7 ms·

The noise level of some US trucks still surprises me. Jet engine like.

LinuxBender 5y ago
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 5y ago
  >>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?)
  - LinuxBender 5y ago
    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.
    - jbkiv 5y ago
      Thanks! I learned something new today.
    - throwaway0a5e 5y ago
      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 5y ago
      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 5y ago
        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 5y ago
        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 5y ago
    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.
- skywal_l 5y ago
  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.
  - LinuxBender 5y ago
    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 5y ago
      > 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 5y ago
        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 5y ago
- jacquesm 5y ago
  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.

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 5y ago
>>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?)
- LinuxBender 5y ago
  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.
  - jbkiv 5y ago
    Thanks! I learned something new today.
  - throwaway0a5e 5y ago
    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 5y ago
    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 5y ago
      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 5y ago
        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 5y ago
  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.
skywal_l 5y ago
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.
- LinuxBender 5y ago
  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 5y ago
    > 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 5y ago
      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 5y ago
jacquesm 5y ago
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.