ON THE EFFICIENCY OF BATTERY-POWERED VEHICLES
Fact: I am able to ride a significantly longer distance on my bicycle than I can drive in my electric vehicle….it can be frustrating to see my electric vehicle run out of juice! So I began to wonder on a recent cycling trip to what extent my Ford Focus Electric vehicle was less energy efficient than my bicycle? or perhaps was my electric vehicle not well designed and the battery appropriately sized? It was time to dig a little deeper.
Curiosity meant that I had to take the thought one step further: how would these two transportation modes compare with nature-provided bipedalism, and ultimately, with the modern gasoline-powered automobile. This is most likely a purely academic exercise in the sense that none of these transportation modes is meant to replace the other, at least not today, but is meant with the hope that we can learn from one method to improve our design and engineering methodologies.
So let’s start by measuring an estimate of energy consumption for each of these transportation modes. First, I know from the dashboard of my electric vehicle that I have averaged 235 Watt-hours per mile (Wh/mi) over the past 20,000 miles of driving. I also know from my bicycle instrumentation that I am averaging approximately 50 kCal per mile — granted, that is at a faster pace than most casual riders but still represents a good starting number. We also know that a medium-sized gasoline-powered sedan has an average fuel economy of approximately 25 miles to the gallon (mpg). And lastly, a wide range of sports publications estimate that a running human burns about 100 kCal per mile.
Next, we need to harmonize these units for a useful comparison. I will spare you the math and give you the conversion factors. I assumed here the EPA’s equivalent figure of 33,700 Wh in each gallon of gasoline. The factors that matter are:
1 kCal/mi = 1.162 Wh/mi = 0.00003449 Gal/mi
Summarizing into one table, we get:
I took the liberty of adding electric bikes and the Tesla Model S to the mix as well as adding an approximate gross weight for each mode, noting that the weight of the bikes do not include the weight of the rider. So what are the results telling us?
First, something we already knew or suspected: An electric car is about 4 to 5x more energy efficient than a sedan with a gasoline-engine. This is primarily due to the fact that an electric motor is more than 90% efficient, whereas a gasoline engine is near the 20% mark…in other words, 80% of the energy in a gasoline engine is lost to heat, and only 20% is used for movement. Go EVs!
My bicycle is nearly 4x more efficient that my Ford Focus Electric. That is a little odd because the human body is not a very efficient machine. So why is the bicycle powertrain more efficient that this ultra-efficient electric motor? The answer is weight! A bicycle with a rider weighs 1/20th the weight of an electric car. The Tesla Model S is considerably heavier than my EV and consequently consumes more energy. The next time you wonder why the Toyota Prius has better fuel economy than a regular sedan, think weight. Geez, we kind of knew that, didn’t we?
But now, we start making observations that are less intuitive. An e-bike is more efficient than a bike, which in itself is more efficient than a human running, yet all three are sufficiently close in weight. The human body is mechanically not very efficient, especially when compared to the power train of a bicycle. The rolling motion of a bicycle lends itself to lesser friction than walking and running, and is thus more efficient. An e-bike replaces the rider with an electric motor that is more efficient than his or her leg muscles — though not as healthy!
So what does it all mean? First, electric-powered transportation is the way of the future — as long as we are not getting the electricity from dirty coal-fired power stations. Second, shed the weight, and that is the weight of your vehicle, and your own weight if you like to ride. Third, a banana provides a human being with about 105 kCal, or equivalently, 120 Wh of energy. That can power a human rider on a bike for 2.5 miles. No power-generator can turn a banana into sufficient electricity for any useful purpose. In other words, while electricity looks green and clean, by the time we consider its cost of generation both in dollar terms and impact on the environment, it probably cannot compete with a healthy banana. Besides, eating a banana beats all forms of fast-charging….Be safe in all your travels!