Efficiency Kings

One of the advantages of electric drive find most compelling is the level of efficiency at which these systems operate.  The immediacy of electron flow through a closed circuit never ceases to amaze me.  Though tempted to write off these phenomena as black magic, I’m told that they are just the so-called “physical properties of electricity.”  A.k.a. “Science,” at least while it’s still legal in this country.

The energy losses you can observe in a car with a traditional internal combustion engine, like the presence of heat, noise and light, all but disappear when you switch to an electric drivetrain.  The puttering noises of an engine that gets so hot you could cook on it.  The light, not as readily observable, but implicit in a car’s own combustion cycle.

As much as I make public claims about EVs, like about how they can run 4 to 5 times more efficiently than gas cars, I thought it might be helpful to explain just how I arrived at such admittedly unbelievable numbers.  If a five percent increase is big news, seventy-five percent gain is revolutionary.

It’s all fine and good to cite efficiencies of different motors and engines and just say, “Gas cars are 17% and electric cars are 85-90% efficient!  See, that’s five times better!”  

But I find practical, observable real-world data to be much more persuasive.  Let’s walk through one scenario using my daily driver, the electric 1968 Saab as an example:

            1.  Online sources seem to mostly agree that one gallon of gas contains about 33.7 kilowatt hours (kWh) of energy.

            2.  The lithium battery in the Saab is about 34 kWh.  (Yes, that means that fully charged, I’m leaving the house with only one gallon of gas worth of energy!)

            3.  In practice, we limit ourselves to using only about 80% of our battery, preventing it from being charged too high, or discharged to low.  This means that we use only .8 x 34 = 27.2 kWh of the battery.

            4.  I have driven 120 miles at around 60 mph in a single charge many times and consider that the maximum range. 

            5.  If I can drive 120 miles on 27.2 kWh, that means 120/27.2 = 4.4 miles per kWh

            6.  If there is indeed 33.7 kWh per gallon of gas, and I can run 4.4 miles for each of those kWh, I have enough information to estimate the “miles per gallon equivalent” (mpge) of my car.   33.7 kWh  x 4.4 mi/kWh = 148.3 mpge

            7.  The stock, gas version of my car is known to get about 32 mpg.  148.3 mpg / 32 mpg = 4.63

            8.  Conclusion: the EV version of the 1968 Saab 96 is 4.63 times more efficient than the stock version.


What is not included in these calculations and why:

            1.  No consideration for charging losses, because a car’s mpg rating also does not consider the energy used to pump gas into a gas tank.

            2.  No consideration for the energy required to produce coal, gas, solar, wind, etc -based electricity, as the mpg rating also is not a “well-to-wheels” number. 

            3.  If either of these factors were considered, it would only strengthen the argument for EVs.

           *4.  One valid exception to the above claim would occur during cold weather driving, with heat running.  An electric vehicle generally must spend additional energy (unofficially estimating 15%) to create heat for warming the cabin, whereas its gas counterpart produces waste heat as a bi-product and must simply run a water pump and a fan to send heat into the cabin.  Or... since there's so much heat, perhaps locomotion is the bi-product here.  So to be fair, yes, accounting for an estimated 15% loss for cabin heat does drop the EV to "only" 3.9 times the efficiency of gas cars, when heat is running.


 

Photos: 
Runs on Fossil Fuels
Runs on Electrons