15. February 2015 · Categories: General

One of the ways to make cars more efficient is to keep the combustion engine running at optimal efficiency, and use an energy storage system to provide the actual, variable demand needed for driving. This works because the efficiency ranges from 10% to 35% depending on the actual load. (The theoretical maximum is around 50%, leaving not much room for further efficiency gains from the motor)

When we use an electrical drivetrain for this, we have a couple of losses:

Efficiency Caused by
92% Mechanical to electrical
60%–95% Battery storage
92% Electrical to mechanical
50%–80% Total

The biggest problem is the battery1, which gets significantly less efficient as we increase the current we are drawing from it.

We have a strong design conflict here: the kinetic energy of 2 tons moving at 226 km/h is 1 kWh, so a small battery is enough to smoothen the load, but to retrieve the 25kW we want for a smooth acceleration efficiently, we can either have a large battery or expensive capacitors. Such a battery would would need to support a lot of cycles: 1KWh are roughly 15 accelerations to city speed, so a thousand battery cycles per year seem a reasonable lower bound.

In essence, we have some expensive tech to improve city mileage to the 60 mpg (4l/100km) range, but otherwise we have come so close to the ideal, that another 50% in efficiency gains can only be had by changing the car to have radically less weight and drag.


  1. some background on batteries can be found here