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Power density

Discussion in 'Tesla' started by nech12, Jan 14, 2016.

  1. nech12

    nech12 New Member

    Jan 13, 2016
    What is a correct power density (kW/kg) methodology: 1) power density = electric motor peak power (kW) / electric motor weight (kg); 2) power density = electric motor peak power (kW) / weight of power train (motor+electronics) (kg)? Does second method possible and applied in the industry?
  2. pvogel

    pvogel Member

    Jun 23, 2015
    Santa Clara, CA
    I would say that power density has nothing to do with the motor (other than it's ability to efficiently use the energy in the batteries). When measuring the energy density you look at the HP or watts per kg of "fuel" (in the case of a BEV that's the weight of the batteries).

    Efficiency comes into play as well though, since energy density of gasoline or methanol is way higher than today's Li-ion or Li-polymer cell tech, but a brushless electric motor is easily 90% efficient vs the best ICE at 30% efficiency or less.
  3. Wilcino

    Wilcino Member

    Nov 5, 2015
    Newcastle upon Tyne, UK
    I'm unsure of your reason for wanting to use the correct unit, which may affect the answer.

    Power density (kW/kg) doesn't make much sense as a description. Density implies a constant that can be scaled. This would apply to Energy density, as in a battery. You can describe a battery technology by kWh/kg (batteries) or J/kg (fuel). As using twice the amount of batteries, or twice the amount of fuel, will give you twice the energy capacity. However people may mistakenly use the term "Power Density" when they mean "Energy Density".

    You can describe car performance in terms of Power to Weight ratio. This is maximum power output vs the weight of the car, and would historically be in hp/ton. Comparing this number between cars gives some indication of relative acceleration performance, but it's not something that scales up and down, a car's weight is set. To compare an EV to an ICE car, you would use EV peak power output (kW) / weight of car. Whether you use electric motor peak power output or drivetrain peak power output (which in the case of a performance Tesla is limited by the battery rather than the motors) is the subject of a loooong debate and I think a few lawsuits. You would then need to convert hp/ton for the ICE car into kW/ton ( x0.7457 ) for a direct comparison.
  4. Paddy

    Paddy Member

    Jan 24, 2016
    Assuming you don't want to know about energy density: Power train people talk about power density. Ie how big the prime mover (electric motor or ice) is relative to its power rating. Pretty meaningless to measure the power density of the entire vehicle thoug.

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