Tag Archives: efficiency

Cars and Cold Weather

Anyone who pays even passing attention to their fuel economy know that regardless of the vehicle driven, colder weather means poorer performance for a myriad of reasons. Important to all vehicles is inefficiency due to rolling resistance and wind resistance. Electric cars have additional inefficiencies due to battery issues.

In cold weather lubricants, which keep parts which move with respect to each other moving, are more viscous and therefore more resistant to movement causing drag. Another source of resistance to motion comes from the lower tire pressure in cold weather. Students in introductory chemistry classes learn Guy-Lussac’s Law: Pressure is directly proportional to temperature. Lower temperatures mean lower tire pressures.

Both friction due to viscous lubricants and lower tire pressure are overcome because both cause friction and friction generates heat. Depending on how cold it is, and how long the vehicle is driven, parts warm up lowering lubricant viscosity and tires gain pressure as they heat up.

Even the air itself conspires in cold weather. Vehicle designers pay careful attention to “slipperiness,” as wind resistance is a big factor especially for a fast moving car or truck. Wind resistance is a function of the density of air and air density is inversely proportional to temperature. Colder air is denser air and provides more resistance.

Internal combustion engines (ICEs) have to be tuned to run rich to get started in cold weather. This means that more fuel is used to get the engine started and up to operating temperature. Additionally, in the winter people tend to start up their engines to warm up the interior of the car before it even hits the road.

Hybrids, plug-in hybrids, and pure electric vehicles suffer an additional problem, because they all are powered to some degree by a battery and batteries in cold weather are a problem. A hybrid electric vehicle like a Toyota Prius has a traditional ICE connected to the drive wheels, with an electric motor which supplements the ICE. Plug-in hybrids are a little different. They have an electric drive train with an ICE used exclusively as a generator to charge the battery when it is discharged. Of course all electric vehicles have only the electric motor and a comparatively large battery to extend the range on a charge.

Two factors contribute to cold weather reduced range in battery powered cars. The colder a battery is the less charge in will accept, thus lowering the vehicles range until the next charge. A factor called internal resistance increases as the temperature decreases. This means you don’t have as much energy stored from the outset. Further reducing range is the process which converts chemical to electrical energy. The distance you can travel on a unit of energy is lowered in colder weather.

Electric cars have reduced range on a given charge in cold weather, but overall are still cheaper to operate than an ICE vehicle. Basically the cost of electricity for a given amount of travel is much less than the cost of gasoline for an ICE vehicle, even at today’s greatly reduced cost for gas.