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What kind of automatic transmission would be the best to connect an electric engine with the rear axle of a RWD car?

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Specifications:

  • 200HP, 3000rpm electric motor
  • top speed 220km/h

The automatic transmission should have higher output RPM and be able to bring the car to top speed of 200km/h.

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  • $\begingroup$ So wheel diameter and final drive ratio will give speed assuming direct drive... $\endgroup$ – Solar Mike Sep 16 '19 at 19:37
  • $\begingroup$ Are you referring to torque converter? $\endgroup$ – Marino Klisovich Sep 16 '19 at 19:39
  • $\begingroup$ No, unless you have one in the final drive... $\endgroup$ – Solar Mike Sep 16 '19 at 19:41
  • $\begingroup$ Why the motor is so far away? Usually electric cars use a planetary gears for transmission directly connected to the HSS and the wheels at the other side. $\endgroup$ – Sam Farjamirad Sep 16 '19 at 20:29
  • $\begingroup$ Question 1: Why do you need a gearbox? $\endgroup$ – Transistor Sep 16 '19 at 20:43
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The essential question here is why or why not use a transmission in an electric car. Here are the underlying issues.

It is commonly asserted that a DC electric motor is a constant-torque device, developing the same torque at standstill (0 RPM) that it does at its rated RPM (in this case, 3000). Advocates of the "no-transmission" position cite this as a reason why electric motors in cars do not need transmissions, but instead can be used in direct-drive mode.

But the flaw in this reasoning is that power- the capacity to perform work at a certain rate- is the product of torque and RPM, and it is this product which accelerates the car.

This means that in the case of a motor which is starting from zero RPM to accelerate a car, its power output at near-zero RPM is near zero, and the motor does not develop its rated power output until it is running at its rated RPM.

This means that to maximize the power output of the motor and thereby maximize the car's acceleration, we need to get the motor all the way up to 3000RPM as quickly as possible, which means gearing the motor down when starting off from zero speed.

Then, when we reach 3000RPM, we upshift the transmission to a "taller" gear ratio, and shift again when the motor comes up to 3000RPM, and repeat until the drag force on the car body is equal to the force applied to the pavement by the rear wheels. this process is designed to keep the motor running at or near its maximum power point for as long as possible.

In fact, the fastest acceleration will be had if the transmission is capable of holding the motor right at 3000RPM throughout the car's acceleration from zero wheel speed to whatever its top speed is. This means that to maximize acceleration, a transmission is necessary, and the best one will be a continuously-variable one that locks the engine speed at 3000RPM at all times.

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  • $\begingroup$ Is the difference between an ICE and an electric motor that an ICE needs to be turning at 3 or 4k, or so, to produce max torque while an electric motor produces its max torque at 0 rpm? See engineeringtoolbox.com/… $\endgroup$ – Solar Mike Sep 17 '19 at 14:19
  • $\begingroup$ Would torque converter do the job? $\endgroup$ – Marino Klisovich Sep 18 '19 at 8:58
  • $\begingroup$ Over certain ranges of RPM, yes. However, torque converters have built-in efficiency losses which require the inclusion of a lock-up clutch to place the motor into direct drive at cruise conditions. $\endgroup$ – niels nielsen Sep 18 '19 at 19:15

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