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Thread: frictional rotary load in transmission

  1. #1 frictional rotary load in transmission 
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    I am modelling a powertrain of a vehicle. I need to extract 16kW of power from the output of the IC engine for auxilliaries (constant throughout). The remaining power is then given as input into the gearbox. The output of the IC engine is torque & angular speed.

    T_Engine x w_Engine - 16000 = T_Gearbox x w_Gearbox

    Without the aux, T_Engine = T_Gearbox ; and
    w_Engine = w_Gearbox

    BUT, how do I distribute the power between torque and angular speed into the gearbox, with the auxilliaries? Because, I know from experience that both torque and angular speed reduce if power is extracted from a transmission. Need help.


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  3. #2 Re: frictional rotary load in transmission 
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    Quote Originally Posted by huria
    I am modelling a powertrain of a vehicle. I need to extract 16kW of power from the output of the IC engine for auxilliaries (constant throughout). The remaining power is then given as input into the gearbox. The output of the IC engine is torque & angular speed.

    T_Engine x w_Engine - 16000 = T_Gearbox x w_Gearbox

    Without the aux, T_Engine = T_Gearbox ; and
    w_Engine = w_Gearbox

    BUT, how do I distribute the power between torque and angular speed into the gearbox, with the auxilliaries? Because, I know from experience that both torque and angular speed reduce if power is extracted from a transmission. Need help.
    The angular velocity of the engine shaft is the same as that of the input of the gearbox, and the torque will also be the same at that point. Ahead of where the engine shaft connects to the gearbox, there will be another gear or something mounted on the engine shaft where the auxiliary power is being extracted. This is where you are losing some torque. The torque of the engine before the auxiliary gear will be the sum of the gearbox torque and the auxiliary gear torque. Hope I haven't totally butchered the terminology. I don't really know about engines.


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  4. #3  
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    An engine produces only as much torque as is required of it. Torque is a meaningless number without knowing the RPM (if you were to describe engine speed as angular velocity around gear-heads, they’d laugh at you) we’re dealing with anyway. Torque doesn’t reduce because power is being extracted via a transmission (unless you’re already at peak output). Torque at a given load and RPM will remain constant. Taking power from the transmission will increase the amount of torque required (and therefore power) at that given RPM, and a subsequent increase in throttle will be required to maintain the same RPM. Long story short; engine torque increases when more load is applied.

    We don’t generally focus much on engine torque, anyway. As long as the engine makes the power you need, you can choose an engine that produces that power at a reasonable RPM to minimize brake specific fuel consumption. If you want more torque than your chosen engine provides, then either get a larger engine or a reducing gearbox.

    Are you asking a theoretical question, or a practical question? If you have 16kW of auxiliary functions you need to power, then why not use a hydraulic setup?
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  5. #4  
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    Quote Originally Posted by Gen1GT
    Torque doesn’t reduce because power is being extracted via a transmission (unless you’re already at peak output).
    That depends on where you are measuring the torque. For a given engine torque, the torque reaching the drive axle will certainly decrease depending on the power being extracted at an intermediate point between engine and drive axle.
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    Quote Originally Posted by Harold14370
    Quote Originally Posted by Gen1GT
    Torque doesn’t reduce because power is being extracted via a transmission (unless you’re already at peak output).
    That depends on where you are measuring the torque. For a given engine torque, the torque reaching the drive axle will certainly decrease depending on the power being extracted at an intermediate point between engine and drive axle.
    Only if a fixed amount of torque is available at the engine's output shaft. If it takes 100 Nm to turn a transmission (at x RPM) with an engine capable of 200 Nm at that same RPM, and you extract 50 Nm at that intermediate point, then torque accumulates, and there is enough to go around.

    If you have an on-road vehicle with a PTO driven hydraulic pump mounted on the transmission, and the vehicle is moving at a given speed requiring specific horsepower (torque at a given RPM), then loading that hydraulic pump will not see a decrease in tractive effort nor engine RPM as long as the engine is under-loaded. (sorry for the run-on sentence).
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  7. #6  
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    Quote Originally Posted by Gen1GT
    Quote Originally Posted by Harold14370
    Quote Originally Posted by Gen1GT
    Torque doesn’t reduce because power is being extracted via a transmission (unless you’re already at peak output).
    That depends on where you are measuring the torque. For a given engine torque, the torque reaching the drive axle will certainly decrease depending on the power being extracted at an intermediate point between engine and drive axle.
    Only if a fixed amount of torque is available at the engine's output shaft. If it takes 100 Nm to turn a transmission (at x RPM) with an engine capable of 200 Nm at that same RPM, and you extract 50 Nm at that intermediate point, then torque accumulates, and there is enough to go around.

    If you have an on-road vehicle with a PTO driven hydraulic pump mounted on the transmission, and the vehicle is moving at a given speed requiring specific horsepower (torque at a given RPM), then loading that hydraulic pump will not see a decrease in tractive effort nor engine RPM as long as the engine is under-loaded. (sorry for the run-on sentence).
    But you'll have to increase power input (push down your foot on the throttle). If you keep the power input constant, you will see a decrease in tractive effort.
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  8. #7  
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    Yes, this is true.

    There are engines and controls that can compensate for load automatically. Kind of like putting it on cruise control.
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