Dear Mrs. Weston, As a resident of Miami where we have no hills and a driver of an electric car that only has one gear, I might not be the best qualified person to answer this question but here goes...
Let me rephrase the question a little. Question: A driver of a gas power car will shift to a lower gear to help his or her car accelerate up a hill. What would the driver of an electric car do? Hypothesis: The driver of an electric car would also shift down to a lower gear to help his or her car accelerate up the hill. Answer: Surprisingly, the driver of an electric car might do better to shift up into a higher gear to help the car accelerate up the hill. The reason has to do with the difference between the torque characteristics of a gasoline engine and an electric motor. If you were to graph out the torque produced by a typical gas engine at the different speed of operation, it would look like an inverted 'V'. Torque would be in pound-force foot (lbf-f) on the vertical axis and speed in revolutions per minute (RPM) along the bottom. At 0 RPM, torque would be 0. At 100 RPM, torque would be a little higher and would continue to increase until, for our example, around 5,000 RPM. After 5000 RPM, torque would begin to drop again. As the gasoline car starts up the hill, the car slows down because it would require more torque to continue up the hill at the same speed. As the speed drops, the motor spins more slowly. If the car was at 5,000 RPMs on the flat road, going up the hill the motor might slow to 4,000 RPMs. Since at 4,000 RPMs, the motor will provide less torque than at 5,000 RPMs, the motor will struggle even more. The RPMs will continue to drop and the motor will be able to provide less and less torque. The driver shifts down to a lower gear which allows the motor to turn more quickly compared to the speed of the car. This means that even though the car is going slower, the motor is back at 5,000 RPMs and able to deliver maximum torque to the system. On an electric car, the graph of torque to RPM will look like a downward slope. At 0 RPM, the electric motor produces it's maximum torque. As the RPMs increase, torque drops. So when the electric car starts up the hill and the RPMs start to drop, the torque the motor can provide will increase. By shifting to a longer gear, the motor will drop to an even lower RPM and be able to provide even more torque. I hope this helps. Cliff Rassweiler Driver ProEV's Kokam Battery powered Electric Imp www.ProEV.com _______________________________________________ Florida EAA mailing list [email protected] http://www.floridaeaa.org
