What are the implications of an electric car being more efficient (~88%) than an internal combustion car (~35%)?

This is, in terms of performance and energy use (energy savings). I am looking for a straightforward very broken down explanation if possible.

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  • 5 years ago
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    First you should understand how efficiency can be measured. There are theoretical calculations, there are bench tests and then there are "real world examples." Also we could be measuring just the motor or we could be measuring the motor and how it performs in the vehicle. Because of the wide variation quotes of efficiency are likely to be misleading.

    For example, to say that a petrol engine is 35% efficient is either a bench test or a theoretical limit. In the vehicle the petrol engine must operate over a wide demand of speed and torque. The engine requires a transmission that additionally saps efficiency from the overall performance. The result is a likely 15% efficiency for the petrol powered car (from tank to wheels) http://www.fueleconomy.gov/feg/atv.shtml Dissimilar measures are a bit of a common fallacy in efficiency comparisons.

    The electric motor used in some ultra efficient solar cars can be 99% efficient. Tesla claimed a 92% efficiency for the roadster from battery to wheels. This is helped by an electric car not needing a transmission for sufficient torque to start and move the car. The electric motor also does not have a narrow range of RPM where it operates in its most efficient mode. Also the electric car often has regenerative braking which allows recovery of some energy that is put into the inertia of a moving vehicle (increasing overall efficiency.)

    With vastly greater efficiency the EV requires less energy to go the same distance as a petrol burner. Although electricity and gasoline are approximately the same price in the US for the amount of energy the greater efficiency of the EV requires less energy storage and is less costly to "fuel." Operating on such a limited amount of stored energy there is a big push to make the EV ever more efficient. This competition has then driven the entire industry to more efficiency.

    With such a big difference in energy costs the petro burner is on the defensive and proponents of gasoline cars look beyond the EV to the generating station for inefficiencies. The average power plants operates at about 30% to 40% efficiency. This seems like a killer argument until we look also at the refining of oil:

    It requires about 4 Wh of electricity to refine a gallon of petrol. (yes the same electricity with all its inefficiency and pollution used to power an EV) This is enough to power the EV further than the average petrol vehicle using the refined gallon. This in turn has caused Elon Musk to say that we would have plenty of electricity for electric cars if we just stop refining oil. It also makes refining oil look like a bit of an energy ponzi scheme. https://www.youtube.com/watch?v=BQpX-9OyEr4

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    While some "mileage" is made in arguments that the EV suffers a degradation of range (less efficiency) in cold weather they seem to maintain an economic advantage over the petrol vehicle as the weather gets colder: http://green.autoblog.com/2014/01/27/ev-savings-ri... Preheating the EV, use of fuels for a separate heating system, and more efficient heating / cooling systems like heated seats and heat pumps will be further improvements as the future unfolds.

    • BriaR
      Lv 7
      5 years agoReport

      Your data is flawed. A gallon of diesel takes my car 50-60miles at 70mph. You say 4Wh (ie WATT hrs) is enough to take an electric car the same distance? NO WAY. 4Wh is 2 cellphone batteries! 4 KILOwatt hours maybe and that is the energy to refine 1000 gallons of fuel.

  • BriaR
    Lv 7
    5 years ago

    In terms of converting stored energy the electric car may be 88% efficient but that forgets the energy loss during charging - in other words the efficiency of the battery to convert electricity into chemical energy.

    It also omits the ~65% loss converting fuel into electricity and in transmitting from power station to charging outlet.

    So when it comes to converting fossil fuel energy onto kinetic energy the internal combustion engine is still more efficient than an electrically powered vehicle.

    • Transmission losses from power station are approximately 5% to 7% not the 65% suggested here. Charging losses may be another 5% or less. I have not seen numbers for the efficiency loss of trucking gasoline to hundreds of thousands of gas stations or the loss of driving autos to fuel up.

  • Mr. P
    Lv 7
    5 years ago

    Well it all sounds wonderful until you start driving it in cold weather, then what do you use for heat ?

    Ok it has a heater, but it is using batteries to make that heat, and they are only 88% efficient (your figure), but to replace that energy you need to plug into the grid, which has transmission losses of 7.7%, and generation losses of 50 to 60%

    So you see there is more energy lost in converting coal to electricity, and transporting it to your battery to convert to chemical energy which you then convert back to rotational energy in the motors than just using a fuel source to provide heat and power.

    It all looks wonderful if you just measure from the point of putting electricity in, but that's not the whole story is it?

    • In a petrol car the engine must get to a certain temperature to reach its maximum efficiency. Cold weather would then tend to decrease the efficiency of an ICE. I have seen one article that suggested that the range of an ICE vehicle declines more than an EV in cold weather.

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