[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.

[skywal_l]

In a 4-stroke engine, throttle or not, intake valves are shut down when in compression so cylinders are sealed off, compression happens anyway, diesel or gas. Indeed, in 2-cycles engines there are not intake valve so LinuxBender's point is valid.

[ska]

I think you misread; I should have been clearer. This is how I understand/remember it although to be fair it's been a while since I've worked on either so might mess it up a bit.

Anyway it has nothing to do with compression or the intake valve in either case. Compression happens in both cases, and doesn't affect anything.

In diesel, Jake type breaks steal energy by opening the exhaust valve right after TDC, e.g. what would be the power stroke. The energy stored in compressed air escapes out the exhaust valve rather than being (mostly) reclaimed by the crank on expansion - this slows down the crank and hence (if not in neutral) the vehicle slows. NB this is not when the exhaust valve would normally open, but rather a cycle earlier.

In gas, on the intake stroke the intake is blocked (not by the valve, further up by throttle) so the intake motion creates vaccuum - this takes energy, which slows down the crank, and hence etc. etc. The exhaust valve doesn't change timing.

The latter approach only works if you have something blocking the intake "above" the intake valve. In a diesel engine the airflow is kept the same and the fuel adjusted (unlike gas) so there is no natural mechanism to do this with the throttle.

[sokoloff]

Most of the energy stored in the cylinder charge during the compression stroke is returned (as if an air spring) on the (what would be the) power stroke. The difference between a gas and diesel engine shows up in the higher pumping losses on the intake stroke (if you're pulling air past a closed throttle plate or not).

[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.