I was recently asked by a friend how I could claim that my EV was so much more efficient than his new gasoline powered car. I told him that my car was more efficient, but it weighed almost 1,000 pounds more and had much more power than his new car. He was skeptical of my claims, to say the least.
I examined the statistics on his car and found that the combined average fuel consumption for both city and highway travel was 9.1 L/100 kM, equivalent to 25.8 miles/US gallon. I then looked at the energy use for my car, for the entire time that I have owned it, and found that it has used 162 Watt-Hours/kM, equivalent to 260.7 watt-hours/mile.
I did a little conversion, to convert the energy in a US Gallon of gasoline to kWh. A US Gallon of gasoline contains approximately 33.7 kWh of energy. A car that burns 25.8 mpg is burning 1/25.8 gallons per mile or 0.0388 gallons/mile. This is equal to approximately 1,306 Watt hours/mile, almost 4 TIMES the energy used by my bigger and heavier EV. My friend was surprised, as he had assumed the cost savings were only a result of taxes on gasoline that are not charged to EV vehicles.
That number also surprised me. A new gasoline engine that is operating well is generally capable of generating mechanical power at up to about 40% efficiency, but the typical vehicle total efficiency is about 20-30% efficient. By comparison, the EV motors are about 80-90% efficient, but the total efficiency is claimed to be about 70%. That would suggest that the average difference should be about 3.5 times different. The ratio of over 4x captured my interest because in fact, I expected it to be less, given that his mileage rating was the manufacturers claim, while mine was based on actual driving, largely in wintertime.
After looking at EV operations, I found the answer. A vehicle powered with a gasoline engine uses power to accelerate the car to a driving speed, but each time the car is required to stop, the driver typically applies the brakes. These brakes convert the kinetic energy in the motion of the car to heat that is dissipated into the air by the brakes. While this is a seldom considered fact, it seems to be a significant amount of energy, in particular in areas where the car is starting and stopping frequently. The EV on the other hand, uses a clever concept called regenerative braking. If you take your foot off the accelerator pedal, the car will slow rapidly and many of the new EVs will come to a complete stop. The use of the brake is applied, only for sudden or unexpected activities. Many EV drivers claim that they use “one pedal driving,” almost never using the brakes. When the accelerator pedal is released, the car is slowed rapidly because the motor turns into a generator and it collects the kinetic energy from the car slowing down and puts it back into the battery. Apparently, this makes a significant difference.
I am aware of one EV club that has a planned a demonstration of this process. They will drive many EVs to a park at the top of a local mountain, and record the amount of energy used going up, and then repeat the measure going back down. In fact, most of the EVs will arrive at the bottom with more energy in the battery than they had when they left the top of the mountain. Several gasoline powered cars will do the same route, with added measurement equipment to accurately measure the gasoline consumption.
One of the key advantages of the EV in the years ahead will be the overall efficiency and the ability to use regenerative braking. It is an unmentioned recycling system that can be used to improve the operation and to reduce wasted energy, that has been lost as heat.
I live near the bottom of a mountain highway, and it now gives me real pleasure to see the battery level increase as I drive down the highway from the summit to my home after a trip to the other side. The comparison with a gasoline powered car, done by the EV club may produce some very interesting results.
In North America, when one looks at the portion of primary energy that results in actual work, the overall efficiency is less than 35%. As we reduce the carbon emissions, to address climate change, one of the easy methods to consider may well be a full-scale program to increase the efficiency of every aspect of energy use. It may be the best way to reduce the impacts of many of the changes that lie ahead.