Are Electric Vehicles Actually More Efficient Than Gas Ones?
How we measure efficiency matters, and the results may surprise you
TLDR; If you need to skip, scroll down to the Maths & Summary Sections
A note from the author:
Before we dive a little too deep here with the numbers, I am not advocating for people to go out and change their cars; we're seeing what vehicle is more efficient at transporting mass. While doing some math on efficiency, we can see how incredible our technology is. I do not include essential arguments about EVs vs. ICE; I do not include charging stations vs. gas stations and lifetime ownership costs; I will address those arguments in a separate article. This article is purely about understanding our technology and the efficiency of it. Let's do the Maths!
Current Combustion Engines (ICEs)
For a long time, around fifteen years ago, I remember thinking that the combustion engine was a technology that should be improved upon. This thought came to me around 2008-2010, when in NY, gas prices were over 4$, let alone reaching beyond the 5$ mark, as we have seen in recent years. I thought this because the combustion engines' efficiency was only 30%; I read somewhere or found it when I was young, so who knows? Today, their efficiency has mostly stayed the same; we are boasting an average of 30-36% efficiency for gasoline and 45-50% for diesel. Incremental improvements are the expectation as the technology matures. The only question I begged was, why can't we scrap what we know and start from scratch? That's what Tesla did, and they effectively refactored, an engineering term for re-did, the engine, and that is what we see in the EV arms race between automakers.
The Numbers Behind Combustion Energy Efficiency
It would be prudent of me to say ICEs have energy efficiency in the mid-30s in terms of percentage. So let us break down where all of that energy is going.
1 chart about fuel energy of combustion engines
Explaining the chart:
Work: transferred energy as a percentage from the engine to the car
Heat Loss: percentage of energy lost due to heat
Exhaust: percentage of energy lost due to having to exhaust the fumes
Looking at the chart above, a study from 1998 (the best I could find) shows that we lose most of the energy in ICEs using fuel due to heat loss and exhaust.
The Cruising Zone
We have all heard of how efficient engines can be at the right speed on the highway, and I decided to go over that quickly.
2009 study produced the above2
RPM makes all the difference in how we drive our cars. Car manufacturers have tried to optimize engines such that the average mph of the vehicle falls within the best zone. Unsurprisingly when we floor it, we use a ton of gas; now we can visualize that.
Tesla’s Approach
We need to dive deep into the history of the electric engine that Tesla is producing to discover how EVs are so efficient.
Where did they start? The Induction Motor (Pre-Model 3)
Tesla's name is a tribute to the 19th Serbian-American inventor, electrical engineer, mechanical engineer, and futurist. Nikon Tesla is best known for contributing to the modern alternating current (AC) electricity supply. When Tesla started making their cars, their engines had a 3-phase AC Motor. Another critical technology was the portable inverter. First invented in 1990 by Alan Cocconi), a device that turns the direct current (DC) in an electric car's battery into alternating current (AC). Using Nikon Tesla's motor designs and a portable inverter, Tesla started to make their first engines.
Pros of the Tesla induction motor:
It doesn't require any expensive permanent magnets
Uses cheaper electro-magnets
PM-free induction
Cons:
Costly and difficult-to-cast rotor fashioned from pure copper
The rotor tends to get hot and can even overheat (wasted energy)
In-efficient at low speeds
How it works
A large magnetic field comes from the stationary portion of the motor, which in turn, induces an opposing magnetic field on the highly conductive copper rotor. When the two opposite magnetic fields interact: they pull toward each other. When the two opposing magnetic fields generated inside the motor of one of their cars collide, it produces an insane amount of power and takes off faster than some of the fastest super-cars.
The Permanent Magnet Engine
Tesla scrapped the induction engine and opted for a permanent magnet engine. While this statement contradicts what I said above, this is their secret sauce.
So, as you know, our Model 3 has a permanent magnet machine now. This is because for the specification of the performance and efficiency, the permanent magnet machine better solved our cost minimization function, and it was optimal for the range and performance target. - Tesla’s Chief Motor Designer, Konstantinos Laskaris
Let's start with an expert, enter engineering teardown master "Ingineerix." In February 2018, Ingineerix posted a series of videos exploring the innermost secrets of the Model 3. In a video titled "The Dark Side," he explores the car's underside and, more specifically, the motor. He revealed that the vehicle has a "Switched Reluctance motor, using permanent magnets."
“Tesla calls it a PMSRM, Permanent Magnet Switched Reluctance Motor. It’s a new type, and very hard to get right, but Tesla did it!” - Ingineerix
The Challenge
If you have a magnet and a piece of iron, the magnet will pull the iron toward it. If you turn on and off the electro-magnets and that iron is on a rotor, it will begin to turn. That is a Switched Reluctance Machine.
The Approach
To succeed at this daunting task, you must look back even further than in 1892 when Nikola Tesla invented the induction motor. You'll have to go back 50 years to 1838, when the first reluctance machine was designed. The reluctance machine is simple, compact, and inexpensive but suffers from Torque Ripple, an infrequency in the amount of power generated.
The Reluctance Machine
The reluctance motor is difficult to control; its RPM and rotor position are among the few difficulties working with it. With modern-day control systems and power electronics, Tesla overcame these challenges, but torque ripple remained the underlying problem. However, in 2011 a researcher found that if you embed small rare-earth metals in the motor alongside the existing electro-magnets, the fluctuations would be smoothed out, and a 30% increase in efficiency.
Final Results
Over the past few years, Tesla has made incremental improvements in their engine; now, they state that it boasts an insane 90% efficiency (improved from 84%) and a peak efficiency of 94%.
Batteries Vs Gasoline
In this section, we see where most of the debate comes in for gas vs. electric vehicles, so we must address it in detail.
Energy Density
Energy density is one of the most critical factors when comparing the two. Gasoline has an energy density of 46 MJ/kg, and 1 gallon weighs 2.754 kg or 6.073 lbs. Diesel fuel has a density of 45.5 MJ/Kg 34.
Gasoline cars: 16.7 MJ /kg or 7.57 MJ /lb
The new Tesla battery has reported 272-298 wh/kg, taking the average of 285 wh/kg. We can derive the current energy density in their batteries like above. I want to point out that Tesla has some of the best battery cell performance, so this is different from the average.
Tesla Battery: 1.026 MJ/kg or 0.465 MJ/LB
We see that gasoline is 16.27x more energy dense than the current Tesla battery cells.
Energy Density Research
Based on the above, it makes so much sense why energy density is such an important factor for electric vehicles or even hybrids. The more energy density auto manufacturers can put into their cells, the further the range of the car can go and the more efficient it can be. As we will explore more below, having more energy density will ultimately make electric vehicles more efficient than gas vehicles. It will not reduce the costs of energy or the amount needed but the weight of the car and the range the vehicle can go. Density will also affect how quickly a car can charge up to a sufficient point. Which merits its own article, and I will do the maths later.
The Weight-To-Cost Ratio Of Energy
It's not close; gasoline has better MJ/lb by over 16x. The Tesla Model 3 long-range battery weighs 1,060 lbs, and I'm using this one for our calculations because I found the most credible source for it, and they claim it can go 358 miles, and the average gasoline car can go 413 miles. I cannot compare hybrids here as there are multiple types (engine -> battery, plug-in); we will add them to our charts later. I am also adding a cargo train here, as it is the most efficient means of transportation as a reference point.
Tesla Battery weight: 1060 lb for 358 miles (3lbs / mile)
Gasoline Car: 84.5lbs for 358 miles (0.23lb/mile)
Cargo Train: 20,195lbs for 500 miles (40.39lb/mile)
Gas is 12.54x more efficient per pound than electric batteries as a fuel source. Re-read that; again, it's only as a fuel source which is one part of the car. An EV engine is much lighter than a typical ICE and their drive train.
So let's compare how much energy it takes to move 1 tonne 500 miles, a metric used in the rail industry to measure energy efficiency.
Tesla 2022 Model 3 (Long Range AWD): 3800 lbs5
Average Gasoline Car: 4094 lbs
Cargo Train: 3000 tonnes (6,000,000 lbs)
Tesla Weight Efficiency Score = 13.157 kwh/100 miles per tonne
Average Gasoline Car Weight Efficiency Score = 64 kwh/100 miles per tonne
Cargo Train Efficiency Score = 0.219 kwh/100 miles per tonne
Electric cars are 4.86x more efficient moving their weight vs. gas cars!
This simple chart reaffirms how great cargo transport is, and I took a conservative number; they can weigh up to 10,000 tonnes (3x more than what I put in). I also took the liberty of throwing in an electric hybrid, getting 50mpg and the average electric car6.
The Most Widely Used Stat, $$ Per Mile
I am writing another article on the lifetime cost of ownership of EVs vs. Gas, but I would be remiss not to mention this stat briefly here.
Gas
The average car got approximately 25.7 miles to the gallon in 2021, according to the Environmental Protection Agency (EPA)7. The national average price for gas for April 2023 is $3.71 per gallon, according to the EIA8
Electric
The average price per kWh in the US is 0.166$, according to the Bureau of Labor Statistics as of March 20239. The average electric vehicle does 0.346kWh per mile, based on 231 electric cars built between 2000 and 2022, and their kWh/100 mi as stated on fueleconomy.gov (the official US government source for fuel economy information)10.
Our results:
Gas: 14.43¢ cents per mile (25.7 mpg)
Hybrid SUV: 10.6¢ cents per mile (35 mpg)
Hybrid Sedan: 8.24¢ cents per mile (40 mpg)
Electric: 5.7¢ cents per mile
I included hybrids here to show how ICEs have been paired with their electric counterpart to create a compelling argument on cost per mile and the performance gain achieved by combining the best of both.
The Maths
All of these calculations are at the time of writing, 5/1/2023
Electric engines are almost 2.7x more efficient at energy transfer than ICEs (90% vs. 33%)
Gasoline is 16.27x more energy dense than the current best battery cells.
Gas is 12.54x more efficient per pound than electric batteries.
Driving an EV vs. a gas car is 2.53x cheaper (at the time of writing) per mile.
To go 500 miles in a gas car would cost 74.28$ vs. $28.50 for an electric.
Electric car batteries take you about 3 lbs/mile for batteries vs. 0.23lb/mile for gas, just a different way of looking at the energy density.
EV engines and hybrid cars are up to 4.9x and 1.9x more efficient than their gas counterparts.
Some hybrid models are more efficient than EVs!
What can we conclude here? Despite the inefficiency of ICEs compared to Tesla's engine, gas cars are less efficient at moving mass even with far superior fuel energy density. However, the average electric vehicle might not be so far ahead of their hybrid sedan in terms of efficiency.
Summary
The energy density of batteries pales in comparison to gasoline; however, because EVs have an incredibly high energy conversion and don't weigh much more than a gas car, they narrow the gap regarding weight efficiency.
In my research, I had to use weight efficiency because batteries weigh a lot, and so does a combustion engine and its drive train, so to compare the two, we needed to consider the entire car, not just one part. The purpose of this article is to figure out what method of transportation via a car is more efficient.
When using this kWh/100 miles per tonne (to ensure we were taking mass into account), we see how much more efficient we have become with our technology. I expected EVs to be more efficient but not almost 5x in energy-to-weight transportation. Yet I was surprised to see how close the average electric was with their hybrid competitor, such as the state car of California, the Toyota Prius sedan. Hybrid sedans stand out as competitors; depending on the EV, they might be more efficient.
When looking at energy efficiency, I was surprised to see how close hybrids and the average electric vehicle come to each other. It speaks to the innovation of the automakers, who are taking some of the best parts of electric and combustion engines and making them work together.
- My name is Dan and I did the Maths
What do you think?
Sources:
https://stacks.stanford.edu/file/druid:cp633zj5935/thesis_BernardJohnson-augmented.pdf
https://cleantechnica.com/2018/03/11/tesla-model-3-motor-in-depth/
https://www.controlglobal.com/home/article/11288233/batteries-or-fuel-cells-for-energy-storage
https://rentar.com/efficient-engines-thermodynamics-combustion-efficiency/
Copyright Dan Flanagan and Sid Technologies LLC
M. K. Anderson, D. N. Assanis, and Z. Filipi, “First and second law analyses of a naturally-aspirated, miller cycle, si engine with late intake valve closure,” Technical Paper 890889, SAE, 1998
R. Reese, “High efficiency ic engines with an emphasis on si engines.” Presented
at 2009 SAE Powertrains, Fuels and Lubricants Meeting, November 2009.
I. Hore-Lacy, "Future Energy Demand and Supply," in Nuclear Energy in the 21st Century, 2nd ed., London, UK: WNUP, 2011, ch.1, sec.6, pp.9