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

0:00 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'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 gonna damage something like the transmission or maybe the engine what the heck is a Jake brake we're getting to it to understand either those methods for slowing down you gotta understand the engine a little bit we've mentioned before that the engines like an air pump

0:46 the more air it can pump the more power you 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 ds1 first let's talk about engine braking in a gasoline-powered manual transmission automobile we're also gonna 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 stroke stirs follow intake throttle open valves open air and gas mix fills the vacuum created by the piston compression vows closed piston moves up and the pressure builds the power strip Pistons driven down by

1:33 combustion generating power finally exhaust where the exhaust valves 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 in a rotational movement and it heads the output shaft which then heads through the clutch into the drive shaft powering the wheels easy all right now you've taken your foot off the gas let's say you're just starting enough sitting in the driveway your engines 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 they're from the throttle that requires less gas that keeps you at those low low revs all right now you're cruising in a

2:19 manual at 55 miles an hour and you let off the gas you're gonna feel the car slow down but the engine rpm will still closely approximate where they were when you're cruising with the gas and that is because the wheels are now driving the engine if you put the clutch in the drive shaft in the engine disconnect and the revs will drop you will be at idling speed and you won't decelerate as quickly so why are you decelerating when the drive shafts engage well that is thanks to the throttle body this syringe will represent our piston when the throttle bodies 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 is at idle this piston is going to be moving more slowly so the throttle body despite not

3:06 being all the way open still left's 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 in 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 foots off the gas the engines creating those tiny little vacuums so it's slowing down

3:52 now if I downshift I've just increased how many of those tiny little vacuums happen each seconds per revolution to the engine and that's why engine braking slows you down I've also increased the availability of torque it's crazy so is it bad for the engine to use the engine to decelerate I'll tell you if you subscribe did don't oh wait all right well not really you're just using air pressure to create a vacuum to slow the wheels but let's not celebrate it okay you could don't you don't you don't want to get the revs up to like a million you'll break it just be smart all right so what about diesels I know you've heard the rumble of a Jake brake if you think this sounds pretty cool

4:39 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 well gasoline engines are more like an air pump we're letting more air in get 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 box 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

5:24 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 some tractor trailers are 73,000 pounds it's a lot of inertia if you think about diesel you're gonna think a big Daddy Rudolf diesel who patented the very efficient torque generating engine cycle and probably one other dude as well classy Lisle Cummins of Cummings diesel yes he's largely responsible for the advancement and mainstreaming of diesel engines but classy Cummins also patented the diesel engine braking system that we know as a Jake brake like James told you me up to speed on Commons in order prove how efficient diesel engines were classy got a team together to drive a

6:09 diesel-powered truck from New York to Los Angeles in an attempt to set a new truck speed record across this great continent these were going great until their descent of the cajon pass into San Bernardino California they're heading down a long steep gravel road 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 well will demonstrate that in another video do truck runway wear amps work myth busted we're dead no no I'm sure they do but if we busted it I mean we're dead so on the edge of calamity classes like me I wish I knew somebody who knows enough about diesel engines to devise

6:56 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 Plessy 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 VAT cams motion to the exhaust valve of a cylinder in April 1960 Jacobs manufacturing company established its class CL Cummins division to manufacture that engine brake the Jake brake is named after the company that manufactured the Jacobs manufacturing company thanks Jacob's ok but here's how it works we briefly said

7:42 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 intakes valve open the piston draws down in the air comes compression stroke all valves are closed the piston comes up air gets super hot as it's compressed power stroke the fuel gets injected in an item the heat which drives the piston down and then the exhaust stroke the exhaust valves open and the bad stuff gets pushed out but when the Jake brake is on 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

8:28 intake valves open piston draws down 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 all right let's grab that syringe so if I could press the air it's gonna want to expand it's full of potential energy if I don't let go you can see the energy returned by the movement of the plunger but if I compress the air and then release it all

9:14 that Energy's lost so we're 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 all right listen hear that pop well this is 20 milliliters imagine this is 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 [Music] so gasoline engine breaking it's like

10:00 you're pulling on the Pistons to slow him 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 but I'm wearing a different shirt that's because I got it from bomb fell what's bombs fell you ask bomb fell is an easier way for men to get better clothes fully personalized every piece handpicked stylist is gonna email you with his or her selections after which you get 48 hours to make any changes or cancel mall together you're in total control my stylist got me the sweet green shirt to match my eyes all right look I recently had a kid I got no time to go shopping yeah I can't tell you last time I slept

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