ELI5: Engine Braking

Brake pads and wheel discs are cheap. Use them.

Its very necessary when towing something though.

Huh, you learn something every day. Thanks anons who answered the question, I was always too embarrassed to ask. Not OP btw

>you don't seem to understand thermodynamics
Er, right back at you. The pressures yielded (and therefore heat) from cumbustion cycles over varying loads do not remain constant.
Unless you're talking about the microscopic difference made by increased friction? In which case is more than offset by your water and oil pump flowing a higher volume of heat transferring fluid.

Even then it doesn't make any real difference on a carburettored car. Throttle shuts post main jet and restricts airflow and therefore vacuum around the main jet. Depending on the carb, off throttle at any RPM is usually equal to fuel rate at idle.
And it's even cheaper to balance the load between the drivetrain to prolong brake life even further.

The engine will run cooler braking at 4k rpm than it would be under load at 4k rpm. The coolant and oil have an easy time taking care of it.

It's when you're next to some asshole driving a tragic and you drop three gears just so you can go
BEEEEEERRRRRRRRHH!

without touching your brakes to show him off, what a faggot

ACKTUALLY
It puts extra stress on the wrist pin because near top of the stroke it goes from slamming the piston up against pressure to yanking it back down under vaccum; normally the usual combustion cycle will allow the pin to transition from pressure to vacuum easily or will be pushing on it consistently during the change from compression to combustion with constant downward pressure.

But who cares as long as you're not in the habit of slapping the throttle closed at redline for miles.

Interesting point, but even then the stress placed on the wrist pin during that transition from cylinder pressure to vacuum in a low compression street engine is like a drop of water in the ocean compared to the outright pressure of a full load combustion event.
Again, interesting point non the less.

thermodynamics? of what?

you shut off the fuel supply going into the engine meaning the only thing left that creates heat is friction by the engine compressing air (a very small amount of air, because of the vacuum that is being drawn) and friction which is present even while the engine is running.

if you really are worried about this, shut the engine off and floor it while engine braking to get maximum engine cooling and braking while going down a hill.

Well the thing is that if it's worn already than rattling it back and forth will get it to wear even faster vs. just placing a smooth massive load on it.

It's really the finer points of autism tho. Unless you're building a race engine that only has to make it until the next rebuild it shouldn't really matter.

this isnt true the wrist pin is under incredible pressure, but the rod itself is what mostly fails or the wrist pin bosses break and pull the pin out of the piston downwards. pistons and rods are designed for extreme pressures pressing down on the rod and the piston, not so much for when they are being flung out of the cylinder in a vacuum.

you see this alot at the end of the 1/4 when guys let off at 8000rpm and engine brake.

That's fair enough in racing applications at high RPM in high compression environments where that sort of force may cause excessive stress (offset with regular rebuilds/component replacement), but on the street presuming you're operating at a regular RPM range, you're not going to see any difference in engine life.
This is however a problem on high performance motorcycles, as they do face mentioned high RPM in high compression environments with throttle actions usually sharper than in a conventional car. But then these engines usually aren't expected to live 200k mile lives without being opened up.

...

i would agree with the motorcycle part, however usually the b/s ratio is pretty low on bikes meaning the piston mean fp/s is usually lower even at redline. such as the cbr600 has a b/s ratio of 1.57 which is generously oversquare compared to a honda d16 which is .833 or a a b16 which is 1.05.

also the static/dynamic compression doesnt really affect the issue with snapping the throttle shut at high rpm's. since the issue is sudden vacuum at extreme piston fp/s.

pulling vacuum on intake stroke after closing throttle
compression of small amount of available air on compression stroke
zero cushion effects on exhaust stroke

and yeah were in agreement on never noticing an issue when engine braking normally as long as its not sitting at redline for minutes on end.

Engine braking is simply the force of compressing air inside the cylinders slowing you down when you're in gear with the throttle closed.

explain then,why does the temp gauge go down on every car that I've ever driven when going downhill?

>prolong brake life
just put new pads in every 2 fucking years like a normal person

Not in a car with a fuel cutoff.

>This is what "L" or the selectable gears are for on an automatic. Because in Drive it picks the most efficient gear all the time and engine braking is at a minimum.
lmao autofags
I went back to my auto winter beater and its fucking unbearable
I almost fall asleep driving and when you go fast the gearbox freaks out and starts doing retarded shit

>eh, it will save you from break wear and break fade, but the heat that would normally go into your brakes is now hanging around in your engine block
>the heat you get from creating friction by rubbing big blocks of static stuff against rotary stuff of brakes disks things is the same as the engine circulating cooling, lubricating oil but not creating explosions that make it go hot

are you one of those fucking retards who brakes for no reason instead of just letting off the gas and waiting?
Learn to fucking drive

found the brainlet

This guy is actually right... that's how sad this sorry board is...

>fuel cuts off
>no combustion taking place
>somehow heat builds up

...

I can't tell if you all are bait or just fucking retarded.

OP, engine braking is where you let off of the gas, thus closing the throttle, causing a vacuum to form, which in turn saps the energy of the crankshaft. Downshifting accelerates this process.

waaait, coasting in neutral doesn't really save fuel? Am I wasting my brakes away? I have a 2007 tsuru if that's relevant

in modern cars (2000+) fuel injection is cut off while coasting in gear. So coasting in neutral actually consumes a tiny amount of fuel in order to keep engine at idle. Plus it's more dangerous

Thank you for the answer.

Also by cut off I guess you mean completely? like the engine keeps going just by inertia?

no, you're just as dumb as him.
I can redline my engine all day and the temp gauge won't change
and you're telling me I'm overheating it by engine braking.

The engine keeps spinning because it's still connected to the wheels and the wheels are spinning.

> gearbox freaks out and starts doing retarded shit
how about fixing it you bloody halfwit

But if I'm engine braking and then press the clutch what happens? I guess the engine keeps going by inertia and then at a certain RPM fuel gets injected again? Or am I wrong?

Yeah once you step on the clutch then the engine will continue to spin due to momentum without having any fuel injected until it slows down enough for the idle logic to kick in and start injecting fuel again to keep it going.

Which is exactly the same as with revving when stationary.

Jah. Revving while stationary will inject fuel to spin it up, but if you take your foot all the way off, it will cut fuel until it slows back down close enough to idle.

yeah? then redline your car for 30 minutes and post results bitch

Wow, you are full of shit.

Dude you need to coast more I've been on my current pads for about 5 years now and they're perfectly fine.
Although living where there are no hills helps

Its not 1970 faggot.

I'd prefer to drive properly and not be fucking retarded.
Maybe if you're insistent on driving your manual like an auto, you should get an auto.
Not in any car.
-9001/10

While I'm sure that person's exaggerating, the truth is unless you're pulling a sustained heavy load for 100% of that time, the cooling system shouldn't be near its capacity... Which I think was the point that person trying to make.
However, pulling massive sustained RPM without any load has its own problems.

Look at how manufacturers test engines, redline for the amount of revs that the engine needs to last before it is supposed to blow up. Who ever designed his engine has redlined such engines with load for literally thousands of hours. It should be perfectly fine to redline a modern car as long as the cooling and lubrication can keep up, which it can if it isn't a pos

Certainly I can see how this would be true on the compression stroke, but don't most failures of this sort happen on the exhaust stroke anyway? There'd be very little difference between on- and off-throttle under those circumstances.

I'm a citycuck
I'm not saying engine braking is wrong, i do it too but mostly when going downhill or on highway offramps

>This is what autotragic fags think

Idling the engine while coasting in neutral is more efficient than the momentum you lose with engine braking, even with fuel cutoff. You'll ultimately use less fuel.
If you're one of those turboautismos, you could also shut off the engine while coasting. Who needs power steering and braking anyway.

B/s? Do you mean rod to stroke ratio?
A b16's r/s is 1.74 which is .01 away from perfect ratio

>car has no power steering

Not necessarily, unless the vehicle in question is rated at a 100% duty cycle @ what ever arbitrary load and output, which even in industrial applications such a 100% duty cycle rarely exists.

Maybe not 100% but plenty of things are good for getting pretty close as steady state operation provided cooling and whatever are good for it. For instance there's a pretty good track record for Subaru EJ25s running at 5kRPM or more in aircraft application with no worry about leaving them there for as long as you want to stay in the air. 6kRPM (not far from redline) for takeoff. So not redline and not necessarily WOT, but a decent modern engine will happily hum along ad infinitum under heavy use if it's not cucked out of an appropriate cooling system or decent maintenance.

Technically 100% or not is nitpicking for the purposes of this thread. We're not talking race engines.

There's a lot of cars that overheat slightly on a racetrack (and some of them overheat majorly). Without air flow I would assume even more cars would struggle with keeping operating temperature. Also, every dyno has at least one fan they put at the grill, and they don't rev the car constantly, just several runs.

It is more efficient assuming you are not willing to stop anytime soon. If the purpose is to stop the car, coasting in gear is the way to go - you have a slightly smaller wear on the breaks and you don't burn the fuel, which are not the case in neutral.

>Subaru EJ25s running at 5kRPM or more in aircraft application with no worry about leaving them there for as long as you want to stay in the air
True, but thos. Engines aren't facing anywhere near maximum load. RPM is only a small part of the equation, it's load that really creates heat.
As a heavily exaggerated example, think of a bulldozer pushing a slot trench with a fully loaded blade for an extended period of time. Then the same machine tramming in top gear with the blade up in the air. In both scenarios the machine will be revving at about 2000rpm, but only one of those scenarios creates an unsustainable ramping up in engine temperature.
Whilst I'm not claiming said EJ25 isn't facing load at cruising altitude, but it's hardly "heavy use", and it's not working as hard as it would be in a motor vehicle loaded to its GCM driving up a constant hill.

And I forgot to mention, that's not even taking ambient temperature into account.
Even any brand new 4x4 without any manufacturing defects can be overheated trying to push through a desert of hungry coarse power sapping sand in low gear on a near 100° day if throttle and load is sustained.

You're pulling a vacuum on the throttle body.
Disgusting white whore I'd never stick my dick in a white woman.

Jah, I'm not going to argue with that. Your typical road car is not really designed to hold up at the track or doing heavy towing all day or other heavy duty application. But it's no worry for non-abusive engine braking that simply happens to keep it spinning fast any more than going WOT on every on ramp. Any motor worth its salt should hold up plenty long when run pretty hard even if it's not 100% hard as long as being taken care of appropriately.

And that in my opinion is an example of not supplying adequate cooling for a situation that goes outside of expected use.

I much prefer clutch braking, much healthier for my car since I'm not straining the engine or the brakes.

>friction
>compression
Still doesn't mean a car designed to run at 6k rpm all day in 110 degree heat is going to have a problem

Where's your typical thermostat, maybe 170deg? Who cares if it's a 110deg day?

Basically my point too that "non-abusive engine braking that simply happens to keep it spinning fast" not only makes absolutely no increase in engine heat, but if anything helps cool with increased oil/water pump speed supplying a higher rate of heat transferring fluid as opposed to idling. We seem to fully agree here.
>And that in my opinion is an example of not supplying adequate cooling for a situation that goes outside of expected use
The problem with the term expected use is that it's quite vague. The example I gave is to the further limits of capability, but as I said earlier there's not much in the way of consumer and industrial vehicles and machinery that's expected to operate at maximum load (and therefore power) with a 100% duty cycle. So I guess you're technically correct with "situation that goes outside of expected use" in that it's not expected that many examples would reach close to their load limits for extended periods.
Kind of going off topic here, but other means of increasing duty cycle includes limiting power outputs, like consumer vehicles and 4x4 that may share a powerplant with something a lot more industrial can get away with a substantially higher output. To use an example a 6.7ISB fitted to a typical HD Ram might make about 400hp, but fitted to a wheel loader it'd be lucky to be rated at 200hp.

...

Combustion creates more heat than friction, which is minimized provided your engine has oil.

So if you're off the throttle, and engine braking, its not creating anywhere the amount of heat it would be on throttle regardless if its the same RPM or not.

In addition to that, your water pump is sniping faster cycling coolant through the block, cooling it even faster.

My first car was a little oil crisis japbox. I'd have it near or at WOT in the highest gear for hours on end because that's all it could give. Purred like a kitten even after 200k when it was started to just fall apart after so many years. Granted that was at around 50hp/L, so not very stressed.

Felt like redline was about another maybe 600RPM above what it could hold down, but it didn't have a tach and I don't know where the redline was anyway. Obviously a fundamentally not-so-stressed engine, but it was correctly designed to be fit for its intended purpose.

>Am I wasting my brakes away?
they're cheap anyways, who cares
are you actually in a situation where you need to engine brake downhill? It's ok to pop into neutral directly instead of downshifting sequentially if it's just level ground to a red light.

Who said anything about need? I mean, there are a few hills where I have to drop down a gear or two if I want cruise control to work because I'll actually speed up in the highest gear.

But who cares? Engine braking is FUN and in many conditions offers superior control and fights brake fade, not to mention if you're going to be driving a manual anyway you can get back on it in the gear of your choice. Nobody cares about cheap brakes, or expensive brakes if you actually do need expensive brakes.

bore/stroke ratio is also used in place of rod/stroke if you dont want to look up the exact rod length.

b16 is square, but a cbr600 is nearly undersquare.

jake brakes are used on diesel engines as they do not rely on air flow for throttle control, and thus have no airflow manifold and can not actually engine brake.

jake brakes work in a completely different manner

intake manifold
jesus christ I'm really not doing well tonight am I