Energy Use in Cars 5: Gasoline Cars vs. Electric Cars

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How do the greenhouse gas emissions of electric and gasoline cars compare?

Big Ideas: 
  • A comparison of the energy cost and greenhouse gas emissions of gasoline and electric cars.

Did you know that electric cars driven in BC produce 40 times less greenhouse gas than gasoline cars?

Our prior analysis has looked at the fuel consumption of gasoline cars. But what about cars that don't use gasoline at all? Electric cars are becoming more and more common, and with good reason: they offer significant energy and pollution savings over traditional gasoline cars.

In order to compare the two types of cars, we need to get away from looking strictly at the fuel consumption, as we can't make a direct comparison along these lines. Rather, we'll look at two other factors: the total amount of energy consumed and the greenhouse gas produced.

Energy Consumption

The average fuel consumption of gasoline cars is about 0.076 L/km travelled1. To convert this figure to energy we multiply by the energy content of gasoline: 32 MJ/L2. Therefore:

<br />
\begin{eqnarray}<br />
\textnormal{Energy consumption of a gasoline car} & =& (0.076 \textnormal{ L/km})(32 \textnormal{ MJ/L}) \nonumber \\<br />
  & =& 2.4 \textnormal{ MJ for each km travelled} \nonumber<br />
   \end{eqnarray}<br />

If we take a typical mechanical efficiency of 0.25, then the total mechanical energy requirement is 2.4 MJ/L x 0.25 = 0.6 MJ per km travelled. Let us assume that the mechanical energy is the same for an electric car and consider the efficiency of the electric car's systems to calculate the total energy consumption of an electric car. In this case, we need to consider the efficiency of charging and discharging the car's batteries and the mechanical efficiency of the electric motor3.

<br />
\begin{eqnarray}<br />
\textnormal{Charging/discharging} & =& \dfrac{\textnormal{Useful Electrical Power}}{\textnormal{Power Required to Charge}} \nonumber \\<br />
& =& 86 \% \nonumber<br />
   \end{eqnarray}<br />

<br />
\begin{eqnarray}<br />
\textnormal{Electric Motor} & =& \dfrac{\textnormal{Mechanical Power}}{\textnormal{Electrical Power Input}} \nonumber \\<br />
& =& 85 \% \nonumber<br />
   \end{eqnarray}<br />

<br />
\begin{eqnarray}<br />
\textnormal{Energy Consumption of Electric Car}& =& \dfrac{0.6 \textnormal{ MJ}}{(0.86)(0.85)} \nonumber \\<br />
& = & 0.82 \textnormal{ MJ for each km travelled} \nonumber<br />
   \end{eqnarray}<br />

This is 1/3 of the energy consumption of an equivalent gasoline-powered car, and it is due to the overall efficiency of an electrical system compared to a heat engine. In addition, electric cars also typically have regenerative braking systems which recovers half of the kinetic energy of the car when it brakes. We can calculate the impact of regenerative braking on the car's energy consumption. From our earlier calculations we know that stopping and starting is responsible for 60% of the energy consumption of a car in city driving. Regenerative braking recovers 50% of that, for a savings of 30%. This gives a total of 0.7 x 0.82 MJ = 0.57 MJ for each km travelled.

This result is pretty close to the average consumption for electric cars which is 0.54 MJ for each km travelled1. This suggests that we are correct in assuming that the effect of the battery efficiency, motor efficiency, and regenerative braking are largely responsible for the savings of electric cars over gasoline cars..

Greenhouse Gas Production

Electric cars certainly consume less energy than gasoline cars, but where does that energy come from? In order to compare the pollution from these two types of transportation we need to understand how much greenhouse gas is produced when the electricity is generated.

In British Columbia, much of our electricity is produced in hydro-electric power stations that have extremely low greenhouse gas emissions. The average greenhouse gas production of electricity generated in BC is 7.8 g CO2/MJ and in Canada is 64 g CO2/MJ4, so the equivalent emissions for an electric car are:

<br />
\begin{eqnarray}<br />
\textnormal{Greenhouse gas emissions in BC for each km travelled} & =& (0.57 \textnormal{ MJ})(7.8 \textnormal{ g CO}_2 \textnormal{/MJ}) \nonumber \\<br />
& =& 4.4 \textnormal{ g CO}_2 \textnormal{ for each km travelled} \nonumber<br />
   \end{eqnarray}<br />

$ \textnormal{Canadian average} = (0.57 \textnormal{ MJ})(64 \textnormal{ g CO}_2 \textnormal{ /MJ)} $

Given that the GHG production due to gasoline is 2.32 kg CO2\L5, the corresponding emissions for a gasoline car are:

<br />
\begin{eqnarray}<br />
\textnormal{Greenhouse gas emissions for each km travelled} & =& (0.076 \textnormal{ L})(2.32  \textnormal{ kg CO}_2 \textnormal{/L}) \nonumber \\<br />
& =& 176 \textnormal{ g CO}_2 \textnormal{ for each km travelled} \nonumber<br />
   \end{eqnarray}<br />


Therefore, greenhouse gas emissions for a gasoline car are over 40 times higher than for an electric car (in BC). Even if we use the Canadian average of greenhouse gas emissions for electricity generation, gasoline cars still emit over 5 times as much greenhouse gas as electric cars.


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