Auto gearbox efficiency
Discussion
I'm curious- what is the most efficient speed for a torque converter gearbox?
At very low engine speed the car doesn't move if there's even a little braking force. Also at high speeds there will be appreciable friction losses.
What sort of speed is the most efficient? Would an efficiency graph be similar to a car's power graph where it rises steadily to a peak then drops off a bit faster than it rose?
No particular reason for asking other than idle curiosity.
At very low engine speed the car doesn't move if there's even a little braking force. Also at high speeds there will be appreciable friction losses.
What sort of speed is the most efficient? Would an efficiency graph be similar to a car's power graph where it rises steadily to a peak then drops off a bit faster than it rose?
No particular reason for asking other than idle curiosity.
jkh112 said:
At certain speeds torque converter boxes lock up and hence gain in efficiency. The exact speed at which this happens varies across a range of factors including gear, revs, box type, car manufacturer.
Specifically ZF 5HP24 but curious as a general concept.Is the lock-up a mechanical device or how does it work?
Rovinghawk said:
kambites said:
I think a simple fluid coupling gains significant efficiency with speed, so the 'box is probably at its most efficient at peak RPM.
Does that compensate for increased frictional losses?As mentioned, modern epicyclic wet disc clutch automatic gearboxes now have a 'fully rated' lock up clutch on the torque convertor. As such, that enables them to completely negate the fluid losses in the convertor when locked. However, due to torsional vibration, and often to minimise the perceived boost threshold on turbo engines, this lock up clutch is not locked until around 1500rpm typically. At any point where the TC is locked, the gearbox geartrain efficiency is not noticeably lower than a conventional manual gearbox using constant mesh helical gears. However the internal brake system used to select the appropriate gear ratio, requires hydraulic pressure to counteract the engine torque and this hydraulic power comes from an internal oil pump driving from gearbox input power. Modern electronically controlled autoboxes are clever in this respect and have pressure controlled oil pumps inside which only generate just enough pressure to prevent slip in the brake system, varying that pressure as the load varries. That means at light load (part throttle driving) they can get close to the efficiency of a purely mechanical manual gearbox.
Max_Torque said:
As mentioned, modern epicyclic wet disc clutch automatic gearboxes now have a 'fully rated' lock up clutch on the torque convertor. As such, that enables them to completely negate the fluid losses in the convertor when locked. However, due to torsional vibration, and often to minimise the perceived boost threshold on turbo engines, this lock up clutch is not locked until around 1500rpm typically. At any point where the TC is locked, the gearbox geartrain efficiency is not noticeably lower than a conventional manual gearbox using constant mesh helical gears. However the internal brake system used to select the appropriate gear ratio, requires hydraulic pressure to counteract the engine torque and this hydraulic power comes from an internal oil pump driving from gearbox input power. Modern electronically controlled autoboxes are clever in this respect and have pressure controlled oil pumps inside which only generate just enough pressure to prevent slip in the brake system, varying that pressure as the load varries. That means at light load (part throttle driving) they can get close to the efficiency of a purely mechanical manual gearbox.
That's a really good post - thank youcologne2792 said:
Max_Torque said:
As mentioned, modern epicyclic wet disc clutch automatic gearboxes now have a 'fully rated' lock up clutch on the torque convertor. As such, that enables them to completely negate the fluid losses in the convertor when locked. However, due to torsional vibration, and often to minimise the perceived boost threshold on turbo engines, this lock up clutch is not locked until around 1500rpm typically. At any point where the TC is locked, the gearbox geartrain efficiency is not noticeably lower than a conventional manual gearbox using constant mesh helical gears. However the internal brake system used to select the appropriate gear ratio, requires hydraulic pressure to counteract the engine torque and this hydraulic power comes from an internal oil pump driving from gearbox input power. Modern electronically controlled autoboxes are clever in this respect and have pressure controlled oil pumps inside which only generate just enough pressure to prevent slip in the brake system, varying that pressure as the load varries. That means at light load (part throttle driving) they can get close to the efficiency of a purely mechanical manual gearbox.
That's a really good post - thank youPablo68 said:
And hence why the term "slush box" doesn't really apply to modern autos.
They still feel "slushy" to drive though, since the clutch doesn't lock until the torque converter has brought the engine speed roughly in line with the prop-shaft speed; you still get that slurring between gears. Gassing Station | General Gassing | Top of Page | What's New | My Stuff