18. February 2013 · Categories: General

In the saga about the NYT Tesla drive one of the problems was the advice given to maximize fuel efficiency: Do not engage cruise control, but travel with short burst of acceleration followed by unpowered coasting.

This advice is actually correct for petrol powered cars. It is based on the fact that petrol engines have peak efficiency when running at roughly 90% torque, and are quite a bit less efficient when under partial load (especially compared to diesels). This means that the improved efficiency under acceleration more than compensates for the increased air friction caused by driving uneven speeds. Actually it is optimal to have the gears in neutral during coasting, and you need to keep the bursts very short, ideally keeping your speed within a 5 to 10 km/h band. It is quite obvious that this is a very stressful way to travel, and modern fuel injection techniques have narrowed the efficiency gap.

But for an electric car this is counterproductive since electric motors have a much more even efficiency, and the uneven load is poison for the battery: The higher the current you are drawing from a battery, the lower the total amount of energy you get. So it is best to draw current as evenly as possible from the battery, and avoid strong acceleration.