Why Are Electric Vehicles More Efficient?

This may be a bit technical, but if you want, just ignore the tables and look at the graphs. The federal government publishes cars fuel economy data for City Driving, Highway Driving, and Combined Driving[https://www.fueleconomy.gov/feg/best/bestworstNF.shtml accessed 11/13/2015]. As of November 13, 2015 the most efficient gasoline car was the Scion iA, the most inefficient was the Aston Martin D89, and the Mercedes Benz S600 is almost as bad. The most efficient hybrid car was the Toyota Prius c. The most efficient all electric car was the Volkswagen e-Golf, and I included the Tesla 85D, for reasons we’ll discuss shortly. Because electric cars don’t use gasoline, there is a calculation to determine an equivalent measure of efficiency, MPGe.

We can learn a lot from these numbers that will guide us in designing an efficient transportation system. First, electric cars are more efficient than gasoline cars, with hybrids in between. The primary reason is that internal combustion engines are inherently inefficient: modern gasoline engines used to power cars are about 25-30% efficient[https://en.wikipedia.org/wiki/Engine_efficiency  accessed 4/8/2016]; by contrast, large electric motors are typically more than 95% efficient [https://en.wikipedia.org/wiki/Premium_efficiency accessed 4/8/2016].

Second, the City efficiency for gasoline cars is worse than their Combined efficiency, while electric and hybrid cars have better City efficiency than Combined. The primary reason for this is that City includes stop and go driving: gasoline cars are inefficient at lower speeds and lose their kinetic energy by braking to slow down; by contrast, electric and hybrid cars use regenerative braking, so most of the energy of motion is recaptured and stored in the batteries.

Third, small gasoline cars designed for efficiency are over twice as efficient as luxury gasoline cars. Interestingly, there is not as big a difference for electric cars, and the larger, luxurious Tesla 85D is even more efficient in highway driving than the smaller, more basic VW.

Fourth, Highway efficiency for gasoline cars is better than their Combined efficiency, while the VW and Prius cars have better Combined efficiency than Highway. This is also due to braking vs. regenerative braking. The Tesla has better Highway efficiency than Combined, in part because it has the lowest drag coefficient of any production car.

Let’s look in more detail at the reasons for these differences, and how these can improve our designs. Power dissipation for gasoline cars si divided among: Engine Losses (mostly heat loss), Drivetrain Losses, Parasitic Losses (such as water pump and alternator), and Power to the Wheels, again for Combined, City, and Highway[https://www.fueleconomy.gov/feg/atv.shtml accessed 4/8/2016].

The comparable figures for electric cars[https://www.fueleconomy.gov/feg/atv-ev.shtml accessed 4/9/2016] show the dramatic difference in Engine and Drive Train Losses. Electric cars use almost no energy when idling, as opposed to gasoline cars. The Battery Charging Losses are as large as the Drive System Losses – we’ll look at that shortly. Note the substantial contribution to efficiency from Regenerative Braking, especially in City driving.

Here is a graphical representation of the comparison.

We have already proposed the innovation of powering Autonomous Vehicles directly from the A-Way, which eliminates the batteries for normal driving. This would eliminate the Charging Battery Losses. The electric power distribution is also more efficient because it can use higher voltages than residential power, and energy recovered through regenerative braking can be sent to nearby Autonomous Vehicles – we will see how effective this can be when we explore Continuous Convoys and En Route Sequencing. Thus if we eliminate the 16% Charging Battery Losses, we might achieve something approaching 98% of the Power going to the Wheels. Clearly, these numbers will be different in an actual system, but this exercise illustrates the potential for excellent efficiency.