[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.