Hybrid Vehicle - Mileage: Don’t fool yourself with ‘free energy’

With hybrid vehicle the most frequently used statistic is the MPG, or the amount of miles you can travel on a gallon. The European equivalent being the kilometers per liter, or km/l. There are many ways to determine this statistic for a vehicle and even for electric vehicles this statistic is used, even though the electric vehicle does not consume liters or gallons of fuel to travel, but electricity (measured in kWh).  

 

The mileage as it is indicated on the display of your vehicle - Link

 

y easy to derive; one simply fills up the tank, drive till the tank is empty and it is a simple equation of dividing the distance driven by the gallons used. Cars have a different efficiency when driving at different speeds; a conventional car is the most efficient on the highway at high speeds and less efficient in urban traffic (especially in traffic jams, with an idling engine you consume fuel and don’t cover a lot of distance). In order to create equal conditions for vehicles we use a driving cycle. There are various driving cycles around, some are country specific, others are specific for highway conditions, urban traffic or a combination of highway and city driving. The main point here is that you want to test a vehicle at the same driving condition as the other vehicle to be able to make a fair comparison. Examples of driving cycles are the NEDC and the Je05. For more information on driving cycles please refer to the site of For a conventional vehicle, this figure is relatively easy to derive; one simply fills up the tank, drive till the tank is empty and it is a simple equation of dividing the distance driven by the gallons used. Cars have a different efficiency when driving at different speeds; a conventional car is the most efficient on the highway at high speeds and less efficient in urban traffic (especially in traffic jams, with an idling engine you consume fuel and don’t cover a lot of distance). In order to create equal conditions for vehicles we use a driving cycle. There are various driving cycles around, some are country specific, others are specific for highway conditions, urban traffic or a combination of highway and city driving. The main point here is that you want to test a vehicle at the same driving condition as the other vehicle to be able to make a fair comparison. Examples of driving cycles are the NEDC and the Je05. For more information on driving cycles please refer to the site of dieselnet.com.

 

 

Where with a conventional vehicle you have just a fuel tank and the distance to consider to derive the mileage figures. With a hybrid there is also a battery involved and when not considering this in the proper way, one could falsely use this ‘free energy’ to get a better mileage. When determining the mileage of a hybrid car, it is only fair to make sure that the battery is equally charged at the end of the cycle as at the beginning. This does not imply that the battery is not to be used, quite the contrary. The battery will (automatically) be used to recoup energy from regenerative braking, it will discharge some on acceleration, etc. In the hybrid vehicle the battery is considered a buffer and if at the end of the cycle the battery would be fully discharged, the mileage has been influenced by this free energy. The fair thing to consider when looking at the mileage is to ‘convert’ some of the fuel to recharge the battery again to the same level as at the start.

 

Don't be fooled - consider electricity in your mileage; it does not come for free - Link

 

As an example, consider a hybrid vehicle which has a fully electric range of 30 kilometers. If you would take it for a trip of 50 kilometers, you could fully drain the battery for 30 and then use your fuel for the remaining 20. If you want to determine the mileage, taking the fuel consumption of the 20 kilometers and combine it with the total distance of 50 is fooling you a bit with the supposedly ‘free energy’. To determine the correct mileage you would then need to run the engine to recharge the battery again, with the bonus that you can have the engine run at its most efficient operating point to do so.

 

Of course, if this would be an actual trip to just get somewhere in the cheapest possible way, you would just plug it in at the end of the trip to recharge the battery and not run the engine. That would not just be cheaper, but also more environmentally friendly.