A Crash Course in How to Drive for Less

Meet Peggie, the battery electric car designed and made by the Energy & Power Group at the University of Oxford.

Description: http://epg.eng.ox.ac.uk/sites/default/files/Pete/Rotterdam/Peggie%20on%20track.jpgThe car ranked highly in the Shell Eco-marathon, and after analysing the data from its three race attempts, the link between energy consumption and driving technique became clear.

Of course, most people don’t own a car quite like this, but similar energy saving tips still apply to help you save money on fuel.

On Peggie’s first race attempt, she was running at 370.2km/kWh (meaning she can travel 370.2km with 1kWh of energy) compared to 564.9km/kWh on the third attempt, an improvement of over 50%.

Graph 1: Peggie’s speed and position on the track for 3 race attempts (Click image for larger view in new window)

This improvement in the efficiency of Peggie’s journey is down to the way the car was driven. The green line representing the third race (best attempt) shows less dramatic variations in speed than the other races, meaning there was less braking events and more gradual speed changes for acceleration and coast-down. The onset of braking events means an inevitable loss of energy – hence minimising braking events as well as sharp increases in speed proves to be a more optimised driving style.  

Now take an average car with an internal combustion engine (the data below is based on a Ford Focus). This engine is about 30% efficient [1], meaning 30% of the energy from petrol is converted into forward motion.

      Diagram: An Internal Combustion Engine [3]

Input       Output
1 litre of petrol Car Engine 15.8 km [2]

An electric car like Peggie is about 88% efficient, 2.9 times more efficient than a normal combustion engine car. This means that if a normal car were to do a lap of the racetrack, controlling its braking in the same way as Peggie, in theory the car should be able to travel 50% more km per litre of petrol. However, because the internal combustion engine is less efficient, the car would only be able to travel 17% more km.

1 litre of petrol Car Engine 18.486 km

Differences in electric v/s internal combustion engine braking and acceleration behaviour exist, however we can in principle calculate the relative money saving due to applying the more

 optimised driving style.  As a litre of petrol costs about £1.40, 17% more km per litre would earn you back £1.29 with every 100km you drive – enough to stop at a service station for

 a coke and a mars bar. [4]