Introduction: The U.S. market for gas-powered cars in 2005.
Source: Analysis of US EPA gas mileage and vehicle specification data, 2005 model year.
Back in 2005, my wife needed a car. And by that I mean, we were interested in a gasoline-powered passenger car. Not a truck, crossover, van, SUV or the like. And not a diesel. We’d done that, and didn’t much like the drawbacks of the diesels of that era. A passenger sedan, in the parlance of the era. Fueled by gasoline.
I was interested in buying something that was efficient. But the raw MPG numbers were a jumble, driven largely by the size of the vehicle. I didn’t really want to try to squeeze into some tiny econobox just because it got better mileage than a bigger car. I was interested in identifying something that was efficient at converting gasoline into movement of passengers and luggage. Not just the tiniest car I could fit into.
I took data from the EPA and calculated fuel efficiency in a way that put large and small cars on more-or-less equal footing. Instead of looking at miles per gallon, I calculated cubic-foot-miles per gallon. Where “cubic foot” is combined interior passenger and luggage room, measured in cubic feet. Under this approach, a car that was twice as big, but got half the gas mileage, would count the same as a car that was half as big, but got twice the gas mileage. They both used the same amount of gas to move a given volume of passengers and luggage.
My “cubic-foot-miles per gallon” for passenger cars is similar to the concept of ton-miles per gallon for freight vehicles. For the simple reason that miles-per-gallon doesn’t tell you how much you were able to haul. Trains, after all, burn a lot of diesel fuel per mile. But they also move a lot of tonnage with that fuel. In some sense, what matters isn’t the amount of fuel burned, it’s the usable carrying capacity that the fuel consumption provides.
The idea was to make this a two-step process. First, I would separate out the vehicles that were efficient, regardless of size. Then, from the lineup of vehicles that rose to the top of that listing, we could try to make an informed choice about what size of car we wanted to buy. Given the choice of efficient vehicles on the market at that time.
Things did not quite turn out as planned. I figured I’d get a “spectrum” of efficiencies, with a few dozen cars to choose from at the high end of the spectrum.
When I did that calculation– and so removed the variation that was driven merely by the size of the car — the data resolved into the amazingly simple picture, shown above. For gasoline passenger cars, the 2005 American market consisted of three pieces:
- The Prius
- The Honda Civic Hybrid
- Everything else.
The real eye-opener, to me, was the extent to which “everything else” was just that. Basically, all non-hybrid cars were roughly equally inefficient.(!) Sure, you had some muscle cars come in at the bottom of the heap. But the point is that there wasn’t some nice, smooth distribution of cars in terms of their efficiency. There was crowded mass of vehicles with little to distinguish one from another. There was the Honda Civic Hybrid, poking up above that mass. And there was the Prius, running about three times as efficiently as the average.
The upshot is that if you wanted an efficient gas car in 2005, the decision was a no-brainer. You bought a Prius. The only other gas car that came close to it was the Honda Civic Hybrid, and that was just a bit too small for me.
The best way I know of to illustrate how different the Prius was from other offerings at the time is to ask this simple question: In the 2005 car market, what was the most efficient gas-powered vehicle capable of moving six people? The answer? Two Priuses. Every six-passenger sedan, van, or SUV got less than half the gas mileage of the Prius. It was that far ahead of its time.
(As a footnote, I never put pickups, vans, or other special purpose vehicles (SUVs, etc.) on this graph because I couldn’t get the interior volume information. The EPA showed the MPG for those vehicles, but in 2005, the EPA only tracked interior volumes of passenger cars. And, as it turns out, that’s still the case. They only note the interior volume of traditional passenger cars.)
Redo: The U.S. market for gas-powered cars in 2022.
Now fast-forward to the 2022 model year, and ask the same question. If you were interested in buying an efficient gas-powered car, what would your options be? (I use 2022, not 2023, because the 2023 model year data from the U.S. EPA are not yet complete).
First, you have to acknowledge how much the market has changed.
In 2005, there were no electric vehicles (EVs) or plug-in hybrid electric vehicles (PHEVs). For passenger cars, 99% of the offerings were straight-up gas vehicles. There were a couple of hybrids, and a couple of diesels. Roughly speaking, 99% of the models offered were standard gas cars.
In 2022, here’s how the U.S. passenger car market shaped up. (Recall, this is just cars, not pickups, vans, SUVs, and the like.) In 2022, three-quarters of the offerings are still standard gas vehicles. But more than one-quarter are alternatives — hybrids, plug-in hybrids, or EVs.
Now let me ask the gas-car-efficiency question, for 2022. I’m ignoring EVs, and for the PHEVs, I’m using just their gas-only mileage. How do gas-powered cars shape up, in terms of my efficiency measure (cubic-foot-miles per gallon)?
Source: Analysis of US EPA gas mileage and vehicle specification data, 2022 model year.
Unlike 2005, the Prius now has quite a bit of company. The 2022 Prius is slightly more efficient than the 2005 model, but now there are a couple of gas-powered cars that top it, and many that are nearly as efficient.
Unsurprisingly, everything at the top of the efficiency charts, for gas-powered cars, is a hybrid of some sort. The entire cluster of gas sedans at the top consists of hybrids and PHEVs (where only the gas portion of the PHEV mileage has been counted.)
For the record, the two Prius-beaters on that graph are the Hyundai Ioniq and Ioniq Blue. For reasons that must make sense to Hyundai, those are no longer being made as gas hybrid versions. Hyundai discontinued those cars as of June 2022 (reference).
So, by the time we get to the 2024 model year, the Prius will be back at the top of the heap. As it should be. But now, at least, it has some close company.
Finally, I have to emphasize that this was for gas-powered vehicles. On paper at least, there are many fully-electric (EV) and partially-electric (PHEV) cars that get MPG-equivalents in excess of the 54 MPG for the Prius Eco (highlighted in the chart above). But here, I wanted to look at gas-powered cars, for comparison to my 2005 analysis. And I don’t want to get into what those MPGe numbers actually mean. It’s not straightfoward.
How does the Prius achieve such efficiency?
You see a lot of bad information about how the Prius achieves its high efficiency, for a gas-powered car. Some is just plain confused, some gets the orders-of-magnitude wrong. Some explanations appear to violate basic physics, such as the law of conservation of energy.
To be clear, Toyota did optimize many aspects of the operation of that car. If you look at a list of where energy gets dissipated in a typical car, the Prius addresses every area.
Source: Energy.gov
Take wind resistance, for example. Once you actually manage to get energy to the wheels of the car (Power to wheels line, above), about half of that energy gets dissipated as wind resistance. If you want decent mileage, you need to keep that to a minimum. The Prius has a “coefficient of drag” of 0.24, among the best ever measured for a mainstream U.S. passenger car (see the extensive list in Wikipedia.) By contrast, your basic brick-on-wheels design — a Jeep Wrangler — has a coefficient of drag of about 0.45. What people perceived in 2005 as the somewhat odd shape of the Prius was all about providing interior volume while minimizing air resistance.
There are other minor contributors. The Prius comes with low-rolling-resistance tires. It minimizes losses from braking by using regenerative (electrical) braking where possible. It reduced the parasitic losses from (e.g.) air conditioning by employing a more efficient compressor design. And so on.
But as you can see from the chart above, most of the energy wasted by a standard gas car is wasted by the engine. As I understand it, the single largest driver of Prius efficiency — and the reason it had to be made as a hybrid in the first place — is that it uses a different type of gasoline engine. It’s an Atkinson-cycle (or maybe Miller-cycle) engine, instead of a standard car Otto-cycle engine.
Why do I think the use of an Atkinson engine is key? Well, here’s a list of the top 20 most-efficient gas cars offered in the U.S. in 2022, listing their specs and the type of engine they use.
Notice anything?
What was unique to the Prius in 2005 is now the standard way to achieve good fuel efficiency in a gas-powered car. But, as explained below, in most cases, you have to add some secondary propulsion (electric motors) to get adequate on-road acceleration.
Here’s the explanation in brief.
The chemistry of gasoline dictates that you need a fairly “rich” gas-air mixture in an internal combustion engine. If you make the mix too lean — too little gasoline relative to the air — the spark plug can’t ignite it and/or it burns poorly and/or it generates a lot of nitrous oxides. To get reliable ignition and a clean burn, you more-or-less need to mix gas and oxygen in the ratio needed for complete combustion of the gas. Too little gas in the mix and the engine stumbles and dies, or runs dirty.
But if you fill the engine cylinder with that relatively rich gas mix, you end up with more chemical energy in that cylinder than you can use. Sure, you can compress it and ignite it. But there’s so much energy that you’ve still got quite a bit of usable gas pressure left when the piston gets to the bottom of the cylinder. (The number I see cited most often is that, under load, the gas pressure in the cylinder is still at five atmospheres when the piston hits the bottom of its range of motion.) At that point — once the piston bottoms out — all you can do is open the exhaust valves and let all that potentially usable energy — that still-usable gas pressure — escape out the exhaust.
And that’s exactly what a standard Otto-cycle car engine does. The intake stroke and power stroke are the same length, the valves open and close within a few degrees of the piston being at bottom dead center/top dead center. At the end of each power stroke, there’s plenty of pressure left in the gas inside the cylinder. And with each cycle, that energy gets tossed out the exhaust port.
This has been known for decades, i.e., that gas engines would run more efficiently if you could put a much leaner mixture into the cylinder. The problem was getting that leaner mix to ignite and burn well. Back in the 1970s, Honda tried to address this with its stratified-charge engine (reference). That used two carburetors — one rich, one lean — and two intake valves. It filled the top of the cylinder with a richer mixture that the spark plug could ignite. And the rest of the cylinder with a leaner mixture that would itself be ignited by the rich mix at the top. It was moderately successful, and I recall that the Honda CVCC with that engine was among the most fuel-efficient cars of its generation.
A better way to put less gasoline into the cylinder, and yet have it burn well, is to fill only part of the cylinder on the intake stroke. Where I need to define “the cylinder” as the full length of the resulting power stroke. That way, the air-to-fuel mix is correct for consistent ignition and clean burn. You simply have less total fuel in the cylinder before you ignite it.
And that’s exactly what the modern Atkinson (or Miller) cycle engine does. And it does that by closing the intake valve well after the piston hits bottom dead center. This allows the piston to push out maybe 30% of the total air-fuel mix, then compress and explode what’s left. As a result, you get both the right chemistry (the right air-to-fuel ratio) and the right amount of fuel to allow the resulting energy to be used efficiently.
In the Prius engine, more-or-less, the intake valve closes when the piston is 30% of the way up the bore. (The engine has variable valve timing, so, well, that varies). That works out to be, in effect, about a 10.5:1 compression ratio, and a 13.5:1 expansion (power stroke) ratio.
And so, while the standard Otto-cycle engine has nearly-identical compression and expansion (power stroke) ratios, the Atkinson-cycle engine has a much lower compression ratio, compared to its expansion (power stroke) ratio. The whole point of which is to allow you to put the right fuel-air mix into the cylinder, just less of it.
But this comes at a cost. With less fuel, and a longer expansion stroke, there’s less power with each power stroke. And part of that power stroke now occurs at pressures much lower than you would get in an Otto-cycle engine.
As a result, the Atkinson engine is efficient, but it has a relatively poor power-to-weight ratio. In the case of the Prius, the older 1.6 liter engine might have produced maybe 140 horsepower configured as run as a standard Otto-cycle engine. But as an Atkinson-cycle engine, I think it barely broke 90 horsepower.
The result would have been a car with unacceptable on-road performance. (Yeah, even for Prius drivers.) So, the engineers at Toyota (and Ford) added electric motors, a big battery to run the motors, and so on. And the hybrid was born.
To be clear, the electrical side of the modern hybrid is more-or-less a necessary evil. It was something tacked on after the fact, to allow the engineers to replace the inefficient Otto-cycle engine with a more efficient, but less powerful, Atkinson-cycle engine.
So, now you know the primary source of Prius efficiency. And you know why the 20 most efficient gas-powered cars offered in the U.S. all use Atkinson-cycle engines.
