Question

What is the efficiency of the most efficient combustion engine? Explain why it is so inefficient. How does the efficiency of an electric motor compare with an internal combustion engine? If this were the selling feature for an electric car, explain why this is terribly misleading.

Answer 1

the efficiency of the most efficient combustion engine ~37% put some numbers to it from a vehicle perspective: Consider the Tesla Model S, which has an available 85kWh battery and a 265 mile range. Consider a similar gas-powered car, which gets 35 mpg. Gasoline contains about 33kwh of energy per gallon. The Tesla uses 320 Wh/mile of energy (85kWh/265 miles) The gas powered car uses 940 Wh/mile of energy (33kWh/35 miles) Once the energy is onboard (not counting the efficiency of the power generation, oil refining, or charging), the Tesla is using only about a third as much energy as the comparable gasoline-powered car.

The biggest issue with electrical power vs combustion power or even hydrogen power is the medium that is used to hold energy in the vehicle. We've seen with electric battery powered vehicles and electric hybrid vehicles that the battery sizes and weights are the primary reason that an electric vehicle doesn't have the same range as a fuel powered vehicle due to the incredible energy density of hydrocarbon fuels. Energy density boils down to the amount of mass needed to store a given amount of energy. In the case of fuel it's in the chemistry of the hydrocarbon molecule and the case of batteries it's in the chemistry of battery plates and acid, and in the case of hydrogen its in the chemistry of the H-H bond. All three are chemical forms of energy storage which are released and translated into work by different methods. Although battery powered motors may be more efficient than combustion, the weight of a battery pack becomes its own worst enemy when trying to extend the range of a vehicle. Even in hybrid vehicles the driving power of the motors is not the primary limitation of the drive system, usually the biggest efficiency issue is the recharge rate of the batteries which limits how much power can be stored during regeneration without melting the battery packs. Hydrogen powered vehicles run into issues with the incredible amount of energy required to produce hydrogen, the low energy storage capability of the H-H bond, the difficulty of storing such small molecules and the lack of infrastructure to support Hydrogen powered vehicles.