The Grim Future of the Naturally Aspirated Engine, Or: The Turbos Are Winning
Over the past 45 years or so, BMW was responsible for some of the very best naturally aspirated engines the world has ever seen. Thumb through its back catalog of powertrains and you’ll find a sublime gathering of sweet-spinning, exquisitely balanced, wonderfully soulful expressions of engineering genius, in four-, six-, eight-, ten-, and twelve-cylinder guises.
And they’re all gone.
BMW no longer offers a naturally aspirated engine. Not one. Neither does its performance-vehicle offshoot, M GmbH. You can buy a BMW today with one, two, or even three turbochargers, but only a single model with none (if you count the i3’s range-extending two-cylinder).
Of course, it’s not just BMW. Over at Audi, the naturally aspirated (NA) engine is now considered a “niche technology”—while you can still buy such an engine in the RS5 and the R8 from Quattro GmbH, they’re no longer available in mainstream models. They’re endangered at Mercedes-Benz, too, with the only significant exceptions being the base versions of Smart’s new Fortwo and Forfour city cars. Even Porsche admits its next 911 Carrera models will go turbo, and the next Ferrari 458 iteration will mark that company’s first mid-mounted twin-turbo V-8 since the F40.
So where did it all go wrong for a dominant engine philosophy that provided power for some of the most astonishingly evocative cars of all time, from Ferrari’s classic Daytona and Enzo V-12s to its current 458 Speciale? From Volkswagen’s enduring air-cooled Beetle four to the McLaren F1’s V-12? From every significant American muscle car to the unmistakable sound and power delivery of Porsche’s flat-engined 911? And is natural aspiration really going away for good?
So What Happened?
The short answer is that modern automotive progress is driven by fuel-efficiency concerns, which are on the agendas of every major government. Witness the ever-evolving European Union legislation on economy and emissions targets. California’s Air Resources Board (and the 16 other U.S. states who copy those regulations) has certainly had its say, as well as the U.S. government’s CAFE mandates, and China’s State Environmental Protection Administration (which typically follows in the wake of the EU) and Japan’s Ministry of the Environment are in the conversation, too. It was the EU, however, that led the procession around this particular corner in the wake of 1992’s Kyoto Protocol.
The Kyoto Protocol insisted the world slice 1990-level emissions back by eight percent by 2012, which led to the birth of the first EU vehicle-emissions regulations of 1993. They covered mostly NOx and particulate emissions, so it seemed primarily like a diesel thing at the time, as did the EU 2 (1996), EU 3 (2000), and EU 4 (2005) regs.
But we should have paid more attention because EU regulation number 443/2009 made life difficult for naturally aspirated powerplants. It demanded carmakers reduce their average CO2 figure to 130 g/km between 2012 and 2015 (the three-year period is to take into account vehicle product cycles). Yet while that one hurt, it wasn’t the end. By 2020, the EU demands that the CO2 emissions number be slashed to 95 g/km for each car company’s fleet average. (The Netherlands has gone even further, demanding 80 g/km by 2020.)
So downsizing has become a thing, with smaller, turbo engines taking over from larger, naturally aspirated ones. BMW’s 328i has a turbocharged four-cylinder in place of its predecessor’s inline-six, for example, but downsizing is not a buzzword that stands alone. There is also “downspeeding,” or designing engines from scratch to operate in lower rev ranges with longer strokes. Most turbocharged gasoline engines today can deliver peak torque somewhere around 1500 rpm.
The Germans Are Leading the Charge—Here’s What They Have to Say
In conversations undertaken for this story, executives at Mercedes-Benz, BMW, and Audi all admitted that the days of naturally aspirated engines are, for them, effectively over. The man recently put in charge of BMW M, Franciscus van Meel, said, “Our model lineup would suggest that we have moved on from them, but we talk systems, not specific technology types.”
M’s engine-development chief, Michael Menn, was more specific: “The reason we changed to turbocharging is fuel efficiency, and that’s all. Turbocharging might be more complicated, but the naturally aspirated engines they are replacing in our cars were not exactly uncomplicated engines. At the moment, the main point is consumption and reduction of CO2. If that stays the main point then I am sure the industry stays with turbocharging.”
It was a similar story at the higher-volume BMW head office, with its engine-development department responding to our questions thusly: “From today’s technical, political, and social perspective, naturally aspirated [gasoline] engines are unlikely to be considered for mass production. Fewer cylinders means less friction, lower revs mean less friction, and turbo engines offer high torque starting at very low revs and for a broad engine-speed range, outperforming naturally aspirated engine concepts. A singular offer of a naturally aspirated engine might be a possibility for limited editions, but it is not otherwise being considered at the moment.”
Over at Audi, it’s more of the same. “Audi was one of the pioneers of the development of turbocharged gasoline and diesel engines, whose advantages are grounded in superior performance and torque yield,” we were told by the head of drivetrain development, Dr. Stefan Knirsch. “The advantages of supercharging have turned the naturally aspirated engine into a niche technology. Nevertheless, there is still a place for it in emotive cars like the R8.” [Worth noting: When Knirsch mentions supercharging, it's not as you might know it, but as a catch-all term for forced induction. All turbochargers can be considered superchargers, but superchargers that are not exhaust-energy-driven cannot be considered turbochargers—Ed.]
For its part, Daimler—which has been making automobile engines longer than anybody else in the world—and its powertrain executives also see turbocharging as the long-term strategy.
In his time as BMW’s development head, newly christened Volkswagen brand boss, Herbert Diess told us a few months ago that an 80-gram target demanded taller gear ratios and downspeeding. “Initially that will mean 1800–2500 rpm [for max torque delivery] for internal-combustion engines, but it will really mean 800–1500 rpm in the longer term. That’s where it has to go. High torque, low revs, higher injection pressures, and probably with electric power to boost it at low speeds.” More on that last bit in a second.
Downspeeding will not help the cause of the naturally aspirated engine. This change started with pulling the average four- or six-pot’s limiter down from 7000 or 6000 rpm to 5000 rpm, and it may be headed even lower, toward 4000 rpm. Peak torque now seemingly arrives even earlier on gasoline-burning turbocharged engines than it does in turbo-diesels. But the promise of the turbocharger in the age of low emissions is that when they’re not being spun fast and enabling the power of an NA engine of larger displacement, they can deliver the fuel consumption of an engine of smaller displacement or with fewer cylinders. On paper, it’s the best of both worlds, although realizing turbocharged engines’ fuel-economy ratings is difficult in the real world, as doing so is almost completely driver-dependent. And few drivers, if any, operate their vehicles in the exact same fashions laid out by federal testing.
The best-of-both-worlds idea is, of course, the marketing line behind Ford’s EcoBoost family of engines. Ford has been the most prominent and aggressive American company in adopting turbocharging, offering U.S. buyers everything from a 1.0-liter turbo three-cylinder all the way to, eventually, a 600-plus-hp twin-turbo V-6 in the GT supercar. But General Motors and Chrysler, the latter with help from its Fiat overlords, aren’t out of the game; GM, for example, is in the process of launching a new family of small-displacement turbocharged engines.
The fate of the naturally aspirated engine is neatly summed up by engineering consultant and former senior Maserati engineer, Paul Fickers: “If you examine the engineering, naturally aspirated engines started with a mean effective piston speed (MEPS) of 5 meters/second and a brake mean effective pressure (BMEP) of 5 bar (72.5 psi). Now, the best NA engines have an MEPS of 24 m/s and a maximum MEPS of 14 bar (203 psi), with a fairly constant factor of correlation between them of around 0.6,” he explained.
“This indicates that quite a lot of the gains in naturally aspirated engines have come from revving, and that doesn’t work with modern legislation. Turbo engines generally start out with a BMEP of 15 bar (218 psi), so nearly every turbo engine is better on specific power than the best NA engines, and now the better turbos are well established at 22 bar (319 psi). There is still huge potential for arriving at up to 50 bar (725 psi).
READ MORE: The 10 Most Unusual Engines of All Time
“The best NA engines make almost 134 horsepower per liter of displacement and 74 lb-ft of torque per liter, but with turbos the figures just depend on the pressure you put in. But,” he agrees, “we might see a return to aspirated performance engines through electric boosting.” Where does that leave the American manufacturers, though? After all, given relatively stable and low gasoline prices over the long-term, they’ve not traditionally been early adopters of fuel-saving technologies. Mercedes-Benz thinks the lot of them will be fully onboard sooner, rather than later, this time around. “Not least because of the increasing CO2 regulations, in the long run, the trend away from naturally aspirated engines is an international one,” Daimler’s engineering division insisted.
Who Will Continue to Fly the Flag?
The possibility remains that someone will try to take a method typically thought of as strictly for green cars or supercars like the LaFerrari and mainstream it, that is to say combine a naturally aspirated engine with an electric motor intended to play the role of turbocharger. But rather than purely seek efficiency or ultimate horsepower, the idea would be to deliver the same blend of power and efficiency touted by proponents of turbocharging.
The firm most likely to push this—and hard—is Toyota, the most well-versed and experienced company in the industry when it comes to electrifying powertrains. In fact, they’re already doing it, and Toyota’s vice president of powertrain for Europe, Gerald Killman, insists the company won’t give up on naturally aspirated engines. “I can see why they’ve moved away in the premium segment [Naturally; Toyota's Lexus luxury division now is rolling out turbocharged four-cylinders—Ed.], but electric boosting is giving us exceptional opportunities to deliver torque to the drivetrain at points where naturally aspirated engines aren’t as strong. That gives the performance people expect and it saves fuel.
“We talk about complete systems to deliver what’s required rather than in terms of naturally aspirated or turbocharged, but we see NA engines playing a large part for us. Yes, we are still developing them, especially for use with electric boosting. We already act directly on the transmission, so we can give the benefits of a turbo engine without using a turbocharger, and there is always more than one way if you look at the whole system of a car and what you want it to be.”
Per Mercedes-Benz: “In the U.S., more and more OEMs are introducing turbo engines and reducing fuel consumption by downsizing.” [Ford's aforementioned EcoBoost engines, for example—Ed.] “Even if the current low fuel prices reduce customer interest, the coming landscape is clear. The same applies to Japan and also Korea, albeit some distance behind. In China, OEMs have a purely local focus and [unless it's in terms of] joint-venture activities with western OEMs, they will still offer naturally aspirated engines for the foreseeable future—not least of all for cost reasons. But short-term changes in the law—especially in terms of emission and consumption requirements—are not uncommon in China, so it may be that also the local manufacturers turn toward turbo engines more rapidly.”
Besides direct boosting, another tantalizing prospect is the electrically driven supercharger, which takes the form of a centrifugal compressor. Audi will almost certainly be first into production with this technology (although, after losing its laser-light tit-for-tat with BMW, we may be wrong), and it has been parading prototypes of both single- and twin-turbo diesel versions using electrically assisted forced induction for some time. These don’t work like an electric motor that’s incorporated with the transmission; instead, an electric motor acts on a compressor to force-feed the engine or another turbo with air—Volvo and Audi have solutions where the electric supercharger directs air into multiple exhaust-driven turbos. (Here’s our detailed rundown on how these sorts of systems work.) As for now, though, it’s unclear whether this method will eventually supplant conventional turbochargers or merely complement them. Because of the turbocharger’s thermal efficiency, we’re betting on the latter, with perhaps the Formula 1–style setup—a common shaft for the turbine, impeller, and electric motor—becoming most prevalent.
Both BMW and Daimler think the two technologies are complementary. “No, they don’t challenge each other,” Daimler says. “Electric and mechanical turbos ideally [work together]. Electric turbos support the mechanical ones, especially in the low range; the upper range is suited for the [super]charger.”
But, aside from Toyota and NA-engine devotee Honda—who, it must be said, is prepping turbocharged four-cylinder engines for production later this year—there is little to no evidence of anybody laying down sketches or resources to design a new, clean-sheet naturally aspirated engine. There might be some upgrading of existing hardware, but don’t expect much more than that.