of these vacuums happening every engine cycle. That's what's slowing down the car. That's engine braking. And remember that we're connected to the gearbox. So now I'm cruising, foot's off the gas, the engine's creating those tiny little vacuums so it's slowing down. Now if I downshift, I've just increased how many of those tiny little vacuums happen each second per revolution of the engine. And that's why engine braking slows you down. I've also increased the availability of torque. That's crazy. So is it bad for the engine to use the engine to decelerate? I'll tell you. If you subscribe, you don't. I'll wait. Alright. Well not really. You're just using air pressure to create a vacuum to slow the wheel. But let's not celebrate it, okay? You could... Don't... You don't want to get the revs up to like a million. You'll break it. Just be smart. Alright. So what about diesels? I know you've heard the rumble of a Jake brake. If you think this sounds pretty cool, wait until we get into what's making that sound. Engine braking is a great way to help out your actual brakes, the one down by the wheels. But a diesel engine on its own can't engine brake. It's crazy, right? While gasoline engines are more like an air pump where letting more air in gets you more power, diesel engines rely more on the energy potential in diesel fuel than the amount of the air coming in. Yeah, turbos help you get more air in for those high compression engines, but diesel engines don't have throttle bodies. They don't. So the engine braking we use in our cars won't work on them big trucks. Letting off the gas won't create those tiny vacuums in a diesel engine the way it does in a gasoline engine. And it wasn't long before people began realizing that the heavy loads that diesel engines can inherently haul would really benefit from a supplemental braking system that could help to do most of the stopping work. Think about it. Some tractor trailers are 73,000 pounds. That's a lot of inertia. If you think about diesel, you're going to think of Big Daddy Rudolph Diesel, who patented the very efficient torque generating engine cycle, and probably one other dude as well. Klessie Lyle Cummins of Cummins Diesel. Yes, he's largely responsible for the advancement and mainstreaming of diesel engines, but Klessie Cummins also patented the diesel engine braking system that we know as the Jake Brake. Like James told you in the Up to Speed on Cummins, in order to prove how efficient diesel engines were, Klessie got a team together to drive a diesel powered truck from New York to Los Angeles in an attempt to set a new truck speed record across this great continent. Things were going great, until their descent of the Cajon Pass into San Bernardino, California. They're heading down a long, steep gravel road, and they kept burning out the brakes. See, brakes stop a vehicle by turning movement energy into heat energy, and if they get too hot, they got nowhere to put that movement energy, and the brakes fail. But we'll demonstrate that in another video. Due truck runway ramps work, meant busted, we're dead. No, I'm sure they do, but if we busted it, it would mean we're dead. So, on the edge of calamity, Klessie's like, man, I wish I knew somebody who knows enough about diesel engines to devise some mechanism to use the engine to slow down the vehicle. And once they survived the harrowing descent, Cummins realized he was that person. In 1955, Klessie began studying what might be done to turn his engine into an effective brake. Diesel engines had an additional cam that activated the fuel injector of each cylinder. He used a simple retrofit mechanism that transferred that cam's motion to the exhaust valve of a cylinder. In April of 1960, Jacobs Manufacturing Company established its Klessie L. Cummins division to manufacture that engine brake. The Jake brake is named after the company that manufactured it, the Jacobs Manufacturing Company. Thanks, Jacobs! Okay, but here's how it works. We briefly said that traditional brakes turn movement energy into heat energy, and in a way the Jake brake does that too. Normally in a diesel, the strokes go like this. Intake stroke. The intake's valve open, the piston draws down, and the air comes in. Compression stroke. All valves are closed, the piston comes up, air gets super hot as it's compressed. Power stroke. The fuel gets injected and ignited by the heat, which drives the piston down. And then the exhaust stroke. The exhaust valve's open, and the bad stuff gets pushed out. But when the Jake brake is on, it goes like this. First, the driver flips a switch from the cab to activate the engine retarding mechanism. That engages a separate arm to control the exhaust valve. The intake stroke happens as usual. Intake valve's open, piston draws down, and the air comes in. Compression stroke is almost normal. All the valves are closed, piston comes up, and gets super hot as it's compressed. But here is where the Jake brake comes in. Right as that piston approaches top dead center, the arm opens the exhaust valve. That lets all that charged air out of the exhaust. You get no real boom, and what were the power and exhaust strokes are mostly just movement from the piston. Alright, let's grab that syringe. So if I compress the air, it's going to want to expand. It's full of potential energy. If I don't let go, you can see the energy return by the movement of the plunger. But if I compress the air, and then release it, all that energy's lost. So where engine braking with a manual transmission and gasoline engine can slow the vehicle down by creating a vacuum, Jake braking in a diesel engine slows the vehicle by compressing the air, slowing the compression stroke, and then releasing that energy through the exhaust. Alright, listen. Hear that pop? Well, this is 20 milliliters. Imagine this was an inline four, six liter diesel engine. That means each of these would be almost one and a half liters, and that's a lot of Faygo. That's why it's got such a big pop, and that is why a Jake brake sounds like this. So gasoline engine braking is like you're pulling on the pistons to slow them down, whereas diesel engine braking, Jake braking, is like pushing on the pistons to slow them down. That's the difference. Engine braking, not breaking your engine. Like the video and subscribe to Donut for more great ones. What? I'm wearing a different shirt. That's because I got it from Bombfell. What's Bombfell, you ask? 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