I’m a mechanical engineering student and I know generally how HEVs operate, but I can’t seem to find anywhere that explicitly explains how torques are “added together” to increase overall torque of the vehicle.
Is there a particular device that allows two shafts to drive one (thus adding their torques), and does that mean that both these shafts would need to have the same rotational velocity to avoid excess torsion on the driven shaft?
Any explanation would be greatly appreciated!
In: Engineering
You would just connect the outputs of the two motors together. The motors would have to rotate at the same speed or at least be geared to the same speed. In order to change gears you wolud use a clutch or torque converter just like a normal car.
This works because both electric and ICE motors can run at different speeds. So they will still work when you connect them together in sync. And in both of them the throttle will control the torque, adding more fuel to a combustion engine will increase its torque and advancing the phase of the field windings in an electric motor will do the same. As the entire driveline is connected together any torque produced by any of the motors will be distributed throughout the entire driveline so everything is at the same speed.
You use something called an overrun clutch. This is already a really common part in automatic transmissions so they’re easy to come by.
An overrun clutch is like a diode or check valve but for rotary motion… it transmits torque in one direction but spins freely in the other. So you put two gears on the driveshaft, each with an overrun clutch between the gear and the shaft (the clutch is a cylinder). Each motor drives one gear. If they try to drive the gear faster than the driveshaft the clutch engages and they transmit torque to the shaft. If they try to go slower (that motor is off) the clutch disengages and the gear just free wheels. It’s a mechanism for summing torques. If they’re engaged, driven shafts have to go the same speed as the driveshaft.
There’s an alternate way to do it with fluid couplings (“torque converters”) where they don’t have to go the same speed, this is how the main “clutch” of an automatic transmission works.
I’m sure that different solutions are used in different applications, but I know that in the Prius they use a planetary gearset to handle the multiple inputs. This allows for the inputs to operate at different RPMS. There are some pretty good youtube videos that show how it works. Here is a (https://www.youtube.com/watch?app=desktop&v=MsvVD0FaF28) of a demonstration model. In the case of the Prius, the combination of inputs and outputs yields an entire transmission that works like a CVT (varying the speeds of the gas motor and electric motors allows for variable speed in forward and reverse without a separate transmission).
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