question about engine braking.
It's hard to find techs who really, really know about transmissions; and that definitely includes myself. I haven't pinned down the answer to this one yet.
For engine braking to work, the wheels have to want to be going faster than the engine, no?
So that the wheels are trying to drive engine compression (rather than engine combustion powering the wheels).
This works a LOT better if you actually have release valves to vent the compressed air, otherwise the compressed air returns a lot of its energy like a spring-- so there's some engine braking cam with engine braking valves on a truck's diesel. A feature that doesn't exist on a gasoline car engine.
Strike one.
But more importantly...
In my manual transmission'd car, when I'm in 5th gear and I let off the gas, the engine rpms stay in line with my vehicle speed. They are mated together with the clutch.
But in my automatic transmission'd car, let's say I need to get to 3000rpm to get to 70mph (and I've stayed in overdrive, and didn't downshift to 3rd). When I let off the gas, that rpm needle drops like a stone to 1200rpm. Try to accelerate again, and it needs to quickly rise back to 3000rpm before I start going faster.
It is NOT tied to the wheel speed.
What I've read/been told, and it make sense, is that the torque converter as designed is also a one-way fluid clutch. I've got a very basic torque converter from 1990 without locking overdrive, if that changes anything.
This is designed such that the pump must always be going faster than the turbine to drive it. The turbine cannot drive the pump: that is to say, if you let off the gas and the drive wheels are still spinning at whatever rpm is 70 miles per hour, they won't be able to drive the engine in the other direction. It's a one-way fluid coupler.
I mean, I'm pretty sure I'm seeing this every time I let off the gas on my automatic and rpms plummet right to idle, and every time I let off the gas on my manual and they don't drop any more than vehicle speed.
But that means that in my automatic, if the rear wheels cannot drive the engine in the first place (not to mention that the engine has no engine braking release valves),
I will get NO engine braking out of my automatic, no matter what gear I choose to force it into? The wheels cannot drive the engine, not in 1st, not in 2nd, not in any gear. Only the engine can drive the wheels.
I feel that this is right.
Going down pikes peak, my one and only experience with mountain driving to relate to, it seemed like engine braking wasn't doing anything and it was all due to the brakes. Let off the brakes, and I sped up as fast in forced-2nd as I did in overdrive, with nary a rise in engine rpms (seemingly untied to vehicle speed).
Maybe more modern, more sophisticated torque converters can do more. My sister feels that her 2010 nissan versa with CVT does do engine braking for her, although she may be wrong. I can't find whether it has a torque converter or not. But even if it does, maybe they lock the torque converter to use it for engine braking. Or maybe she's just imagining it. So I also wouldn't mind knowing about CVTs, or, more modern automatics that have locking converters and maybe other functions. In theory if you lock the converter, maybe it acts like the direct clutch on a manual, with none of that one-way fluid clutch. On the other hand, if you force the engine rpms to be tied to the wheel speed and run as fast as the car is, it DOES burn more gas, the opposite intention of a locking torque converter-- so it's entirely possible that even modern automatics could be incapable of any engine braking.
Prius of course uses its electric motor/generator to engine brake, that's one exception.
But it is it correct that my old 1990 ford with its 4 speed AOD and very basic non-locking torque converter, is by design incapable of any engine braking?
Apologies for the wall of text.
I wanted to lay out all of the information I believe I have, so you can follow my train of logic-- and if I can in fact engine brake, pinpoint where I've got some bad information that took me to the wrong conclusion.
And if I'm correct and I can't engine brake, well, next time I come down pike's peak in my wagon [never], I won't bother putting it into 2nd because it won't make a difference. More a point of curiosity than anything that will grossly affect my driving.
It's hard to find techs who really, really know about transmissions; and that definitely includes myself. I haven't pinned down the answer to this one yet.
For engine braking to work, the wheels have to want to be going faster than the engine, no?
So that the wheels are trying to drive engine compression (rather than engine combustion powering the wheels).
This works a LOT better if you actually have release valves to vent the compressed air, otherwise the compressed air returns a lot of its energy like a spring-- so there's some engine braking cam with engine braking valves on a truck's diesel. A feature that doesn't exist on a gasoline car engine.
Strike one.
But more importantly...
In my manual transmission'd car, when I'm in 5th gear and I let off the gas, the engine rpms stay in line with my vehicle speed. They are mated together with the clutch.
But in my automatic transmission'd car, let's say I need to get to 3000rpm to get to 70mph (and I've stayed in overdrive, and didn't downshift to 3rd). When I let off the gas, that rpm needle drops like a stone to 1200rpm. Try to accelerate again, and it needs to quickly rise back to 3000rpm before I start going faster.
It is NOT tied to the wheel speed.
What I've read/been told, and it make sense, is that the torque converter as designed is also a one-way fluid clutch. I've got a very basic torque converter from 1990 without locking overdrive, if that changes anything.
This is designed such that the pump must always be going faster than the turbine to drive it. The turbine cannot drive the pump: that is to say, if you let off the gas and the drive wheels are still spinning at whatever rpm is 70 miles per hour, they won't be able to drive the engine in the other direction. It's a one-way fluid coupler.
I mean, I'm pretty sure I'm seeing this every time I let off the gas on my automatic and rpms plummet right to idle, and every time I let off the gas on my manual and they don't drop any more than vehicle speed.
But that means that in my automatic, if the rear wheels cannot drive the engine in the first place (not to mention that the engine has no engine braking release valves),
I will get NO engine braking out of my automatic, no matter what gear I choose to force it into? The wheels cannot drive the engine, not in 1st, not in 2nd, not in any gear. Only the engine can drive the wheels.
I feel that this is right.
Going down pikes peak, my one and only experience with mountain driving to relate to, it seemed like engine braking wasn't doing anything and it was all due to the brakes. Let off the brakes, and I sped up as fast in forced-2nd as I did in overdrive, with nary a rise in engine rpms (seemingly untied to vehicle speed).
Maybe more modern, more sophisticated torque converters can do more. My sister feels that her 2010 nissan versa with CVT does do engine braking for her, although she may be wrong. I can't find whether it has a torque converter or not. But even if it does, maybe they lock the torque converter to use it for engine braking. Or maybe she's just imagining it. So I also wouldn't mind knowing about CVTs, or, more modern automatics that have locking converters and maybe other functions. In theory if you lock the converter, maybe it acts like the direct clutch on a manual, with none of that one-way fluid clutch. On the other hand, if you force the engine rpms to be tied to the wheel speed and run as fast as the car is, it DOES burn more gas, the opposite intention of a locking torque converter-- so it's entirely possible that even modern automatics could be incapable of any engine braking.
Prius of course uses its electric motor/generator to engine brake, that's one exception.
But it is it correct that my old 1990 ford with its 4 speed AOD and very basic non-locking torque converter, is by design incapable of any engine braking?
Apologies for the wall of text.
I wanted to lay out all of the information I believe I have, so you can follow my train of logic-- and if I can in fact engine brake, pinpoint where I've got some bad information that took me to the wrong conclusion.
And if I'm correct and I can't engine brake, well, next time I come down pike's peak in my wagon [never], I won't bother putting it into 2nd because it won't make a difference. More a point of curiosity than anything that will grossly affect my driving.
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