Why are electric cars so efficient?

It is true that electric car batteries at present are not at a level that can outperform an internal combustion car. It is also true that range anxiety is still the biggest impediment in the spread of these new machines. However one area where electric cars cannot be beaten is their efficiency and emissions ( or lack of them).

Why are electric cars efficient?

Following are the reasons why electric vehicles are more efficient and in turn more economical.

Responsive Transmission

In an electric drive, energy is supplied to the motor when needed. The electric motor stops when the car brakes or comes to a stand still. In a Diesel or Gasoline powered car, the engine keeps running even when the car is standing still (idling during the journey). Although in modern IC engine cars, the engines is stopped if the car senses that its idle, nonetheless, stop- starting  of engine frequently increases its wear and tear.

Propulsion Efficiency

From the time fuel is burned to the time power is delivered on wheels, almost 75-82% is lost in heat and other areas. This data is for combined drive cycle i.e. Urban and Highway conditions. The situation worsens if only urban driving is considered where only 14%-20% of power in the fuel is available to the wheels.

In case of electric cars, the propulsion efficiency can be as high as 85% (electricity to torque conversion). In an internal combustion engine, auxiliary components such as water pump and alternator also take up 5-7% energy of the fuel burned.

Even for Hybrid Electric Vehicles (HEV) that has an electric generator but utilizes motor for propulsion, the overall propulsion efficiency is higher than IC engine car. The benefits of diesel-electric propulsion are well documented compared to direct diesel engine drive. For its efficiency and flexibility, this system has been the choice of railway engines and boats. Diesel engines if solely used at optimal speeds for the purpose of electricity generation can reach efficiencies of 40%. This means propulsion efficiency of 32% can still be reached in a Diesel-electric drive (at 80% motor efficiency).

In modern electric vehicles, BMW-i3 with Range Extender option is an example of Diesel-electric drive when its diesel electric generator is in use.

BMW i3 concept car exhibited at the 2011 International Motor Show Germany

Regenerative Braking

Electric car can offer regenerative braking. The portion of energy that would have been lost as heat in warming brake discs now recycles through the alternator and charges the batteries. Similarly when going down a slope, potential energy can be captured. Note that regenerative braking is mostly available for electric cars with AC drive.

Do electric cars lower emissions even if they are  powered by electricity from coal-fired power plant?

Modern coal-fired power plants as dictated by the law inside EU have to be at least 44% efficient. In worst case scenario, the coal plants produce electricity at 33% efficiency. If using this electricity, the electric motor operates at an efficiency of 80%, than the overall propulsion efficiency would be 0.33*0.8 =0.264  i.e. 26.4% . In terms of energy efficiency it still works out better than I.C engine figures of 18% mentioned above. For finding out CO2 emission levels, CO2 factors  for Diesel and  Coal will have to be observed. According to the data by IPCC, the emission factor for EU diesel is 73.9 tCO2/TJ and anthracite (coal) is 98.7 tCO2/TJ. This data suggests that coal is 33.5% dirtier than diesel. However the propulsion efficiency of coal powered -electric car as noted earlier is  46% better than pure diesel drive. This means that electric cars are still lower in emissions, even when they are powered by an old coal-fired power plant.

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Reference values used for calculations:

Calorific value (Energy content) in Diesel: 44800 kJ/kg

Density of Diesel =0.832 kg/lt

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