6 ms·

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

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
      > 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....
      - BenjiWiebe 5y ago
        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).
        frosted-flakes 5y ago
        TDIs have turbos.
      - mcguire 5y ago
        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. :-( )
pwg 5y ago
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 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 5y ago
  > 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 5y ago
    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 5y ago
      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.
schwap 5y ago
There's nowhere near enough inertia in the rotating assembly of an engine to significantly slow down a vehicle.
- jacquesm 5y ago
  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 5y ago
    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 5y ago
      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.

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
    > 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....
    - BenjiWiebe 5y ago
      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).
      - frosted-flakes 5y ago
        TDIs have turbos.
    - mcguire 5y ago
      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. :-( )

pwg 5y ago

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 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 5y ago
> 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 5y ago
  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 5y ago
    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.

schwap 5y ago

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

jacquesm 5y ago
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 5y ago
  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 5y ago
    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.