Friday, November 21, 2014

Eco versus Drive

Although the electric car comes with a transmission (and a shifter), this is not your traditional transmission.  A gasoline engine cannot stop turning, and its range of efficient operation is more limited than an electric motor.  To work around this, gasoline cars had 2-speed transmissions in the 1950’s, 3-speed transmissions in the 1960’s, 4-speed transmissions in the 1980’s, and so on to the point that Chrysler has released a 9-speed automatic transmission.  All of these transmission speeds (or gears) are designed to improve how a smaller gasoline engine can work harder when needed and slack off while cruising.  To back up, the transmission includes a gears with reverse threading to turn the wheels backwards.  The automatic transmission also includes a slippage mechanism that allows the vehicle to remain at rest while the engine continues to run, costing it low-speed efficiency. 

By contrast, the electric vehicle has just one speed that is always engaged.  This is possible because of several factors.  The electric motor can start turning from a standstill (without a slippage mechanism), and it requires no complicated change of gears to get up to speed.  Also, direction can be controlled by changing electric polarity to the motor, so there is no need for a separate gear for backing up.  So, in an electric car, most of the transmission functionality is handled virtually via an electronic controller.

The "shifter" on my Mistubisih I-MiEV in its usual position (Eco)
Of course, maintaining a familiar set of levers and knobs to operate the electric car makes it easier for most people to adopt.  The Mitsubishi i-MiEV includes a traditional shift lever for selecting the different operating modes (just like a conventional transmission selector) and includes positions for drive (D), economy (E), and brake (B).  These three modes affect the programming of the electric motor controller and how much electricity flows between the motor and battery.  In drive (D), the motor is delivered maximum electric current, but regeneration when not braking is minimized.  In economy (E), the power delivery is slowed, smoothing out accelerator pedal response and reducing energy consumption while increasing the regenerative effect when not braking.  Finally, brake (B) combines the drive (D) mode responsiveness with stronger regeneration than economy (E) and is most useful when driving on long hills.  Note that regeneration is the same in all modes when using the brake pedal.

I usually drive using economy (E) because my route is mostly level and this mode results in a smooth delivery of power.  In addition to increasing my driving range slightly, it makes for very comfortable acceleration and makes it easier to maintain a steady pace on the freeway.  In drive (D) mode, the motor response is more punchy, with more immediate response at slower speeds and making it harder to maintain a steady pace on the freeway.  For nearly all driving situations, economy (E) is the more comfortable, predictable, and economical mode for driving.  This changes the moment I get into stop-and-start traffic on the freeway.  Because regenerative slowing does not light the brake lights, I do not feel safe relying on regenerative slowing in heavy traffic, so I switch to drive (D) whenever traffic gets slow on the freeway.  This way, I communicate my slowing clearly to the drivers behind me.

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