[LinuxBender]

If it's when they are slowing down it's likely the jake-brakes j-brakes you are hearing especially if they have straight pipes. Diesel engines don't have engine braking by design so a mechanism was added to the heads to create artificial engine braking that can be toggled on per head.

[ Edit for clarification: ] I have created some confusion with this statement. For clarification diesel engines never had engine braking due to the lack of a throttle plate but this has been worked around with add-ons using different techniques. On a big-rig this is jake-brakes. On smaller modern vehicles this is usually a small turbo or an exhaust baffle. The operator of a modern diesel vehicle will effectively experience engine braking when they let off the throttle. On older diesel pickups and cars there was no engine braking.

[gambiting]

>>Diesel engines don't have engine braking by design

First time I hear about this. I've driven and owned plenty of diesel vehicles in my life and diesel engines definitely do have engine braking(unless it's different in semis? but I don't see why it would be - just leave it in gear and let it slow down?)

[garaetjjte]

Technically diesel engines do not strictly "engine brake" because of lack of throttle plate, and thus lack of pumping losses. However that doesn't mean that it won't slow down: friction losses, heat loss to cylinder walls, etc. still occur. Surely diesel passenger car will decelerate stronger when left in gear than in neutral. Given how many pages and pages of discussions you can find people arguing whether petrol or diesel engines brake stronger, it seems pumping loss doesn't make that much difference.

[LinuxBender]

The engine itself has no braking due to the lack of the variable air-intake that gasoline engines have that would otherwise starve the engine for air especially when downshifting and closing the throttle.

Specifically on non-big-rigs, modern diesel cars and pickups create engine braking using a small turbo and tighten the spline or in some cases have an exhaust feedback baffle or flap, varies with year/model. Big rigs still use jake-brakes.

[tomxor]

Interesting. I was only taught engine breaking from the practical perspective of down-shifting, but not the details of why it works. I understood the implicit effects of shifting down - maintaining the same high RPM with the same high resistance as a vehicle slows... but never gave much thought to what exactly those resistances were, I just assumed it was a combination of friction, compression, driving an alternator, other arbitrary mechanical losses etc.

Would there really be no significant braking effect without that "high manifold vacuum"? I suppose the engine does have a lot of mass so I could believe the effect could be too slow to be useful.

[mcguire]

Gasoline engines have a throttle plate that, when you let off the throttle, prevents intake air from reaching the cylinders. The pistons try to draw air into the cylinders and create a pretty decent vacuum. (Respect to the throttle plate. :-))

Diesel engines don't; the throttle controls fuel flow into the cylinders. Let off the throttle and air flows through the intake, cylinders, and exhaust just without producing any power.

The effects of friction are roughly the same on both engines, and they are what engine designers and builders want to minimize to maximize fuel efficiency and power.

[throwaway0a5e]

It doesn't have "no" braking. But it has a hell of a lot less than it would if there were some restriction on it, e.g. a throttle.

[spookthesunset]

I've done what feels like engine braking in "consumer" diesel trucks. Since I never had to flip switches or anything, how does the engine know how to enter into this "engine braking" mode?

Never even crossed my mind that diesels don't natively engine brake. Then again how diesels work is a bit of a mystery to me... mostly because I never bothered to look into it much.

[LinuxBender]

Newer diesel engines use a turbo or baffle. Most commonly a turbo to create effective braking. This is operationally superior to jake-brakes in that the mechanism is tied into the ECM and transmission allowing for things like cruise control to function as expected. Jake-brakes on the other hand require a bit of technique by the driver to use correctly and avoid jack-knifing the vehicle with its trailer, especially on ice. Some modern pickups can even be put into "towing mode" to make better use of the add-on braking mechanism and allow cruise control to work downhill.

I suppose this the right time for an important PSA. If anyone tows something heavy in an older diesel pickup be aware the only braking you have is what your brake pedal provides. Glaze those brakes and you are going on an exciting adventure.

[robocat]

For anyone wishing to experiment:

You can test the petrol-car-vacuum braking theory if you have an older manual petrol car with a cable from the accelerator to the butterfly valve of the throttle. While driving at 50kph, put into neutral, turn off the ignition, engage a lower gear, release clutch. Test pressing and releasing the accelerator pedal while using engine braking and feel for a difference.

SAFETY: 1. Don’t turn off the ignition all the way and lock the steering (although I admit that is very exciting to have steering locked into one direction, I don’t recommend trying it). 2. Be mentally prepared to lose power steering and power brakes. 3. Only on wide straight roads with no other traffic and safe ways to stop. 4. Probably other warnings specific to your vehicle, and situation. 5. I recommend against trying it on an automatic trans.

If your diesel has turbo vanes controlling the braking, you could probably test it out the same way (presuming electronics are disabled when ignition is off).

Another way to test things is to remove relevant fuses.

Disclaimer: there are lots of ways to screw up even being careful - I do not recommend learning by failure in deadly situations.

[garaetjjte]

Diesels might have anti-shudder valve which closes air intake when shutting off ignition.

[jbkiv]

Thanks! I learned something new today.

[skywal_l]

Diesel engines do not have engine breaking? Are you sure? For me, engine breaking is just the fact that the engine, without power, have moving pieces which, by inertia, is going to slow down the vehicle. Diesel engine being heavier than "regular" engine, the engine brake effect is more important.

At least that's my experience with the cars I used to own.

Edit: For the record, my experience is for 4-strokes diesel engines. Apparently, 2-strokes are still in use in the US.

[pwg]

Diesel engines have no throttle plate that controls the airflow into the engine.

The closed throttle plate in a gasoline engine is what creates a gasoline engine's brake effect, by pulling a vacuum in the intake below the closed throttle plate, which produces the brake effect.

With no throttle plate, the remaining mechanical components in a diesel engine provide minimal friction, certainly not enough to produce any brake effect.

The jake brake (https://en.wikipedia.org/wiki/Jake_brake) converts the diesel engine into a huge air compressor when activated, which provides an engine brake effect. Unfortunately it also often creates a very distinctive, and often loud, sound from the exhaust as well.

[seszett]

> With no throttle plate, the remaining mechanical components in a diesel engine provide minimal friction, certainly not enough to produce any brake effect.

I'm not sure if maybe we have different definitions of "braking", but a diesel engine definitely slows down a car when one throttles down. The vehicle slows down faster than when on neutral, and the braking power depends on which gear is engaged, which seems to indicate very much that there is engine braking going on.

[ska]

Posters point wasn't that the vehicles you drive didn't effectively have engine breaking, but that in diesel designs this is something that had to be added intentionally - with [edit gas engines] you get it whether you want it or not.

Fun fact - the effect can be strong enough on a high compression motorcycle engine to break your rear tire free (obviously lots of other parameters there).

[skywal_l]

Interesting.

But what do you mean by 4 cycles. The diesel engines I know all have 4 cycles. I though 2 cycles engines were found on old tractors from the 50s no?

Edit: Looking at [0], assuming this is true, I understand the confusion now. It seems, in the US, heavy duty diesel engines are 2 strokes which, apparently, do not have engine braking.

[ska]

Me being sloppy, of course you can have 2-stroke or 4-stroke diesels. Edited to improve.

The main thing going on here isn't the cycles, it's the lack of a throttle plate. With these designs the amount of air entering cylinder doesn't relate to your throttle position.

If you come off the throttle every compression cycle a "full" cylinder of new air gets compressed, then decompresses and pushes against the piston. In normal operation the energy is re-transferred to the crank (with some loss). It sort of "bounces". But with a compression brake, you force the engine to do the work of compressing that air, but then full open the exhaust valve to let the pressure escape... much more energy lost each cycle, which transfers through drive train and slows you down.

In comparison to typical ICE: in that case when you come off the throttle, the intake is sealed off, so the cylinder on intake stroke is "sucking" against a closed path, which loses energy. Similar effect, different cause.

[roelschroeven]

> With no throttle plate, the remaining mechanical components in a diesel engine provide minimal friction, certainly not enough to produce any brake effect.

I don't understand. I've driven multiple diesel engine cars throughout the years, and they most definitely have a brake effect. I'm not even sure they brake less than the gasoline cars I've driven. Easily enough to slow down for taking an exit from the freeway, for example, when shifting down appropriately. To the point that there regularly are situations when I lightly press the brake pedal not to brake but to simply light the brake lights, if there are cars behind me.

It does sound plausible that the lack of a throttle leads to less or no brake effect, but it simply doesn't fit my observations.

I'm talking about regular cars here, both recent and less recent (the oldest one was built in 1989).

Maybe there are different diesel engine types with different brake capabilities? Or do some gasoline engines brake much more than what I'm used to, and my reference for what is and isn't significant braking is all wrong?

[mcguire]

https://www.youtube.com/watch?v=o8Cta2cC2Co

https://en.wikipedia.org/wiki/Engine_braking#Diesel_engines

[pwg]

See @ska's comment at https://news.ycombinator.com/item?id=30195348

[skywal_l]

Diesel engines might not have throttle plate but they use injection which certainly do not inject air when acceleration is released, so the cylinders will act exactly the same way. Reading the web I see conflicting account on this subject. Strange...

Also, I though that modern petrol engines did not have throttle plates anymore and use the same injection system than diesel engines (no more carburetors).

[ska]

> use injection which certainly do not inject air when acceleration is released,

FWIW injectors don't inject air; the airflow is separate, get's compressed (and hence heated) then the fuel is injected, then bang (in diesel)

[garaetjjte]

>work the same way as 4cyl

I'm not sure what you mean, both Otto and Diesel cycles are four-stroke.

In petrol engines power is usually controlled by throttle plate which limits volume of air going into cylinder, and enough fuel is added during the intake stroke (either by injection or carburetor) to have combustion close to stoichiometric.

In diesel engines there's no throttle plate and engine always runs on lean mixture, and power is controlled only by amount of injected fuel, which is done after air is already compressed and hot.

[ska]

Point was injectors inject fuel not air...

I think we cross-edited, remaining confusion I think was about 2 vs 4 stroke but it's not really relevant so I had adjusted with a nod to when diesel injection occurs in 4.

[throwaway0a5e]

The fuel system doesn't provide restriction on the air going through the engine.

A diesel engine that's not dumping in fuel (because your foot isn't on the pedal) has about as much engine braking as a gas engine that's run out of fuel but the operator has floored the pedal.

A gas engine has a throttle that can restrict airflow. A diesel can either be equipped with an exhaust brake or compression brake. The latter is tons more effective but louder.

[LinuxBender]

Diesel engines themselves have no engine braking. Each personal vehicle implementation of diesel engines have worked around this using different techniques. The most common outside of big-rigs is a turbo that tightens a spline or closes a feedback baffle.

To the operator of the vehicle it will appear there is engine braking on modern diesel engines. Older pickups and cars have no engine braking.

[spookthesunset]

> Older pickups and cars have no engine braking.

So what did they do on long downhill mountain passes? Just ride the brakes? Were the brakes designed to accommodate being ridden for so long?

Asking 'cause I downshift all cars I drive when going down mountain passes...

[LinuxBender]

They would drive slowly and carefully and take alternate routes when possible.

Just ride the brakes?

No that will overheat and glaze the brakes. That is why long steep hills initially had run-away ramps created. The run-away ramps are still used but not nearly as much as they used to be. In many places alternate routes were created for people towing heavy things. A good example of this is the grapevine on I-5 in southern California. There is a truck route and the main route. That also has many run-away ramps.

[spookthesunset]

> In many places alternate routes were created for people towing heavy things.

That, uh, sounds pretty inconvenient!

So without engine brakes if you downshift in an older diesel does the engine just rev right up and the car doesn't even bother to act like it is slowing down? That has to be pretty weird....

[mcguire]

Eastbound on Interstate-40 on the eastern slope of the Appalachians the truck speed limit at the top of the pass is 35mph and there are very, very many warning signs including radar-activated lights. There are also three or four runaway-truck ramps (filled with loose gravel) that are somewhat frequently used, and often trucks pulled over to the side to let their brakes cool.

(The Rockies have even more of this sort of thing, but I haven't been out there in quite a while. :-( )

[BenjiWiebe]

It very much slows down. Just not quite as much as a gas engine. You still have friction losses (especially as you get higher rpm), losses from alternator, water pump, engine fans, oil pump, etc.

One of my vehicles is a VW Jetta TDI (diesel, ALH engine).

[schwap]

There's nowhere near enough inertia in the rotating assembly of an engine to significantly slow down a vehicle.

[jacquesm]

It's not the inertia that does the job (that keeps things going, actually) but the compression and shedding the compressed air that will slow things down. But for a big rig doing that idling it won't be enough, especially not on a descent with 25 tons pushing you downhill.

[BenjiWiebe]

It's also the friction of everything turning. And you've still got your alternator, oil pump, water pump, fan etc that are removing energy.

[jacquesm]

A Jake Brake is essentially using the engine as a compressor and then venting the air at TDP, that's why they make such a racket.

[jacquesm]

A Jake brake is for long descents, it essentially uses the engine to slow down instead of the brakes to avoid overheating them.

Normal diesels do engine braking just fine, but not aggressive enough to shed speed on a long descent without over-revving, and you really don't want to do that with a diesel engine.