The noise level of some US trucks still surprises me. Jet engine like.
[ 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.
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?)
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.
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.
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.
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.
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.
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.
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.
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.
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...
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.
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....
One of my vehicles is a VW Jetta TDI (diesel, ALH engine).
(The Rockies have even more of this sort of thing, but I haven't been out there in quite a while. :-( )
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.
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.
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).
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.
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.
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.
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).
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.
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)
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.
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.
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?
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.